<VV> compression test vs Cylinder leakdown test?

[djtcz at comcast.net]

Original message 
----- Original Message -----
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. 
> 
================================================ 


For a thorough "leak down" (differential compression) test the pressurized air needs to be fed thru an orifice ( 0.040" diameter for engines under 1000 cubic inches ) and the pressure needs to be read on both sides of the orifice. 
http://www.mytractorforum.com/attachment.php?attachmentid=7700&d=1144805048 
Setting the inlet pressure at 100 psi means the engine side pressure is reveals the "percent leak down" directly ( 80 psi = 20% leakdown) 


I'm thinking even at a fairly functional sort of healthy 10% there may be audible leakage past the rings, and without the gage reading 


A compression test did not identify terminally sludged up oil rings on a good running but smokey Chevy 6. I don't know if a LD tester would either. 


Dan T