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)

[r_towle]

if you had the case "checked over" they should have tested and measured each cylinder register in the case to ensure all four are the same distance from the centerline of the crankshaft...its kinda important and its honestly not to late if your measurements have a variation....better now than 100-400 miles after you finish.

Are you bolting down each cylinder and using a plate on top to get an accurate reading?

What is your measuring technique?

This motor requires a partial assembly...measure for shims (one ring, or all rings to keep the piston centered) and then take it all apart to put in the proper parts...

torque down the cylinders each time.
Check before and after you put in the shims to make sure you are right....
Then do final assembly.

So, really its two assembly times, but sometimes three if you are cautious.


RIch