Cylinder Pressure vs Cylinder Size?


Well-Known Member
Mar 3, 2019
I "designed" my own cylinder head for this little 40cc motor I'm using and figured my 1st clue to if I got it right would be a compression test. Uh-oh. Only 75 psi. Even after I oiled up to simulate the rings being broken in it only got up to 95 psi. Based on past experience below 100 psi they won't even start!

This led to the inescapable conclusion that I must have got the dimensions wrong in the combustion chamber. That led to days of double and triple checking the math that I used to predict the volumes that I cc'd all over again in the head itself. I have another head for a 44mm piston that I was able to get centered and sealed with a very similar 5.1 cc chamber and got the same results.

Now I'm thinking there's gotta be something wrong with the piston. After lots of practice getting a different number every time I tried with my telescoping bore gauges, I finally settled on there's got to be less than 2 thou clearance in the cylinder. Checked again and did the math that proves there's only 27cc's above the exhaust port. Minus 2.5 cc's of piston dome in a measured 5.1 cc chamber means it has to be a 9 to 1 compression ration. Anything else is physically impossible.

I was stumped. Didn't know what to do.

But then, during a routine phone call with my brother he told me about a PW50 he recently got running. It had a bad switch but while he was working on it he did a compression test and found something like 50-60 psi. A new Chinese top end's only $25 so he popped one on thinking to get it in tip top shape ... and it still had the same compression! But it runs just fine.

It's finally starting to dawn on me that my "below100 psi won't start" rule wasn't on anything smaller than a 125. Could it be smaller cylinders make less pressure ???

Yeah. It would seem so.

I plugged in on a 70cc Honda clone I have in the herd that runs like a champ and found 75 psi. At 95 my little 40's gonna pop!

So, after a week or more of chasing my tail thinking I must have made a mistake ... I found out I was wrong. :)