I would think that volumetric efficiency would be a factor ( as far as your
compression gauge reading) only at engine speeds in excess of those achieved
during cranking. The flow rate through the engine at 300 rpm is low enough
and the time between TDC-TDC long enough to allow a relatively complete
"fill " of the combustion chamber. By the way, you are holding the throttle
open during the test, right? This restriction can affect the combustion
chamber 'fill' and subsequently the final pressure developed during the
compression stroke. Even more obscure is the consideration of the volume of
compressed air which is contained within the gauge during measurement. This
compressed air is essentially bled off the combustion chamber pressure,
reducing the observed measured pressure. Taken to the absurd, an enormous
gauge could bleed off a large fraction of the true compressed air within the
chamber.
I'm very much in agreement with Dan B- The real value in a
compression test is the determination of whether the cylinders are evenly
matched. The absolute pressure is of less concern, unless of course you see
25 psi across all of 'em! That being the case, you might consider listing
the car on E-Bay!
Bob Westerdale
>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.
>
>
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.
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
|