Just wondering about using regular aviation fuel instead of low octane pump fuel?

The two grades I know are 100 and 100LL (low lead). Will these damage the 4-stroke engine or just provide more rip? Anything you know will help. Thanks.

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 making sure that light end hydrocarbon fractions don't boil off too early, and the lowered rpm ranges 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.

WOW RICH! Thanks for the info. I am wondering why you say Amoco pump is so good(you say). I have has outstanding results with Mobil 93 in the past. Is it really that much better and why? Also, when you say jetting, are you saying jetting to a "T". By this I mean gauges, changing slides and modifying internals? I can jet very well with only changing needle position, air screw, pilot, and main. Is this enough or should I experiment with needle changes and slide changes? Will I see the benifits of Amoco if I don't mess with the needle size(not clip position) and the slide??

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Aaron's web site

Wow, thanks Rich. Lots of info there. So now I'm wondering about race gas? Where do you buy this stuff? What are the different types?

Just looking for the best performance from my new thumper toy while taking good care of her. Thanks.

There are lots of good fuels available. Best bet would be to grab a copy of Cycle News, National Dragster, or RoadRacing World and check the ads in back to see which companies have distributors in your area. The specific distributors will be able to recommend the appropriate fuel for your application.

I just want to add some observations from the "other side" of the issue. In a low compression, piston powered airplane, 100 and 100LL aviation fuels tend to foul spark plugs, particularly at idle, with their huge amounts of lead. Don't be fooled by the LL (100LL is about 8 grams lead, I think), it's really lots of lead, just less the straight 100 (sometimes referred to as 100/130).

The petroleum industry is leaning toward a 82UL aviation fuel replacement for 100LL and 100. Don't confuse this new fuel with the old leaded 80 (red dye). The relatively few planes that genuinely need 100 octane will either need to lower their manifold pressure settings to comply with lower octane, or buy their gas in Mexico :-)

I personally would not want to see lead deposits all over the combustion chamber in my dirt bike.

Tony

A little info from the Phillips MSDS sheet
AVIATION GASOLINE, 100LL
PHILLIPS 66 COMPANY General MSDS Information:
Product No.: 1014050(21223)
Tetraethyl lead < 2.1 g/gal

That's less than half the lead in VP C12, and 1/3 the lead in fuels like Phillips B37, and VP C18.

I usually use the high lead B37 or C18 as the base stock when I roll my own fuel and have never found it to leave much in the way of deposits. Actually the MMT additive that is commonly used in octane boosters has a much bigger problem with deposits. If we were to put huge miles on our bikes lead content might be a bit of an issue but for our applications it's pretty much a non-issue. AvGas is still a poor substitute for race fuel regardless of the lead content.

Tony is a pilot and part of the aircraft industry so you can count on his info on the other changes listed.

It will be a sad day for me when they stop making 100LL. As for fouling plugs, I have a B8EGV that has seen many new pistons come and go. Yes, I do carry a spare. But have not needed it in over 3 years, running 100LL!

[This message has been edited by flyinzuki (edited 03-16-2001).]

I always like the smell of avgas.... except, in the cockpit!

Mayday, Mayday, Mayday !!!!!!

Tony

That's interesting on the lead content for that company's product. I was sure that a fellow flier and Chevron company employee told me 8 grams for their 100LL. That's been stuck in my memory for some time, so it's concievalbe that this company uses a different amount, or that amount has been reduced industry wide in the last few years, or that I'm moving into senility.

Yes, my dirt bikes will never match the 3980 hours on my little Cessna, and their engine operating profiles are quite different, (not to mention the compression ratio).

I always drained 100LL fuel out of my sump drains (because it was convenient) to fuel my son's first bike, and never had any problems. I've never even tried anything but Chevron unleaded premium (and some occasional Mexican sewer gas) in the YZ400F.

I'm certain that the EPA will make the lead question mute in the next 10-20 years.

Tony