Leak down test
Leak down test I have a 383 stroker in a 72' chevelle that has around 120-140 miles on it since build...it burns oil excessively and i decided to do a leak down test to determine its problem. I've done the drivers side bank and the results are pretty bad all of the cylinders were at 16-25%. So I believe that the rings were probably not installed properly, however there is apart of me that thinks that maybe with more miles they will seat better. Though I think that 16-25% is really bad. The problem is that I am pulling the motor because my flexplate is tore up (never by advanced auto brand starters!!) and I have another motor (LT1) that I was building up for my camaro that I am thinking about throwing in it. I was wondering some other peoples thoughts about the situation...give the motor more time to seat the rings...or throw in a new motor that is already to go...and that has less than 5% leak down. just tryin to get some feedback thanks
ps. I did determine that the leak was through the rings and not the valve seals
ps. I did determine that the leak was through the rings and not the valve seals
Staging Lane
Joined: Jun 2006
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first off if the cylinders are oil soaked from the rings not seating its worst than you think, years ago it was first do a dry compression test than add oil to see how worn a motor is. ---- a leak down test is used to under stand the relationship between the cylinders 16% to 25% difference between them is not that bad --- 1 question what sort of valve seal was used and were the heads re done but a machine shop ---- 2 did you re bore it and if so what sort of rings did you use ----- 3 does it still have a PCV valve ---- 4 does the oil you broke the motor in with regular or syn type and what weight ---- 5 did you drive it like you stole it or was it driven like a human ?????
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Joined: Dec 2004
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You need to drive the **** out of a new motor, the best way is to take it to 4-5K rpm and let the car engine brake down to 1500 or so and do it over again. This will seat the rings. idle the car up to temp before hand, this should let most of the other stuff seat in, then go seat the rings!
8 second club
Joined: Aug 2005
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From: Greenwood, IN
You don't need to drive the **** out of a motor to seal it up, if the rings don't seal up shortly after it's first started they ain't going to. Either the hole is round or it ain't. Either the hone job was done right or it wasn't. Maybe the rings were installed upside down. If they were not file fit rings maybe they were the wrong size and have excessive gap. Could be a number of things, but they should seal up pretty darn fast.
Now sealing up and being fully broke in are different things, when I say seal up I mean a new motor should not use oil after the first few minutes of running or at all.
I rarely leak down my motors. On a friend of mines 355 street rod motor with just a regular set of speed pro moly rings. We leaked it before it was ever started just for the hell of it. It had around 15-20% leakage. After being ran for 20 minutes it had 2-4%.
The only motor I've ever seen use excessive oil for the moment it was started, never got any better. After taking it apart and rehoning it sealed up instantly.
Now sealing up and being fully broke in are different things, when I say seal up I mean a new motor should not use oil after the first few minutes of running or at all.
I rarely leak down my motors. On a friend of mines 355 street rod motor with just a regular set of speed pro moly rings. We leaked it before it was ever started just for the hell of it. It had around 15-20% leakage. After being ran for 20 minutes it had 2-4%.
The only motor I've ever seen use excessive oil for the moment it was started, never got any better. After taking it apart and rehoning it sealed up instantly.
does anyone know how important it is to have the cylinder at top dead center while doing the leak down test b/c it may not have been exact. In addition from what I understand a leak down test determines the the percentage of lost compression not the difference between the cylinders right?? The The break in cycle was done gently and with regular straight 30 weight oil..however it may have over heated during break in.
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WE broke my car in by beating the **** out of it pretty much. As far as blowby, I have NONE. I ran it without crank case ventilation b/c the PCV was stopped up cold for 5K miles (I know, not good) and it didnt even pop the dip stick, just weeped a little from the front of the manifold gasket. Now when I pull the PCV and breather out of the valve covers and plug 1 hole and hold my hand over the other, i feel NOTHING. Engine sealed up great. Compression low was 147 compression high was 155, 7psi difference at the MOST between 8 cyl...not bad.
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Its actually kinda high I would think, considering most stock 5.0's make 135-140 and I have 10.5:1 compression, BUT i also have a good bit of cam overlap compared to stock with my 224/224 .542 cam.
150 sounds about right to me. Especially considering someone sent in 122psi avg. cranking compression with the same cam/heads as me to MMFF and MMFF did not comment on it being low.
All I know is when I had EFI it did quite nicely beside LS1 cars. This carburetor has killed me but thats getting tended to this week.
150 sounds about right to me. Especially considering someone sent in 122psi avg. cranking compression with the same cam/heads as me to MMFF and MMFF did not comment on it being low.
All I know is when I had EFI it did quite nicely beside LS1 cars. This carburetor has killed me but thats getting tended to this week.
Staging Lane
Joined: Jun 2006
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A LEAK DOWN TEST IS DONE AT ( BDC ) USUALY CANT KEEP THE MOTOR STILL AT ( TDC ) THE LARGER THE CAMS DURATION THE LOWER THE STATIC COMPRESSION WILL BE THE #s YOU POSTED ARE FINE I STILL THINK ITS THE VALVE SEALS BUT THATS ME JZ
ok the compression ratings i got on the cylinders were between 135-150 dry and almost all around 150 wet. From what I've heard anything above 15% leak down is an indication of a potential problem. I removed the carb and the headers and didn't hear anything coming out of them when performing the test but did hear a whoosing sound from within the motor. Also when the motor is running the valve cover breather expells some smoke/oil, wouldn't that indicate crackcase pressure??
Ok well I went back out to the garage and performed a test again on the same bank and got different results....should they vary based on the position of the piston in the cylinder?? In theory I wouldn't think it would matter since it is measuring the % of leakage and that throughout the cylinder the rings should seal the same...alittle confused at the moment
Last edited by Chevyguy358; Jul 19, 2006 at 07:17 PM.
it doesnt matter as long as the valves are shut. % should be the same whether at TDC or BDC. Check and see if you can hear any air coming out of the exhaust, oil cap, carb, ect. anywhere air could possible come out. you are really supose to check it on TDC on the compression stroke so the rings are at the highest they can be.
I loosened the rocker arms so that the valves would be closed and tested the cylinders at various positions. I removed the carb and the headers and didn't hear anything coming out of the intake end nor the exhaust end...did hear some whoposing noise in the motor thaty I could hear from the oil cap
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I bet "whooshing in the motor" means out the exhaust. I think your rings are decent, valves sound like the issue, just my stupid guess though.
PS, all motors have a LITTLE vapor coming out of the breather when no PCV is attatched. ALL MOTORS HAVE CRANK CASE PRESSURE!!!!
PS, all motors have a LITTLE vapor coming out of the breather when no PCV is attatched. ALL MOTORS HAVE CRANK CASE PRESSURE!!!!
well i placed my hand over the exhaust port on the head and felt nothing and tried to put my ear as close to it as i could and couldn't hear anything. i did hear from a knowledgable friend that stroker motors tend to burn more oil than normal motors due to the angling of the connecting rod during the compression stroke...also I used 6 inch rods instead of 5.7...don't know if that would have anything to do with it at all.
both the piston and connecting rods are suppose to by designed for this application...i guess what I'm goin to do is take the whole longblock to a shop and have them do definitive diagnostics on it...the results that I have gotten in my oppinion indicate the rings are at fault but since I spent a lot of money on the buildup I might as well get a professional oppinion on it before I replace the motor or rebuild it.