Compression test?

[art leong]

It's been almost 30 years since I did a compression test. I vaugely remember turning the motor 5 full cycles. The gauge I just bought tells me to crank till it doesn't go up any more.
Now at 5 cycles it shows 225, If I crank till it reaches the highest point it goes to 300.
HELP

[Dragsinger]

Art,

to me, the number is not as important as the consistency between cylinders. With all plugs removed, throttle blocked open, (throttle open so cylinders can receive all the air they want) 5 compression spins is plenty.

You did not say what engine you are testing but 225 is very good for anything. I have never seen a 300 reading?

A couple of notes, as a gauge test you can compare the compression gauge to a air compressor gauge. Example, say your shop air compressor gauge ready 135 pounds, rig a chuck to put that same air pressure to your compression gauge. Both gauges should show the same number.

Also, in an ideal world, engine compression reading would be equal in all cylinders. ( +or - 2% is reasonable for a race engine)

[Pat Cook]

Art, that engine must have short cam timing and decent compression ratio......?

[art leong]

Originally Posted by Pat Cook

Art, that engine must have short cam timing and decent compression ratio......?

14.5 to 1 compression ratio, 256 degrees duration at .050, 44 degrees overlap at .050

[Dean Feiock]

There is a simple formula to calculate cranking compression:

Atmosperic pressure multiplied by compression ratio

14.7 X 14.5 = 213.15-------------I would say your 225 is fairly correct

[art leong]

Originally Posted by Dean Feiock

There is a simple formula to calculate cranking compression:

Atmosperic pressure multiplied by compression ratio

14.7 X 14.5 = 213.15-------------I would say your 225 is fairly correct

This equation leaves out cam overlap and duration. Isn't that a factor?

[Pvt Parts]

Originally Posted by art leong

This equation leaves out cam overlap and duration. Isn't that a factor?

Yes it's a factor. You can run those numbers but they don't mean much of anything, just that they are close to the same. Change overlap, lobe centers or valve lash and the number will change. You do have all of the plugs out right? Has the motor been run yet or is it new?? What does it leak???

[art leong]

Originally Posted by Pvt Parts

Yes it's a factor. You can run those numbers but they don't mean much of anything, just that they are close to the same. Change overlap, lobe centers or valve lash and the number will change. You do have all of the plugs out right? Has the motor been run yet or is it new?? What does it leak???

All plugs are out. Motor has about 15 passes plus 6 or 8 dyno pulls. Haven't leaked it down yet.
I'm running hydraulic followers I wonder if they are pumping down a bit causing less duration?

[Dean Feiock]

Originally Posted by art leong

This equation leaves out cam overlap and duration. Isn't that a factor?

Yes, it is a factor, but a very small one. At cranking speeds the air is just moving to slow for the cam to have that much effect on compression.

In the past, I have even forgotten to block open the throttle, only to go back and redo the test with no change in numbers. I assume the engine pulled what air it needed from around the throttle plates, intake manifold, and the other cylinders.

To put it simply, you have a cylinder full of air and you are compressing it "X" number of times as the piston moves upward. That's why you will see most street car engines are in the 125 to 145 PSI range and most diesel engines in the 300 PSI range. As stated by others, it's more important that all cylinders be equal than your total number. But if you need to reference your number against something, the formula is accurate.

If you really want to see what effects the cam has on compression, you need to work with a running compression test, not a cranking one. With a running compression test, cam timing, volumetric efficiency, and reversion all come into play.

[Pvt Parts]

Sounds like you are looking for a problem. Did it make good power on the dyno?? Did it not perform to your expectations?? If you are looking for a problem, leak the motor.... even then, if the numbers you are posting are consistant across the cylinders, you are looking in the wrong place. Even leak testing it's hard to tell because of the variable ring seal at lower pressures. If you have a problem and the pressures are consistant across the cylinders, look somewhere else.

Keep in mind that things move around in a new motor, especially valve seats. I've had a new motor (less than 15 passes) shift a valve seat so bad that you could hear it when you cranked the motor and it only slowed down .02 to .03. If it leaks good, move on.