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compression ratio formula and opinions

Moderators: Curt!, Pa

Post Wed Dec 16, 2009 3:59 pm

Posts: 83
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.


Last edited by panic on Sun Nov 28, 2010 6:44 pm, edited 1 time in total.

Posts: 83
Thanks Panic
Glad to know my math was right.
Also thanks for your suggestions.
I'll talk to him about the mods you listed.
Best regards.

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