Hitting the exact Compression Ratio Options

[malcolm2]

I am to the point on my rebuild that I need to make a decision. (IMG:style_emoticons/default/confused24.gif)

I am sure that this type of discussion can become very technical and fill volumes of books. I am not asking for that.

I am the kind of person that wants to know "why" a certain cam needs a certain CR. In order to make the best decision, I need answers. Or if it is a huge secret, that is fine too. I can go blindly along with the crowd.

Maybe it's really what. Has anyone experienced What an engine will do, or what will I see or feel if my cam mfg recommends 8.0:1 CR and my normal state (no shims) is 8.35:1 CR? To get to just about 8, I have calculated that I need about 2mm of deck height. I am currently sitting at 1.62mm average DH with a variance between cylinders of 0.15mm. So I need two 0.4mm shims and two 0.5mm shims and I will be dead nuts on 8:1. (IMG:style_emoticons/default/stirthepot.gif)

So the mfgr's value can be reached (if I can round up the shims), but I am curious to know how differently the engine will function if I go shim-less or even only get half way, maybe 8.2:1. (IMG:style_emoticons/default/WTF.gif)

[mark21742]

Most cam manufacturers give you minimum compression ratio recomendations....if you go higher your engine will be a tad peppier....for every full point of compression (going from 10:1 to 11:1) you can expect a 3-4% gain in torque and HP acrossed the rpm range.
With longer duration cans you need higher static compression to get the same dynamic compression you would get from a shorter duration cam because, or the longer duration cam the intake valve stays opened longer after bottom dead center, and you don't start to build compression till the intake valve closes...

For instance my 408 in my 04 GTO had a static compression of 12.49:1, but since the intake valve doesn't close till 85 degrees after bottom dead center it comes out to only 8.22:1 dynamic compression.

Hope this helped a little