Race fuel questions

[Supracentral]

Jdub,

I've been racing for nearly 20 years, and while I've always been able to find a fuel that meets the requirements of my engines and in the correct specific gravity allowed by the racing classes, but I've never really understood all the technical details.

For example, in the drag car, I run VP Import which has a Specific Gravity of .744 @ 60°F, a lead content of 6.0 g/gal, is Oxygenated and has a Motor Octane of over 120+. VP does not provide a Research Octane or an R+M/2 rating for this fuel.

For the street cars, I've always run VP Motorsport 109 which has a Specific Gravity of .722 @ 60°F, is unleaded, is Oxygenated and has a Motor Octane of 101, a Research Octane of 109 resulting in an R+M/2 of 105...

I know in the US we measure fuel at the pump with R+M/2 octane scale. I'm aware that's the Research Octane rating + the Motor Octane rating divided by two.

I guess I'm trying to understand why Research Octane is used at all... And why the R+M/2 value is considered a "standard"?

My understanding (and correct me if I'm wrong) is that the Motor Octane test method is highly accurate and is done under controlled conditions for both intake temperature and fuel temperature and is tested with varying timing and different compression ratios. However Research Octane testing is done with fixed timing and that fuel temp is not controlled with the only variable being the compression ratio. This seems like it would be a fairly non-representative test of how fuel behaves in the real world.

Why would you average the results of two wildy different tests when one of them seems more representative of what a real engine would do and the other is not?

 

[jdub]

This Chevron Tech Review might help:
http://www.chevronwithtechron.com/products/documents/69083_MotorGas_Tech_Review.pdf

From that article:

ANTIKNOCK PERFORMANCE
Knock-free engine performance is as important as good driveability. Octane number is a measure of a gasoline’s antiknock performance, that is, a gasoline’s ability to resist knocking as it burns in the combustion chamber. There are two laboratory test methods used to measure the octane number of a gasoline (see page 47). One method yields the Research octane number (RON); the other results in the Motor octane number (MON). RON correlates best low-speed, mild-knocking conditions; MON correlates best highspeed and high-temperature knocking conditions and with part-throttle operation. For a given gasoline, RON is always greater than MON. The difference between the two indicates the sensitivity of the gasoline to changes in operating conditions. The larger the difference, the more sensitive the gasoline.

RON and MON are measured in a single-cylinder laboratory engine, so they do not completely predict antiknock performance in multicylinder engines. The Modified Uniontown procedure (see page 48) involves using an actual vehicle to measure the antiknock performance of a gasoline. The resulting value is called Road octane number (RdON). Because vehicle testing is more complex than laboratory testing, there have been several attempts to predict RdON from RON and MON. The equations take the following form:

RdON = a(RON) + b(MON) + c

A good approximation for RdON is a = b = 0.5 and c = 0, yielding (RON + MON)/2, commonly written (R + M)/2. This is called the antiknock index (AKI). The U.S. Federal Trade Commission requires dispensing pumps to be labeled (posted) with the AKI of the gasoline they dispense. In addition, owner’s manuals for vehicles in the U.S. must specify recommended fuel by AKI. (R + M)/2 is voluntarily posted in Canada.

Neither the AKI nor any of the other single-value indices that have been developed forecast the performance of a gasoline in all vehicles. In some vehicles, performance correlates better with either RON or MON alone rather than with a combination of the two. Also, for a given vehicle, the correlation can vary with driving conditions.

As the formula indicates, gasolines with the same AKI can have different RONs and MONs. This may explain why a vehicle knocks while running on certain brands of gasoline or even between fill-ups of the same brand. Of course, for a comparison to be valid, a vehicle must be operated under identical conditions, which is not easy for the typical driver to arrange.

Aviation gasoline is rated using MON - it is a better predictor of engine performance under load and before 100LL often you would see it expressed a 2 numbers...i.e. 100/130. The first rating is under lean conditions used at cruise, the second under rich conditions used for takeoff and other full power circumstances. Both are in an engine that requires more demanding requirements above that of a normal car. In this sense, a race car engine would be very similar...MON would be the better indicator

 

[Eman14j]

:aigo: