<VV> Fwd: Compression test vs Cylinder leakdown test?

[Ken Pepke]

In addition, the 'leak down' test provides one the opportunity to listen at the tail pipe for valve leaks.
Ken P

*****************

> From: Mark Durham <62vair at gmail.com>
> Date: August 14, 2011 4:40:58 PM EDT
> To: John Gull <vairdude at gmail.com>
> Cc: virtualvairs at corvair.org
> Subject: Re: <VV> Compression test vs Cylinder leakdown test?
> 
> 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