Berry Kercheval wrote:
> > Bought my first compression guage. It is calibrated in psi. How do I
> > determine the compression ratio? Ie. how do I convert x psi to n:1?
>
> Well, atmospheric pressure is 15 psi (close enough for this calculation) so
> 10:1 compression ratio will give you 150 psi.
Does this mean the gauge should start at 15 psi?
>
>
> However, only a perfectly sealing engine will give you 150. figure at least
> 10% leakage, which would be 135 psi. If you can get 130 on all cylinders of a
> 10:1 engine you're doing fine.
>
> The calculation of absolute expected pressure like this really isn't useful
> except as a gross check that the gauge is working more or less OK. It's more
> important that you get repeatable, uniform readings across the cylinders. If
> your 10:1 engine gives you 115, 120, 120 and 125 that's pretty decent and not
> cause for worry.
I getting close to these numbers for an 9:1 engine, so I'm in good shape.
Thanks.
On a side note, I bought a screw in model with a rubber hose. The fitting has
a
stepped 14/18mm thread. Unfortunely the 14mm thread is only about half an inch
and the thread for the plug starts a half an inch inside the spark plug hole.
The
documentation for the gauge recommends buying a special extender. Of course,
none
of the stores that sell these types of gauges has any of the extenders for sale.
So I bought a device called a no foul adapter (do these work?) and ground off
enough of the gauge end to be able to thread the stepped adapter. The no foul
adapter also had the theard starting 1/2" in. This adds the added volume of the
hose and the volume of the adapter to the volume of the cylinder at full
compression. Will this affect the reading or am I worrying about nothing?
Thanks again,
Peter Thomas
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