Bad compression on a new rebuilt engine!!!

[Jeanluc]

My engine is a complete rebuilt one. The compression ratio should be 9.8-1 cr. The leakdown test is ok on all cylinders.... but my compression is only 120 Psi :ugh ?????!!!
My camshaft is degreeing with 4° advance.
can a incorrect degreeing cam explain a low compression test????

Thanks

JeanLuc

 

[redc4corvette]

Compression is eather rings or valves you need to figure out which it is.

 

[G Winter]

If valve job is bad I would think compresion would be uneven. Do you have all plugs out when you take the test ? The speed that it cranks has an effect on the reading you get.

also you want wide open throttle while cranking.

 

[Hib Halverson]

Station compression pressure does not always equate to compresion ratio.

What are the specs of the camshaft in the engine?
Was the compression test done with the engine warm, the throttle blocked wide open and engine cranked for three revs during each test?

If the leakage test was ok, you have no problem with compression rings or valves.

 

[Ken]

Station compression? Or do you mean "static"?

 

[c4cruiser]

Do all cylinders show 120psi on the compression check? Check compression again using the method mentioned here. Remove all plugs, let the engine spin at least 3 revolutions and the TB open.

A rebuild using new pistons, rings and a properly machined piston bore should show around 170+ psi. The cam advance of 4 degrees is correct for a stock motor. Is there any binding in the valve train? How was the valve lash (lifter pre-load) set?
Was the rebuilt motor the one from your car or did you buy a motor already assembled from some company?

 

[Jeanluc]

I rebuilt myself the engine.

the camshaft is a Comp-cam hydrau-roller 258HR (206-212/480-487 lift.114 lobe separation.
I use a complete CC-kit for spring, pushrod, rocker, lifter.....etc.....
pistons are KB forged (I do not remember reference but I choose a bigger one than stock.....for have 9.78-1cr)

The pushrod adjustement is: zero lash + 1/2 turn.

When I take compression, all sparks are removed, TB open, 3 or 4 revolutions, engine hot.....
but I'm septic about camshaft degreeing. I use a CCA-5100 wet belt. I adjust it with C-cam degreeing instructions:confused .

If I have error with more advance than 4°.:confused ....so the intake valve closed later ...and the compression start too late for have good value!!! :ugh
are you agree with me?????

 

[G Winter]

If the cam was retarded you would lose cyl. pressure. Intake valves would be open as piston came up on compresion stroke.Which they are to some degree anyhow. Usually if that is the case your engine wouldn't perform as it should. Might have low vacuum readings with a pulse in the gauge, even poor idle. If cam advanced to far not sure it would effect air intake enough to cause a lower compression reading,but might give you a very high vacuum reading.

 

[jmccloud]

Low compression

Have you tried another compression gauge? I had a bad one many years ago and went through a lot of unnnnn-necessary grief.
Good luck.

 

[Jeanluc]

centerline control....

I just control this afternoon the intake centerline. Result: +1° advance.....
I had a little hope that my problem was a retard cam.....!!!

About Comp cam wet belt, ( mine is perfect, after a control today) is it possible that is the problem???

Effectively, I going to change compression gauge....

thanks again

 

[Hib Halverson]

Station compression? Or do you mean "static"?

Yep.
Hell of a typo, eh?