Dynamic compression ratio

[MalcolmV8]

I'd never heard of this till a few days ago. I was wondering how this one 03 Cobra got rebuilt with 9.7:1 compression ratio from the stock 8.5:1 compression ratio and was still running high boost. The guy said because of his aftermarket cams the dynamic CR was still fairly stock. Around 7.7 if I remember correctly.

Interesting read.

http://cochise.uia.net/pkelley2/DynamicCR.html

[87ranger]

thats true and very few people really catch the concept of it, while your mechanical compression maybe 9.7 to one, that is assuming 100% cyclinder filling and trapping of those gases in the cylinder,obviousely that doesnt happen in the real world, pumping losses and camshaft centerline/duration/ overlap and blowdown time on the exhaust valve cycle reduce the eficiency of the engine by 1.5- 2 points, thats why in the 60's and 70's when a lot of guys put a monster cam in theyre engine they had to bump the compression a couple of points to ragain lost efficiency from the new cam install, your cobra motor that might have 10.1static right now only has 8.1 dynamic, hope this helps

[MalcolmV8]

Yeah it was something I never realized until recently myself.