Compression test results.

[strtrcr50]

What are acceptable compression test results?
I have
1 - 185 psi
2 - 190 psi
3 - 195 psi
4 - 198 psi
5 - 183 psi
6 - 187 psi
7 - 193 psi
8 - 195 psi
Is this ok?

 

[347HO]

Probably need your camshaft events
Where you dialed in your camshaft
On a stock engine, no new parts?

 

[strtrcr50]

stock engine. hci to go on soon.

 

[347HO]

stock engine. hci to go on soon.

Your numbers are for a lack of a better word... extremely high.
May I ask how you cranked the motor over to get these readings?

When "hand" cranked CCW two complete turns I would expect between 160psi to 180psi on a well maintained 5oh.

 

[Fast63]

maybe his gauge reads high. Either way, good numbers because they are relatively close together in reading. Slap on the HCI

 

[strtrcr50]

spun the motor on the starter with the accelerator pedal all the way down.

 

[347HO]

maybe his gauge reads high. Either way, good numbers because they are relatively close together in reading. Slap on the HCI

So if you use your starter to crank the motor and you get high numbers, then you turn it over by hand and you get 90's and 80's... that's good because they're close together?
But this test does not know whether it's rings or valvetrain, so in that respect I agree with you... install the heads, cam and intake and do it again, correctly to see where you are at.

 

[795.0pacecar]

Those are very good numbers, I have seen them that high before, in fact, mine were in the 180s-190s and my car runs like a champ and doesn't burn a drop of oil.

 

[347HO]

Those are very good numbers, I have seen them that high before, in fact, mine were in the 180s-190s and my car runs like a champ and doesn't burn a drop of oil.

Okay... those numbers are ON par with an almost "full out" race engine with near to zero miles on it.
These numbers only tell me the engine is being cranked over with the starter, and more than two complete cycles when results are documented... or the engine has an aftermarket camshaft or heads or gaskets or a re-indexed camshaft.

When doing diagnostic tests on the engine... they are only effective when the tests are done correctly.

If you've done this test correctly... then you have the first "used" stock engine I have heard with these numbers
and that aint a bad thing either.

 

[dcurtis]

looks good to me

 

[795.0pacecar]

I've never seen a stock mustang with numbers quite that high, but I have seen as high as 170-180 on a stock mustang. My engine obviously isn't stock.

Compression tests are done with the engine warm, spark plugs out, throttle open, and using the starter.

 

[Fast63]

So if you use your starter to crank the motor and you get high numbers, then you turn it over by hand and you get 90's and 80's... that's good because they're close together?
But this test does not know whether it's rings or valvetrain, so in that respect I agree with you... install the heads, cam and intake and do it again, correctly to see where you are at.

Honeslty, I don't know why anyone would turn the engine over by hand to check for compression results. You don't run the engine that slow, it idles approximately how fast the starter can turn it. Plus the starter is consistent, doing it by hand is not.

No compression test ever can say if rings or valvetrain are bad, only that there is a loss of compression. You need to do a leakdown test to find what the problem is. The psi being close between all the cylinders means that either all your cylinders have a problem, or none of them do. With numbers that high, basically it means none of them do.

As far as the psi, I think race engines have well over 200 if they are naturally aspirated. It also does depend on the cam as previously mentioned. My dad owned a comet when he was young that had a 260 with 220psi because of his pop up pistons. Total street car though. My 1985 5.0 had 110psi stock, and the book said that is normal. WHen I upped the compression, I am only getting 130 now, but I know there are no problems with my engine and that is normal psi for it.

The "correct" way to test compression is as strtrcr50 said, turn the starter with the gas pedal all the way down. Make sure you remove the coil wire first!!!!

 

[347HO]

Honeslty, I don't know why anyone would turn the engine over by hand to check for compression results. You don't run the engine that slow, it idles approximately how fast the starter can turn it. Plus the starter is consistent, doing it by hand is not.

No compression test ever can say if rings or valvetrain are bad, only that there is a loss of compression. You need to do a leakdown test to find what the problem is. The psi being close between all the cylinders means that either all your cylinders have a problem, or none of them do. With numbers that high, basically it means none of them do.

As far as the psi, I think race engines have well over 200 if they are naturally aspirated. It also does depend on the cam as previously mentioned. My dad owned a comet when he was young that had a 260 with 220psi because of his pop up pistons. Total street car though. My 1985 5.0 had 110psi stock, and the book said that is normal. WHen I upped the compression, I am only getting 130 now, but I know there are no problems with my engine and that is normal psi for it.

The "correct" way to test compression is as strtrcr50 said, turn the starter with the gas pedal all the way down. Make sure you remove the coil wire first!!!!

See... this is one example of someone who "assembles" and engine and another who "builds" an engine.
I'm not saying anyone elses method is wrong... it's just not correct in my opinion.
As I and probably many others do when building an engine, nothing is taken for granted and no measurement or clearance goes unchecked.
You can be sure I will check my final assembly of my build and crank it over by hand and check multiple items. One of them being cranking psi. During the build other items checked are PTV, roller tip to valve stem alignment, crank to block clearance, rod cap to block clearance, rotational torque are only a few... there are many more.
Those of you who wait till the engine is installed... well then it's too late when you have something wrong.
And anyone bringing up the subject that the engine is already installed and has miles on it... yep, I still crank it by hand. This way I KNOW where I am from where I WAS.
Again, not saying you're wrong... there are just different ways getting to the same goal, and some methods are better than others.

 

[Fast63]

See... this is one example of someone who "assembles" and engine and another who "builds" an engine.

It is ignorant to say my example is of someone who "assembles" an engine. I have "built" my own and "assembled" my own before. I have checked all the things you listed before myself as well as other things. I just think it is overkill to check compression before the engine is in the car because if you REALLY ARE meticulous about checking things, knowing that the rings, valves, and gaskets, etc are in good shape, means you know your compression will be fine. I don't even check compression on an engine that I built or assembled unless there is a problem with the way it runs or I just want to know how many psi I have.

I still do not see the point of testing it by cranking it by hand. You turn it so slow that the compression is bleeding out through the ring gaps while you are turning it.

 

[347HO]

It is ignorant to say my example is of someone who "assembles" an engine. I have "built" my own and "assembled" my own before. I have checked all the things you listed before myself as well as other things. I just think it is overkill to check compression before the engine is in the car because if you REALLY ARE meticulous about checking things, knowing that the rings, valves, and gaskets, etc are in good shape, means you know your compression will be fine. I don't even check compression on an engine that I built or assembled unless there is a problem with the way it runs or I just want to know how many psi I have.

I still do not see the point of testing it by cranking it by hand. You turn it so slow that the compression is bleeding out through the ring gaps while you are turning it.

My point is in your post above.
You will not know your rings, valves or much anything else is correct till you have the engine installed. Not the best time to find something is amiss.
If you've never made mistakes or overlook small details with all the engines you've "built" over the years... you're doing very well.
Myself... I cannot claim that perfection.

I realize this has not much to do with an engine installed from factory and "we" going into it after the fact.

The method I described allows you to check compression numbers down the road, and compare them.