Detonation is caused by two things.

1) Increase cylinder pressures. Which can be caused by either forced induction or higher compression pistons or both.

2) Gasoline that burns too fast and lights to easily. Race fuel is harder to light off, and burns slower. Cheap gas burns easier.

As you increase cylinder pressure you have to introduce fuel that burns slower. You have to do this because the molecules are closer together and the reaction will travel faster, thus concentrating the explosion time wise. What happens is explosion happeneds before you get to TDC, and the piston is trying to come up, and the explosion is trying to stop it from coming up. The piston then cocks side ways, and that is what you hear, the piston slamming into the cylinder wall. If you put a slower burning fuel (i.e. race gas) in the engine then the bulk of the explosion will take longer time wise and occur after TDC and stop the detonation, and you make more power.

You can also stop deonation be retarding the timing. What happens here is you light the mix later, or closer to TDC. By the time the explosion ocurrs fully, you have crossed TDC, and the explosion happens late enough not to cause detonation.

Timing and fuel are everything. As long as you control detonation, you can run whatever compression you want, but the higher the cylinder pressures go the more timing and fuel become critical.

Boost; 14.7 psi is the pressure of the air we breath at sea level. So we an engine that will draw in 2.0 litres of air at atomosphereic pressure. So if you put 7 pounds of boost on your engine, the engine thinks it is a 3.0 liter, because that is the amount of air you are moving through it. At 14.9 pounds of boost it thinks it is a 4.0 liter and so on. Ever notice that at 7 pounds of boost engines make about a 50% increase in power, that is because they think they are 50% bigger.

Compression ratio; Every 1 point you increase in compression gives you a 4% increase in horsepower. 9.0 to 10.00 on a 100 horse engine will yeild you 104 horse engine if everything else remains the same. And the same holds true for every point you increase. That is why a 12.5 engine runs better than a 9.5 engine. Now that holds true no matter what atmospheric pressure is. So a 12.5 to 1 engine will make more power than a 9.5 to 1 engine at 7 pounds of boost, at 14 pounds of boost and at 30 pounds of boost. BUT... the increase cylinder pressures require you to control detonation. As long as you can control detonation, you can run these boost levels.

Lets make this simple. We have a 100 hp engine with 8.5 to 1 compression. We put 15 pounds of boost on it, now we have a 200 hp engine. Now lets build a second engine with 12.5 compression. The first jump we goto 9.5 and get 4% to 104 hp, now we got to 10.5 anf get 108.16 horse, now to 11.5 and 112.48, and finally 12.5 we get 117 hp, now we add our 15 psi of boost and we got 234 horse power engine. So at 15 psi a 12.5 to 1 engine will make ruffly 17% more horsepower. That wins races.

Now because of detonation you may need to pull more timing out, and may loose part of that horsepower at high boost levels, but you will get there quicker, and on the bottom end of the race track you should make enough of a difference that the low compression car can't run you down on the big end of the track. With the electronics in timing we have a 12.5 to 1 15 psi street motor is a reality. Now if you go all out on a race engine. the 12.5 motor may be limited to 28 psi, where a 8.5 to 1 may be able to go to 30 psi without detonation, but I would put my money on the 12.5 engine.

My buddy has a drag bike running 35 psi on 13.0 to 1, but he is running on methanol which burns much slower than any gasoline. High compression and turbos do work.

gary

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