On Wed, 10 Mar 1999 Speedo79@aol.com wrote:
> Matt,
>
> According to my calculations, if you are running 8.7-1 cr., and atmospheric
> pressure at sea level is 14.7 psi, you should get an average of 128 psi across
> all cylinders. Since you rebuilt your engine, I figure you had the head and
I'd be very interested to see the formula for this...
14.7 psi x 8.7:1 would give you 128 psi, but unfortunately, that doesn't
work here... if you had a sealed piston and compressed it 8.7:1 >and
allowed it to cool< then you'd get your result. The air doesn't have a
chance to cool while measuring with a compression tester, and so the
numbers will be higher. It's also complicated by valve timing, as another
responder mentioned.
The equation above is a good minimum value though. If you have
compression more than 10% less than that, you have a questionable piston.
The old rule of comparing the compression between cylinders on the same
engine is best, though, since they are (theoretically) effected the same
way by valve timing and cooling.
I asked this same question during my first go-round on this list <g>.
For what it's worth, my 8.0:1 engine gives readings in the 140-145 psi
range.
--
John M. Trindle | johnt@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire H Stock | '88 RX-7 C Stock
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