On 25 Dec 2006 at 13:01, Michael ONeill wrote:
> I'm thinking of changing the pistons from the dished out type that have a
> 7.5 to 1 compression ratio to the flat type that have a 9.0 to 1 ratio. Do
> you think this is a good idea?
Don't know nuffin 'bout pistons, but the 9:1 ratio is nice if you
don't mind having to be particular about the gas you run. The '76
engine was 9:1 but the difference was the heads,so they tell me. I
say this because my own engine is a '76 in an '80 car. It was
spec'ed to use 95 octane gas back then. That's not available easily
today, though there are so many other differences in the gas that I'm
not sure how much stock to put in that spec specifically. Over the
years I've gone through great pains to set my timing to prevent the
engine from pinking. Under full throttle in some conditions it still
does and I don't think any amount of spark retard will prevent it
except unacceptably late. With roughly 20 years since its rebuild
this engine is still going strong so it must not be too bad.
Of course, that higher comp ratio must be giving me more power, and
more power is always good. I don't have any standard by which to
judge it, never had a different Spitfire. But as far as the comp
ratio is concerned, I'd say it's a good tradeoff. Someone else will
have to comment on the pistons.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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