OCTANE NUMBERS AND WHAT THEY REPRESENT

[DaveInDaytona]

I've been asked this a bunch of times and thought this would be interesting to all. Content courtesy of VP Fuels


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

To answer these questions, we need to first explain the machines that do the testing. These machines were made in the 1930's and were designed to test for octane numbers from the 0-100 range. Any number above 100 is an extrapolation. Both of these machines are dinosaurs and are not adequate for today's high technological fuels or engines, but they are all we have for testing fuels. These machines are one-cylinder engines that have an adjustable head on them 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 Machine___________Research Machine

RPM________________900____________________600

INTAKE TEMP.____300 DEGREES F___________120 DEGREES F

TIMING_________VARIABLE BASIC_________FIXED AT 13 DEGREES
SETTING_________26 DEGREES____________(DOES NOT CHANGE)

As the comparison above shows, the Motor Octane Machine is run at a higher RPM, Hotter Temperature and More Timing. This machine will put more stress on any fuel and more accurately represents a Racing Engine. VP Racing Fuels always uses the Motor Octane Numbers when advertising our fuel because our fuels are used primarily 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 he/she is getting a better fuel. The R+M/2 Octane Number is the average of the Research and Motor Octane numbers and is displayed on the 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 applications.

[NC30]

<some more info>

The four most important properties of racing fuel

1. OCTANE - The rating of fuels’ ability to resist detonation and/or preignition. Octane is rated in Research Octane Numbers (RON), Motor Octane Numbers (MON), and Pump Octane Numbers (R+M/2). Pump Octane Numbers are what you see on the yellow decal at the gas stations and represents an average of RON and MON. VP uses MON because this test method is more prevalent in racing. Most other companies use RON because it is higher, easier to come by, and sounds better in marketing messages. Don't be fooled by high RON numbers or an average -- MON is the most important for a racing application. However, the ability of the fuel to resist preignition is more that just a function of octane.

2. BURNING SPEED - The speed at which fuel releases its energy. In a high-speed internal combustion engine, 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 is not contributing to peak cylinder pressure, which is what the rear wheels see.

3. ENERGY VALUE - An expression of the potential in the fuel. The energy value is measured in BTUs per pound, not per gallon. The difference is important. The air fuel ratio is in weight, not volume. Remember, this is the potential energy value of the fuel. This difference will show up at any compression ratio or engine speed.

4. COOLING EFFECT: The cooling effect on fuel is related to the heat of vaporization. The higher the heat of vaporization, the better its effect on cooling the intake mixture. This is of some benefit in a four- stroke engine, but can be a big gain in two-stroke engines.

http://www.kartweb.com/TechArt/Fuel/vp_tech.htm
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In testing various grades of pump gas I consistently found that 87 octane fuel makes 1-2 more horsepower than those exact same bikes ran on Premium 93 octane. We tested five liter class motorcycles (97 CBR900RR, 02 Honda 919, 2000 RC51, 2000 GSXR750 & an 02 R1)

Standard Race Fuel (VP C12, C14, Sunoco 104, 100LL AvGas even high octane pump gas 96 or 100) ran straight on a stock RC51 motor will actually lose horsepower.

This is argued by many because the throttle response becomes crisper & is often mistaken for more performance when in truth the bike is making less power on the dyno sometimes by as much a 4-6hp

<from the same site>
Oxygenated Race Fuels (Nutec #4, VP MR1 etc) can add 3-5hp without any fuel or mapping changes at all & 5-10hp sometimes even more on a stock motor with proper mapping & lots of ignition retard.

The catch is that A. the stuff is really expensive usually about $15-$20 a gallon & B. it is highly corrosive & must be drained from your tank after each race weekend to keep it from eating parts of your fuel system.

http://www.rc51.org/fuel.htm
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Have to say I do love the smell of C14 & castor oil.
cya