Race fuel is for racing? Of course it is.
Anyone who’s heavily involved in motorsports is likely to have a collection of cars and parts along with a supply of five and 55-gallon fuel containers. But, why does racing require race fuel? Does higher quality race fuel make more power? Absolutely. In fact, race fuels can produce more power and reliability when combined with the proper engine calibration. The tune for the engine must be optimized to match the burn rate, specific gravity, and stoichiometry of each fuel. The ideal ignition timing, air-fuel mixture, and boost levels depend heavily on the type of fuel used.
Text by Tim Bailey // Photos by DSPORT Staff
DSPORT Issue #131
Real World Tuning
To illustrate the performance differences between different fuels, the DSPORT Project R35 GT-R will be dyno tuned on three different fuels using COBB’s AccessTuner tools for proper engine calibration. The three test fuels (91-octane, 100-octane unleaded and E85) represent a broad range of differing fuel qualities and characteristics. Octane, a measure of detonation resistance, is familiar to everyone. Most people understand that higher-octane fuels are more resistant to detonation. Detonation, something every tuner wishes to avoid, is the uncontrolled explosion (rather than controlled-burn-rate of combustion) of the air-fuel mixture in the cylinder. Detonation creates incredibly high cylinder pressure spikes that produce an audible knock. Detonation can destroy engine components.
It is the instruction set for the motor, the calibration or “tune”, which is responsible for maintaining controlled combustion and preventing detonation. Through combustion, the energy created in the cylinders can be converted into maximum mechanical force by the engine. An ideal engine calibration delivers the best possible power and economy while avoiding detonation. Engine calibrators have a limited number of control parameters to adjust for fuel types and their unique detonation resistance and burn speed. In this Tuning Tech installment, we will present three core engine control parameters (ignition timing, Air-Fuel ratio, and boost pressure) and demonstrate how they vary with fuel type.
Ignition timing: It Starts at the Spark
Ignition timing is normally described at the point in crank rotational degrees before top dead center at which the spark plug is fired to initiate combustion. The point at which the air-fuel mixture is ignited must be timed so that combusting gasses push down on the piston through its downward stroke. When riding a bicycle our leg naturally starts to push down and create torque just after the pedal begins its downward stroke. An engine works in the same fashion. Hence, the combustion or power stroke of a four-stroke engine must be “timed” properly. When correct, the forces of expanding gasses push down on the piston through the longest range of its downward stroke. Earlier or “more” ignition timing generally creates more torque and power to a point. However, timing must be matched to the burn rate of the fuel and is dependent upon a vast number of factors. Two of the most predictable factors are engine load and speed. Higher engine speeds often require earlier ignition timing. The reason is that burn rates remain relatively constant at a steady load, but the total time for combustion decreases as engine speeds increase. As such, spark timing must be advanced at higher RPMs and decreased at lower RPMs.The opposite is true for engine load. Higher engine loads (generally higher boost pressure) result in a cylinder mixture that burns at a higher rate. This faster rate of burn at higher load means combustion can start later to extract maximal mechanical force.
Ignition advance timing controls the start of combustion to best create mechanical force and torque and control cylinder pressure. Timing increases with RPM because total time for combustion decreases. Timing decreases with increased engine load because more densely packed air and fuel mixtures burn faster compared to lower load (lower boost) conditions.