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mooney265's Avatar
 
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Compression test or Leakdown?

Would a good 'ole compression test tell me if all my cylinders were up to par. Or, do I need to do a leakdown?

And, if a compression test would do, what kind of compression reading would I be looking for? From my reading, it looks like consistancy is the key.

As far as tools required: Any pics of the tools needed/created to perform the leakdown/compression test?

Thanks!
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.

Last edited by mooney265; 06-18-2010 at 06:56 PM..
Old 06-18-2010, 06:04 PM
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I've never done it myself, but you need a compression tester (looks like a gauge you hook up to a sparkplug hole). Engine is supposed to be warm when doing the test. Leakdown is not just whatever numbers you calculate, but also it makes a difference from where you hear the leaking.

When I had my PPI, they obtained both numbers. I have no idea what the "factory specs" are supposed to be. I've never seen them printed anywhere. Has anyone else? So I just went with the fact that the numbers were all clustered together. (Maybe that just means the engine is entirely worn out in all 6 cylinders?) On Pelican, I've seen people post numbers in the 110-120 range, and others in the 160 range for turbo motors.

For leakdown, the lower the better. Again, clustering of the values together is a good thing. Some guys have posted values of 3 or 4%. I think mine were closer to 10-12%; I realize those figures seem very high, but that was 20K miles ago and the car still scoots, so who knows...
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Old 06-18-2010, 07:56 PM
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I've done compression tests before on anything from lawnmowers, mercury outboards to nissan 2.0 liters... So, I've got the compression test thing figured: Disconnect coil and fuel pumps, pull spark plug, insert compression gauge, turn engine over, read gauge...

The one thing that I can't seem to wrap my head around, however, is how do I pressurize each cylinder for a leakdown? And, how do I align each cylinder for the test?

When folks use numbers like 3% leakdown... What does that mean:

3% leakdown after 1-minute? or,
3% leakdown after 5-minutes? or, 5-seconds???

Thanks!
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.

Last edited by mooney265; 06-19-2010 at 05:02 AM..
Old 06-19-2010, 04:59 AM
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do a compression test, if all within spec forget the leakdown. I like to use a leak down only after a bad compression test to figure out problem. I know a couple people who have done a leakdown and torn a motor apart to find out there was nothing wrong with it. I recenly had a 3.2 sitting on shop floor, bad leakdown numbers on one cylinder but i was confident that motor was fine. I put the motor in, ran it and rechecked and compression and leakdown numbers they were fine. The smalllest bit of carbon can fool you.
Old 06-19-2010, 06:32 AM
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Quote:
Originally Posted by mooney265 View Post
I've done compression tests before on anything from lawnmowers, mercury outboards to nissan 2.0 liters... So, I've got the compression test thing figured: Disconnect coil and fuel pumps, pull spark plug, insert compression gauge, turn engine over, read gauge...

The one thing that I can't seem to wrap my head around, however, is how do I pressurize each cylinder for a leakdown? And, how do I align each cylinder for the test?

When folks use numbers like 3% leakdown... What does that mean:

3% leakdown after 1-minute? or,
3% leakdown after 5-minutes? or, 5-seconds???

Thanks!
During a hot or cold compression test you should also hold the throttle wide open while cranking the starter so there is no intake air resistance.

A leakdown test tells a lot more about the motors condition. You use an air compressor with the leakdown tester. There is a regulator on the leakdown tester air inlet and you adjust that to 100psi. The piston is TDC on the compression stroke in it's cylinder or it will be forced down by the air pressure.

There is an air hose from the leakdown tester outlet screwed into the spark plug hole of that cylinder and in the body of the leakdown tester there is a tiny orifice in between the pressure gauge and the leakdown gauge to the right of it that the air has to flow through if any is leaking out of the combustion chamber.

Because of the very small orifice, the right side leakdown gauge which is a 100psi gauge shows how much air is leaking out of the motors combustion chamber if it is leaking out faster than the air can flow through the orifice and replace it.

Being a 100psi gauge on the cylinder side of the orifice, if enough air is leaking out of the combustion chamber to make that gauge read say 93 psi because air can't flow through the small orifice in the leakdown tester body fast enough to replace the air leaking out of the cylinder than thats considered 7% leakdown on that cylinder.

Then you listen for hissing air leak with a hose in your ear and the other end in the throttle body, exhaust header or tailpipe, and oil tank or engine case drain plug to figure out if the leak is piston rings, intake, or exhaust valve.

Compared to that a simple compression test tells little.
Old 06-19-2010, 08:06 AM
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Being a 100psi gauge on the cylinder side of the orifice, if enough air is leaking out of the combustion chamber to make that gauge read say 93 psi because air can't flow through the small orifice in the leakdown tester body fast enough to replace the air leaking out of the cylinder than thats considered 7% leakdown on that cylinder.

Then you listen for hissing air leak with a hose in your ear and the other end in the throttle body, exhaust header or tailpipe, and oil tank or engine case drain plug to figure out if the leak is piston rings, intake, or exhaust valve.



Ok, that makes sense to me.... Does anyone sell a leakdown testing device/tool? I'm sure I can makeup the hose/megaphones you discussed...

Oh, and what's the easiest way to get each cylinder at TDC [or, rotate the crank]? I figure the starter would move the crank in too large of an increment... If I remove all the plugs, could I rotate it at the alternator pulley??
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.

Last edited by mooney265; 06-19-2010 at 02:36 PM..
Old 06-19-2010, 02:24 PM
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Do a Google search. All kinds of places sell the leakdown testers (Pelican?, Jegs, Summit, etc.). Longacre is one mfg.

Yes, remove all the plugs to make it easier, as you have to remove them all anyway. You might be able to use the alternator nut, but a wrench on the crank nut is better. This will help fine tune it to TDC.
Old 06-19-2010, 03:09 PM
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I hot one from Harbor Freight for $30. Very solidly made, good hoses, and the regulator works fine.

The leakdown gauge on the one I got was calibrated in percentages but it was actually a 15psi gauge... don't know why they did that because 15psi is not enough to seat the top compression ring.

One possible use is with 15psi in the cylinder you can hold back the crankshaft from turning with a wrench while the piston is halfway down the cylinder and check the leakdown there if you want. I can't hold the crank from turning when 100psi is in there and the piston is halfway down the cylinder..

So... just buy a cheap 100psi gauge with the same standard threads for around $5 at Harbor Freight and screw it in in place of the origonal one so you can adjust the regulator to pressurize the cylinder with 100psi.

Then while testing leakdown at 100psi, each pound below 100psi on the right side gauge would be 1% leakdown.
Old 06-19-2010, 03:32 PM
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Thanks Rocket and Fairman... I had just gotten off of YouTube looking at leak down tests... It makes sense to me now.

I know I can't get a wrench on the crank [I tried many moons ago] when the bolt began to come loose after a rebuild. My wrench removed the engine bracket and re-torqued it...

So, I guess I'll try the alternator pulley. The belt is pretty tight, so, hopefully it does the trick.

I'm still a little confused about the firing order... I know it's 1,6,2,4,3,5. So, I guess I just get 1 at TDC [by the mark on the crank] and start from there - Moving 60-degrees at a time to the next cylinder in the order. 12-o'clock, 2-o'clock, 4-o'clock, 6-o'clock and so on...

Right?
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Old 06-19-2010, 03:44 PM
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