Interesting. I've always heard to check compression with the plugs OUT ...
In fact, I believe my Chevy manual says to check with the plugs out and
the throttle open.
Anyway, the compression gauge reading varies a fair amount with things not
related to engine condition, like the gauge itself (my Sun gauge has at
least a cc of dead space, which will lower the reading), altitude, etc.
The variation between cylinders is the thing to look at. IMO 10% from
highest to lowest is about the best you'll see on a stock (non-blueprinted)
engine. Even 20% probably isn't anything to tear down an engine for.
Somewhere around 40% I start planning on a rebuild REAL soon <g>
Randall
On Monday, March 22, 1999 9:24 AM, Andrew Mace [SMTP:amace@unix2.nysed.gov]
wrote:
>
> On Mon, 22 Mar 1999, George Hahn wrote:
>
> > What is typical compression (lbs) on an unmodified 1991cc TR3A engine?
>
> The figure quoted in the original "Service Instruction Manual" (so
> probably for TR2-spec. engine) is 120 lbs./sq. in. The complete
> description of how this is measured fascinated me in reading it: "(With
> three Sparking Plugs fitted and compression gauge in fourth cylinder
> engine warm, throttle set at tick over, using 20 SAE oil and operating
> the starter)"
>
> Anyway, those numbers seemed a bit low for a TR3A, so I looked up the
> figures for the "Special Order" engine for TR4 (which, of course, is the
> 1991 cc. engine). Spec's for that are 145 lbs./sq. in. -- this from the
> TR4 factory workshop manual.
>
> > Any opinions on how wide a variance I should see among cylinders before
> > I become concerned that there's a problem?
>
> Rule of thumb is around 10% or more, although <5% would be better still.
>
> --Andy
>
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> * 10/Herald/Vitesse (Sports 6) Consultant *
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