AN INFORMAL GUIDE
TO RACING FUELS
Note: This article is furnished for informational purposes only. It is not meant to constitute an endorsement of any Sunoco products.
submitted by Dick Moritz
Marcus Hook, PA. . . . It wasn't supposed to be like this.
The other drivers weren't supposed to beat you easily, week after week. You've asked around, and it sounds like they're using the same parts you are. Your driving isn't that bad. What's their edge?
Gradually, a suspicion is sneaking into your mind. . . could it be the fuel they're using? And come to think about it, what do you really know about your fuel - where it comes from, how long it's been sitting in someone's storage tank, what's actually in the stuff?
Filling in the Blanks About Fuel
CIRCLE TRACK obviously can't answer all of these questions. But we can give you a start. We figured many of our readers could use a bit more understanding about the magic stuff you put into your tank each week - whether you're driving a modified racer on the track, the tow vehicle in the paddock, or your own Chevy Blazer on the street.
We talked to the experts at Sunoco Race Fuels, headquartered in Marcus Hook, PA. Sunoco Performance Products is the largest manufacturer of racing gasoline in the world. They gave us a ton of information about gasoline and race fuels.
The key point is this: detonation is the enemy. Anyone who's torn down an engine which suffered from detonation will not soon forget the signs: smashed rings, burned and even broken pistons, and bearings prematurely destroyed. Everything you do to fuel a race engine should be aimed at preventing detonation and its first cousin, pre-ignition.
But what are these demons, exactly? In the properly set up internal combustion engine, mixed fuel and air don't explode in the cylinders - they burn, with a steady flame front moving across the combustion chamber as the engine manufacturer intended. This controlled burn produces a strong, even power stroke.
Detonation - observed by the driver as knock or ping - occurs when ignition takes place other than at the spark plug. Suddenly there are multiple flame fronts in the combustion chamber instead of just one; collision of the flame fronts causes detonation and can seriously damage an engine in a very short period of time. The causes can include bad timing, the wrong fuel, changes in compression ratio and many other things.
Pre-ignition is just what it sounds like: the fuel ignites somewhere in the cylinder BEFORE the spark plug fires, possibly due to a hot ember of carbon buildup, a hot spot on the cylinder head, or other causes. Again, multiple flame fronts collide and cylinder damage can result.
The most common cause of detonation or pre-ignition, whether in a race car or on the street, is the wrong octane fuel. This leads us to a discussion of octane. The chemistry can get a little thick here, so bear with us.
Octane Is More Than a Number
You know octane primarily as a number on the pump where you buy gas for your street vehicle - 87 (regular), 91, 94 (premium) and so forth. Most people think of octane as a measure of how readily gasoline burns. Actually, it's a measure of its RESISTANCE to detonation. The higher the octane number, the greater its resistance to detonation or pre-ignition. Get it?
Octane is more than one number, also - and those multiple numbers can tell you a lot. Octane is calculated by one of two prescribed methods, each involving running a standard one-cylinder test engine with known compression ratio under carefully specified conditions. One method produces the Research Octane Number, or RON; the other results in the Motor Octane Number or MON. The single octane number you see on the pump is a result of averaging the two: (R+M)/2. This number is known as the Anti-Knock Index (AKI) and is regarded as the best overall measure of how well a specific fuel will resist detonation.
But let's look even closer. Sunoco explains that the RON is calculated with the test engine running at 600 rpm and the intake temperature set at 83 degrees F. MON raises the speed to 900 rpm and intake mixture temperature is 300 deg. (both use standard atmospheric pressure). Which of the two do you think more closely resembles racing? Right, the MON.
Two fuels can have the same AKI, but far different RON and MON. For example, an RON of 118 and an MON of 104 will give you an AKI of 111. You can get the same AKI from a fuel with RON equalling112 and MON at 110. If you've got a choice, for racing, Sunoco advises that you choose the fuel with the higher MON - it will likely give you better resistance to detonation under race conditions. Similarly, Sunoco spokesmen add, fuels with a smaller gap between RON and MON will generally fight off detonation better than fuels with a big spread.
The biggest caution when you buy race fuel: be sure you know what octane figures you're being quoted. Some fuel suppliers may quote only the RON because it's usually higher. Try to get both RON and MON numbers and do a little analysis.
So Many Fuels, So Little Time. . .
Now that we understand octane, what sort of octanes and fuels are available for the circle track racer to buy? In fact, where the heck does he buy them?
The first point we should make here is that today's race gasolines are highly sophisticated products. They are carefully formulated for specific racing needs and are quite different from normal gas. To cite an example, one of Sunoco's unleaded, 104 octane race fuels consists of about 120 different chemicals, the refiner says. Only 60-80 percent is traditional gasoline; the rest is additives put into the mix to improve performance under race conditions.
Let's study Sunoco Race Fuels' offerings to illustrate the choices racers have before them. Sunoco offers five different grades of race fuel to circle track racers and enthusiasts, two of them unleaded and one of them street legal:
As if that weren't enough, Sunoco also produces several of what might be called "specialty fuels:"
That's quite a range of choices. And they don't even touch on other fuels, such as methanol/alcohol and nitromethane, which are not gasolines and for which the octane ratings don't really apply.
Finding the Fuel You Need
Where do you get these fuels? Well, in the case of racing gasolines, sometimes you can buy them at your local service station! Sunoco owns a number of retail gas stations across the country where you can buy its racing gases, often pumping them just like regular. To locate these stations, Sunoco says you can call (800) RACE GAS or visit the Web site www.racegas.com. Other sources for racing fuels include race parts distributors, speed shops, and race tracks themselves.
You can also buy (or borrow) race fuel from your buddies and your competitors. The problem is, do you know what you're getting? We've demonstrated how important it is to know the AKI, the RON and the MON of any race gas you purchase. If you beg, borrow or steal from others, you might not be getting the full story on what the gas contains. Or, even worse, how long it's been sitting around - gasoline degrades with time (see sidebar). So, if you're concerned about your race fuel, it's best to buy a proven product from a supplier you know.
Which brings us to the part of the story you REALLY want to know about - how do you figure out what's the right fuel for your engine? When you pull into the paddock at your local track, what do you do?
The answer is surprisingly simple, according to Sunoco Race Fuels' Custom Fuels Manager, Art Brown: "Most race tracks and race series specify the fuel that must be run in their events," he explains. "In many cases they will designate an official fuel supplier, which will have tanks at the event for competitors. This fuel will be properly refined for the compression ratio and other engine specifications defined by the track or series. If you've built your engine properly, you should be able to use the spec fuel and have nothing to worry about."
If for some reason your engine is pinging on the spec fuel, Brown advises, the first step is usually to retard the timing. Taking timing out should let your motor perform under race conditions without detonation or pre-ignition worries. Then, after the event, try to figure out where your engine building and the track's specified fuel failed to match up.
In rare cases, there may be no spec fuel and you may have no idea where to start. For these situations, Sunoco's Brown supplies the following guidelines - emphasizing that they are ONLY guidelines:
"These are really misleading figures, because there are so many possible variations," Brown notes. "First of all, they are subject to any engine modifications you might have made. If you've altered the valve timing or overlap or modified the head, higher octane may be required."
"In other cases, you may be able to get away with lower octane than might be expected. For example, I know of some dirt track late model stock cars running 800 hp, 15:1 compression engines on 110 octane fuel very successfully. It all depends on the specific application. The above chart is only a very rough starting point."
The other technique you can use, Brown adds, is blending fuels to determine what your engine will be happy on. Start with an octane that you know will not cause knocking in your engine. Blend it 50-50 with the next lower octane fuel available, and see if knocking occurs. If things seem OK after a couple of tankfuls, drop to the lower octane fuel and try that for awhile.
"Keep blending your way downward until you find a fuel just above the point where knocking begins. If you do that, you'll be meeting your engine's needs, and paying less for fuel at the same time," the Sunoco manager advises.
Wrapping It All Up
So there you have it -- more than you probably thought you would ever need to know about racing fuel. To sum up:
The bad news is. . . if you're using the right fuel and still losing, there aren't any simple reasons for your lack of success. Back to the drawing board!
Online Editor: Christopher Kintner
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