Correct way to do a compression test?
Thread Starter
Racer
Joined: Dec 2000
Posts: 347
Likes: 1
From: Walpole MA
Correct way to do a compression test?
What is the best and/or most correct way to do a compression check?
IE: all plugs removed or 1 at a time, hot or cold engine, etc.
What is a good PSI range for a L-48?
IE: all plugs removed or 1 at a time, hot or cold engine, etc.
What is a good PSI range for a L-48?
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: Correct way to do a compression test? (74Coupe)
all plugs removed to help the engine spin faster will give you a higher reading usually. Also prop the throttle in the wide open position. Cold or hot it dont really matter. If theres a serious problem youll see it either way. Not sure of the desired reading pressure. Whats most important is that the pressures in all cylinders are relitivly the same. It can vary a little though. Crank the engine for several seconds on each cyl. The pressure reading will usually go up a bit on each compression stroke until it peaks. Its good to have a helper crank the engine so you can watch the gauge and have then stop cranking when you see it peak. Write the readings on a chart as you proceed so you can compare any differences that may come to light.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: Correct way to do a compression test? (74Coupe)
Just did mine, take all the plugs out and put the tester in one cylinder. You crank it over 6 - 8 times to get the highest reading. I forget but there is a limit something like 15 or so psi difference between the cylinders usuallly indicates a problem. If you have a low cylinder you squirt some oil in there and if the compression reading comes up that cylinder could have bad rings.
All eight in mine were between 235 and 240 psi and I think that is an indication that it passed the test. However I am using oil and now I'm on to a leak down test thanks in part to a loaner gauge from ddecart.
All eight in mine were between 235 and 240 psi and I think that is an indication that it passed the test. However I am using oil and now I'm on to a leak down test thanks in part to a loaner gauge from ddecart.
Racer
Joined: Mar 2001
Posts: 372
Likes: 1
From: Ellicott City MD
Re: Correct way to do a compression test? (MotorHead)
...and now I'm on to a leak down test thanks in part to a loaner gauge from ddecart.
Burning Brakes
Joined: Oct 2001
Posts: 1,146
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From: Milford NH
Re: Correct way to do a compression test? (74Coupe)
Is there a way to relate the PSI reading back to a compression ratio? I am curious to what compression I currently have and don't think I will be removing the heads to measure the volumes.
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: Correct way to do a compression test? (fireball)
...and now I'm on to a leak down test thanks in part to a loaner gauge from ddecart.
How long do you wait after injecting air to measure the "leakage" on a leak down test?
How long do you wait after injecting air to measure the "leakage" on a leak down test?
If you hear air in the muffler then it is the exhaust valve.
If you hear air in the carb then it is the intake valve. If you hear air through the diipstick tube then the rings are bad and finally if you hear air in the adjacent cylinder then it is the head gasket.
I am just a rookie at this, I have owned a few hotrods before but never done this test. :D
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Pro
Joined: Jun 2002
Posts: 544
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From: La Jolla California
Re: Correct way to do a compression test? (69stingray)
You can get a rough idea of compression ratio from a compression test. You have to know all your cam specs , engine displacement, engine internals, etc. You can find some formulas if you search around on the net (I've lost the link). The one I used had a LOT of math involved but it caculated my compression ratio pretty closely.
Race Director
Joined: Nov 2000
Posts: 11,872
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From: Waterloo ontario Canada
Re: Correct way to do a compression test? (MotorHead)
In a leak down test the air is continually pumped into the cylinder. It is maintained at a continous say 100 psi and the amount that continuously must be added to maintain that pressure is the leak down.
On my high precision leak down gage I dial the desired pressure in the cylinder, I use 125 psi and it has a flow meter on it that reads the flow in letres per minute that is added to maintain the 125 psi. I have charts which show how much leak down assossiates with each reading on the flow meter.
This is not a cheap gage, but consists of a regulator and flow meter.
On my high precision leak down gage I dial the desired pressure in the cylinder, I use 125 psi and it has a flow meter on it that reads the flow in letres per minute that is added to maintain the 125 psi. I have charts which show how much leak down assossiates with each reading on the flow meter.
This is not a cheap gage, but consists of a regulator and flow meter.
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Re: Correct way to do a compression test? (norvalwilhelm)
Mine has two gauges and you are supposed to adjust one about 10psi lower than the supply and the other measures the leakdown. I guess if you set the first one for 100psi and the second shows 95psi then you have 5% leakdown. Anything over 15 % is trouble. Like I said I am a rookie at this and will have more answers after I actually do the test.
I just wanna know how you prevent the pistion from being pushed down from TDC when you apply the air, do ou put it in gear ?
[Modified by MotorHead, 8:09 PM 11/8/2002]
I just wanna know how you prevent the pistion from being pushed down from TDC when you apply the air, do ou put it in gear ?
[Modified by MotorHead, 8:09 PM 11/8/2002]
Melting Slicks
Joined: May 2000
Posts: 2,125
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From: Crescent City, CA
Re: Correct way to do a compression test? (69stingray)
Cranking cylinder pressure has nothing to do with compression ratio.
Cylinder pressure is relative to the cam timing opening and closing of valves and the duration of the cam.
A 13 to 1 compression racing engine might have lower cylinder pressure than a stock engine because of the cam duration or the opening and closing of the valves.
Anyway, thats how I understand it.
Cranking compression should be within 10% of all the cylinders, if not there is a problem with the low cylinder.
Terry
Cylinder pressure is relative to the cam timing opening and closing of valves and the duration of the cam.
A 13 to 1 compression racing engine might have lower cylinder pressure than a stock engine because of the cam duration or the opening and closing of the valves.
Anyway, thats how I understand it.
Cranking compression should be within 10% of all the cylinders, if not there is a problem with the low cylinder.
Terry
