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]

Compression ratio and camshaft selection go hand in hand, although it is an extremely complex topic and one that can't be thoroughly explained in an internet forum.

The first thing to consider is that the static compression ratio is directly related to where in the rpm band peak torque will be made. The higher the compression ratio, the higher in the rpm peak torque will be made in a properly designed engine. If you have an engine with a relatively low redline, say 6000 rpm, a high static compression ratio will do you no good. You will make better power by keeping the static compression ratio low. Conversely, if you have a relatively high redline, say 9000, you need a correspondingly high static compression ratio. Determining how fast you can spin your engine is another topic entirely (search 'piston speed').

Keep in mind that most of these normal engines with normal valve trains have a peak RPM of 5500...really nothing left after that.

Proper valve geometry setup will result in a higher RPM range..and a more matched setup for the motor...cylinder versus cylinder.


'static compression ratio'. The formula for this is:

cylinder displacement + combustion chamber volume / combustion chamber volume

This is the 'normal' compression ratio you will hear guys talk about (i.e. 9:1, 10:1, 13.5:1, etc.).

'dynamic compression ratio' which is the effective compression ratio multiplied by the volumetric efficiency. This is the actual 'running' compression ratio the engine sees while it's doing its thing, hence the term 'dynamic'.


So what ends up happening is, the higher your static compression ratio, the later you want those intake valves to close to keep the effective compression ratio around 8:1. This means by definition your cam will have more duration. So the higher the static compression ratio, the more duration your cam will need. The lower your static compression ratio, the less duration it will need.

There is alot more involved...effective compression ratio is the Cr when the intake valve closes...to when the exhaust valve opens...and as the name implies...its what you really get for a CR...but its very tied to the camshaft selection.


When you buy a cam for your build you need to talk to the cam manufacturer and ask them what is best for your engine. Picking it yourself based on how slick the marketing terminology is, is a recipe for disaster and unfortunately that is how most cams are chosen.

You have other options if you do not want to shim your cylinders that much.
redo the chambers on the heads...make sure they match and CC them very accurately.