Fuel Ratings

So, let's break it down a bit; how is fuel rated? We've all seen octane numbers on pump and race fuel, but what does "octane" actually mean? Simply put, the octane rating is still just a knock threshold value of the fuel, or its resistance to detonation. Many fuels are referred to using the R+M/2 number. That's the Motor Octane Number (MON) + the Research Octane Number (RON) divided by 2. You've likely seen this Anti-Knock Index (AKI) or Pump Octane Number (PON) on any given fuel pump, usually right next to the octane number on the selection button.

What the heck does that mean? We'll use one of our favorite fuels from VP, MS109, as an example. Below is the Typical Values chart you'll see next to any VP fuel.

Color: Despite octane or intent, fuel looks more or less the same, so coloration is added simply to help differentiate the different blends both for easy identification by hot rodders and racers, as well as tech inspectors.

Research Octane: This is the octane rating given by a Research Octane Engine (ROE). Designed in the 1930s, this single cylinder engine resembles an ancient Briggs & Stratton and operates at 600 rpm with an intake temp of 120 degrees F, and timing fixed at 13 degrees. It uses an adjustable cylinder head that can move up or down to increase or lower the compression ratio while the engine is running. The point at which the fuel being tested begins to detonate is its Research Octane Number.

Motor Octane: This is octane rating given by a Motor Octane Engine (MOE). This machine, also designed in the 1930s, is similar in design to the ROE, except that it runs at a steady 900 rpm, uses an intake temp of 300 degrees F, and features variable timing with a base setting of 26 degrees. It also uses an adjustable head to vary compression ratio to determine the point of detonation. The MOE test puts much more stress on the fuel and can more closely resemble the loads on a performance or racing engine, so it will always be lower than the RON, and is the value racers pay the most attention to.

R+M/2: This is the Research Octane number added to the Motor Octane number and divided by two. This is the standard rating method for all street pump gasoline that you will find at any given U.S. station (in Australia and most of Europe, the octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the pump number is R+M/2). Why? Mostly for marketing purposes. The RON will always be significantly higher than the MON, so the two are averaged together. Per VP in-house testing, standard pump 92-octane has a MON of about 86, while 87 is really more like 79 MON. Think about that next time you ponder saving a few cents and hose down your pride and joy with 87!

Specific Gravity: A fuel's specific gravity is its weight compared to water at 60 degrees F. Water is 1.00 on the scale, so since fuel is less dense the number will always be a decimal. Knowing this is vital to tuning the engine because it provides reference point for tuners to base fuel flow. For example, MS109 is .722, while the 50-state legal VP100 is .746. If you were to tune your engine to run VP100 on the street, but decided to switch to MS109 for a more aggressive tune on a track day, the lower Specific Gravity denotes that it's a lighter fuel, which would cause the engine to run leaner with the same tune. To compensate, more fuel should be added in to get the air/fuel ratio (AFR) back into the target zone.

Stoichiometric Value (Stoich): The "stoich" is related to the AFR since a fuel at a stoichiometric mixture has just enough air to completely burn. Factory EFI tunes use the value to control emissions, while aftermarket EFI controllers use it as another tuning parameter. The stoich value is really only valuable under light-load cruising conditions, though, since the 14.7:1 (14.7 grams of air per 1 gram of fuel) established by the EPA (gasoline's stoich is truly closer to 13:1) burns extremely hot and is too lean for acceleration or any other loading on the engine. Attempting to maintain a stoich value under hard acceleration would induce a great deal of detonation, which is why oxygen sensors are required in the exhaust of an EFI-equipped engine; they send data to adjust the AFR by adding fuel to compensate for engine load, or even different fuel blends. Carburetor-equipped cars have to manually adjust to compensate, of course.

Motorsport 109
Typical Values
Color: clear
Research octane: 109
Motor octane 101
R+M/2: 105
Specific gravity: .722 at 60° F
Stoichiometric value

Detonation & Compression

Speaking of detonation and compression, we'd like to clarify that the octane rating is not an indicator of the energy content of fuel, it's only a measure of the fuel's tendency to burn in a controlled manner rather than exploding in an uncontrolled manner. For example, the octane number of a fuel is raised by blending in ethanol, but its energy content per volume is reduced. So yes, you can run higher compression with E85 fuel, but it will not produce as much energy per volume of an equivalent race fuel.

Of course that begs the questions, will my car make more power if a higher-octane fuel is used? Maybe. The main reason to run higher-octane fuels is to ward away detonation as the compression ratio is raised and the timing gets more aggressive. On a straight octane-to-octane comparison, there is a point at which higher octane will not benefit your car. Just as it's very possible to "under-octane" an engine and lose power or cause mechanical damage, you can also "over-octane" an engine and create a situation where the fuel isn't responding to the conditions your engine presents.

Picking A Fuel

So now that you will have a basic grasp of what you're looking at when studying an extensive list of fuel offerings such as those aforementioned 70-plus fuels on VP's website, we'll get to the real question we know you want answered: What fuel do I need for my car? The good news is that unless you are a hard-core racer or run in a class with a spec fuel, you can ignore most of the offerings that are tailored to those guys. Many blends were created by VP specifically to meet a certain race class' standards, while many others exist simply as lower cost alternatives for racers on tight budgets.

Using the advice of Rueckert, we narrowed the choices down to a few that should encompass the vast majority of the rodders out there, as well as a few representative applications. Unleaded: VP100, MS109. Leaded: VP110, VP113, C16, and C25. Check VPRacingFuels.com for the full spec sheet on these or go to PopularHotRodding.com. The biggest rule of thumb when choosing? Learn to read your plugs, and always err on the side of safety. The second biggest? Call the tech experts at VP and discuss your platform.

VP Racing Fuels
P.O. Box 47878
San Antonio
TX  78265