<VV> compression test vs Cylinder leak-down test?

[Ken Pepke]

Neither a compression test nor a leak-down test will point a finger directly at a problem most of the time.  Both give only clues as to what to think about.

Because a compression test looks at each cylinder on an individual basis, problems which affect only one cylinder such as a bent or burned valve or holed piston will show up more clearly.  But this test requires the cranking system to be in good condition.   So, is it the valve or the piston?  A vacuum gauge may add some food for thought here if the engine will run.  One is never sure if uniform low static compression is a ring problem, cam timing issue,  or just a characteristic of that engine.

A leak-down test does not require cranking the engine but it can be a pain to get each cylinder to top dead center and to keep it there.  It is kind of a noisy operation so listening for leaks is kind of limited to getting your ear close to the tailpipe.  You will hear a burned or bent exhaust valve but bent intake valves don't show up to well.  

Stuck rings are a different problem.  Because both the tests give results for one cylinder at a time, we tend to look at the differential between cylinders. Stuck rings can be the same for all the cylinders, so will worn or broken rings and probably bent intake valves.  The leak-down will be affected so it may start one to thinking about stuck or worn rings.  And it does remove cam timing issues from the thought process but not the bent valve issue. 

Ken P
Wyandotte, MI
Worry looks around; Sorry looks back, Faith looks up.

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

> From: djtcz at comcast.net
> Date: August 18, 2011 9:07:42 PM EDT
> To: virtualvairs at corvair.org
> Subject: <VV> compression test vs Cylinder leakdown test?
> 
> 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