Compression test numbers vs compression ratio

130 is low.

 

More important is how even the #'s are from cylinder to cylinder. A number of things can affect compression numbers, including the person doing the test, the equipment and atmospheric conditions. Here in Colorado, compression #'s always look low because of the altitude. If you have any doubts, have a leakdown done. It's a better test of a motor's health.

Dave

 

From a thread "Dino Compression"

There is an old “rule of thumb” that allows one to estimate a range of cranking compression in a stock gasoline engine if one only knows the piston compression ratio. The “rule of thumb” is that the cranking compression should fall within a range of 15.5 times piston compression ratio and 21.5 times the piston compression ratio.

The 21.5 times the piston compression ratio would be for a perfect engine
The 15.5 times the piston compression ration would be for a well worn engine

Pressure calculator if you have certain specs:

http://not2fast.wryday.com/turbo/co..._pressure.shtml

Best wishes

 

I would consider a test that showed exactly the same number on each cylinder to be highly suspect, regardless of the numbers.

Dave

 

Careful though, cam timing REALLY effects cranking compression.

 

Yes!

Also, thye may have forgotten to open the throttle or something silly.

Redo it and also have a leakdown done.

But if the car is running well, does it even matter? What ever it is, it's fine....unless this is you PPI.

 

The inlet valve closing will affect the "dynamic compression" even if two engines have the same "static compression".
Early closing = higer reading
Leter closing = lower reading

130psi seems low however, maybe the throttle was closed as MK E suggested.

 

There are lots of threads on how people get bad #'s because so many people don't know how to to a correct compression test. The engine must be hot for one thing....and if the engine was flooded it can make the #'s way low. My car was tested at like 130 and when I had another guy redo it, he got 168-170 across. The factory spec was 170.

If all the #'s are the same but low then it's more likely a messed up test than if most are high and a couple aren't.

Ken

 

Cylinder pressure is really the number they are givving you when they do this test.
First thing, is ensure that the tester is clean, that the valve has been checked for any debris.( I have seen mechanics pull out a tester and not bother to clean it and have poor results that have made the customer cry)

The engine must be HOT.

Use WOT when cranking.

A very goog battery helps too. It ensures that the crank speed is consistant.

The results should be there in 2 to 4 revolutions. It is even great when the pressure is there two revolutions.


Cam timing, ( if the camshaft are retarded, the cylinder pressure
will be lower at cranking speed, but will increase at higher speed. If the cams are advanced, the pressure will be higher initially)

The static compression ratio is given number, cylinder pressure changes
drastically during the operational range of the motor.

Good luck.

 

Another mistake is not threading-in the hose to the spark plug hole tight enough. Because the spark plug hole is so recessed, you can't really use a tool to tighten. I hold it as tight as I can by hand, while I have a helper crank the starter.
As a point of reference, my low mileage 308 engine gave well over 200psi on each cylinder and it was cold and had been sitting for over a year.
Vince

 

3 or 4 years ago there was a thread here about how to check compression. Two people argued for pages as to whether or not you should just keep cranking to motor over or just to do it a particular number of times. That debate came close to getting out of hand.

I have no idea how it should be done, but now why are we getting 20 less then before?

 

If they added oil and it took you back up to 130, no matter what the cam timing is, that would seem to indicate something towards the ring area. (because the oil won't affect your valve seat area).

Right. In theory your only in/out sealing areas are the valves, spark plug area, and around the piston.

If oil was added (which again is a great way to narrow it down), and no change, then your back up towards the valves leaking around the seats. (although remember what was said previously based on cam timing and the potential effect). Go for the leakdown test if at all possible.


Bob

 

did they do a leakdown?

 

I didn't that there was two different compression test, which way is standard, wet or dry ? Andd it should be at 185 water temp ?

 

i think consistency is the most important between cylinders. 5 lb -10lb max spread max between all cylinders is a good sign in my book, so many other variables w oil additives, flooding etc, can effect individual cylinder numbers...
but im no auto mechanic. thats how i check boat engines before a purchase...

 

I would call the guy and ask him what the plugs looked like when he pulled them out. Something is fishy about those compression numbers to me. They are low but so even with one another. I understand your point, if the rings are bad (IF) who cares about the valves by then? Have you spoken directly to the service guy? What shop did it?