ive seen as much a 30psi of increased compression through retard cam timing a few times. it only take a few 100ms for the whole combustion process to occur as it is anyways. when you have you timing not in sync with the stroke of the piston you can get a increased pressure due to a compress extra air but releaseing it late.
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if you havent been in a small plane its worst, cause your eyes and brain say we are moving and your body says oh no we are not. eyes and brain go asshole we are moving, cant you see? body says fuck you we are not moving at all. so they fight and fight and eventualy your body says, you know what fuck you brain, see if you can stop this!! then you throw up.