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Discussion Starter #1
Could someone please tell what a normal percentage is on a leak down test on a rebored motor (30 over) with new pistons rings and heads redone? Is it necessary for the motor to be warm when doing the test? I first did a compression test on motor as per PW's posts and all cylinders were within 1-2 lbs of 160. This is a 383 with KB dished pistons @ 10:1 and a Crane 282H cam.
Thanks in advance.
Wally
 

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It depends on the size of the orifice in the tester. If you have a small orifice the downstream gauge will read lower than if you have a larger orifice.
 

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Discussion Starter #3
The guage is seperated into 3 sections , 0-40 low, 40- 60 moderate and 60 + is high. I have about 500 miles on the motor and was wondering whether 38% was about average as I have not done this test before. This was with 95 pounds of air input from the compressor as per the instructions that came with it.
Wally
 

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I'm not sure what gauge you have. Does you set up have one or two gauges?
Some use a single gauge that reads directly in percentage, but those numbers are very high for percentage.


The leak down tester measures the difference between input pressure and cylinder pressure. For example if you have a steady 100psi line pressure and the gauge reads 90 psi that's 10% leakage.

If your gauge is reading PSI then 95-40=55 55/95x100= 58% not exactly low. Is it calibrated in metric units or something?

I'm not sure what you have. It's something with the pressure... or the compressor. It's very important that the compressor can maintain air pressure. Most little compressors aren't sufficient.

Are you checking at TDC with both valves closed? You also need to have a breaker bar on the crank bolt to rock the piston back and forth to settle the rings. Be careful as it can hit you if you don't have firm grip.
 

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Discussion Starter #5 (Edited)
The unit I have is a two guage unit and is in psi (proform is the brand name). Input guage is 0 -200 psi, on the left side , the leakage gauge is on the right and is marked in percentages ,0-10% is set, 10-40%low, 40-70%moderate, 70-100% high. My compressor is a big shop unit so supply is not a problem. I was reading the leakage as a percentage and not doing the differential(or should I?).Thanks for the info on the breaker bar and the safety warning! Is it necessary for the engine to be warm? What would be a normal percentage?
Wally
 

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Those are crazy numbers on that gauge. An engine will run with 50% leakage but not very good. I wouldn't call 60% leakage "moderate".

10% or less is good. 10-15% is acceptable. 20% or greater the engine needs refresh.

It's also helpful to find where the leakage is occuring.

If you hear air rushing in the exhaust, then exhaust valve(s) are leaking.
Air rushing through the carb, intake valve(s) are leaking.
Air in the valve cover then the rings are leaking.
 

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Discussion Starter #7
I agree that the % numbers are crazy. All of the leakage is from the valve cover holes, there were none from the carb or the exhaust. I was not believeing the numbers on the guage and want to look into it further. I was not thinking that 38 % leakage was a good thing at all! These are new pistons and rings and the heads are basically new. Thanks for the info and I'll investigate further.
Wally
 

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What I don't get (maybe because I'm dumb) is...Wouldn't the air compressor keep it at 100psi even if there were "leaks"?
 

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i believe once the cylinder is pressurized, it's locked down so the only leakage would be going by the rings or the guides... the compresser is used to only charge the cylinder.


maybe i'm off a bit, but that's basically it
 

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So the valve would have to have a 1 way flow to hold the pressure in. If thats the case then it may not be a bad idea.
 

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Leak Down Tester?

Hey Now to all, "Bill's" II here. Yes this does work and very well. The only thing I do different is loosen the rockers and pressurize the cylinder. I also use this method to change broken valve springs without removing heads. Sincerely, "Bill's" II :D
 

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Leakdown is a very subjective topic. Reading the linked article brings up more questions than it answers. We take a regulator, set it to some no flow pressure, pressurize our cylinder then read how much pressure the regulator can maintain in the cylinder. That tells me that a really compliant regulator would make your engine seem better than a non-compliant regulator. If we set the regulator to 100# and it can only maintain 80# in the cylinder, is that good or bad? If I got a larger regulator that supplies more air and can maintain 95# in the cylinder, is that better?
The same question can be asked of the orifice-style leakdown testers - what's 10% leakdown and what does it mean? 10% of what?
 

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An engine is an air pump and performance is directly linked to the ability to pump air efficiently.
Air is squishy and subject to the laws of physics. Absolute values can be misleading therefore interpreting the results is just as important as having a good test setup and procedure.

Here's some good reading on vacuum, compression and leakdown testing. (Mike: you might like the clever voltage drop method of compression testing!

ASA Tech to Tech

I use a nitrogen cylinder to maintain stable, dry input "air". 100 psi input over 90 psi cylinder pressure is considered 10% leakage. While the value is relative to the laws and argueably subjective it is much better than scratching your head and wondering if the cylinder seals or not!
Listening for cylinder leaks will tell you where the air is being lost. This is very valuable test to do right after the heads are installed on the engine stand.
You don't even have to install the valvetrain. This will pay for itself the first time you find a problem. Much easier to fix at this point than after it's in the car.
 

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The regulator is adjusted manually, by the knob. If I have air supplied, put a air blowing attachment on the end, and press the button, the gauge on the regulator will drop to a consistant value. So no, the regulator doesn't self adjust.

I tried this tool and method on a small block last night, I got about 5% leakdown on all cylinders. So I guess the rings and valve seats are sealing.

Not sure what to try next, I'm getting smoke out the back, and the engine is consuming oil. Maybe I'll look into a leaking intake gasket. Engine is a less then 50 miles fresh 383. Very fustrating, :( .
 

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That regulator method is a pressure test not a leakdown test.
If you didn't hear air rushing into the pan and out the breather holes then it's not pressure related but don't assume the rings aren't the problem.
I'm guessing you have a ring oil control problem or you are sucking oil from the galley into the intake.
Are you sure you didn't install your rings upside down? They often have a tapered face that if installed wrong can pump oil into the cylinder.
 

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isnt it something like this.

If the tapered face is on the inside of the ring then the taper faces up. If the tapered face is on the outside of the ring then the taper faces down?
 

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Hi guys,

In my opinion a compression test or leakdown test will not show a bad oil ring problem. In my experience most engines with an oil ring sealing problem will actually have better compression and less leakage, because the extra oil leaking into the cyl helps seal it.


Does your engine have a lot of blow by?


If not I would look at valve seals first. I have seen a lot of "fresh motors" that have a similar problem that ends up being improper valve seal installation.

If you use the "0" rings on the stems they must be installed after the spring assembly is compressed. IF if the seal is put on first it is usually pushed down on the stem and will bottom out on the top of the guide. This is much worse than using no seal at all (in my opinion any way). The misplaced seal now acts as a pump that forces oil into the guide. It kinda pressure feeds the guide. The same can be said for press on seal as well if they come lose.

You guys will think Im crazy (again) but I really like the little (0) ring seals as long as you use the proper retainer.

Just my opinions

Thanks

Jeff
 
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