>Rod MacLeod wrote:
>
> > Can anyone tell me the proper compression in pounds per square inch for a
> > healthy 1976 carburater model tr6. All I ever see is a ratio (7.75 :1) and
> > I'm no math jeanie ass.
>
>Can it be as easy as multiplying atmospheric pressure (14.7 psi) times
>compression ratio? For 7.75 that would be 114 psi. That sounds low but so
>does 7.75.
>
>I assume that whatever the formula is it will vary based on your altitude.
>
>Geo Hahn
>59 TR3A
>Tucson, AZ
You're right, Geo ... with a small "but". You also have to consider
what's called 'volumetric efficiency', i.e. the ability (or
inability) of the internal combustion engine to completely fill the
combustion chamber with the air-fuel mixture during the intake stroke.
Now, I'm recalling from a long-ago memory of reading a couple of
books on turbocharging, so my number may be off, but I believe that a
"normally aspirated" engine (not supercharged or turbocharged) works
at about 85% volumetric efficiency.
Basically, the engine just can't quite fill itself up in the short
time the intake valves are open due to the flow restrictions of the
air filter, carbs, manifold, etc.
That being said, my Bentley manual quotes 8.5:1 for the compression
ratio on my TR3 and also quotes 120 psi as the compression pressure.
If you do the math, those numbers would indicate 96% volumetric
efficiency based on 14.7 psi atmospheric pressure. (so maybe I'm
full of it)
What's more important than the "correct" reading is consistency of
the readings across all your cylinders. Readings much higher or
lower than the rest may indicate carbon build-up, broken rings, etc.
I've heard from some folks that you should be concerned if your
readings are more then 10% different between cylinders; others say 10
lbs.
Hope this is helpful and that I'm not just babbling!
Dan
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