Cam Speed Characteristic Compression Pressure Effect
|Installed Centerline||Advanced ||Higher|
Building Detonation Tolerance
It is widely recognized that higher compression builds more power, and this is true up until the point the engine begins to detonate, which is highly destructive. In the years following the phase-out of high-octane leaded pump gas, most of the advice we've run across in regard to ratio is to wind it way down to avoid detonation. The idea of avoiding detonation is still valid, but dropping the ratio always costs efficiency and power. OE manufacturers have been steadily raising compression ratios, and in fact many current production cars carry compression ratios much higher than the actual ratios used in the '60s. Many things can be done when building an engine to increase its detonation tolerance and to allow a higher ratio to work without detonating. Build an engine with steps to reduce the tendency to detonate, and you can run more compression than the next guy. Here are some of the most effective ways.
Quench: We have already talked about quench as a separate topic, and it is nothing new in the world of engine design. The desirable effects of an effective quench area have been documented since early in the last century. One of the key reasons quench affects detonation tolerance is because the most likely part of the charge to auto-ignite and begin detonation are the gasses remaining in the far end of the chamber away from the plug. Again, about .040-inch piston-to-head clearance works really well in a street application. As the clearance is increased, the effect diminishes, and by about .060-.070 inch, the effect is lost. In fact, an engine with a dead quench area with a large volume of end gas can actually induce detonation.
Thermal Barrier Coatings: Coating technology has come a long way, and these days, thermal-insulating metallic ceramic-coatings are fairly common. In terms of detonation tolerance, the biggest bang for the buck is to have the chamber face of the valves coated. The hot valves, particularly the exhaust valve, impart a large degree of detonation inducing heat to the mixture. Coating the valves greatly reduces this heat transfer and increases the detonation tolerance. A set of coated valves can be worth as much as running fuel with three points higher octane.
Heat: Another key component in detonation tolerance is induction-charge temperature. Taking in a cooler intake charge makes the engine less likely to detonate. A cold-air intake is a step toward this goal, but it's important not to neglect how much heat is added to the charge on the way into the engine. The exhaust crossover in the intake manifold can add tremendous heat to the mixture. Blocking it-especially when using an aluminum intake manifold-can dramatically lessen the potential for detonation. Coolant temperature has a similar effect, though to a lesser degree. A high-capacity cooling system and cool thermostat reduces the likelihood of detonation