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Hi all, just wondering about compression in engines, what's better, high or low compression?

Edit: also what would be considered a good compression for my DOHC?

<small>[ June 11, 2004, 03:00 PM: Message edited by: SpaceWiz ]</small>

Compression is the ratio of the piston at BDC (bottom dead center) to TDC (top dead center). In other words, 9:1 means every nine units at BDC are being compressed to 1 unit of equal amount.

The higher the compression on NA motors, the more power they will create per cylinder. However, the higher compression is, the higher an octane is needed to prevent early detonation. On forced induction motors, lower compression will be capable of takinf a higher psi boost, but high compression engines will provide a greater gain in torque and hp per psi increase. They will also require more frequent, precise tuning then low compression set-ups.

Also, I believe a high compression set up will provide much greater torque then any low compression engine. I'm going by the 4 cyl volkswagen TDI, which has a ration of about 19:1. It produces 100hp, and 161 ft lbs of torque. Of course, other characteristics of desiels engine may also explain the extreme torque...not quite sure on that one.

As for the DOHC, I would say stock compression is optimal unless you plan to do major work on it.

Ok, thanks for the detailed answer
The reason i asked in the first place is because i crashed the main bearings on my stock motor and got a new DOHC very cheap, and that one is tested to be at 14:1 on all cylinders. Getting that one installed this week so i can finally get out on the road again.