compression question

[racerdad33]

was wondering how to convert compression psi to ratio sauch as 10:1 and so on.anybody know how to do this?

 

[David Trustrum]

No sorry not directly related.

To get the ratio you need to measure the volume of the head with piston at TDC. Add that to the cylinder size to get total volume (ie what the engine draws at BDC) then divide that number by the first (head) volume.

ie: 124cc engine with 10cc head = 13.4:1

The so called 'corrected' ratio takes into account the height of the exhaust port to get the lower numbers manufacturers often quote.

I can hear you tuning out from here. :confused:

 

[FLBob]

Say atmospheric pressure is 14.69 psi (29.91 in.Hg. (inches of mercury))(It's different everywhere and always changing a little.) 10:1, 14.69 X 10 = 146.9 psi, to answer your question. I doubt the direct conversion has much relevance in the real world.

 

[FLBob]

Sorry-re-read your post and it won't let me edit. I gave it to you backwards.
Divide your psi by 14.69. You got 160 psi, divide by 14.69, you got compression ratio of 10.89. (Perfect world, no leakage etc.)