Daze

reason being is that with compression in all 8 cyl the starter can not turn the motor as fast as running idle would be and the slower it cranks the more time there is for the cyl to decompress as the the piston strokes, soooooo by removing the drag on the cyl not being tested you get a much more accurate test. This is the information that was originally give to me when I was learning and I have found it to be very helpful.

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. Also increases cranking speed.
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.


One other thing I have found for checking a bad cyl is take an old spark plug and bust the ceramic and electrode out of it and cut off the bottom electrode. This will give you a hallow tube with the correct size and threads to thread in to the cyl. Weld an air fitting to the spark plug body where the ceramic once was. (I have a whole bunch of these with all different sizes, I make one every time I am working on something new) Rotate the crank until both valves are closed on the cyl in question. Insert new tool and hook up you compressor, set at about 40 PSI. if a hiss occurs out the carb than it is an intake valve leak, if a hiss occurs out the exhaust than it is an exhaust valve leak, and if it comes out one of the other spark plug holes than it is a head gasket leak.

That same spark plug air tool can be used to change valve springs with the heads still on the car.

kalli

all very true and good read. shoudn't that go into the FAQ if not already there? :-)

At the same time I have another one for ya:
when you have no or little compression on the cylinder for a few turns and then it suddenly jumps up to full compression, this may indicate a sticky valve.

Kalli

fastbackford351

Thank you Daze. Gonna have to print that out and stick it in my shop manual for future reference.

Good stuff.

jlg2002

Daze, comes thru again- great posts! Regarding the compression readings. In my experience, 120 - 150 +/- 20lbs from highest to lowest are good numbers for a stock engine with good ring and valve sealing (specs from the 1966 shop manual, show 130-170 @ SLwith no more than 20 PSI diff). Although, the engine is showing signs of wear,but it's time to start thinking about a rebuild in the next year or two if you want top performance.

gothand

FWIW, my GT40 crate motor (9.0:1) gave me readings between 138 - 145 psi, cold with only a few hundred miles on the engine. 110 - 120 soundslike a well worn engine to me if we are talking stock (9.3:1 C code or 10.0:1 A code).

Jeff

Daze

Your low and inconsistent readings were for a totally different reasons. 1. you said the motor was cold and that will always reduce your compression. 2. the other issue, and this is a big one for your case, is a new motor will burn more oil and have worse compression than a motor with 20K-75K miles on it because the rings have not seated yet. Give the motor 5000-10,000 miles and then do the test with a hot motor and the compression will probably be above 150 to 175 and have no more than 5# difference between cyl

gothand

I agree, though I wouldn't call a range of 138-145 psi "inconsistent". My point in listing my numbers on a cold, new engine with only 9.0:1 was to let the O.P. know that his numbers were a bit low for an early V8 (9.3:1, 10:1 or even 10.5:1 for the K-code). The onlyreason I did the testing was to see if I tweaked a head gasket.

Jeff

67mustang302

My compression readings cold prolly suck *****. I have fat ring gap because my pistons have a ring pack that sits high up, and they have extra gap for nitrous/blower. That and I'm at about .0045 clearance on all the cylinders. Also has a bit of a cam. If I checked mine cold I wouldn't be at all shocked if it came in under 130. There's a lot of expansion in them rings when they's hot. That's why it's the same with a leakdown test, it's best done with a bit of oil in the cylinder on a hot engine, to get an idea of where it's at when it's actually running.

kalli

sorry to hijack,
i bought myself a compression tester as I never trusted the push on rubber compression tester. have a screw in version now.
readings are:
175, 175, 170, 165, 170, 182, 165, 175.

it's a 289 and I have not the foggiest idea what year from. I'm very reluctant to pull the starter to check numbers.

seems all ok or would you say I should try adjusting valves or anything ... ?

Kalli

scuf23

OK - you are hijacking the thread ... Can I hijack your engine?

That is all good stuff guys!!! Thank you for all the feedback and info - the garage did the test for me, so I may do like kalli and buy myself a compression tester so I can do it myself- I mean I really do not know if they removed all the spark plugs, if the engine was cold or warm ...

This being said, while I am not racing and just cruising at 80 mph on the interstate, I kind of felt that down the road I would have to do something with the engine ...