Compression test

96GT281 · May 26, 2008

i just did a dry compression test on my 96 with 84k on it.

FIREWALL
------------------------------------------------------
190---------------------------------190

190 ----------------------187

150---------------------------------195

185---------------------------------190

Im concerned with the cylinder only putting out 150 and the cylinder with the smiley next to it; the plug was hand loose and there was a lot of junk on it from blowby.

BurningRubber · May 26, 2008

190 is high, the 150 is lower than it should be related to the others but still an acceptable number.

Mine were 160/170ish at the highest, and 2 cylinders was 120psi... but it still runs like a raped ape.

96GT281 · May 26, 2008

we did it on 98COBRA281's car and 6 were 195 and 2 were 200

98COBRA281 · May 26, 2008

yeah... i was rely surprised my car was that high, and its got 102k on it

the98stang · May 27, 2008

96GT281 said:
i just did a dry compression test on my 96 with 84k on it.

FIREWALL
------------------------------------------------------
190---------------------------------190

190 ----------------------187

150---------------------------------195

185---------------------------------190

Im concerned with the cylinder only putting out 150 and the cylinder with the smiley next to it; the plug was hand loose and there was a lot of junk on it from blowby.

The cylinder showing 150 psi is 33% lower than the others, which isn't good. It could be the piston rings or even the valves. I would just drive the *******cker till she blows. Run some 10w-40 dyno oil in it to reduce blow-by and oil consumption.

BTW- that plug was most likely loose because it wasn't torqued properly or someone used too much anti-seize on the threads. Torque all plugs to 13ft-lbs on a stone cold engine and use a very VERY light coat of anti seize on the threads.

the98stang · May 27, 2008

96GT281 said:
we did it on 98COBRA281's car and 6 were 195 and 2 were 200

You probably did the compression test with the spark plugs installed. This will give you high numbers like that. All plugs must be removed for the test, the engine must be at operating temperature, and the TB must be open for an accurate test.

Stan Weiss · May 27, 2008

the98stang said:
You probably did the compression test with the spark plugs installed. This will give you high numbers like that. All plugs must be removed for the test, the engine must be at operating temperature, and the TB must be open for an accurate test.

That is interesting. I have always found that the numbers were lower with the other plugs in than with them removed.

CobraRed_96_GT · May 27, 2008

Stan Weiss said:
That is interesting. I have always found that the numbers were lower with the other plugs in than with them removed.

+1

98COBRA281 · May 27, 2008

yeah we had the other plugs in, we did it when it was cold,

BurningRubber · May 27, 2008

the98stang said:
The cylinder showing 150 psi is 33% lower than the others, which isn't good. It could be the piston rings or even the valves. I would just drive the *******cker till she blows. Run some 10w-40 dyno oil in it to reduce blow-by and oil consumption.

BTW- that plug was most likely loose because it wasn't torqued properly or someone used too much anti-seize on the threads. Torque all plugs to 13ft-lbs on a stone cold engine and use a very VERY light coat of anti seize on the threads.

I wouldnt run that thick of an oil in a 4.6, tolerancces are pretty tight and thats a bit heavier than whats reccomended.

Mine were reading like this at the worst... 167psi and 120psi... it uses no more than 1 quart of oil between changes if I drive it hard, and it runs down any other comprable Stang with just as much power.

No smoke out of the exhaust either.

I did my compression test dead cold, with all plugs in except that cylinder. COP's and fuel injectors unplugged.

I wouldnt do a hot test on a 4.6... asking for plug trouble, lol.

the98stang · May 27, 2008

BurningRubber said:
I wouldnt run that thick of an oil in a 4.6, tolerancces are pretty tight and thats a bit heavier than whats reccomended.

Mine were reading like this at the worst... 167psi and 120psi... it uses no more than 1 quart of oil between changes if I drive it hard, and it runs down any other comprable Stang with just as much power.

No smoke out of the exhaust either.

I did my compression test dead cold, with all plugs in except that cylinder. COP's and fuel injectors unplugged.

I wouldnt do a hot test on a 4.6... asking for plug trouble, lol.

10w-30 will be fine, especially sense his rings are most likely shot. Many people I know have used 10w-30 in the 4.6 for years with no problems.

You can do a compression test on a cold engine or with the plugs in. Your readings may differ, but only a little bit.

Compression test

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