So I sold for a friend a pair of low-compression U heads (iron). They are stamped 10.5 and have an enormous chamber (the deepest U chamber i’ve ever seen). Heads were taken from a sidecar U bike, never been milled down. He called me today asking what compression ratio for these heads. My calculations led me to 4.5:1 for a standard 73.79” displacement.

I used the formula:

Combustion Chamber(or 10.5) = displacement of 1 cylinder(or 36.895)/Comp. Ratio – 1

My questions are:

Is that the correct formula?

What should he expect from this compression ratio for a solo bike? Too slow?

Looking for some opinions on this. Thanks.