AVGas: The Truth
Posted by Rich Rohrich
Posted 04-21-2005
An excerpt from Fuel for Thought - Part 2
by Rich Rohrich
The Avgas question comes up all the time and there are a huge number of silly myths associated with it.
To the question will Avgas hurt my engine?
The simple answer is 100LL Avgas usually isn’t the best choice but it won't hurt anything. 100LL (Blue) Avgas seems to be the most readily available version so that's what we'll concentrate on.
For general high speed purposes Avgas has a couple of problems:
1) The 90% distillation point for 100LL Blue Avgas is set at 275 degrees F and the end point is set at 338F, 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 VP C12 has a 90% distillation point around 228F and an end point set at 233.3F.
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 isn’t right for many applications. 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 distillation 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 distillation 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 bear 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 there is a possible exception. The closer you get to the thermal limits of a particular engine design the more useful Avgas becomes. The end point distillation temperature of Avgas is high enough that some of the fuel can make its way into the combustion chamber in liquid form. While this would normally prove to be a liability in most engines it could be useful when you are tuning to the ragged edge. The liquid fuel droplets entering the combustion chamber will leech some heat and help to cool the piston crown. In an engine that is pushing the thermal limits this can be the difference between a win and a holed piston. Some specialty karting fuels like Philips P45 are designed with a large jump at the end of the distillation curve to specifically provide this cooling effect. It’s not uncommon for a fuel with higher octane than Avgas but lower end point temperatures to experience preignition problems long before Avgas. It's the tuning equivalent of making lemonade out of lemons. For engine combinations that aren't near their thermal limits like MX applications this high end temperature just ends up as oil spooge dripping out of the silencer even when the jetting is lean. As I've pointed out time and again, the octane rating of a fuel is just one part of the tuning equation.
The sad fact is given the sorry state of pump fuel today Avgas is looking better all the time even when you aren't pushing the thermal limits.
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 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.
- If you are running near the thermal limits of your engine combination Avgas may provide a margin of safety that isn't available even in a high-octane fuel with a flat fuel curve.
- 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.
