Loosen Cam shaft compression goes up!!

[jazzyjay]

I am trying to diagnose a compression problem. I loosen the camshaft bots and my compression goes up. the short block is a new rebuild because I thought problem was there. But the problem is in head. What I am thinking is the seats where cut wrong in turn the valves need to seat deeper in head. Can I remove some metal from valve stem above keepers to make up the difference or take it from the bottom of the lifters. The car runs and actually made a 11 sec pass whith me getting out of it after the 1/8 mile mark but notice compression was low. I have Crower dual springs and Comp cams

 

[jeremy19984g63]

Did you bleed the lifters before installing them?

 

[jazzyjay]

If I bleed the lifter they compress and the same results as if I loosen the cam caps. The problem with that is as soon I crank up the car they pump back up and don't compress

 

[95AWD_TSI_TALON]

I am so confused at what you are trying to say. You are talking about backing out the bolt that is holding on your cam gears? How are you checking your compression with the cam gear in this stateWTF.? If you are talking about adjustable cam gears and you are changing the "degree" that they are set. It will alter your compression aswell.

Please elaborate on what you are doing to get the difference aswell as the compression numbers.
Matt

 

[TunaTalon]

I loosen the camshaft bots and my compression goes up.

You have discovered the difference between static compression ratio and effective compression ratio. The short story is that compression starts when the intake valve closes, not when the piston passes BDC. The more aggressive the cam, the later the intake valves closes and the lower the tested compression.

See page 31 of http://www.kidzuku.com/StrokeOrNot.pdf for the long story.

 

[92awddsm]

You have discovered the difference between static compression ratio and effective compression ratio. The short story is that compression starts when the intake valve closes, not when the piston passes BDC. The more aggressive the cam, the later the intake valves closes and the lower the tested compression.

See page 31 of http://www.kidzuku.com/StrokeOrNot.pdf for the long story.

In short, loosening the cam cap bolts effectively lower overall cam lift and duration, therefore, static compression goes up. This is because the valves open later, open less, and close sooner. If you were able to crank the car up like this and keep reasonable oil pressure in the head and to the lifters, they would pump up to the same point and give the same compression as would a head with cams torqued properly.

Anyway, what is your static compression and how much does it change when you loosen the cams? Have you noticed that compression drops more when you retighten the cams?

And yes, the valve stem tips can be cut and usually are(or should be) any time a valve job is done. I wouldnt suggest doing it yourself because Im sure you cant cut them square and an accurate amount without the proper machine.

 

[Defiant]

You're running a compression test with loose cam caps?

I'm running shrieking into the night, overcome by sheer terror.

 

[jazzyjay]

You have discovered the difference between static compression ratio and effective compression ratio. The short story is that compression starts when the intake valve closes, not when the piston passes BDC. The more aggressive the cam, the later the intake valves closes and the lower the tested compression.

See page 31 of http://www.kidzuku.com/StrokeOrNot.pdf for the long story.

Ok now I am understanding make sense because since my last post I check each cylinder on the compression stroke and had no leak by (air escaping) valves

when I retighten the cam caps the compression is 118-120 when they are loosen it is 140-150 WTF

 

[TunaTalon]

You're running a compression test with loose cam caps?

I'm running shrieking into the night, overcome by sheer terror.

Come baack Defiant, come baaaaaak.

Should there be a FAQ or wiki on the effect of camshaft profile on compression testing?
My search of "compression test" and "intake valve" found no reference to anything like effective compression ratio.

 

[Defiant]

Perfect testament to a leakdown test's superiority over compression testing.