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Discussion Starter · #1 · (Edited)
I know that the engine should be run prior to leakdown as when the engine is warm everything expands and the leakdown/compression should be better. But Ive also read getting cold numbers and hot numbers is a good idea. However all the searches i do on this forum and google dont show the difference between a hot and cold leakdown test.

My 521 with 10.7:1 compression and only has less than 200 miles on it not including breakins and idling. Forged flat top MAHLE pistons with moly rings. I gapped them alittle loose for a mild nitrous/forced induction per MAHLE's instructions. Nitrous to 150HP.
Ring Gaps:
top ring=bore X .0060" Gapped rings to .026"
2nd ring=bore X .007" Gapped rings to .031"
Oil ring rail= minimum .015", all rings were around .021"

I borrowed a friends brand new harbor freight leakdown tester and got the following results. Engine is COLD! and its got alot of condensation so the outside temps dropped a good bit, its 65*F outside and pouring. The harbor freight tool was pretty hard to get dialed in as everytime it tested i got different results but all within the numbers im posting.

Bold numbers are ones that seemed more accurate but i always got one different number on a test.

Cylinders:
#1= 27%
#2= 24-35%
#3= 30%
#4= 28-33%
#5= 27%
#6= 30-35%
#7= 25-33%
#8= 33%

Tests started at #1 TDC, then rotated engine by hand until cap rotor lined up with plugs for firing. I got no bubbles in coolant, most of the hissing could be heard through the valve cover breathers. Once the rain came pouring down i couldnt hear much.
 

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Discussion Starter · #2 ·
These seem high and im sure its bc the engine is dead cold. But how much of a difference can i see from a hot and cold engine? and how does the spread looK? They seem pretty good as they are averaged around eachother.

I cant find my compression tester so ill see if i can get a whole of one and get some of those numbers as well.
 

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While mine was on the dyno, the next day I ran a comp test on it dead cold. Highest was 225 lowest was 210.
 

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Well...at least your numbers are consistent. I would warm the engine to it's normal running temp and retest. Your top ring gap is on the big side and high leakage rates will show the worst when cold. I also would get more miles on the engine before passing final judgment. Cylinder leak-down is a great tool to diagnose problems but if the engine doesn't have any problems other than what you are testing for I'd let it go and drive the thing until it is completely broke in. One final point: how you roll the engine over to TDC will also have an effect on leakdown Do not back up the crank and don't go even slightly past TDC, that will unload the rings.
 

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Discussion Starter · #5 ·
I have to ask my friend for his compression tester. What is a good range?

Im gald they are consistent! I know the ring gaps are alittle big but thats bc i was planning on juicing it alittle like a 150shot at the track... alittle planning ahead will allow me to get that little extra UMPH!

Reason i did the leakdown is that I had oil in the spark plug holes but not in the chamber. Plus i had smoke out the back of the car at 4500rpms but i was unsure of the cause. The engine spuddered giving it gas as i went around the highway off ramp. I figured the oil in the chambers could cause some spuddering. Thats when i found the oil in the spark plug holes but the plugs looked fine. I now think running 2 breathers hooked to vaccuum port on the carb ALL THE TIME without a PCV valve was sucking major oil into the engine and at 4500 rpms i had oil shooting out of the K&N open filter media breathers I had on before. Plus i had an oil leak for the spin on adapter at the block for my oil filter relocation which could have caused some smoke.

I was sure that if i went past the TDC that i backed off enough to rotate the engine back a good but so the rings were set right. Setting the engine to TDC on all the cylinders is rather hard without a piston stop lol.
 

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WEVE ALWAYS DONE our leak down cold, and have had 0 percent to 5 percent leak down on a good engine
 

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Actually used the harbor freight leakdown tool last night and checked a few cylinders. A post about the tool on another forum, mentioned 10psi being a good point for air supply to the leakdown tool.

Someone else took % gauge to the right off, added a real PSI gauge and did the math for leakdown with good results compared to a more expensive professional tool.

Problem with the harbor freight gauge is questionable accuracy on the % percentage leakdown.

I was more concerned with a consistent measurement between cylinders to identify a potential bad valve. So the tool worked well for that check.

I'm no expert but given the tool and your consistent results, I would be content with the readings.
 

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Discussion Starter · #9 ·
Im pretty sure its impossible to have a 0% leakdown evenwith gapless rings, so 1-5% and would that encompass larger ring gaps? Or a difference in hyper and forged pistons? I know that the hyper KBs require insane large ring gaps compared to most others as well as forged pistons. I will have to check leakdown on a couple of cylinders after the engine is run. Ill also do a compression test hot at that time as well.

Its just that 1-5% seems like too little leakdown... or is it me?
 

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Discussion Starter · #10 · (Edited)
Actually used the harbor freight leakdown tool last night and checked a few cylinders. A post about the tool on another forum, mentioned 10psi being a good point for air supply to the leakdown tool.

Someone else took % gauge to the right off, added a real PSI gauge and did the math for leakdown with good results compared to a more expensive professional tool.

Problem with the harbor freight gauge is questionable accuracy on the % percentage leakdown.

I was more concerned with a consistent measurement between cylinders to identify a potential bad valve. So the tool worked well for that check.

I'm no expert but given the tool and your consistent results, I would be content with the readings.
What readings were u getting? I always question the quality of any cheap chinese made tool. But my friend paid like 100$ for the leakdown tester lol... i figured its gotta be "ok".

What is the math for leakdown? i might even be able to make my own leakdown tester lol.

EDIT: i noticed that my PSI was always around 12-14psi on the leakdown gauge. I kept hearing that the gauge would normally operate around 10-20psi. It was very difficult to get the needle to stop at the "0" mark on the leakage gauge as the regulator would always increase after it was locked down, if you reversed the regulator knob it would stop the needle but then the leakdown results would always be different.
 

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Discussion Starter · #13 ·
I like zero gap too. But CNCmotorsports when they were putting my rotating assembly together advised against the zero gap rings. I cant remember why now tho...
 

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I tried the Harbour Freight leakdown tester as well and got variable results that could not be trusted with the low pressure zeroing method and the regulator drift. Bought the ATC tester and it worked well because it was higher pressure and it has a quality regulator.
A good tool for $115, why make one that wont be as good.

Dave
 

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I tried the Harbour Freight leakdown tester as well and got variable results that could not be trusted with the low pressure zeroing method and the regulator drift. Bought the ATC tester and it worked well because it was higher pressure and it has a quality regulator.
A good tool for $115, why make one that wont be as good.

Dave
"higher pressure" that's the deal.
 

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In my opinion static leakdown tests numbers are largely meaningless. But they do often times allow you to find a problem with a valve, scored cylinder etc. If the engine runs fine and has a low amount of blowby it's probably just fine.
 

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What readings were u getting? I always question the quality of any cheap chinese made tool. But my friend paid like 100$ for the leakdown tester lol... i figured its gotta be "ok".

What is the math for leakdown? i might even be able to make my own leakdown tester lol.

EDIT: i noticed that my PSI was always around 12-14psi on the leakdown gauge. I kept hearing that the gauge would normally operate around 10-20psi. It was very difficult to get the needle to stop at the "0" mark on the leakage gauge as the regulator would always increase after it was locked down, if you reversed the regulator knob it would stop the needle but then the leakdown results would always be different.
The 3 holes i checked were around 20% with the Harbor freight tool. I supplied the tool with about 10psi.

As i understood it.
Math on the tool would be a percentage of supply air into the motor to what the leakdown portion of the gauge reads.
ex. 100psi supply to tool (supply gauge,) leakdown gauge reads 90psi then 10percent leakdown
 

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I just did a cold test on my fresh off the dyno engine, over the past 20 yrs I've gotten use to zero gap second rings that read 0-4% cold most of the time and 0% when warm.

We went a different direction with the re-build:



4.606 bore x 4.25 stroke = 565 actually 567"
Arias pistons, lateral gas ported
Akerly & Childs ring pack .043, 1/16th, 3/16th with a Tool Steel top ring ( I don't recall the end-gap on the top ring)

The cold leak-down looked like this:

% leak
# 1- 4%
#2- 3%
#3- 5%
#4- 12% ( rings)
#5- 3%
#6- 6%
# 7- 3%
# 8- 5%

Engine did it's 10 minute break in and 10 power pulls with Kendall 30wt then I switched to Mobil 1 15-50. I'm not sure what going on with the one out of wack hole.

I'm interested in seeing what they'll look like hot.

G

Jon
 

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Discussion Starter · #20 ·
hey guys, finnally got the engine and trans running, let it all warm up and shut her down and pulled the #1 plug. I put the leakdown on it and did a quick test... i still got 28% leakdown with the harbor frieght tool, I got 27% leakdown with the engine cold. I only did one cylinder just to see if there was a difference and there wasnt.

I then checked compression on the same #1 cylinder and got 175/180psi with alittle air pushing past the oring seal on the tester. I believe that 180psi is pretty darn good! which seems to contradict the leakdown test results... hmmm
 
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