How do you get the compression ratio of a motor? Is there some kind of formula?

 

2-stroke are harder to figure compression on since they have the ports in the cylinder and the intake charge is somewhat pressurized.
4-strokes aren't that hard. You must know the cylinder displacement, the size of the piston dome, the thickness and diameter of the torqued head gasket, and the size of the combustion chamber. Take the combustion chamber size in cc and add the volume of the head gasket. Subtract the dome size in cc from this number. Divide the cylinder volume by the new number. This is an estimated compression ratio. To be exact you must calculate ring height, valve pocket size, and many other factors.

It took me an hour to write this due to interuptions, so if someone has posted before I finished, I appologize.

 

Thanks for the info. Seems like alot of trouble.

 

why not use a compression gauge and thread it into your spark plug hole and kick the motor over until the little needle no longer moves and that till tell uyou your compression in pounds per square inch that sounds a little easier huh?

~Eric

 

buy doing this it will give you a psi number which you can divide by 14.7 to give an estimated CR. the problem is that if your rings or seals are not good it will read a low psi and then give you a false CR. this is a mickey mouse way of doing it. If you are doing this because you are building you engine and want to see if you will need high octane fuel than the results will lead to failure. try finding out as much info as you can then go here.http://www.csgnetwork.com/compcalc.html

 

When checking the compression with a guage like MX265X said, you must open the throttle completely while kicking it over or you'll get a false reading!

 

My compression is 220, I have a compression checker. I was wondering how to get your compression ratio.