if a set of pistons will give me a 10:1 compression ratio with 60 cc combustion chamber'd heads, what will a 65 cc head give me? how can I calculate this?

 

you would need to know the internal displacement of 1 cylinder, both at TDC and BDC

add the displacement of the cylinder at BDC to the 65cc chamber volume, then divide it by the displacement of the cylinder st TDC with the 65cc chamber added

this will tell you your new CR

 

you're the man!

 

the displacement at tdc would be smaller than the displacement at btc right?

 

way smaller

 

The piston maker should be able to tell what the compression would be, look in his literature about the pistons........

 

they said with a 60cc head the ratio would be 10:1

I want to find out how to make that smaller, like 9:1 or 8.5:1

 

yea.. i know


however, running a 65cc chamber isnt going to drop your CR too much from a 60cc chamber

 

how would i lower that compression ratio without buying new pistons?

 

You have to factor in the volume created by the thickness of your head gasket, the cc of your pistons and the "drop" of the pistons (in the hole) to both numbers (which is what he aluded to).

If you take all those things together and call it "X", then the ratio is:

(625 + 60 + X) / (60 + X) = 10.0
for the manufacturer's cited chamber volume where 625 = 5000cc/8cylinders.

Solving that we get X = 9.444444.

By using a 65 cc chamber, you end up with (625 + 65 + 9.4444) / (65 + 9.4444) = 9.39553 CR.

So you dropped to 9.4:1 by my quickie calculations.

If you were to add 5cc to X by using a thicker head gasket (on top of the 65cc chamber), you would end up with 8.867:1.

To figure that, you'd need pi * radius^2 * h = 5cm^3 where the radius is in centimeters and the thickness is in centimeters. A google on the engine shows a straight 4" bore, which is 102mm or a radius of 51mm, or 5.1cm.

So, h = 5/(pi * 5.1 * 5.1) = .06122 cm or .6122 mm or .0241 inches. By that calculation, you'd need a 25 thou thicker headgasket to get an ~8.8:1 compression ratio.

Does that sound right to you guys?