<VV> Compression test vs Cylinder leakdown test?

[Mark Durham]

John, a cylinder leak down test is a way to see how much air leaks out
of a cylinder, versus how much you put in. I'm an aircraft mechanic by
trade, and this is how we test compression on aircraft engines. You
put the cylinder to test at top dead center compression stroke, add 80
psi air pressure thru the spark plug hole, and move the prop back and
forth to get the best reading. Anything over 60 psi (above 75%) is OK
but must be consistent between all cylinders, within a few pounds. As
an engine ages, its compression may drop but should stay in a 5 to 10
pound range. We use that check because it is dangerous to have
propellers swinging around.

A compression test measures the cylinder pressures. Again, you want
consistent numbers across the cylinders. Generally a spread of 25 psi
is too great, and if they all are in the 100 to 110 range, they are
getting too low.

Cylinder pressures will be different for different compression
ratio's. A 8.5 to 1 engines compression pressure will be lower than
for a 10.0 to 1 engine. I believe the average for a 8.0 or 8.5 is in
the 125 to 135 range. My engine, which has had head work done, is in
the 150 psi range.

If you have a low cylinder, you can squirt a few squirts of oil into
that cylinder and retest. If the compression goes up, you have a
ring/cylinder problem. If the compression does not come up, you have a
valve leak, leaking head gasket, broken rings, hole in a piston, that
sort of thing.

Rather than think about how much pressure is normal for your engine,
just take the readings on the six cylinders and compare them. If
compression is even across the board, within 5 to 10 psi spread for
the 6, you are probably OK. If pressures get below 110, even if even
across the board, performance may start to suffer. I would use a
thicker oil to compensate for low pressure and slow oil use, until I
could take the engine apart. Some engines can be run for years that
way.

What's better? The compression check is the easiest for automotive
engines. In both cases, you look for consistence across the cylinders
and that when pressures go below a certain value, we see poor
performance and a rougher running engine.
Regards  Mark Durham

On Sat, Aug 13, 2011 at 4:59 AM, John Gull <vairdude at gmail.com> wrote:
> Well, which test is better? What do the results of these tests tell you?
> Thanks for any help! John Gull in Asheville NC
>
>  _______________________________________________
> This message was sent by the VirtualVairs mailing list, all copyrights are the property
> of the writer, please attribute properly. For help, mailto:vv-help at corvair.org
> This list sponsored by the Corvair Society of America, http://www.corvair.org/
> Post messages to: VirtualVairs at corvair.org
> Change your options: http://www.vv.corvair.org/mailman/options/virtualvairs
>  _______________________________________________
>