Well I have decided to send my motor to Eric Gorr for the porting and 144 job. It went out Friday. I have sent him an e-mail for him to call me at his discretion to discuss what porting would be best for what I am looking for, and if I should opt to have the head modded for race gas. I do want to speak to him about this but I am hoping that some people that have had these things don't to their motors could comment. I am looking for an increase in low - mid end, but I do not want to loose the top end or over rev. Would the low-mid port job make my bike feel like a stock 95 kdx200 (I don't want that). Or, would the more everywhere port job increase the bottom and mid enough for me to be happy with my investment. As for the race gas question, I have year round access to sunoco cam2 110 oct leaded. Can I use this gas? Will this gas decrease the reliability of the motor and increase top end replacement intervals? Can I leave the gas in a can (unmixed) for upto 1 month? And finally, does race gas increase performance enough to have the extra inconvenience of getting it. Thanks in advance, and I look forward to hearing from you guys.
What style of porting and use race gas?
- Thread starter burgunder
- Start date
robwbright
Member
- Apr 8, 2005
- 2,283
- 0
Eric told me the difference between head work/race gas and no head work/pump gas is about 1 HP. Your total gain will probably be in the 6-8 HP range. I figured that since I was spending almost $500 on the motor - and I only use 2-4 gallons a week, why not get the head work.
I have run AVGAS 100LL (briefly because I didn't have race gas on hand), 105 and 110 in the 144 with no detonation issues that I've noticed. I wouldn't recommend the AVGAS, but 105-110 should be fine. The octane isn't so much the issue - you will want to look into distillation curves and all sorts of other stuff.
Low-mid porting is generally not recommended for 125s - you're not going to get much actual "low end" no matter what you do - there's just not enough displacement even at 144cc.
Mid-top is for experts. I'm not an expert, so I went with mo betta everywhere. The best way I can describe it is pretty much the same power spread as stock, but 20% more everywhere.
The link to my review of the 144 after I got the jetting pretty close:
http://www.dirtrider.net/forums3/showthread.php?t=126800
First race report:
http://www.dirtrider.net/forums3/showthread.php?t=133418
Here's a thread on fuel you might find useful.
http://dirtrider.net/forums3/showthread.php?t=125942
And there are a couple articles from Fuel genius Rich Rohrich. I don't have the links anymore, so I'm pasting in the next response. . .
See attached pic showing distillation curve of race gas vs. pump gas - it's obvious why the race gas makes more power. . .
And dyno chart of a CR133 vs stock CR125. The 133/134 and the 144 make similar peak HP, but the 144s have a bit more bottom.
I have run AVGAS 100LL (briefly because I didn't have race gas on hand), 105 and 110 in the 144 with no detonation issues that I've noticed. I wouldn't recommend the AVGAS, but 105-110 should be fine. The octane isn't so much the issue - you will want to look into distillation curves and all sorts of other stuff.
Low-mid porting is generally not recommended for 125s - you're not going to get much actual "low end" no matter what you do - there's just not enough displacement even at 144cc.
Mid-top is for experts. I'm not an expert, so I went with mo betta everywhere. The best way I can describe it is pretty much the same power spread as stock, but 20% more everywhere.
The link to my review of the 144 after I got the jetting pretty close:
http://www.dirtrider.net/forums3/showthread.php?t=126800
First race report:
http://www.dirtrider.net/forums3/showthread.php?t=133418
Here's a thread on fuel you might find useful.
http://dirtrider.net/forums3/showthread.php?t=125942
And there are a couple articles from Fuel genius Rich Rohrich. I don't have the links anymore, so I'm pasting in the next response. . .
See attached pic showing distillation curve of race gas vs. pump gas - it's obvious why the race gas makes more power. . .
And dyno chart of a CR133 vs stock CR125. The 133/134 and the 144 make similar peak HP, but the 144s have a bit more bottom.
robwbright
Member
- Apr 8, 2005
- 2,283
- 0
Fuel Terminology
by Rich Rohrich
Air/Fuel Ratio - The ratio of pounds of Air to pounds of Fuel needed for combustion in an engine. Air/Fuel ratio is based on pounds of AIR to pounds of FUEL but carbs are metered (jetted) by volume so changes in fuel can change A/F ratios. A/F Ratios range from about 2:1 for NitroMethane to about 16:1 for gasoline, with 14.7:1 considered the stoichiometric or chemically correct ratio under perfect conditions with normal (non-oxygenated) gasoline. Gasoline A/F ratios for best power tend to be in the 13.25:1 - 13.75:1 range.
Anti Knock Index or AKI - The octane number you see at the pumps in the US equal to (R+M) /2 . This is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON) .
ASTM - "American Society for Testing and Materials" This organization is widely recognized as the authority on specifications for petroleum products.
Auto Ignition Temperature - The temperature at which a combustible mixture will ignite on its own. This is sometimes called the "Spontaneous Combustion Point". This is closely associated with detonation.
British Thermal Unit - BTUs are used to measure the energy content of a fuel, usually in BTU/lb. The higher the BTU value the greater the potential energy available. Gasoline is in the range of 20,200 BTUs while Methanol has an energy content of only 9,700 BTUs.
Detonation - An uncontrolled pressure rise and the associated heat rise caused by the reacting of the fuel in the combustion chamber. This is usually associated with the pinging sound you here from engines as multiple flame fronts collide in the combustion chamber.
Flame Front - The advancing of fuel reacting from a source of ignition. During normal combustion there is only one flame front which advances from the spark plug to the outer edges of the combustion chamber. During abnormal combustion multiple flame fronts can start, which leads to detonation.
Flame Speed - Flame speed or burn time is the time in milliseconds from 10% to 90% fuel burned. Flame Speed is a function of fuel chemistry, and engine design . The component make up of a racing fuel will influence the burn time whether it's a high octane fuel or not. Racing fuels usually have the Flame Speed adjusted based on the intended application (i.e. engine rpm, bore size , etc.)
Hydrocarbons - The building blocks of gasoline made up of chemical compounds consisting of hydrogen and carbon atoms only . Gasoline is made up of a combination hydrocarbons with different molecular weights and structure. The various components that make up gasoline are classified according to the number of carbon atoms in the molecules. These components form chemical groups called Aromatics, Paraffins, and Olefins. Aromatics are the components that most people are familiar with. They include compounds like Toluene, Benzene, Aniline, Benzene, and Xylene among others. They are commonly used to increase octane and to raise BTU content. They can often times be found in commercial octane boosters. The amount of energy a gallon of gas will produce is a function of the proper combination of Aromatics, Paraffins, and Olefins. These combinations are designed to produce a specific distillation curve depending on the application.
They fall into three basic groups:
Light Fractions - which vaporize from 85 to 130 degrees(F)
Medium Fractions - which vaporize from 130 to 250 degrees(F)
Heavy Fractions - which vaporize from 250 to 400 degrees(F).
Octane Number - A measure of the anti-knock characteristics of a given fuel. The octane number you see at the pumps is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON). This number is sometimes referred to as the Anti Knock Index or AKI. Each of these octane numbers is determined by ASTM laboratory tests. Low-speed and low load knock characteristics are determined by the RON test method. The MON method tests high-speed, high load, high temperature situations, in practice these conditions exist during periods of high speed power accelerations, hill climbing, or any period of high power output (sounds like racing to me). Obviously the MON number will be lower, but it's the one racers should concern themselves with.
Oxygenates - Oxygen bearing chemicals that can be added to fuel that bring additional oxygen to the combustion process. Currently two types of oxygenated compounds - ethers and alcohols - are used in gasoline at levels higher than 2 percent.
The ethers generally consist of :
TAME - Tertiary Amyl Methyl Ether
MTBE – Methyl Tertiary Butyl Ether
ETBE – Ethyl Tertiary Butyl Ether
While the alcohols are generally :
Methanol - (MeOH)
Ethanol – (EtOH)
Isopropanol – (IPA)
t-Butanol – (TBA)
Mixed C1 to C5 alcohols
Nitromethane, Nitropropane #2, Propylene Oxide, Piric Acid are also used for their power producing qualities. All of these compounds vary in the oxygen content from a high of 50% for methanol to a low of about 15% for TAME. Some of these compounds are better than others at producing additional power and are very often used as gasoline extenders. Oxygenates are also being used in an attempt to reduce exhaust emissions.
Pre-Ignition - The starting of ignition by any source other than the spark plug before normal firing of the spark plug.
Reformulated Gas – RFG - The 1990 Clean Air Act (Act) requires the Environmental Protection Agency (EPA) to issue regulations that would require gasoline to be "reformulated" so as to result in significant reductions in vehicle emissions of ozone-forming and toxic air pollutants. This cleaner gasoline is called reformulated gasoline (RFG). RFG is required to be used in nine major metropolitan areas of the United States with the worst ozone air pollution problems. In addition, several other areas with ozone levels exceeding the public health standard have voluntarily chosen to use RFG . The major differences between RFG and conventional gasolines are :
RFG has lower levels of certain compounds that contribute to air pollution - notably Benzene.
RFG will not evaporate as easily as conventional gasoline – lower RVP.
RFG will contain "chemical oxygen" (oxygenates) – causing lower stoichiometric Air/Fuel ratio
Reid Vapor Pressure (RVP) - A standard indicator of gasoline volatility or how quickly a fuel evaporates. Oil companies vary RVP seasonally to correspond to the weather. Gasolines have an RVP range of 5 to 15. High RVP fuels are used in cold climates for easier starting, because they will tend to evaporate easily. But a high RVP fuel can easily vapor lock in hot weather, or under most racing conditions, regardless of the air temperature. Most racing fuels have low RVP ratings that are fixed year round, usually in the 5.5 to 8.0 range although some special purpose fuels have RVP set as low as 1.5.
Relative Air Density - RAD - A working number based on barometric pressure, air temperature, altitude and humidity. RAD is used by tuners to help determine the jetting changes required by changes in weather or air temperature. Although usually measured with a RAD gauge, it is possible to calculate relative air density using a chart or calculator.
Sensitivity – The difference in octane numbers between RON and MON. A high sensitivity fuel would have an MON much lower than it's RON. This is a general indicator of the sensitivity of the fuel to changes in severity of the engine operating conditions. Standard US pump fuel usually has a sensitivity of 10, so a 92 octane US unleaded pump fuel will usually have an RON of 97 but an MON of only 87.
Spark Lead Time - The time before TDC needed to fire the spark plug to ensure complete combustion of the trapped mixture. Different fuels and engine setups will require different spark lead times. As a general rule, the more efficient the combustion the less lead time (timing) the engine will require. This is especially true of high compression four stroke engines.
Specific Gravity - A measure of the density of a liquid relative to the density of water, with water having a specific gravity of 1.0. Given the fact that Air/Fuel ratio is based on pounds of AIR to pounds of FUEL, but carbs are metered (jetted) by volume changing the specific gravity of your fuel can have a profound affect on the A/F ratio of your engine. In short you've changed the jetting by changing the density of the fuel. Octane boosters and alcohol based fuels can change the density of the fuel a large degree.
Surface-Ignition – The igniting of the fuel air mix by any source other than the spark plug before or after normal firing of the spark plug. This is usually caused by hot spots in the chamber like, carbon deposits, too hot a plug.
Volatility - The tendency of a fuel to evaporate {to change from a liquid to a gas state}. This is one of the most fundamentally important qualities of fuel in carbureted engines because it has a major influence on the vapor-air ratio in the cylinders at the time of ignition. The greater (higher) the volatility of a fuel the greater the tendency to evaporate. In a normal engine nearly all the fuel needs to be evaporated before ignition. So for any Engine/Air Temperature combination there is a minimum volatility that is required for proper running.
Volatility Curve - Also know as the "Distillation Curve". The ASTM Distillation test provides a measure, in terms of volatility, of the relative proportions of all the hydrocarbon components of a gasoline. The ASTM distillation curve designates the maximum temperatures at which 10%, 50%, and 90% of the fuel will be evaporated as well as the maximum end point temperature. These distillation characteristics, define and control, starting, warm-up, acceleration, vapor lock, and crankcase dilution. The significance of any point on the ASTM volatility curve depends on the temperature range in question. Under low temperature conditions as in cold weather starting, when only a small portion of the fuel is evaporated, the low temperature end of the curve is the most. While for higher temp ranges like those in the intake of a hot engine the shape of the low temp end of the curve is much less important since all of this part of the fuel is evaporated anyway. Oil companies vary the 10% distillation point seasonally to correspond to the weather. Different parts of the country will have their fuels blended to compensate for regional elevations as well. Most Racing fuels have Distillation curves that fall into a much more narrow temperature range that is held consistent year-round.
Vapor - Liquid (V/L) Ratio - The ratio of volume of vapor formed at atmospheric pressure to the volume of gasoline . The V/L ratio increases with temperature for any given gasoline. The temperature at which the maximum V/L ratio is specified for each ASTM volatility class is based on the air temperatures and the altitude associated with the use of the class. V/L ratio for Avgas will be much different than the ratio used for Racing or Pump gas.
Vapor Lock - Rapid formation of vapor in the fuel lines or carb, that causes a restriction in flow. Vapor formation begins to occur in fuel lines, pumps, etc. when the fuel reaches a temperature where the vapor pressure of the fuel is equal to the pressure in the system. Gravity feed fuel systems (positive pressure) tend to be less prone to vapor lock than snowmobile type vacuum pump systems where negative pressures exist.
by Rich Rohrich
Air/Fuel Ratio - The ratio of pounds of Air to pounds of Fuel needed for combustion in an engine. Air/Fuel ratio is based on pounds of AIR to pounds of FUEL but carbs are metered (jetted) by volume so changes in fuel can change A/F ratios. A/F Ratios range from about 2:1 for NitroMethane to about 16:1 for gasoline, with 14.7:1 considered the stoichiometric or chemically correct ratio under perfect conditions with normal (non-oxygenated) gasoline. Gasoline A/F ratios for best power tend to be in the 13.25:1 - 13.75:1 range.
Anti Knock Index or AKI - The octane number you see at the pumps in the US equal to (R+M) /2 . This is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON) .
ASTM - "American Society for Testing and Materials" This organization is widely recognized as the authority on specifications for petroleum products.
Auto Ignition Temperature - The temperature at which a combustible mixture will ignite on its own. This is sometimes called the "Spontaneous Combustion Point". This is closely associated with detonation.
British Thermal Unit - BTUs are used to measure the energy content of a fuel, usually in BTU/lb. The higher the BTU value the greater the potential energy available. Gasoline is in the range of 20,200 BTUs while Methanol has an energy content of only 9,700 BTUs.
Detonation - An uncontrolled pressure rise and the associated heat rise caused by the reacting of the fuel in the combustion chamber. This is usually associated with the pinging sound you here from engines as multiple flame fronts collide in the combustion chamber.
Flame Front - The advancing of fuel reacting from a source of ignition. During normal combustion there is only one flame front which advances from the spark plug to the outer edges of the combustion chamber. During abnormal combustion multiple flame fronts can start, which leads to detonation.
Flame Speed - Flame speed or burn time is the time in milliseconds from 10% to 90% fuel burned. Flame Speed is a function of fuel chemistry, and engine design . The component make up of a racing fuel will influence the burn time whether it's a high octane fuel or not. Racing fuels usually have the Flame Speed adjusted based on the intended application (i.e. engine rpm, bore size , etc.)
Hydrocarbons - The building blocks of gasoline made up of chemical compounds consisting of hydrogen and carbon atoms only . Gasoline is made up of a combination hydrocarbons with different molecular weights and structure. The various components that make up gasoline are classified according to the number of carbon atoms in the molecules. These components form chemical groups called Aromatics, Paraffins, and Olefins. Aromatics are the components that most people are familiar with. They include compounds like Toluene, Benzene, Aniline, Benzene, and Xylene among others. They are commonly used to increase octane and to raise BTU content. They can often times be found in commercial octane boosters. The amount of energy a gallon of gas will produce is a function of the proper combination of Aromatics, Paraffins, and Olefins. These combinations are designed to produce a specific distillation curve depending on the application.
They fall into three basic groups:
Light Fractions - which vaporize from 85 to 130 degrees(F)
Medium Fractions - which vaporize from 130 to 250 degrees(F)
Heavy Fractions - which vaporize from 250 to 400 degrees(F).
Octane Number - A measure of the anti-knock characteristics of a given fuel. The octane number you see at the pumps is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON). This number is sometimes referred to as the Anti Knock Index or AKI. Each of these octane numbers is determined by ASTM laboratory tests. Low-speed and low load knock characteristics are determined by the RON test method. The MON method tests high-speed, high load, high temperature situations, in practice these conditions exist during periods of high speed power accelerations, hill climbing, or any period of high power output (sounds like racing to me). Obviously the MON number will be lower, but it's the one racers should concern themselves with.
Oxygenates - Oxygen bearing chemicals that can be added to fuel that bring additional oxygen to the combustion process. Currently two types of oxygenated compounds - ethers and alcohols - are used in gasoline at levels higher than 2 percent.
The ethers generally consist of :
TAME - Tertiary Amyl Methyl Ether
MTBE – Methyl Tertiary Butyl Ether
ETBE – Ethyl Tertiary Butyl Ether
While the alcohols are generally :
Methanol - (MeOH)
Ethanol – (EtOH)
Isopropanol – (IPA)
t-Butanol – (TBA)
Mixed C1 to C5 alcohols
Nitromethane, Nitropropane #2, Propylene Oxide, Piric Acid are also used for their power producing qualities. All of these compounds vary in the oxygen content from a high of 50% for methanol to a low of about 15% for TAME. Some of these compounds are better than others at producing additional power and are very often used as gasoline extenders. Oxygenates are also being used in an attempt to reduce exhaust emissions.
Pre-Ignition - The starting of ignition by any source other than the spark plug before normal firing of the spark plug.
Reformulated Gas – RFG - The 1990 Clean Air Act (Act) requires the Environmental Protection Agency (EPA) to issue regulations that would require gasoline to be "reformulated" so as to result in significant reductions in vehicle emissions of ozone-forming and toxic air pollutants. This cleaner gasoline is called reformulated gasoline (RFG). RFG is required to be used in nine major metropolitan areas of the United States with the worst ozone air pollution problems. In addition, several other areas with ozone levels exceeding the public health standard have voluntarily chosen to use RFG . The major differences between RFG and conventional gasolines are :
RFG has lower levels of certain compounds that contribute to air pollution - notably Benzene.
RFG will not evaporate as easily as conventional gasoline – lower RVP.
RFG will contain "chemical oxygen" (oxygenates) – causing lower stoichiometric Air/Fuel ratio
Reid Vapor Pressure (RVP) - A standard indicator of gasoline volatility or how quickly a fuel evaporates. Oil companies vary RVP seasonally to correspond to the weather. Gasolines have an RVP range of 5 to 15. High RVP fuels are used in cold climates for easier starting, because they will tend to evaporate easily. But a high RVP fuel can easily vapor lock in hot weather, or under most racing conditions, regardless of the air temperature. Most racing fuels have low RVP ratings that are fixed year round, usually in the 5.5 to 8.0 range although some special purpose fuels have RVP set as low as 1.5.
Relative Air Density - RAD - A working number based on barometric pressure, air temperature, altitude and humidity. RAD is used by tuners to help determine the jetting changes required by changes in weather or air temperature. Although usually measured with a RAD gauge, it is possible to calculate relative air density using a chart or calculator.
Sensitivity – The difference in octane numbers between RON and MON. A high sensitivity fuel would have an MON much lower than it's RON. This is a general indicator of the sensitivity of the fuel to changes in severity of the engine operating conditions. Standard US pump fuel usually has a sensitivity of 10, so a 92 octane US unleaded pump fuel will usually have an RON of 97 but an MON of only 87.
Spark Lead Time - The time before TDC needed to fire the spark plug to ensure complete combustion of the trapped mixture. Different fuels and engine setups will require different spark lead times. As a general rule, the more efficient the combustion the less lead time (timing) the engine will require. This is especially true of high compression four stroke engines.
Specific Gravity - A measure of the density of a liquid relative to the density of water, with water having a specific gravity of 1.0. Given the fact that Air/Fuel ratio is based on pounds of AIR to pounds of FUEL, but carbs are metered (jetted) by volume changing the specific gravity of your fuel can have a profound affect on the A/F ratio of your engine. In short you've changed the jetting by changing the density of the fuel. Octane boosters and alcohol based fuels can change the density of the fuel a large degree.
Surface-Ignition – The igniting of the fuel air mix by any source other than the spark plug before or after normal firing of the spark plug. This is usually caused by hot spots in the chamber like, carbon deposits, too hot a plug.
Volatility - The tendency of a fuel to evaporate {to change from a liquid to a gas state}. This is one of the most fundamentally important qualities of fuel in carbureted engines because it has a major influence on the vapor-air ratio in the cylinders at the time of ignition. The greater (higher) the volatility of a fuel the greater the tendency to evaporate. In a normal engine nearly all the fuel needs to be evaporated before ignition. So for any Engine/Air Temperature combination there is a minimum volatility that is required for proper running.
Volatility Curve - Also know as the "Distillation Curve". The ASTM Distillation test provides a measure, in terms of volatility, of the relative proportions of all the hydrocarbon components of a gasoline. The ASTM distillation curve designates the maximum temperatures at which 10%, 50%, and 90% of the fuel will be evaporated as well as the maximum end point temperature. These distillation characteristics, define and control, starting, warm-up, acceleration, vapor lock, and crankcase dilution. The significance of any point on the ASTM volatility curve depends on the temperature range in question. Under low temperature conditions as in cold weather starting, when only a small portion of the fuel is evaporated, the low temperature end of the curve is the most. While for higher temp ranges like those in the intake of a hot engine the shape of the low temp end of the curve is much less important since all of this part of the fuel is evaporated anyway. Oil companies vary the 10% distillation point seasonally to correspond to the weather. Different parts of the country will have their fuels blended to compensate for regional elevations as well. Most Racing fuels have Distillation curves that fall into a much more narrow temperature range that is held consistent year-round.
Vapor - Liquid (V/L) Ratio - The ratio of volume of vapor formed at atmospheric pressure to the volume of gasoline . The V/L ratio increases with temperature for any given gasoline. The temperature at which the maximum V/L ratio is specified for each ASTM volatility class is based on the air temperatures and the altitude associated with the use of the class. V/L ratio for Avgas will be much different than the ratio used for Racing or Pump gas.
Vapor Lock - Rapid formation of vapor in the fuel lines or carb, that causes a restriction in flow. Vapor formation begins to occur in fuel lines, pumps, etc. when the fuel reaches a temperature where the vapor pressure of the fuel is equal to the pressure in the system. Gravity feed fuel systems (positive pressure) tend to be less prone to vapor lock than snowmobile type vacuum pump systems where negative pressures exist.
robwbright
Member
- Apr 8, 2005
- 2,283
- 0
Fuel For Thought
By Rich Rohrich
Part 1 - The Basics
It's getting so you can't read the newsgroups or walk through the pits at a racetrack without bumping into a group of racers discussing fuel, and the problems associated with it. It's obvious to everyone that the fuel we use is a vital link in the performance chain, yet it is surrounded by more than the usual share of myth and misinformation. Given the sweeping changes on pump fuels mandated by the government, and the vast number of race gas offerings the subject is a hot topic. Through this series of articles I hope to shed a little light and hopefully squish a myth or two along the way.
What does the Octane Number at the pump mean?
The octane rating of a fuel is what most people are familiar with, but there seems to be a lot of confusion surrounding it. In simple terms the octane number you see at the pump is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON) or (RON + MON) / 2. This final octane number is sometimes referred to as the Anti Knock Index or AKI. This pump octane number is a measure of the anti- knock characteristics of a given fuel.
MON and RON are determined by standardized ASTM laboratory tests. The details of the tests are not as important as what they mean in terms of performance. Low to medium-speed knock characteristics are determined by the Research (RON) method, while high-speed and partial throttle heavy load knock characteristics are determined by the Motor (MON) method. MON testing is conducted under more stringent conditions with the timing on the test engine advanced and run with a higher inlet air temperature, so the MON number tends to be lower but also more valid for high-performance applications. There are a number of more valid tests that have been developed to determine the anti-knock characteristics of fuels used in high performance engines, but the aren't in general use at this point so we are stuck with the old reliable pump octane number.
So what's that knocking sound coming from my engine?
The Knocking sound you hear when your engine is in trouble are the result of abnormal combustion. The most common combustion problems are detonation and pre-ignition. In simple terms detonation is the uncontrolled burning of the fuel in the combustion chamber, while pre-ignition can be defined as the starting of the burning process by any source other than the spark plug usually before the plug has fired.
To truly understand what detonation is, its important to understand that if you raise the temperature of any combustible mixture high enough, it will ignite on its own. This is sometimes called the "spontaneous combustion point" or the "auto ignition temperature". Detonation is a rapid uncontrolled rise in cylinder pressure caused by all or part of the fuel mixture reaching this auto-ignition temperature.
Following the ignition process through a cycle should help complete the explanation. As the piston rises and compresses the trapped mixture the pressure and temperature begins to rise. The spark plug fires somewhere before the piston reaches Top Dead Center (TDC) and starts burning the compressed mixture in the cylinder and as a consequence raises the combustion chamber temperature. While this burning is taking place, the piston is still rising and still compressing the air/fuel mixture which raises the cylinder pressure, and combustion chamber temperature even higher.
At this point, the pressure rise in the cylinder is very rapid, but it generally proceeds at a fairly even controlled rate. The remaining unburned mixture and the end gases at the edges of the combustion chamber are being raised to extremely high temperatures as the advancing flame front compresses and heats up the mixture directly in front of it. This activity before the flame front reaches the end gases at the edge of the chamber are sometimes called pre-flame reactions. The longer it takes for the complete burning to take place the greater the chances that these pre-flame reactions will force the end gases to reach the auto ignition point and cause a rapid uncontrolled pressure rise, along with a huge increase in cylinder temperature. If brought to the auto ignition point the end gases of the combustion chamber can cause a pressure and frequency rise that is high enough to be audible. That's the KNOCK or PING that you hear. Ideally, the burning of the mixture will be completed before any of these end gases have an opportunity to reach the point of auto ignition. If the ignition timing is set correctly this should happen around 15-20 degrees After Top Dead Center (ATDC).
It's hard to visualize the immense pressures we are talking about in the combustion chamber. In a normal combustion cycle the pressures can easily reach 100 times the trapped compression ratio, that's 800-1300 psi banging away at the piston crown and cylinder head and bearings. Once an engine starts to detonate the pressures can reach 3 to 4 times that high. The pressure rise during detonation can be almost instantaneous, so it's easy to see why the edges of the piston can be broken away during these cycles. It's like having a small bomb go off in the engine.
As you may have guessed from the earlier discussion of octane numbers, high octane fuels have a considerably higher auto ignition temperature to keep these pre-flame reactions from causing sudden uncontrolled pressure rises. If the charge burns fast enough or the fuel is resistant enough to auto ignition (high octane) then all is well and the pressure rise isn't too extreme. Hopefully it should be fairly clear that if you can shorten the burn time (10% to 90% burned) enough then the octane requirement of the engine will be reduced. As a general rule, the first half of combustion 0-50% burned, speeds up in direct proportion to rpm, while the 50-100% burned time speeds up exponentially with rpm. So all other things equal, the faster you spin an engine, the faster the charge will burn and the more knock resistant the engine will be. Small bore, high rpm motors are by design, very knock resistant.
We defined pre-ignition previously as the starting of the burning process by a source other than the plug. This has the same effect as advancing the timing. This causes the engine to be subjected to huge amounts of heat, because the piston and cylinder walls are subjected to the burning process for a longer period of time, this in turn raises the combustion chamber pressure. It's possible for pre-ignition to melt the top of pistons because of the extreme temperatures that this advanced timing causes. Keep in mind that any time you raise the temperature in the cylinder you get a corresponding rise in pressure , or conversely raising the pressure also raises the temperature. So it's easy to see how pre-ignition and detonation are very closely linked.
So it pretty much boils down to this. If you can control the pressure and temperature in the combustion chamber , things will go along with out too many problems. But once you cross that temperature/pressure threshold a number of interrelated actions can take place that causes all hell to break lose. High octane fuel is one way to keep the carnage in check.
How much octane do we need?
Cylinder pressure is one of the key factors in determining the octane requirement of an engine. Intake valve closing time on four-stroke engines, and exhaust timing on two-strokes will have a major influence on the dynamic cylinder pressure . It's a commonly held misconception that higher Octane fuel slows down the flame speed which keeps the engine from knocking. Flame speed is a function of fuel chemistry, not the Octane rating. The component make up of the fuel will determine the flame speed whether it's a high octane fuel or not. . Racing fuels designed for high rpm applications tend to have higher flame speeds than normal to help reduce burn time. There isn't much time available to complete the combustion cycle at 10,000rpm, so choosing the right fuel can really make a difference. Choosing a faster burning fuel will allow you to run less ignition advance, and ultimately make more power at higher revs.
Every engine can have radically different requirements. Even two similarly modified engines can have requirements as different as 5 to 8 MON numbers in some cases. The factors affecting octane requirements should be of great interest to every racer. By changing these factors around you can raise or lower the octane requirement of your engine. Some of the more obvious factors are :
D E S I G N F A C T O R S O P E R A T I N G C O N D I T I O N S
Compression ratio Outside air temperature/ Intake air temperature
Trapped Compression Ratio Altitude
Ignition timing Humidity
Combustion chamber shape Barometric pressure
Charge Motion in the cylinder Premix ratio (two-strokes)
Air/fuel ratio Engine RPM
Cooling efficiency Engine load
Scavenging efficiency
Spark plug location
Spark plug heat range
In looking at this list, it should be apparent to you that a number of these factors/conditions are pretty much out of our control. We will concentrate on those that are more easily changed.
Is it better to raise fuel octane or lower the octane requirement of the engine? The answer to that question is yes. You want to lower the octane requirement of the engine as much as possible without lowering engine performance. You also want to use a fuel with an octane rating just high enough to keep your engine from ever detonating.
Engine timing is one of the factors on the list, but on most modern engines the timing is fixed or electronically controlled. You have more to lose than gain if you play with the timing of these ignitions, it's best to leave them alone.
Another way to fend off detonation is to make sure the engine's cooling system works as efficiently as possible. That means clean radiators at all times, and unrestricted airflow across the cooling fins on air cooled engines. It's important to keep the water temperature between 50 and 70 degrees Centigrade. This range produces the most power while keeping the octane requirement low.
Most two-cycle engines have a squish band machined at the outer edge of the cylinder head, this band serves two purposes . The close proximity of the piston to the cylinder head helps to cool (or quench) the end gases that were heated by the pre-flame reactions, but more importantly the squish band helps to add motion to the burning charge, which helps speed up the burning and limits the amount of time available for pre-flame reactions to heat the end gases. A fast burning chamber will tend to be very resistant to detonation . This is part of the reason you've seen manufacturers switching from domed pistons, to flat top pistons and back again. They are always looking for that magic combination of scavenging efficiency and charge motion. Unfortunately, on most stock engines that squish band doesn't serve much purpose. In theory squish bands work very well, but production line tolerances leave squish clearances so great that the only thing being squished is horsepower. Cutting the cylinder head to bring the squish clearance into spec while still retaining the original cylinder head shape and volume is not the easiest thing to do, though it will definitely pay dividends .
Four-stroke engines usually have a quench area designed into the cylinder head to aid in lowering the temperature of the end gases, while charge motion is often generated as a combination of intake port velocity, port angles, piston shape and combustion chamber shape. We'll go into the specific interactions in a future part of this series. Large bore engines tend to have a higher octane requirement then smaller bore engines given the same compression ratio, because of the longer distance the flame front has to travel and the additional time available for pre-flame reactions to take place. It is possible to use two spark plugs per cylinder to lower your engine's octane needs in large bore engines. Adding a second spark plug shortens the fuel burn time and decreases the distance the flame front needs to travel.
One of the single most important things you can do to lower the octane requirement and save your engine some self destruction via the detonation express is to LEARN TO JET! Running too lean causes excessive heat build up that can pave the way for Detonation and Pre-ignition. Check out the Technical Articles section for a good jetting tutorial.
Benzene Toluene
This all brings us back to the octane question : How much do you need. We'll go into more detail in Part II of the series but here are a few suggestions to get you started:
Start simple and work your way up. Try a good grade of premium gas that doesn't contain ANY ALCOHOL. In most states they will have a sign on the side of the pump warning you about the percentage of alcohol (ethanol or methanol) in the fuel. Most well modified normally aspirated engines can run on 95 -100 octane gasoline. Good porting with flow matched transfer ports can significantly lower octane requirement on two cycle engines. If your engine detonates try one (or all) of the measures to lower the octane requirement of the engine. If all these measures fail, try mixing pump gas 50/50 with a good of good quality race gas (Phillips 66, Power Mist, UNION 76, Sunoco, VP, ELF, etc..) with your gasoline. Make sure you use race gas specifically designed for you your type of application. The fuel manufacturers can make recommendations based on your engines rpm range, bore size, and the type of riding you'll be doing. It's best to stay away from AvGas for your bike. We'll go into the specifics of AvGas in the next installment. Keep in mind that octane requirement is lower at high altitude and high humidity. An engine that ran fine at 10,000 feet could very easily detonate at sea-level, or a sudden drop in humidity on a hot day can cause knocking that never appeared before.
By Rich Rohrich
Part 1 - The Basics
It's getting so you can't read the newsgroups or walk through the pits at a racetrack without bumping into a group of racers discussing fuel, and the problems associated with it. It's obvious to everyone that the fuel we use is a vital link in the performance chain, yet it is surrounded by more than the usual share of myth and misinformation. Given the sweeping changes on pump fuels mandated by the government, and the vast number of race gas offerings the subject is a hot topic. Through this series of articles I hope to shed a little light and hopefully squish a myth or two along the way.
What does the Octane Number at the pump mean?
The octane rating of a fuel is what most people are familiar with, but there seems to be a lot of confusion surrounding it. In simple terms the octane number you see at the pump is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON) or (RON + MON) / 2. This final octane number is sometimes referred to as the Anti Knock Index or AKI. This pump octane number is a measure of the anti- knock characteristics of a given fuel.
MON and RON are determined by standardized ASTM laboratory tests. The details of the tests are not as important as what they mean in terms of performance. Low to medium-speed knock characteristics are determined by the Research (RON) method, while high-speed and partial throttle heavy load knock characteristics are determined by the Motor (MON) method. MON testing is conducted under more stringent conditions with the timing on the test engine advanced and run with a higher inlet air temperature, so the MON number tends to be lower but also more valid for high-performance applications. There are a number of more valid tests that have been developed to determine the anti-knock characteristics of fuels used in high performance engines, but the aren't in general use at this point so we are stuck with the old reliable pump octane number.
So what's that knocking sound coming from my engine?
The Knocking sound you hear when your engine is in trouble are the result of abnormal combustion. The most common combustion problems are detonation and pre-ignition. In simple terms detonation is the uncontrolled burning of the fuel in the combustion chamber, while pre-ignition can be defined as the starting of the burning process by any source other than the spark plug usually before the plug has fired.
To truly understand what detonation is, its important to understand that if you raise the temperature of any combustible mixture high enough, it will ignite on its own. This is sometimes called the "spontaneous combustion point" or the "auto ignition temperature". Detonation is a rapid uncontrolled rise in cylinder pressure caused by all or part of the fuel mixture reaching this auto-ignition temperature.
Following the ignition process through a cycle should help complete the explanation. As the piston rises and compresses the trapped mixture the pressure and temperature begins to rise. The spark plug fires somewhere before the piston reaches Top Dead Center (TDC) and starts burning the compressed mixture in the cylinder and as a consequence raises the combustion chamber temperature. While this burning is taking place, the piston is still rising and still compressing the air/fuel mixture which raises the cylinder pressure, and combustion chamber temperature even higher.
At this point, the pressure rise in the cylinder is very rapid, but it generally proceeds at a fairly even controlled rate. The remaining unburned mixture and the end gases at the edges of the combustion chamber are being raised to extremely high temperatures as the advancing flame front compresses and heats up the mixture directly in front of it. This activity before the flame front reaches the end gases at the edge of the chamber are sometimes called pre-flame reactions. The longer it takes for the complete burning to take place the greater the chances that these pre-flame reactions will force the end gases to reach the auto ignition point and cause a rapid uncontrolled pressure rise, along with a huge increase in cylinder temperature. If brought to the auto ignition point the end gases of the combustion chamber can cause a pressure and frequency rise that is high enough to be audible. That's the KNOCK or PING that you hear. Ideally, the burning of the mixture will be completed before any of these end gases have an opportunity to reach the point of auto ignition. If the ignition timing is set correctly this should happen around 15-20 degrees After Top Dead Center (ATDC).
It's hard to visualize the immense pressures we are talking about in the combustion chamber. In a normal combustion cycle the pressures can easily reach 100 times the trapped compression ratio, that's 800-1300 psi banging away at the piston crown and cylinder head and bearings. Once an engine starts to detonate the pressures can reach 3 to 4 times that high. The pressure rise during detonation can be almost instantaneous, so it's easy to see why the edges of the piston can be broken away during these cycles. It's like having a small bomb go off in the engine.
As you may have guessed from the earlier discussion of octane numbers, high octane fuels have a considerably higher auto ignition temperature to keep these pre-flame reactions from causing sudden uncontrolled pressure rises. If the charge burns fast enough or the fuel is resistant enough to auto ignition (high octane) then all is well and the pressure rise isn't too extreme. Hopefully it should be fairly clear that if you can shorten the burn time (10% to 90% burned) enough then the octane requirement of the engine will be reduced. As a general rule, the first half of combustion 0-50% burned, speeds up in direct proportion to rpm, while the 50-100% burned time speeds up exponentially with rpm. So all other things equal, the faster you spin an engine, the faster the charge will burn and the more knock resistant the engine will be. Small bore, high rpm motors are by design, very knock resistant.
We defined pre-ignition previously as the starting of the burning process by a source other than the plug. This has the same effect as advancing the timing. This causes the engine to be subjected to huge amounts of heat, because the piston and cylinder walls are subjected to the burning process for a longer period of time, this in turn raises the combustion chamber pressure. It's possible for pre-ignition to melt the top of pistons because of the extreme temperatures that this advanced timing causes. Keep in mind that any time you raise the temperature in the cylinder you get a corresponding rise in pressure , or conversely raising the pressure also raises the temperature. So it's easy to see how pre-ignition and detonation are very closely linked.
So it pretty much boils down to this. If you can control the pressure and temperature in the combustion chamber , things will go along with out too many problems. But once you cross that temperature/pressure threshold a number of interrelated actions can take place that causes all hell to break lose. High octane fuel is one way to keep the carnage in check.
How much octane do we need?
Cylinder pressure is one of the key factors in determining the octane requirement of an engine. Intake valve closing time on four-stroke engines, and exhaust timing on two-strokes will have a major influence on the dynamic cylinder pressure . It's a commonly held misconception that higher Octane fuel slows down the flame speed which keeps the engine from knocking. Flame speed is a function of fuel chemistry, not the Octane rating. The component make up of the fuel will determine the flame speed whether it's a high octane fuel or not. . Racing fuels designed for high rpm applications tend to have higher flame speeds than normal to help reduce burn time. There isn't much time available to complete the combustion cycle at 10,000rpm, so choosing the right fuel can really make a difference. Choosing a faster burning fuel will allow you to run less ignition advance, and ultimately make more power at higher revs.
Every engine can have radically different requirements. Even two similarly modified engines can have requirements as different as 5 to 8 MON numbers in some cases. The factors affecting octane requirements should be of great interest to every racer. By changing these factors around you can raise or lower the octane requirement of your engine. Some of the more obvious factors are :
D E S I G N F A C T O R S O P E R A T I N G C O N D I T I O N S
Compression ratio Outside air temperature/ Intake air temperature
Trapped Compression Ratio Altitude
Ignition timing Humidity
Combustion chamber shape Barometric pressure
Charge Motion in the cylinder Premix ratio (two-strokes)
Air/fuel ratio Engine RPM
Cooling efficiency Engine load
Scavenging efficiency
Spark plug location
Spark plug heat range
In looking at this list, it should be apparent to you that a number of these factors/conditions are pretty much out of our control. We will concentrate on those that are more easily changed.
Is it better to raise fuel octane or lower the octane requirement of the engine? The answer to that question is yes. You want to lower the octane requirement of the engine as much as possible without lowering engine performance. You also want to use a fuel with an octane rating just high enough to keep your engine from ever detonating.
Engine timing is one of the factors on the list, but on most modern engines the timing is fixed or electronically controlled. You have more to lose than gain if you play with the timing of these ignitions, it's best to leave them alone.
Another way to fend off detonation is to make sure the engine's cooling system works as efficiently as possible. That means clean radiators at all times, and unrestricted airflow across the cooling fins on air cooled engines. It's important to keep the water temperature between 50 and 70 degrees Centigrade. This range produces the most power while keeping the octane requirement low.
Most two-cycle engines have a squish band machined at the outer edge of the cylinder head, this band serves two purposes . The close proximity of the piston to the cylinder head helps to cool (or quench) the end gases that were heated by the pre-flame reactions, but more importantly the squish band helps to add motion to the burning charge, which helps speed up the burning and limits the amount of time available for pre-flame reactions to heat the end gases. A fast burning chamber will tend to be very resistant to detonation . This is part of the reason you've seen manufacturers switching from domed pistons, to flat top pistons and back again. They are always looking for that magic combination of scavenging efficiency and charge motion. Unfortunately, on most stock engines that squish band doesn't serve much purpose. In theory squish bands work very well, but production line tolerances leave squish clearances so great that the only thing being squished is horsepower. Cutting the cylinder head to bring the squish clearance into spec while still retaining the original cylinder head shape and volume is not the easiest thing to do, though it will definitely pay dividends .
Four-stroke engines usually have a quench area designed into the cylinder head to aid in lowering the temperature of the end gases, while charge motion is often generated as a combination of intake port velocity, port angles, piston shape and combustion chamber shape. We'll go into the specific interactions in a future part of this series. Large bore engines tend to have a higher octane requirement then smaller bore engines given the same compression ratio, because of the longer distance the flame front has to travel and the additional time available for pre-flame reactions to take place. It is possible to use two spark plugs per cylinder to lower your engine's octane needs in large bore engines. Adding a second spark plug shortens the fuel burn time and decreases the distance the flame front needs to travel.
One of the single most important things you can do to lower the octane requirement and save your engine some self destruction via the detonation express is to LEARN TO JET! Running too lean causes excessive heat build up that can pave the way for Detonation and Pre-ignition. Check out the Technical Articles section for a good jetting tutorial.
Benzene Toluene
This all brings us back to the octane question : How much do you need. We'll go into more detail in Part II of the series but here are a few suggestions to get you started:
Start simple and work your way up. Try a good grade of premium gas that doesn't contain ANY ALCOHOL. In most states they will have a sign on the side of the pump warning you about the percentage of alcohol (ethanol or methanol) in the fuel. Most well modified normally aspirated engines can run on 95 -100 octane gasoline. Good porting with flow matched transfer ports can significantly lower octane requirement on two cycle engines. If your engine detonates try one (or all) of the measures to lower the octane requirement of the engine. If all these measures fail, try mixing pump gas 50/50 with a good of good quality race gas (Phillips 66, Power Mist, UNION 76, Sunoco, VP, ELF, etc..) with your gasoline. Make sure you use race gas specifically designed for you your type of application. The fuel manufacturers can make recommendations based on your engines rpm range, bore size, and the type of riding you'll be doing. It's best to stay away from AvGas for your bike. We'll go into the specifics of AvGas in the next installment. Keep in mind that octane requirement is lower at high altitude and high humidity. An engine that ran fine at 10,000 feet could very easily detonate at sea-level, or a sudden drop in humidity on a hot day can cause knocking that never appeared before.
robwbright
Member
- Apr 8, 2005
- 2,283
- 0
burgunder said:
I am no expert on these things, but this is my limited understanding
1: Only in the sense that you're making more power, which I suppose results in more wear. I didn't notice any difference from 2005 (stock) to 2006 (144).
2: Don't know whether it's a good idea or not, but I used it for 3 or 4 months with no problems. I had it in dark blue plastic cans.
3: Again - when you're talking about a 6-8 HP increase and you can add an extra HP, I thought it was worth it. If you want to save a bit of $$, you could mix high test or 100LL AVgas 50/50 with the 110. I've done that, as well.
- Thread starter
- #6
robwbright,
WOW!!!, I really don't know what to say. That is one of the best responces that I have ever recieved. Printing it, and its going in my bike file. I know alot of it you didn't write, but if it weren't for you I wouldn't have read it. Thanks.
I am pretty sure I am going with the "more better"porting, and head modded for race gas.
WOW!!!, I really don't know what to say. That is one of the best responces that I have ever recieved. Printing it, and its going in my bike file. I know alot of it you didn't write, but if it weren't for you I wouldn't have read it. Thanks.
I am pretty sure I am going with the "more better"porting, and head modded for race gas.
robwbright
Member
- Apr 8, 2005
- 2,283
- 0
I do tend to try and be thorough - but I suppose some would say I'm a bit too wordy. . . [inside joke] ;)
No problem - I have that stuff saved on my comp, so it didn't take much time at all. I went through all that about a year ago.
And BTW, as to the wear issue on the crank, Eric's 144 piston only weighs one half ounce more than the stock 125 piston.
Also, FYI, most stock 125s and stock 250f's post about 30-33 HP on the dyno. One of the guys here had an E.G. 144 that put up 39.6 HP. A stock 250 2 stroke is in the mid 40s.
Of course, even a 144 with race gas won't have anywhere near the usable power spread of a stock 250 four stroke. You're still going to have to work harder than them.
No problem - I have that stuff saved on my comp, so it didn't take much time at all. I went through all that about a year ago.
And BTW, as to the wear issue on the crank, Eric's 144 piston only weighs one half ounce more than the stock 125 piston.
Also, FYI, most stock 125s and stock 250f's post about 30-33 HP on the dyno. One of the guys here had an E.G. 144 that put up 39.6 HP. A stock 250 2 stroke is in the mid 40s.
Of course, even a 144 with race gas won't have anywhere near the usable power spread of a stock 250 four stroke. You're still going to have to work harder than them.
- Thread starter
- #8
Thanks again for all the information. As far as the 250s go, I have a 03 250f. It's a great bike but not nearly as fun as my 125. The light weight and handling are incredable. I looked into the 250 2stroke, but for what I paid for the 125, I can have the 144 with everything (guards, pipe, ect.) for less. The 125 just needs a little more "ummpf" for the woods and I think the 144 is perfect. Thanks again.
Last edited by a moderator:
Solid State
Member
- Mar 9, 2001
- 492
- 0
robwbright said:
When I first read this, I got the impression that the 144 was nearly as strong as a stock 250 2 stroke (usable power spread and all). It, of course, is not even close. I like the weight differential, but the power ain't there. Go ahead and post the dyno's just for reference.
robwbright
Member
- Apr 8, 2005
- 2,283
- 0
Solid State said:
The peak power to weight would actually be pretty close at 39.6 HP - especially if the rider were 20 pounds lighter.
Of course, what I said about the 250f. . .
"Of course, even a 144 with race gas won't have anywhere near the usable power spread of a stock 250 four stroke."
. . . would also apply to a 250 2 stroke. The power peak on any 125 or 144 is VERY narrow. Any 250 is much wider.
I'm not sure that I have the 39.6 dyno anywhere, but I believe it was meber marcusgunby's. There are various dynos in here for stock and mod bikes from 85s to 500s:
http://www.dirtrider.net/forums3/showthread.php?t=97637
and here for two 144s and a 139.
http://www.dirtrider.net/forums3/showthread.php?t=89861
Note that dynos vary considerably, so the numbers aren't necessarily consistent. If I recall correctly, Marcus had dynoed stock bikes at 30-33 and got 39+ on the same dyno from his properly tuned 144.
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