YZ 125 Fuel

Joined
Sep 23, 2006
Messages
14
Likes
0
#1
Hey guys
ive been talking to many different people about types of fuel to use in your yz 125

is their any tests or proof to show which gas is best to run?

im hearing 94 octane is great
im also hearing aviation gas (100) is great too

is their any Major difference?
 
Joined
Feb 26, 2007
Messages
187
Likes
0
#2
run premium fuel. It's perfectly fine. Avgas is a little overboard in my honest opinion. I am a pilot and I fly cessnas as of right now and I know that my cessna can take normal automobile gas if needed. It's actually approved to do so. It's just that avgas is an inconvenience to go out and get unless you live out at an airport which sometimes I feel I do. But honestly I've never used 100LL (LL just means low lead) in a bike before, only aircraft. But in my honest opinion is run whatever is most convenient for you. Just don't run regular because the fuel savings of about 50 cents is not worth the risk of maybe something going wrong.
 
Joined
Sep 23, 2006
Messages
14
Likes
0
#3
alright so aviation gas is a definite "No"
i decided to go with synthetic oil at 33:1 Ratio
And 91 octane gas
I went to 3 gas stations. and the highest octane i could find was 91

Am i missing something?
 
Joined
Apr 18, 2006
Messages
1,534
Likes
8
#6
Unless your engine has been modified to increae its compression ratio then standard premium pump gas should be fine. You need to run a high enough octane to avoid detonation, but running a higher octane than necessary will provide no additional benefit.

Depending on the year and model of your bike even regular gas could be fine. The critical factor is the compression ratio of your engine. A "high performance" engine will typically run a compression ratio around 12:1, which requires an octane of 91. The more standard "non performance" compression ratio run around 9:1, which can run on regular gas.

Most late model race bikes will have the higher compression, requiring premium. Some racers will modify the heads or install taller/dome topped pistons to increase the compression ratio and then run really high octane race gas.

The probability is that your YZ has the 12:1 compression ratio, especially if it is a later model. If you are not sure then it would be safe to run the premium, as it certainly won't hurt anything and the cost difference isn't worth the risk.

Rod
 
Joined
May 9, 2006
Messages
4
Likes
0
#7
100LL is 3.17/gal at the local airport, while 93 is 3.59/gal at the pumps...

I thought that was interesting.
 
Joined
Feb 10, 2007
Messages
436
Likes
0
#8
Amoco/BP has 93 unleaded at all their stations here in the US. I believe Sunoco has a high octane unleaded at the pumps too, although the Sunoco stations are pretty few here in Georgia.

I ran Union 76 race fuel in my bikes for years. 100 octane unleaded. Ran good, smelled even better!

Av gas is not really meant for land based use.

Rotor
 
Joined
Apr 2, 2005
Messages
73
Likes
0
#10
The Truth About Avgas

Got this off another forum years back... not sure where but I hope it clears up some misconceptions!!!

100LL OCTANE is closer to 91/96 (lean/rich)

AVGas: The Truth
Posted by Rich Rohrich
Posted 04-21-2005

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.
 
Joined
Oct 5, 2003
Messages
5
Likes
0
#12
I have a yz125 also. The guy that I bought it from had always run VP because he raced and theres been a good deal done to the engine. When I got it I asked if I could just run high octane gas and he said he wasn't sure. Since then I have been running AVgas and have had no problems at all.
 

Rich Rohrich

Moderator / BioHazard
Joined
Jul 27, 1999
Messages
22,763
Likes
557
Location
Chicago
#13
SKIDOOrider800 said:
Got this off another forum years back... not sure where but I hope it clears up some misconceptions!!!

AVGas: The Truth
Posted by Rich Rohrich
It came from here on DRN originally.
 
Joined
Oct 19, 2006
Messages
8,130
Likes
2
Location
Merrillville,Indiana
#15
SKIDOOrider800 said:
Got this off another forum years back... not sure where but I hope it clears up some misconceptions!!!

100LL OCTANE is closer to 91/96 (lean/rich)

AVGas: The Truth
Posted by Rich Rohrich
Posted 04-21-2005

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.