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Avation gas?

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Posted by: Buzz Bomb

A local rider told me he buys gas from the local airport for less than a dollar a gallon. I know its not made for dirt bikes, but could you have the head modified specifically for aviation gas? I would think it's more consistent than pump fuel and is a higher octane. The rider's bike seemed very crisp running, too.


Posted by: Rich Rohrich

The question comes up all the time and there are a huge number of silly myths associated with it.

The simple answer is:
100LL (Blue) Avgas seems to be the most readily available version so I'm assuming that's what we are talking about. 100LL Avgas USUALLY isn’t the best choice but it won't hurt anything.

** For those of you in a hurry, or just sick of me rambling on about this crap skip down to the bottom of the thread to the >>>>>> for a summation.

For those of you still with me, here are some details.
Contrary to popular belief this isn't 100-octane fuel. Aviation fuels are rated on an ASTM Lean/Rich performance number system. 100LL is rated at 91/96 By comparison; Unocal Leaded race gas that is used in lots of spec fuel racing classes has performance number of 112/160. 100LL is closer to 91 octane (MON); by comparison VP C12 is rated at 108 (MON).

For our purposes Avgas has a couple of problems:
1) The 90% boiling point for 100LL Blue Avgas is set at 275 degrees F, which in an engine that turns over 7000 rpm will likely make less power than a fuel that has it's 90% point lower. Pump gas has similar problems, but most good race gas will have 90% Point MUCH lower. As an example Phillips B32 has a 90% boiling point around 235 degrees F and VP C12 has a 90% boiling point around 220 degrees F.
2) Depending on the refiner 100LL can have fairly high aromatic hydrocarbon content, in the 30% by weight range. This level of aromatics will tend to make the throttle response mushy and flat in applications that see big throttle opening transitions on a regular basis. It's similar to what happens when you dump a lot of Toluene based octane booster in your fuel. Throttle response becomes a distant memory.

3) The vapor pressure and distillation curve of Avgas just doesn't seem right for our purposes. The distillation curve or Volatility curve of a fuel determines to a large degree the warm-up, transitional (on & off) throttle response, and acceleration characteristics of an engine.
Here's the simplified version:
A fuels distillation curve designates the maximum temperatures at which various points between 10% and 90% of the fuel will be evaporated as well as the maximum end point temperature. So for any Engine/Air Temperature combination there is a minimum volatility that is required for proper running. As you probably know gasoline is made up of different hydrocarbons, with different boiling points. By combining these
Hydrocarbons together you get a Distillation/Volatility curve. Some hydrocarbons (light ends) boil off at low temps some at much higher temps. Depending on the intended application, a petrochemist will blend hydrocarbons to get a curve that matches the rpm range, temp, altitude, and acceleration characteristics for the application. The problem with avgas as a race fuel is the fact it is blended for an application where
Acceleration and throttle response is not a high priority. If you think about the average light airplane application, you're talking about a fairly low compression engine that runs in a fairly narrow rpm band, and is rarely called on to provide the type of transitional throttle response that a high rpm, acceleration critical application like motocross does. What's more important to the Avgas designer is controlling mixture strength by eliminating the possibility of vapor lock and icing while making sure that light end hydrocarbon fractions don't boil off too early. The lowered rpm ranges used in these engines allow them to push the boiling point up on the upper end as well. As you can see, by using straight Avgas or by mixing various types of fuel together you are modifying a number of important fuel design parameters. You may hit on a combination that works well, but more likely you'll have an engine that doesn't detonate, but doesn't accelerate very well either. So Avgas is SAFE, but not a very good choice. The high paraffinic hydrocarbon content of 100LL makes a very good base stock if you want to play back yard petrochemist, and I believe this is how some of the smaller race fuel blenders start out. I can tell you from experience that it's a ***** to document and test various changes unless you have a lot of time and patience, so trying to come up with your own Super Fuel is probably more trouble than it is worth.

So it sounds like Avgas is really bad for our purposes, and for the most part it is, but given the sorry state of pump fuel today, Avgas is looking better all the time.

>>>>>>
Here's my short course take on things based on my experience and personal biases, (keep in mind this is pretty generalized)

- In almost every case 100ll Avgas is a better choice than alcohol pump fuels

- If you don't need the additional octane that 100LL provides, then MTBE based pump premium (especially Amoco) will tend to provide better throttle than Avgas assuming you have any jetting skill. If you can't jet you're just wasting your time worrying about any of this stuff on a stock bike.

- Mixing 100LL Avgas with a good race gas designed for your application and rpm range is a reasonable way to save some money.

- Mixing alcohol based pump fuels with ANYTHING in an attempt to make it BETTER is just a chemical circle jerk, and if you're that cheap or that ignorant you deserve the crummy performance and the insurmountable jetting problems that you will invariably be blessed with.

- Milspec Avgas is a different animal entirely, but isn't readily available so we won't worry about it.

- The correct race fuel for your application will outperform ANY of the above, regardless of whether the engine is stock or modified. The more demon tweaks hiding in your engine, the more you have to gain.


Posted by: KawieKX125

Rich, what is the, "90% boiling point"? Also, is the amoco gas really all that much better? If so, I will maybe try it over the expensive C-12 and see how it performs.


Posted by: Rich Rohrich

Quote:
Originally posted by KawieKX125
Rich, what is the, "90% boiling point"?


It's the temperature at which 90% of the fuel will be evaporated.


Posted by: backtoracin

Buzz Bomb, there is no way your bud is buying 100LL for less than a buck, if he is, I wouldnt put it in my bike, Id start looking for bent airplanes at the end of the runway . Here in Denver, 100LL runs 2.20 out of the self serve. The plane that I fly uses it, and really, to run it in a bike, its pretty crappy. Stay with the race gas, more consistent jetting, better buring fuel, etc. Aviation gas is made for airplanes, remember these aircraft engines that 100LL is designed for are air cooled, and severly outdated in terms of technology, etc. Good luck.


Posted by: spanky250

Avgas is designed for airplanes. I haven't seen any wings out on the track or in the woods lately.


Posted by: TexKDX

Besides its qualities in the bike, I experienced two ways that you can tell race gas is very different from pump gas: one, the smell. I got some on my hands. It evaporated quickly and left no smell. Two, I spilled some on the concrete in the sun. Man did it evaporate quicky, almost like brake cleaner.

No question this stuff is different from pump crap. This is Phillips B32 I am speaking of BTW.


Posted by: Eddy V

Hallo Rich,
I am a motocrosser from belgium riding a RM125 2001.Because the stock compression ratio of this bike is on the limit of detonation i have been running on 50:50 unleaded 98 pump fuel and VP C-12.Now i have no race fuel anymore and it is almost impossible to get VP in Belgium anymore.While i'm loocking for an other race fuel I have two choices.I can use 50:50 unleaded 98 pump fuel and AVgas or i can mix 5 or 10%(or more,tell me your opinion)pure MTBE with the pump fuel.My bike is modified without a raise of CR.Nevertheless i had a serious detonation problem this WE (piston,cylinder ,gearings)Do you have any idee of MSV for this bike and use for motocross.?


Posted by: Rich Rohrich

Quote:
Originally posted by Eddy V
While i'm loocking for an other race fuel I have two choices.I can use 50:50 unleaded 98 pump fuel and AVgas or i can mix 5 or 10%(or more,tell me your opinion)pure MTBE with the pump fuel. My bike is modified without a raise of CR.Nevertheless i had a serious detonation problem this WE (piston,cylinder ,gearings)


Depending on what type of AvGas is available to you it should stop the detonation up to a point. How well it stops detonation will depend in large part on what is causing it in the first place..

Even though the octane rating of MTBE is high, it causes a rapid pressure rise in the combustion chamber so mixing it with standard road fuel is LIKELY to make your detonation problem WORSE.

Quote:
Originally posted by Eddy V

Do you have any idee of MSV for this bike and use for motocross.?


I've never had one apart so I'm afraid I can't be of any help on that. Eric Gorr might be able to help you with this.


Posted by: Malden

I had a detonation problem with my 99RM 125 and Eric said it could be fixed with a simple head mod. He explained the problem to me in detail but i dont remember the specifics. I ended up having the overbore kit done and i never experienced detonation again. I would have the head done before it causes seious damage.


Posted by: Irish MXer

My local airport (Lakeway Local) has a avation club that sells the gas there for what they buy it for. I fill up with the 100LL fuel and I pay about $10 per 5 gallons of gas. I usually bring a 55 gallon industrial size drum to fill up but whatever you use works. I think it is a much better value than filling up with the $6 per gallon c-12. Even though c-12 is some really nice stuff offering greater throttle responce the value on the AV gas is sooooo much better. I mix mine 50:1 with the AMSoil 100:1 pre-mix. For me it offers much more throttle responce than just pump and it smells much much better Hope this helps


Posted by: cujet

From time to time this subject comes up, so I wanted to share some favorable results.

Some of you may remember that I am trying to extend the life of my Husky engine. I have learned that in some applications the use of leaded fuel helps out 2 stroke engines espeically the crankshaft. In addition there is quite a bit of factory and personal experience bearing this out.

So, I have switched to 100% Avgas in my Husky 360WXC. Here is what I think I have learned. BTW, the previous fuel was S. Florida spec Amoco 93.

1, Jetting needs to be leaner all the way around.
2, Throttle response is considerably sharper.
3, Spark plug deposits are siginficant, in other words, I should expect more fouling.
4, Starting is easier, especially when cold.
5, Fuel seems very consistent from week to week, jetting req do not change.
6, Price is good, at $1.80 at the local airport, $2.99 at work (major airport).
7, power is easily on par with other race fuels and Amoco.

Rich makes good points about Avgas, however I believe he is mistaken about the octane. I did the research and found that Avgas 100LL is 100 octane fuel, and compares properly to pump Amoco. Example, 50% 100LL mixed with 50% Amoco 93 will achieve at least 96.5 octane. Possibly better due to the lead response of the unleaded fuel.

Avgas is designed to vaporize at low temps and therefore can cause and is prone to vapor lock. No attempt is made to limit vapor lock with Avgas as vapor lock is not a problem in aviation due to required designs.

Race teams have tested Avgas for years with good results. There are even race fuels that are just repackaged Avgas!

By no means am I claiming it is right for everyone, however my big bore 2 stroke runs better on Avgas than Amoco or other pump gas. And certainly runs as well as the more expensive race fuels I have tried. Rich may have a good point about the RPM limitations of Avgas. However my Husky runs to 9500RPM, with the torque peak at 7500.

I suspect leaded fuels may help the CRF450 valve problem also.

Chris


Posted by: cujet

6.19 Are aviation gasoline octane numbers comparable?

Aviation gasolines were all highly leaded and graded using two numbers, with
common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is
the Aviation rating ( aka Lean Mixture rating ), and the second number is the
Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade,
100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to
replace the 80/87 and 100/130. Soon after the introduction, there was a
spate of plug fouling, and high cylinder head temperatures resulting in
cracked cylinder heads [110]. The old 80/87 grade was reintroduced on a
limited scale. The Aviation Rating is determined using the automotive Motor
Octane test procedure, and then converted to an Aviation Number using a
table in the method. Aviation Numbers below 100 are Octane numbers, while
numbers above 100 are Performance numbers. There is usually only 1 - 2
Octane units different to the Motor value up to 100, but Performance numbers
varies significantly above that eg 110 MON = 128 Performance number.

The second Avgas number is the Rich Mixture method Performance Number ( PN
- they are not commonly called octane numbers when they are above 100 ), and
is determined on a supercharged version of the CFR engine which has a fixed
compression ratio. The method determines the dependence of the highest
permissible power ( in terms of indicated mean effective pressure ) on
mixture strength and boost for a specific light knocking setting. The
Performance Number indicates the maximum knock-free power obtainable from a
fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine
designed to utilise the fuel can obtain 150% of the knock-limited power of
iso-octane at the same mixture ratio. This is an arbitrary scale based on
iso-octane + varying amounts of TEL, derived from a survey of engines
performed decades ago. Aviation gasoline PNs are rated using variations of
mixture strength to obtain the maximum knock-limited power in a supercharged
engine. This can be extended to provide mixture response curves which define
the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost
( weak about 16:1 stoichiometry ) before knock [110].

The 115/145 grade is being phased out, but even the 100LL has more octane
than any automotive gasoline.

The previous is copied from this very informative site on gasoline:

Make sure to select full page view.

http://www.faqs.org/faqs/autos/gasoline-faq/part1/


Posted by: the bigfella

I have been working in the R&d side of Motorcyle racing for over 14 years and can tell you in the past Pleanty of world motocross and road race championship have been won using 110 A-gas, even some with fuel sponsors logos on the side of the bike ........

I know i sounds wrong (those race fuel guys wouldn't make stuff up just to earn a $$$ would they?)

The benifits of race fuel (some of them) are there but rarely out weigh the cost of the freight to get the fuel to the races. these teams will pay $28,000 for a lighter dohicky.... you think if there was any thing in a fuel the wouldn't have been sending guys out to local air ports for fuel...........

The only thing that stopped us from using it now is the no lead rule for racing here and in europe............. bikes engine life is @1/3 of what is was with out the lead...........you not know how good you have it ............
I hear that AMA will be unleaded i the next few years fit it if you can..........


Posted by: Andy891

This is a great thread and please excuse me if this sounds like a dumb question after the previous technical info provided.....but i have been running 100/130 avgas (purchased in australia) in my ktm 300exc 2-stroke as it was running it when i bought it. I offered to give a friend a tankful as his yammy 2-stroke was and he was adimate that to run it he would need to change his jetting...is this true? or can he run it with his standard jetting?
thanks
regards
Andy


Posted by: bwalker

Cujet, I always thought avgas had high low end boiling points which made throttle response pretty crappy.


Posted by: jmics19067

Andy I believe your friend would be on the right track.



Cujet & BWalker; How much would you think the outside air temp would affect throttle response& av gas. you havta figure that Fla is considerably warmer that MIch. most of the time


Posted by: cujet

A little non technical observation here. When I preflight Avgas powered aircraft I generally used to take the sumped fuel and throw it on the ground. (now recycling is required). Avgas evaporates very rapidly (within seconds) and leaves little residue. If compared to pump gas for cleaning parts, Avgas evaporates many times faster, with a very dry feeling to the parts being cleaned. Pump gas leaves all sorts of residue, sometimes taking a very long time to evaporate.

I believe (and learned in school) the avgas is designed to vaporize readily at very low temperatures. Aircraft regularly fly at extremely low temperatures. For instance, 30,000 feet over florida results in about 40 degrees below zero C. Sometimes as low as 50 degrees below C. There are small a number of piston powered aircraft that can operate at this altitude. In addition, there are a few new high altitude certified engines that use Avgas. Certainly, Avgas is used in Alaska and other cold weather areas also.

My personal experience with Avgas is quite favorable. It seems to allow easier starts. jetting is considerably leaner. I can only guess that the additives in local pump fuel tend to require richer jetting. Power is certainly on par with pump gas. Others on the Macdizzy two stroke forum are finding that Avgas works well for them also. It may be that pump gas quality has dropped (or changed) to the point that a consistent fuel such as Avgas is necessary in certain engines.

I do know that using local Mobil 93 results in sluggish RZ350 performance with no wheelies, while Chevron, Texaco and to a lesser extent Amoco result in crisp running and wheelies in 1st and 2nd. Avgas results in a required 2 jet sizes smaller and crisp running with no loss of power or RPM. The RZ350 pulls hard to 10,000 and signs off at 11,500.


Chris


Posted by: jmics19067

"I believe (and learned in school) the avgas is designed to vaporize readily at very low temperatures. Aircraft regularly fly at extremely low temperatures"

My question is what is atmospheric pressure doing to the fuel at that time though. less pressure lower boiling temps.

one would have to figure that a lot of pump gas has been designed with extremely high boiling temps to prevent city chores driving. If you look at a an older car with a mechanical pump <usually pretty darn close to an exhaust manifold> quite a bit of fuel line that is wrapped around a hot engine and then then the carb getting hot and boiling out its fuel when shut off. I can see that there could be quite a few problems with vapor lock and cars not starting easily while somebody shut it off to head into a store. Plus the fact that cleanliness and isolation of the different products <gasoline diesel ,kerosene, home heating oil.....> is probably not a very high priority when it comes to shipping and storing


Posted by: cujet

jmics, Atmospheric pressure has little to do with the situation I described, as all high altitude Avgas powered aircraft use some form of boost. In addition they are well intercooled. So, the charge air is vary cold and often at or slightly above atmospheric.

Chris


Posted by: Humai

Avgas is the only readily available form of "Race Gas" we can get in this country. Packaged by BP as "Race Gas" it's 100LL by any other definition.

So, it's a case of "it's all we've got therefore we work with it as best we can". The best thing about it is that it is consistent. These are my observations compiled while trying to adapt jetting to it on two strokes and four strokes:

1) Throttle Response. This is a very subjective area, but throttle response when jetted correctly still greatly exceeds my demands anyway. Many's the time "throttle response without commensurate mental and physical counter-response" has got me and my buddies into a power of crap in offroad situations, if you know what I mean. I've noticed more propensity for "lean bog" issues at low revs with the YZ/WR fourstrokes with 100LL.

2) Spooge. 100LL will definitely spooge a little more on bikes with a tendency to spooge, such as KTM's 250E, despite jetting work. Many bikes run perfectly dry on 100LL though.

3) Octane. 100LL is definitely better than our unleaded pump premium for reducing pinging.

4) Jetting relative to pump gas. 100LL needs at least two main jet sizes leaner than pump gas in our part of the world.

5) Starting. Seems better than pump, but this feeling is subjective.

6) Stability in storage. Our 98 (RON) octane unleaded premium (same 98 as you Aussie blokes, brewed in Perth I believe), which goes under the name of Optimax or Ultimate, seems to deteriorate very quickly, making for hard starting and unexpected pinging when it's old. It also requires several sizes richer mains to run. By comparison, 100LL seems pretty stable and consistent.

7) Evaporation. Seems to evaporate very quickly, leaving an oil-free whiteish residue.

Summary: 100LL is OK if it's all you've got. At least it's consistent, so you're not trying to jet to a moving target.

&nbsp;


Posted by: PMK

Chris,
If the Husky is together let's try and get a ride in before 01.03.
With all the homework you did on the AvGas, did you find a definite 100LL rating? I found in maybe AC65-9A where they stated 100LL was to replace 100/130, not 91/96. Does North County have 100LL for 1.80?
PK


Posted by: cujet

PMK, the Husky is back together in all it's former glory. Runs very well! I fount the best price for Avgas ar Indiantown (X60)? They had it as low a $1.70 for a while! North County is about $3.00!

The best information I could find about the actual octane rating of Avgas is that it is 100+- 1. However in some situations it can be comparable to a slightly higher race gas. Ex, Avgas may provide as much detonation resistance as a 103+ octane race gas in certain applications. I suppose that would depend on the individual engine design and whether or not it is boosted.

I am running slightly advanced timing for more torque and using my torque pipe. This in an attempt to keep the revs down and crankshaft life up! The bike feels great, however it has lost some of that screaming top end speed. Maybe only 90MPH instead of 102!

I am looking to ride Ocala National Forest soon, are you interested? Otherwise, any local area is OK with me. Call me.

Chris


Posted by: PMK

Chris, I sumped 4 gals from the SX300 as a precaution. Can't be too safe you know. I ran it in the YZ this afternoon, I wasn't let down as it ran pretty good with only changing the idle mixture, but still has a flat spot right off the bottom. One thing I did notice is that when riding the canal road (sugar sand)to the track by the house, with pump gas I could never bounce the rev limiter in 5th. This is with the modified MAJ passage and MJ up and down. First try with the 100ll hit the rev limiter in 5th no worries, good midrange too as lifting the front wheel in 5th by coming back into the throttle was not a problem. If you're at PBI, I'll be back on 12.2. I'm thinking to start moving the TPS again to remove the off idle lag. I'm not sold on a jetting problem. Thoughts? Glad to know the head job and powervalve bushing mod worked, use good oil so as not to overheat those main bearings, maybe oil wouldn't have helped.
PK



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