PART 1: AV gas in a turbo 2 stroke

adam5187 said:

I am just wondering if running pure av gas in a 2 stroke turbo is a cheaper alternative to a 50/50 race fuel/premium mix. I can get it here in Alberta for 1.46 per litre, so any info or problems would be appreciated, thanks. I will be running 12lbs of on d7 with a silber kit.

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adam5187 said:

I meant a 50/50 mix of race and pump gas. A 5 gallon drum of race fuel here is about 60-70 bucks and our airport will sell us av gas, just pull up to the pump. Thanks for the info, I am going to be running pure av gas this winter, can't wait for snow!!

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What does octane really measure and how important is it?

One of the most frequently asked technical questions we get at VP involves the difference between Motor, Research and R+M/2 Octane Numbers. The next most frequently asked question is why some fuel companies represent their fuels with Motor Octane Numbers, while other companies use Research or R+M/2 Octane Numbers.



Realize first that octane is a measurement of a fuel’s ability to resist detonation—nothing more. The two types of machines used for testing octane—a Motor Octane machine and a Research Octane machine—were designed in the 1930s. They were designed to test for octane numbers from the 0-100 range, therefore, any number above 100 is an extrapolation.

Both of these machines are dinosaurs and are not adequate for today's high tech fuels or engines, but they’re the only means available for testing fuels. These machines are one-cylinder engines with an adjustable head that can move up or down to increase or lower the compression ratio while the engine is running. The Motor and Research machines are the same in this respect, but they differ in several other characteristics. 



The following is a comparison of the two machines used for testing octane numbers:
---------------------- Motor Octane Machine --------- Research Octane Machine
RPM --------------------------900------------------------------ 600
INTAKE TEMP.---------------300 DEGREES F -----------------120 DEGREES F
TIMING --------------------VARIABLE BASIC------------------FIXED AT 13 DEGREES
(DOES NOT CHANGE)
----------------------------SETTING 26 DEGREES

As you can see, the Motor Octane machine runs at a higher RPM, higher temperature and more timing. This machine puts more stress on the fuel than a Research machine and more accurately simulates a racing engine. VP Racing Fuels always includes Motor Octane Numbers when promoting its fuels because our fuels are used exclusively for racing applications.

The Research Octane machine will always produce a higher number for the obvious reason that it does not put the same amount of stress on the fuel. This number is used by some fuel companies to trick the racer into thinking the fuel is rated higher, i.e., higher quality, than it really is. The “R+M/2” Octane Number is the average of the Research and Motor Octane numbers for a fuel and is the number displayed with yellow labels on retail level gas pumps.

When comparing fuels for racing purposes make sure to compare Motor Octane Numbers because these are the ones that count in your racing application. Focusing on the MON of each fuel will help ensure you’re comparing apples to apples with regard to octane.

But bear in mind, a fuel’s ability to prevent detonation is a function of more than just octane. For example, VP’s fuels—oxygenated or nonoxygenated—vaporize much better than competitive fuels with comparable octane ratings. This means VP fuels cool the intake charge, burn faster and yield more efficient combustion. As a result, the “effective” octane rating of VP’s fuels is even higher than the rating generated by the octane test. As a result, VP fuels will prevent detonation more effectively than competitive fuels with comparable MONs.

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Besides octane, what else do I need to consider when selecting a fuel ?

Too often, racers focus only on octane when evaluating the quality of a fuel. Octane is certainly important, but it’s just one of several key fuel properties that should be considered when evaluating and selecting a fuel. It’s entirely possible to generate more horsepower with a lower octane fuel if it’s designed properly with respect to its other key properties. It’s also possible for two fuels to have the same octane rating, but perform very differently due to their other key properties.

1. OCTANE: Octane is simply a rating of a fuel’s ability to resist detonation and/or preignition. It is not so much an indication of a fuel’s ability to make power, but rather a fuel’s ability to make power safely, i.e., without blowing your engine. Octane is rated in Research Octane Numbers (RON), Motor Octane Numbers (MON) and Pump Octane Numbers (R+M/2). A Pump Octane Number is the number you see on the yellow decal at gas stations, representing the average of the fuel’s RON and MON. (See below for a more detailed explanation of how octane numbers are derived and what they represent.)

VP relies on MON numbers because the MON test more accurately simulates racing conditions. Don't be fooled by high RON or R+M/2 numbers. Many companies use these simply because they look higher and are easier to come by because of the test methods. Also bear in mind that the ability of fuel to resist detonation is more than just a function of octane.

2. BURNING SPEED: This is the speed at which fuel releases its energy and is partially a function of a fuel’s vaporization qualities. At high RPMs, there is very little time (real time - not crank rotation) for the fuel to release its energy. Peak cylinder pressure should occur around 20˚ ATDC. If the fuel is still burning after this, it’s not contributing to peak cylinder pressure, which is what the rear wheels see. Because VP’s fuels are designed with a particular focus on vaporization characteristics, most of VP’s fuels—oxygenated or nonoxygenated—vaporize much better than comparable competitive fuels. This means it cools the intake charge, burns faster and yields more efficient combustion. As a result, the “effective” octane rating of VP’s fuels is even higher than the octane test indicates, and they will prevent detonation better than competitive fuels with similar MONs.

3. ENERGY VALUE: Energy value is an expression of the potential energy in fuel. The energy value is measured in BTUs per pound, not per gallon. The difference is important as the air/fuel ratio is in weight, not volume. Generally speaking, VP’s fuels measure high in BTUs per pound and thus, have a higher energy value. This higher energy value will have a positive impact on horsepower at any compression ratio or engine speed.

4. COOLING EFFECT: The cooling effect of fuel is related to the heat of vaporization. The higher a fuel’s heat of vaporization, the better its ability to cool the intake mixture. The superior vaporization characteristics of VP’s fuels make cooling effect one of their key advantages. A better cooling effect can generate some horsepower gains in 4-stroke engines and even bigger gains in 2-stroke engines. VP’s superior cooling effect can also ensure circle track racers maintain power in the longest races and harshest conditions. In virtually any application, the cooling effect of VP’s fuels will help extend engine life.

Understanding these key properties of fuel will better equip you to evaluate fuels for your application. By clicking on your application to the right,
you’ll find a list of VP fuels designed with you in mind, along with the fuels’ relevant characteristics.
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Can you "Over-Octane"an engine? If so, what happens?

Yes you can over octane an engine.

Generally you will get sluggish throttle response. Also you may get fouled plugs and deposits in the exhaust ports and headers.

When an engine is trying to burn a fuel that has too much octane, the burn rate is incorrect and all of the fuel won't be used up. This excess usually gets left as a deposit or gets pushed out the exhaust.

It is important to match your engine's needs with the correct fuel.

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Anyways, I have not noticed decrease in throttle response running avgas.

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mountainhorse said:

Sincere questions..

No decrease compared to running what other fuel?

In what kind of sled/motor/mods/compression ratio?

Are your "before" and "after" throttle response impressions made on the same day?

Thanks for that link to http://www.airnav.com/fuel/local.html... Very easy to use!! $4.45 Self Serve at my local airport.

I really want to "believe" in AV gas... especially since it seems to be not only a higher Octane fuel, but a higher Quality fuel compared to today's "pump" fuels.

With the Radically different Evap temps of AvGas (90% evap @ 275°F) compared to VP110 (90% evap @ 211°F)...Throttle response and piston cooling would be different to SOME degree.

Also, 100LL has a lower heat of combustion at around 18,715 BTU/lb (about the same as E-10 gas)... compared to VP100 or Pump91 at around 18,800 BTU/lb...

100LL and Ethanol added pump 91 have less engergy per unit burned... in comparison, VP C-12 has about 20,500 BTU's/lb.

I need to do more digging to find out about the stability of AV Gas as well... meaning storage longevity.

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