HOW TO DO AN ADVANCED SEARCH.

Mon Jul 08, 2013 3:56 pm by Admin


For the benefit of 944Hybrids users there are two search functions available for you to use.
The purpose of this sticky is to explain the "Advanced Search" function because it is much more powerful and is the best choice when researching information.

When you log on to the site a list of options is shown in a line at the top of the page. One option is labelled "Search", use this option (NOT the search box lower down on the right).

After you click on the upper search option, a drop down box appears. At the bottom of this box is a radio button marked "Advanced …

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Cylinder Leakage Testing

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Cylinder Leakage Testing

Post  Bluemach1 on Sun Dec 06, 2009 1:45 pm

If I do a cylinder leak test with the engine cold (on the stand) will the results I get be of any use? I would expect that there would be a bit more loss past the rings. But if I get a similar reading across all 8 cylinders could I extrapolate a good / bad condition assumption?

I was also curious about compression testing. Same condition as before, cold on the stand. If I hand crank the engine, is any compression reading I get of value?

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Re: Cylinder Leakage Testing

Post  944-LT1 on Sun Dec 06, 2009 3:19 pm

You wont get the same number at hand crank speed. You can however compression test the engine. You need to pump air into the cylinder which causes the piston to seat at the bottom of its strock (BDC). Both valves should be closed at this point just like TDC and see what the leak down is. Im sure theres data for any specific engine out there somewhere.

EDIT: did a google search and found this. [You must be registered and logged in to see this link.]

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Re: Cylinder Leakage Testing

Post  Bluemach1 on Sun Dec 06, 2009 3:47 pm

I just tried the compression test by hand crank... No way that you can get this to work.

I am going to go fire up the compressor and do the leakage test. I just dont know what results will be with everthing cold...

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Re: Cylinder Leakage Testing

Post  gt1scca on Mon Dec 07, 2009 1:20 am

100 PSI minimum...

125 - 140 is common for average wear / mileage.
All cylinders should be within a range of ~10%.

Example: Cylinder 1 = 125 PSI / Cylinder 3 = 95 PSI
(Cylinder 2 has problems...)

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Re: Cylinder Leakage Testing

Post  sbwrench on Mon Dec 07, 2009 7:44 am

GT1, you stay up too late! Actually leak down is read as a percentage. Air leaking out vs. air put in. On a good race engine it is not unusual to see only 1% hot. That will only be 2 or 3% cold and the increase is usually at the rings which shows up at the oil fill cap or crank breather. The other two places for leakage is intake valves and exhaust valves. You can hear an unsealed intake valve at the intake manifold and a leaking exhaust valve at the exhaust port. Anything under 10% on a used street engine is acceptable. There won't be a big difference hot to cold. Most important is consistency from cylinder to cylinder.

Also, it is done with the piston at the top of the bore and the valves closed, not at the bottom of the bore. Be careful. If you are just coming up on, or just past TDC, when you add air through the spark plug hole it sometimes wants to drive the piston down. If you have a wrench on the crank snout it can smack the hell out of your fingers as it quickly rotates!

Go buy a decent leak down tester. They are invaluable. I like the snap-on brand. Mine is going on 20 yrs. old and works like a champ.

If you see a really high leakage through the rings, squirt a little oil in the spark plug hole and check again. If the leakage goes way down it's probably time for rings.

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Re: Cylinder Leakage Testing

Post  944-LT1 on Mon Dec 07, 2009 8:59 am

I thought you have the piston at the bottom of the strock for leak down. I mean, its gonna go down when you compress air into the bore anyway unless you lock the engine somehow? Oh well. Never done it before so im probably wrong. Laughing

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Re: Cylinder Leakage Testing

Post  sbwrench on Mon Dec 07, 2009 9:54 am

It's done at the top of the stroke to eliminate the variable of piston rock and its effect on ring seal. You probably won't see much difference on a production engine. When you do stuff of a higher level of performance you tend to see pistons with pretty short skirts for friction and weight reduction. A short skirt piston can have a fair amount of rocking at the bottom of the bore. Depending on the stroke, the skirt can actually come a fair bit out of the bore at BDC. Anyway to get a fair representation of ring seal, TDC when leaking down. I haven't done a million race engines but sometimes it seems like it!

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