> Now I am confused. I was told that there are two types of pistons, the
> flat top variety prior to 1974 1/2 that yielded a stock CR of 8.7 to 1, and
> the later dished that gave a standard 8.0 to 1. Have I been misinformed?
>
> Regards,
>
> Chris Delling
Chris,
If you're not dealing with original parts, be aware that the volume
of the combustion chamber in the head changed with the introduction
of the 18V engine. The older pre-18V heads have larger combustion
chambers, and so the dished pistons used with them had a shallower
dish to keep the compression ratio up. The newer heads had smaller
combustion chambers, so the pistons had to have larger dishes to
keep the compression ratio down. If you mix and match parts, bad
things can happen. Small-chamber heads and small-dish pistons will
give you too high of a CR, leading to pinging or worse.
Large-chamber heads and large-dish pistons will give you a too-low
compression ratio. This is the setup I have. Not harmful in and of
itself, but you don't get full power from the engine. On the other
hand, I can run 87 octane gas all day with no pinging!
So, we now have large-dish, small-dish and flat pistons. Any other
varieties out there?
Scott
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