> Compression Ratio (CR) = V1 + V2 divided by V2. V1 is the
> volume of the cylinder with the piston at bottom dead center (bdc).
Not quite. V1 is actually the volume swept by the piston as it moves from
TDC to BDC. But your calculations were correct, so I think this is just a
mis-statement.
> V2 is the volume of space above the piston at top dead center (tdc).
Right.
> Wait a minute, that's exactly what you get on a stock TR4!
> This means when I figured out the CR on my own car I was either
> way off, or
> I am way off now. I was always under the understanding that the TR4 had a
> different head with a bigger combustion chamber but what I have
> figured out
> here would indicate that it's identical.
I believe part of the confusion is that the modified cylinder head didn't
come out until late TR4 and 4A. Most TR4s used the same head as the late
TR3A.
Another part is that the factory didn't update the CR specification when
they changed the head. That may be because they thought it would look bad
to lower the specified CR, or may have just been because the late head
didn't really change the volume all that much. I haven't measured one
(foolishly sold the only one I've ever had) so I don't know for sure. But
they fact they don't list a different CR for the 83mm option (even though it
undoubtedly used the same head as the 86mm and hence had to have had a lower
CR) suggests that they weren't entirely forthcoming.
> OK now I am copying the list for input - could someone help out on this?
> Now I have serious doubts as to whether I really do have 10.3 CR
> on my TR3 which is now 2200 cc.
Calculating backwards from factory numbers is risky business. If you really
want to know your CR, best to measure it yourself. But if the engine is
already built, and you can find gas that makes it run OK, then why not just
enjoy ?
The one I built to 10.5:1 knocked pretty bad on pump gas unless I also added
MMT. But I neglected to measure deck height and evidence suggests that my
block is a bit short and misshapen (like it's owner), so I may have overshot
the 10.5.
Randall
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