Gasoline

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What octane gas should I use?

What's the point of running an SHO on 87 octane gas? Why do people buy hot rods, then replace the tires with cheapies, run gas that will make it under-perform, etc. Could saving 20 cents a gallon really make a big difference in the annual costs when you consider oil changes, tires, insurance, plates, depreciation, brake rotors and the clutch budget?

From Gary Morrell:

Economics, personal style, and availability have a lot to do with this behavior. The SHO is only part hot rod, the rest is conventional 4-door sedan, and that's how most of its buyer's use it. Folks like me whose SHO is modified to the point of barely being roadable are a tiny, tiny minority. I see plenty of SHOs with 2 baby seats strapped in the back, which would get in the way of the attachment points for my driver and passenger 5-point harnesses. ;-)


At high altitude, fuel runs from 85 to 91 octane, not the 87 to 93 that flatlander's are used to. The 20% thinner air reduces the engine's effective compression ratio so high octane fuels simply aren't useful.

If you look in the SHO owner's manual, on the dash, and on the gas door sticker, it says "Premium fuel recommended", not required. All Ford engines, regardless of the intended performance level, are calibrated on regular fuel (87 octane), not premium. The reason for this is to obtain a calibration that will give optimum drive-ability under a wide variety of environmental conditions and available fuels.

Because the Yamaha engine is not particularly octane-limited, meaning that the compression ratio is fairly reasonable and the engine can tolerate large amounts of spark advance, (which helps to make more power) the difference between 93 and 87 octane would probably mean an approximate 3% decrease in low RPM torque under adverse conditions: like a fully loaded car and 95degF ambient temperature. Remember, higher octane fuels do not produce any more power when burned; octane is a measure of a fuel's ability to resist premature ignition when subjected to heat and pressure. The higher the octane number, the less likely a fuel is to pre-ignite. High octane fuels allow engines to produce more power because they allow more spark advance before pre-ignition occurs. It is of no benefit to put 100 octane fuel into an engine that doesn't have the spark advance dynamic range to take advantage of it.

Under warm cruise (closed-loop) strategy, EEC will push the spark advance out to the point where trace knock is detected, and then back the timing off slightly, to keep away from pre-ignition. This strategy makes sense to get the most efficient burn and the most power. Under these conditions, you may hear some knock if you mash the gas, because it takes EEC a few CPU machine cycles and crankshaft rotations to drop out of closed-loop strategy and retard the timing.

Under wide-open-throttle (WOT) conditions, EEC is not looking at the knock sensor because the engine is simply making too much noise for the knock detection strategy to filter out the knock signal from the noise. Timing and fuel for WOT is derived from lookup tables that are vectored primarily by crankshaft RPM and engine coolant temperature, and to a much lessor degree by the mass air signal.

Under most conditions, EEC should be able to get the most out of any reasonable fuel that you pump into the tank, however, an especially bad load might be beyond the range that the spark lookup tables can compensate for, so pulling the octane shorting bar invokes a new set of tables, with less aggressive spark advance curves for WOT operation.

Here's another good discussion on octane level:

GH: Gerald Harmon, GM: Gary Morrell.

GH: Most tracks offer unleaded race gas. It's usually 100 octane but I've heard some tracks have 104 but I've never seen any.

Used on occasion, leaded race gas will not hurt the O2's. Use it a lot and their life is shortened quite a bit. I used it in my Mustang quite a bit with good results.

GM: Leaded fuel will also foul the catalytic converters, and right quickly I might add, a few tanks of leaded fuel and the cats will be toast. The long term effect of lead poisoning a cat isn't pretty, the ceramic monolith eventually starts to fall apart, and we all know what that leads to in SHO's...

GH: Lastly, when race fuel is not needed it will actually slow a car down. It does burn slower.

GM: Actually flame front travel isn't all that different for various fuel octane grades, but difficulty of ignition is the culprit, especially if high octane fuel is run in a low or moderate compression engine. (The end result is that running high octane fuel in a low compression motor has the same effect as retarding the timing... less power.) Also keep in mind that the Yamaha's 4-valve pent-roof combustion chamber with its centrally located spark plug is about near-perfect, which means it can extract more power out of the burn than a poorly-designed combustion chamber. Some of the venerable small and big block high compression V8's of yore had absolutely rotten combustion chamber designs, making their octane requirements higher than the more efficient designs of today. Additionally, modern engine management systems are able to detect trace knock and manage spark timing much more effectively than older systems with fixed mechanical and vacuum advance curves.

GH: For those in doubt, Steeda (Mustang shop) actually did a dyno test on this a long time ago and use some 92 vs. some unleaded race gas (100 or 104) and the car lost 6-7 hp. You should use the lowest octane you can get away with.

GM: This is excellent advice, I've compared 92 PON street fuel to 100 PON race gas in my SHO at the local road course, and average lap times on the race fuel were just a tad slower, <0.5 sec for a 2-mile course. We even tried advancing the timing for the race fuel, all it did was raise the engine coolant and oil temperatures. Disabling the knock sensor in software made no difference either.

A good FAQ on gasoline in general can be found here. (Broken Link. ShoAdmin 11:44, 28 February 2011 (MST))