My Thoughts - Saab Performance Bulletin Board - Saabnet.com
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My Thoughts
Posted by Eric [Email] (more from Eric) on Wed, 11 Apr 2001 23:35:37
In Reply to: Good point, so what is the term for that? n/m, Jeff Cunningham, Wed, 11 Apr 2001 18:08:23
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FIRST, Your compression RATIO always stays the same i.e. 9:1. This means the engine sucks in 9 parts of matter when the piston is at BDC, Those 9 parts will be compressed into the volume of 1 part when the piston reaches TDC. Atmospheric pressure is 14.7 psi. If your compression RATIO is 9:1 on a N?A engine, at BDC pressure will be that 14.7 psi, but when the piston reaches TDC, that 14.7 psi will become 132.3 psi.
SECOND, I thought that pre-detonation is when the timing is advanced too much to compensate for the increase in the volume of matter that is compressed, which as it becomes more dense, the combustion of that matter becomes more rapid and violent, causing the peak of the combustion to occur before TDC. Under normal conditions, the spark is triggered before the piston reaches TDC to allow for the combustion process to proceed so that the peak energy to be released when the piston is at or right after TDC. When you retard the timing, the spark occurs closer to TDC, compensating for the increase in volume/shorter combustion process and preventing pre-detonation.
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