On Mon, 14 Oct 1996, CBrown1500@aol.com wrote:
> In the April '96 issue of Classic Cars, in the Technical Queries column,
> comes this chart:
>
> Compression
> ratio 6:1 7:1 8:1 9:1 10:1 11:1
> Pressure
> (psi) 108 123 131 152 167 181
>
> These pressures are based on a fully charged battery, warm engine, and
> throttles wide open.
Add to that: all other plugs removed.
> And, they state this is an "estimate"--there are so many
> variables.
Exactly.
My experience with Triumphs indicate that they usually register higher
than this. I don't know why, but my 2.5 with 10:1 compression seems to
consistently register almost 190-200 PSI on all cylinders (with a TR5 cam,
BTW) (and two different meters, one of reasonable quality). Having asked
around, this seems to be fairly consistent with what others are getting in
a similar setup.
Does anyone know why the difference compared to the "static" number is so
high? Gas dynamics, yes, but we are really talking about a couple of
houndred of RPM from the starter again here. The cam timings surely
aren't optimized for that sort of RPM?
The reading definitely "builds up" over a number of strokes, but
that would have to do with having to fill the internals of the pressure
meter with air first, or? I have a screw-in type of meter that might take up
more space in the compression chamber than a spark plug. But surely not
enough to account for this?
Egil
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