difference between leakdown and compression tests?

[boatster]

Long story short I ran a motor without oil for a mile and it scored the pistons. Limped home and emery clothed the walls and ran her again. She still reaches the same top speed (maybe 1 mph slower) and runs smoothly. Thing is a leakdown test showed a good amount of leaking in 2 of the 3 cylinders. If it were leaking wouldn't my boat run alot slower? This leads to my question, what does a leakdown test tell you? How does it differ from a compression test information wise? I've heard you can get high compression even with a bad leakdown. I don't understand what this implie s

 

[Northern Eclipse]

Re: difference between leakdown and compression tests?

A compression test is just a measure of pressure for a cylinder and will give you a rough idea on the condition of that cylinder, a Leakdown test is the process of inducing compressed air into the cylinder via spark plug hole, with piston at tdc and on compression stroke, a leakdown test will inform you were a cylinder may be leaking resulting in a loss of compression and is usally done after a low compression reading has been found. The results of a leakdown are diagnosed as follows. Air hissing from exhuast exhaust valve or seat damage, air leaking up into intake manifold up into carb or throttle body, damaged intake valve or seat. Air leaking up thru dipstick tube or valve cover could be worn or broken rings damaged piston or cylinder. Air bubble's appearing in a cooling system could be a bad head gasket or cracked head. Any results found in a leakdown my not also be true to what actually is the damage, but will almost always require a teardown to inspect further. So in a nutshell a compression test will not tell you whats broken, but will indicate whether or not a leakdown test is needed. Hope this helps.

 

[JB]

Re: difference between leakdown and compression tests?

Moving to Outboard Non-repair.

 

[walleyehed]

Re: difference between leakdown and compression tests?

Leak-down shows the wear at the point it is most usually present...the top of the stroke.<br />It's the ability of the rings/cylinder fit to hold a certain pressure. A standard compression test is biased due to the fact the piston and rings are moving rapidly throughout the entire travel of the piston.

 

[skyguy59]

Re: difference between leakdown and compression tests?

"Differiential" compression testing is the way we do all aircraft piston engines. 60/80 being considered the minimum serviceable limit with no more than 10%(8 PSI) between highest and lowest cylinders.. In other words 80 PSI at the regulator gauge and no lower than 60PSI on the cylinder gauge. Cylinders are usually checked at operating temp. You have to interpret the results before thinking about disassembly and repair. I have seen cylinders read 50/80 and you go fly it for an hour and they will read 70/80. If you read a little low on a differiential, run it for a bit and check it again.<br /><br /><br />Russ In Texas

 

[DES]

Re: difference between leakdown and compression tests?

Lycoming is the only one using 60lbs...Continental says the lowest cyl. must not be more than 5lbs below the ave. of all.<br />If they're all 40lbs., that's great...compression affects the dike rings differently in an air-cooled aircraft engine.<br /><br />BTW, yes, this is the original walleyehed visiting parents.

 

[skyguy59]

Re: difference between leakdown and compression tests?

That was right out of AC43-13.2 and is industry standard ( read used by most all mechanics as a guideline) for TCM, Lycoming P&W and Franklin.I have dealt with TCM alot. If you use their calibrated orfice it will show 40 PSI is the minimum. If it isn't below 40 TCM won't give you warranty on the cylinder. I won't return a cylinder to service at 40/80. 5 PSI is in the ballpark of the 10%. If you look at a compression tester you'll know what I mean. Also there are different size orficies for the tester depending on cubic inch displacement of the engine. Using too small of orfice on a large engine will give you false readings on the high side and vise-versa<br />i.e a TSIO 520 and an R4360 don't use the same orfice.<br /><br />Russ In Texas

 

[DES]

Re: difference between leakdown and compression tests?

Continental rep for 7 years after extensive training from all current av engine powerplant manufacturers....know it well, also AC 43-13 is RECOMMENDATIONS, not rules.<br />Adapting leak-down to marine use is tricky at best due to the fact that if you don't know the displacememnt of the engine in question (each cylinder), and the piston ring design, you can't make accurate judgement on orfice size and must live with the fact you are not looking for "high" numbers, just "even" numbers.<br />"Industry standard" doesn't cut it on the engines we manufacture for unlimited aerobatics, and experimental cat.<br />Don't get me wrong, I belive in a leak-down check, but only a few understand what the results indicate, and even fewer would understand the procedure to do it correctly....as far as applying this in a major way for Marine use.

 

[skyguy59]

Re: difference between leakdown and compression tests?

Pilot, A&P, IA, FAA DAR for 30 years, Director of Quality for an FAA Repair station. What area do you cover? I used to talk to a TCM rep by the name of John Black for the Texas Area. Haven't seen him in quite a few years. True most people don't know how to do the check correctly,thats why I said you have to interpret the results.<br />I would be interested in some of your experimental engines. The last thing I got off into was in the 80's when Ram did the 550 liquid cooled engine on the 414.<br /><br />Russ In Texas

 

[DES]

Re: difference between leakdown and compression tests?

Still got a 414 in Hays with L.C. engines.<br />I originated in western Kansas, covered Arizona for a while, then back to Kansas, most of the northern half. Moved into turbine engine conversions about 5 years ago-TPE-331 mostly...some PT-6A work. also about the time I started spending way too much money on boats and homebuilt aircraft. I went to Texas for the marine business and not aviation, so I don't claim to know any of the Texas Reps.<br />3 gentlemen and myself have been assembling Lycoming 360's, and 540's for several years now for restricted and experimental cat.