compression ratio??

[tretz]

Can someone figure my comp. ratio for me ?

4.030 bore
3.00 stroke
-.028 piston position
frpp gasket set head gasket
58cc chambers

not looking for the exact # but somthing close
If you need more info, ask

 

[John Z]

Compression ratio = Compressed volume/(compressed volume + swept volume)
Compressed volume = head + gasket thickness + "piston position"
head = 58 cc = 3.54 ci
gasket thickness volume = T*pi*(d/2)^2 = 0.04*3.14*(4.03/2)^2 = 0.51 ci
Assumed 0.04" thickness
"piston position volume" = same as above but use 0.028 for T = 0.51ci
swept volume = same as above but use T=3, 38.27

So the compression ratio is about 9.7:1

 

[tretz]

thank you, how much do I lose to the valve reliefs ?

 

[D.Hearne]

He's not going to get 9.7 with the piston that far down the hole. Multiply the bore radius times itself (2.105x 2.015) then times pi (3.1416) then times the stroke ( 3.00) then times 16.387 ( to get the swept volume to ccs) Gasket volume is going to be 9.5 ccs. volume of the .028 deck clearance is about half that ( 4.5 ) Volume of the piston top is about 5 ccs for flat tops with 4 valve reliefs. I don't have my calculator handy but I can tell you that the ratio is going to be close to 9 to 1 give or take a few tenths of a point. To get 10 to 1, you need zero deck pistons which will have a pin height ( comp height) of 1.610.

 

[tretz]

didn't mean to start any arguements I just was curious, I just wanted to end up between 9 and 9.5 and it looks like i'm close, thanks again for the response

 

[John Z]

My original calc did not include the valve relief. With a 5 cc valve relief the compression drops to 9.1. The valve relief was not mentioned in the first post, so I left it out. I'm suprised with how much it affects compression ratio.

 

[D.Hearne]

It doesn't take a lot of ccs to affect the ratio with small displacement engines. Not many take the time to figure it out, and wind up with less than they thought with 289-302's. The key here is piston choice, not all 302 flat tops have the correct compression height to get the ratio up to 10 to 1. Many have a 1.585 comp height, but you want a 1.610 to get it up there for aluminum heads.