compression test and #'s ? - MustangForums.com


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Old 03-03-2008, 09:55 PM   #1
SalikDDD
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Default compression test and #'s ?

i'm seriously considering doing a compression test because this is getting absurd with this car, it's down on it's power i know it, i just wanna know what #'s i should be looking at, the engine as far as i know was rebuilt at least within 1000 miles ago due to the crosshatching still being perfect on each of the bores, along with new pistons freeze plugs, bearings etc and that it holds 80psi of oil pressure (little extreme huh?)
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Old 03-03-2008, 10:02 PM   #2
Gun Jam
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Default RE: compression test and #'s ?

oil @ 80 psi is what im getting on 10-40w

the maximum compression reading you get really isnt as important as the difference between cylinders. You should be getting anywhere from 115lbs to 150+ compression i would think is about normal. But if you have much more then a 10psi difference between any two cylinders that could be trouble.

-Gun
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Old 03-04-2008, 01:19 AM   #3
67mustang302
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Default RE: compression test and #'s ?

Is it a flat tappet cam, and was it broken in properly? Is there evidence of metal in the oil?
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Old 03-04-2008, 01:25 PM   #4
SalikDDD
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Default RE: compression test and #'s ?

Quote:
ORIGINAL: 67mustang302

Is it a flat tappet cam, and was it broken in properly? Is there evidence of metal in the oil?
yes, and no there was no metal in the oil.
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Old 03-04-2008, 02:58 PM   #5
jlg2002
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Default RE: compression test and #'s ?

Compression on most newly rebuilt motors is a little tricky since the rings haven't fully settled in yet. But as a rule of thumb, 135-180 (it depends on the CR of the pistons that are installed) and not more than a 20% variance between highest and lowest cylinders.
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Old 03-04-2008, 06:13 PM   #6
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Default RE: compression test and #'s ?

here is some info on the best way to do your test

Compression Testing Basics
1. Make sure your battery is at full charge. Best way is to run it for an hour first if possible.
2. Disconnect the coil wire. (very important)
3. Remove ALL the spark plugs. This will allow the engine to only COMPRESS the cylinder you are testing and saves on battery power since the starter can just turn the engine easily.
4. Remove the air filter from the system and take off the air cleaner so that the carb is open to fresh air
5. Use a screwdriver, hammer handle or something to open the carb or throttle body to full throttle position. Try to jam the linkage, not the butterfly valve, but sometimes you have no choice.
6. Install the compression gage on Cylinder #1
7. Install a remote starter across the starter solenoid. (recommended) Or have a friend crank it over for you.
8. On each compression stroke you will see the needle jump up. You want to run each test for FIVE bumps.
9. Write down the reading after the initial jump. Usually around 70-90 psi or so. Then write down the reading on the 5th bump. Typically from 120-160 psi.
10. Repeat this for all 8 cylinders (assuming a V8). Best way is to draw 8 circles on a piece of paper with the cylinder numbers labeled as you look down at the engine. Then write down the readings. Don’t be afraid to run each test several times and take the most consistent readings.

How to interpret readings.
· Look for any discrepancies between the numbers. All readings should be within about 10% of each other.
· If any one cylinder seems low (or high) then you have an idea where to go next.
· High pressure in one cylinder indicates a carbon buildup that is actually decreasing the chamber volume and raising compression. Very common on old high mileage engines where a lot of oil has been burned over the years.
· Low pressure indicates that there is a problem with the cylinder. Could be any of the following.
o Head Gasket
o Intake or Exhaust valve
o Piston rings or worn cylinder walls.
o Cracked block
o Hole in the piston head
o Faulty spark plug threads

· Here’s a good tip. If any TWO ADJACENT cylinders show identical LOW readings then the head gasket is blown out between the two and they are sharing air with each other.
· After you have identified a low cylinder, try squirting 1 teaspoon of oil into the cylinder through the spark plug hole. This will temporarily seal the piston rings. If the pressure suddenly comes back to a normal reading then you have a ring or cylinder wall issue.

· Burned exhaust valves will allow air to be sucked back into the tailpipe instead of only blowing it out. A quick check for this is to use a $1 bill (or a piece of paper) and hold it in the end of the tailpipe. If you feel it being pulled into the pipe there is an exhaust valve problem.

· Checking the radiator for bubbles can indicate a coolant leak into the cylinders- which causes WHITE smoke.

· Check your oil level also and look for evidence of foaming or coolant sludge. A cracked block can allow coolant to get into the oil system. Also a common culprit is intake manifold seals.

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Old 03-04-2008, 06:22 PM   #7
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Default RE: compression test and #'s ?

Daze, I just saved that to a word doc for future reference. Thanks!
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Old 03-04-2008, 06:32 PM   #8
ninteen66mustang
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Default RE: compression test and #'s ?

Wow, good post daze. I need to do a compression test. My cousin said to do it two ways, something like, with oil somewhere and without or something. anyone know what that is? is that the thing where you squirt it in the cylinder?
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Old 03-04-2008, 06:40 PM   #9
scuf23
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Default RE: compression test and #'s ?

Maybe that should go to the FAQ section

That is at least the 2nd or 3rd time I am seeing this kind of post and great input from Daze everytime

Thanks man!
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Old 03-04-2008, 06:47 PM   #10
Daze
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Default RE: compression test and #'s ?


Quote:
ORIGINAL: ninteen66mustang
My cousin said to do it two ways, something like, with oil somewhere and without or something. anyone know what that is? is that the thing where you squirt it in the cylinder?
the squirt of oil goes in the spark plug hole as a way to seal the rings, if compression goes up a lot than than the rings are worn or not seated properly. IMHO if readings are consistant and at a decent # there is no need to do the "oil test"
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