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Good for a chuckle... The AV Gas Truth

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Posted by: JTT

Rich should enjoy this one. Found this on another site the other day and though immediately of Rich

Anyways, Aviation gasoline or "av gas", is blended specifically for use in small aircraft. It's also commonly used by many high performance engine owners because of it's high stated octane rating (usually 100-110) and the relatively low price compared to racing fuel. Unfortunately this fuel is not all it appears to be. Avgas octane is rated on a different scale than gasoline's intended for ground level use. What is 100 octane "av", is not necessarily 100 octane "ground level". Besides this, there is also a big chemical difference. Normal ground level race fuels are made up of gas molecules that have a "light end" and a "heavy end". The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process.

Small aircraft are constructed as very weight conscious vehicles. That's because their somewhat weak engines often have difficulty taking off with any extra weight. To help reduce this weight problem, aviation gasoline's are blended with no heavy molecule end. This makes a gallon of avgas weigh substantially less than a gallon of ground level fuel. Since small plane engines turn very low rpms and produce so little power, the omission of the heavy end is not a horsepower issue. However, for high output MX racing engines, there is defiantly a compromise in power. This, despite the fact that many mx owners experience the desirable cooler operating temperatures that avgas offers. In addition, some blends of avgas will quickly separate from some oils used in premix situations.


Posted by: biglou

Nice, lol. The "defiant power compromise" is pretty funny too. "Power be damned!"


Posted by: RM_guy

Looks like they got themselves a real rocket scientist there


Posted by: Rich Rohrich

Welcome to the internet, where everyone is an expert ...... just ask 'em .


Posted by: cujet

Interestingly enough those weak aviation engines make more actual power than converted (for aircraft use) auto engines. It just could be that the aircraft engines are designed to do a particular job well. I am not really a fan of big air cooled pushrod piston engines, but I must admit the BMEP figures are quite good on those aircraft engines.

Chris


Posted by: matt-itude

On the topic of aviation fuel i have heard many say not to use it and they all say there is a reason, but i have never found that reason (reading not trying the fuel) I figure there is probably a fairly simple explanation. I'm not as versed on fuels as this group is so just the basic would be appreciated.


Posted by: Rich Rohrich

Quote:
Originally Posted by matt-itude
I figure there is probably a fairly simple explanation.


There are at least 100 different threads on race fuel & AvGas that will explain all this in excrutiating detail.

Here's just one of mine that I found.

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.


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: matt-itude

That was very much appreciated and helpful. I have been using some 105 leaded that they have out of the pump at a nearby amoco station. I guess I better ask them exactly what it is and see if its worth the extra 1$ a gallon or not. I have just liked the smell much better and have no idea how it performs in my bike compared to other things. thanks!


Posted by: Oldschooldeathwish

so thats how planes fly their gas is lighter than a cars damn now i feel like an idiot


Posted by: matt-itude

Yeah just wait until these light wheight freeks hear about this . I think ill buy some stock in av gas.


Posted by: JTT

Thought you all might enjoy that one. And thanks Rich for reposting your avgas article...it's one that should be stuck to the top of the forum as it comes up so much


Posted by: 2strok4fun

Well, I am sure we could fill this forum with stupid cut and paste on fuel alone.

here is one I found. The "secret" big oil dosent want you to know.

http://www.lubedev.com/smartgas/additive.htm


Posted by: Rich Rohrich

Quote:
Originally Posted by 2strok4fun
The "secret" big oil dosent want you to know.

http://www.lubedev.com/smartgas/additive.htm


That might be the dumbest thing I've ever read. issed:

I want my 5 minutes of life back


Posted by: Studboy

Me too.

He doubled his gas mileage just by adding acetone!

How come nobody every told me about this? :silly:


Posted by: RM_guy

Me thinks he was sniffing the acetone


Posted by: velosapiens

Quote:
Originally Posted by Rich Rohrich
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.



i tried avgas since race gas is difficult to get and expensive where i live. i can vouch that the throttle response went south. it felt like i had a rubber band instead of a throttle cable. like i would turn the throttle and then the power would slowly build. i read something rich wrote a while ago about aromatic content, and just went back to pump gas, which works just fine in my bike.


Posted by: Jasle

Rich iI stole your post but I gave you credit. Having the same debate at Gomxracing.com


Posted by: GPracer2500

Quote:
Originally Posted by Rich Rohrich
There are at least 100 different threads on race fuel & AvGas that will explain all this in excrutiating detail.

Here's just one of mine that I found....

I realize this is an old thread but I wanted to make a comment. Very nice information. That's exactly the kind of technical analysis that really scratches me where I itch.

One thing though about the octane rating of 100LL as mentioned here:

Quote:
Originally Posted by Rich Rohrich
....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)....


100LL is defined by either ASTM D910 or DEF STAN 91-90. The USA (and most of the world) uses the ASTM standard. DEF STAN is a British standardization organization. These two defining standards are so close to being identical they can be considered one and the same. So, I won't bother citing DEF STAN standards from here on out....

ASTM D910 requires 100LL have a minimum octane rating specification of 99.5 MON (Motor Octane Number). This is determined using the same ASTM D2700 testing procedure that all gasoline MON testing uses. Within ASTM D2700 there is a chart that's used to convert MON ratings to Lean Mixture Aviation Ratings. Near 100 MON they are very close to one another. Below about 100 MON, the conversion yields a Lean Mixture number a shade under the MON (maybe 1 or 2 points?). MON ratings between 100 and 110 will convert to a Lean Mixture number up to about 20 numbers higher.

Decades ago there was a specific test for the Lean Mixture rating scheme, ASTM D614. But it was done away with after a correlation was found between D614 and the then new D2700 method used for MON. The Rich Mixture rating still uses a separate method for testing, ASTM D909 (which for 100LL is required to be 130 or higher).

A British Petroleum product handbook published in 2000 lists their 100LL gasoline as having a "typical" value of 105 Lean Mixture rating. That indicates the typical MON value for their 100LL is slightly above the minimum 99.5 MON required by ASTM D910.

Point is, we can mostly ignore Lean and Rich mixture ratings since we already have the rating we want to be using anyway--MON. 100LL will (at a minimum) test at 99.5 MON.





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