Compression testing a 4.6 3 valve.

[Born To Run]

What psi should i see roughly in each cylinder to determine all is ok? 200,160,180? What is the low end of exceptable psi in each cylinder?What % of difference between the cylinders is ok,10% 20%??
Any info would be appreciated.
Also should the compression test be done wet or dry & what do the results mean?

 

[hamish]

10% max difference from highest to lowest.
A wet test is usually done to help diagnose a bad ring seal.
If the compression reading is significantly better with oil added new rings will probably be needed
I don't know what our engines standards are as far as expected compression level.

 

[Born To Run]

Compression test

10% max difference from highest to lowest.
A wet test is usually done to help diagnose a bad ring seal.
If the compression reading is significantly better with oil added new rings will probably be needed
I don't know what our engines standards are as far as expected compression level.

I got (dry test) 200 psi on cylinders 2,4,5,6,7,8 & 190psi on #1 & 180psi on #3.

The wet test results gave compression readings of 2.5%-5% increase in each cylinder.
On another motor i had last year it tested 200 psi as well.
I think its safe to say a used 3 valve with 30,000-50,000 miles should compression test around 200 psi.

This engine seems to be within good useable limits.

 

[Flapjack]

That #3 is borderline acceptable, per OEM standards. I'd say it's ok, though.

As for your average numbers (for anyone else seeing this thread), altitude is a huge factor, as well as what your static compression ratio is. For my altitude, I see around 150-160 per cylinder on a stock engine.

 

[hamish]

One of the key things to watch is how each cylinder pressures up.
If you crank the engine over for 3 compresson cycles to hit 200 psi on say cylinder 1
and have to crank the engine through 5 compression cycles to hit 200psi on a different cylinder then there is definitely an issue.

 

[Born To Run]

That #3 is borderline acceptable, per OEM standards. I'd say it's ok, though.

As for your average numbers (for anyone else seeing this thread), altitude is a huge factor, as well as what your static compression ratio is. For my altitude, I see around 150-160 per cylinder on a stock engine.

Good to know,i did not even think about Altitude when doing the test,it never dawned on me,but when racing my weather station is my friend.

Also what makes a difference is the battery used to crank it over,make sure it is fully charged with good cranking amps,this engine is out of the car in a unheated garage (35-40F),which may also make the readings slighty lower in my opinion.

One of the key things to watch is how each cylinder pressures up.
If you crank the engine over for 3 compresson cycles to hit 200 psi on say cylinder 1
and have to crank the engine through 5 compression cycles to hit 200psi on a different cylinder then there is definitely an issue.

All cylinders jumped to 150 psi on the 1st crank & by the 3rd crank maxed out the psi.

 

[retfr8flyr]

#3 is a little weak but should be alright. I would do a leak down test and see if it's rings or valves in that cylinder.


Earl