Fuel Tech Tips – Straight Talk about Gasoline
(and a few words about “exotic” fuels)
Bob Kennedy likes to say that “fuel makes horsepower”. This truism must be kept in mind. While it’s true that hp is usually limited by airflow, to make power you need fuel. Fuel system upgrades are especially crucial for a supercharged car or a car using nitrous oxide. The purpose of this edition of ” tech tips” is to help you learn about fuel systems for high performance cars, especially those that have been modified for greater performance.
Gasoline – General Information
This site has an excellent general discussion of gasoline. If you read through the FAQ you will have a good basic knowledge of gasoline. The characteristic of gasoline that is most often discussed is octane, but there is much more to fuel than octane, as you can see from reading the FAQ or from the following paragraphs.
The Four Primary Properties of Fuel
OCTANE: The rating of fuels ability to resist detonation and/or pre-ignition. Octane is rated in Research Octane Numbers, (RON), Motor Octane Numbers, (MON), and Pump Octane Numbers (R+M/2). Pump octane is what you see on the yellow decal at the gas stations and represents an average of the two. The MON is the test method is more prevalent in racing. Don’t be fooled by high RON numbers or an average. MON’s are most important for a racing or other high performance application, however, the ability of the fuel to resist preignition is more that just a function of octane.
BURNING SPEED: The speed at which fuel releases it’s 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. Thus for maximum power, ignition timing must be tailored to the fuel being used. We can do this for you at Kennedy’s using Dynotuning. We have found that the correct timing can add 20-30rwhp to most supercharged vehicles without any additional risk of engine damage.
ENERGY VALUE/SPECIFIC GRAVITY: An expression of the energy potential of the fuel per pound. The energy value is measured in BTU’s per pound, not per gallon. It is therefore closely related to the specific gravity of the fuel at a given temperature. The difference is important in tuning. Say you tune with a fuel with a high specific gravity, say 0.75gms/gallon and achieve a perfect air-fuel ratio. Then, you switch to a fuel with 0.70gms/gallon specific gravity. The fuel mixture will now be way off and the motor very lean – you are delivering ~7% less fuel. This gets an important point that we wish to emphasize. Once you have your car dialed in using a specific fuel, do not change to another fuel.
HEAT OF VAPORIZATION: 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.
However, for purposes of our discussion here, octane is the most important of these characteristics. It is a truism that using fuel of a higher octane than needed will not produce any performance gain. On the other hand, using fuel of too low an octane will definitely cost power, and in an extreme case can cause severe engine damage.
This brings up the question of “how much octane is enough?” This is not a simple question. Even for stock motors, the often stated advice to “follow the manufacturers recommendations” may not be optimum. Cars equipped with knock sensors, which includes essentially all current GM, Ford, and almost all imports, will run often fine on “regular” gas. But with regular, especially in hot conditions or under heavy load, the computer will very likely pull timing. This will usually be imperceptible to most drivers, but it will rob you of horsepower and fuel mileage. Using a fuel with a couple of additional octane points may make a noticeable difference with these vehicles. If you own one, you should give it a try.
If you have a supercharged vehicle, use the highest octane fuel you can get! Why is this? A Supercharger increases the dynamic compression ratio of the operating motor. By forcing more fuel and air into the cylinders, the cylinder pressure is increased. That is why a SC makes power. But the increased pressure increases the tendency of the motor to detonate. Combined with the hotter intake charge, detonation becomes a real possibility. Detonation robs horsepower, but worse, it can break a motor. At Kennedy’s we use a number of ways to tune around this. A richer AF ratio cools the intake charge, as does intercooling. Retarding timing also helps reduce detonation. A Kennedy’s Custom Water Injection System is one of the most effective ways to prevent detonation and does not rob boost and is easier to install than an intercooler. But the easiest technique for reducing detonation is simply to use good gas! Another essential element in preventing your supercharged motor from detonating is cam timing. The intake closing point needs to be chosen correctly to avoid excessive cylinder pressure and detonation. A Kennedy’s custom cam is carefully selected with this (and other considerations) in mind.
Kennedy’s street supercharger kits are designed to work best with Premium Unleaded. Do not skimp and try to get by with low octane fuel.
Some Fuel Facts
- Each additional pound of boost over our base kits requires approximately an additional 1.5-2 point of octane.
- One degree of ignition advance over base requires 1 point of octane.
- Water injection has equivalent anti-detonation effect of ~4-6 points of octane.
- Lower compression ratios will decrease octane requirements. Power will drop off with lower compression unless more boost is added. For maximum horsepower, run a low compression/high boost combination.
- A cam with a later intake closing point (further from BDC) will lower the dynamic CR and decrease octane requirement. However, it will also cost hp unless accompanied by increased boost.
We hope you found these fuel tech tips informative. To learn about “alternative” fuels, click here.