Yes it is a design compromise, DA, usually based on the detonation margins existing for that engine design. Fuel octane, quality etc all have a part to play, plus ultimate output from the engine. Any time you drop compression ratio, you lose power and efficiency. Manufacturers use different "tricks" to change compression ratios, including piston deck heights, but sometimes they will also vary the combustion chamber size too. Another trick is to use "dished" pistons rather than flat topped ones. All methods vary the combustion space available, at TDC, not the actual cylinder volume in any appreciable way. If you want to vary the cylinder volume you either have to change the bore or the stroke. That's much more expensive!!!!!
Cheers.
Actually, that is a bit simplistic. I am of course referring to the static compression ratio. Turbo or Supercharging does change the dynamic compression ratio, it has to, since you are compressing a bigger volume of mixture. In a dynamic sense, the compression ratio in every engine varies somewhat in practice, according to many variables. Static ratio is what is normally quoted as a comparison. That is of course as you said, the comparison of combustion space with the piston at TDC compared with the cylinder volume at BDC.
Last edited by Dale Harris; 8th Sep 2004 at 02:02.