Reading Georges post on the wide torque curve got me thinking. There seems to be some misconceptions on timing and the ability to run high compression motors on pump gas. There are many things that affect the anti-detonation properties of any motor. Port velocity, squish, chamber size/shape, plug placement, and bore size are some of the most important. Squish alone can make a motor with 11 to 1 compression have better anti-detonation properties than the same motor with 10 to 1 and poor squish. We can alter most of these with cylinder head design and gasket thickness. What they are trying to promote with these is a faster burn speed. A faster burn is much more resistant to detonation. Chamber turbulence is a MUST. The old cleveland heads have large valves for lots of airflow but do not have the best chamber design. Some are better than others but regardless there is lots of room for improvement. With bigger valves and larger runners the air entering the combustion chamber (particularly at low rpm's has a poor swirl because of low velocity air. Newer designs utilize smaller runners, more central placement of spark plug for better flame front propagation and specifically designed quench pads to aid in keeping the air turbulent as the piston comes up. With this significantly faster burn speed peak cylinder pressure happens sooner in the cycle. It has been found that peak cylinder pressure should happen around 15 degrees ATDC for peak torque production. If our burn is happening faster and peak pressure is happening sooner we have to reduce timing to make best power. We tune A LOT of modified muscle cars and have found that we end up pulling timing not because the cars are detonating but because they are plain and simply over timed. We have seen power gains in excess of 20HP by LOWERING timing by several degrees. Aluminum heads also have a higher detonation threshold because of their ability to shed heat faster. Remember, more timing does not always = more power.
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