Compression lower after rebuild

[sleepyvr4]

Ok - went from 155-170psi on all 4 (checked twice) with a head warped 9 thousandths pushing coolant...

Had head trimmed 8 thousandths and cleaned, new topline composite headgasket installed and now:

145-150psi on all 4

bottom end and everything else the same. Head torqued to 75 ft/lbs on stock head bolts.

I'm no rocket surgeon, but I was at least expecting the same numbers...


???

Thanks fellas

 

[Rausch]

Timing correct? Did you have the valves lapped or something (valve sealing issue)? Headgasket thicker than what you expected?

Stock compression? Was the previous test dry as well?

 

[sleepyvr4]

timing is fine. machine shop guy said everything seats like a champ. 7.8:1 except the shaved head. All comp tests done after car warm/driven.

 

[Rausch]

Coolant/oil in the bores during the old tests maybe?

 

[turbowop]

Latest test done with motor full warmed up and throttle held wide open?

Either way, at least compression between cylinders is nice and close. The stock bottom end in 1051 only puts out about 145psi across as well.

 

[iLLeffeKt vr-4]

Could the head bolts be streched? Or headgasket too thick?

 

[grocery_getter]

Did you clean the carbon deposit on top of the piston when the head was off?

 

[prove_it]

If you were getting a reading of 170 before, then you had something in the cylinder like water or oil. No way a stock 7.8:1 engine builds compression like that. I get about 165 even on #715 with a new engine with 8.6:1 compression. I think your 145-150 reading is right on the money for a good strong stock engine especially a 4g63. IMO anyway.

 

[gvr4ever]

165psi is brand new stock compression. I'd want more then 145 on a rebuild. I've gotten 150-155 on cars with over 100K miles on them. I think 125psi is near or at the service point.

Try to gently break it in a bit, and try again in a month.

I thought a taller head gasket created more compression, not less. No?

 

[kartorium]

Getting the head decked usually offsets any extra thickness the headgasket provides.

 

[prove_it]

Quoting gvr4ever:

I thought a taller head gasket created more compression, not less. No?



No, a taller gasket adds height in the combustion chamber, which lowers compression. 4G63's have blow-by, it's hard to say how good your rings and cylinders are.

 

[GVR-4]

Similar to Mark, my stock 7.8:1 ~120,000 mile engine gets right at 150 psi on all cylinders when at operating temp.

 

[iLLeffeKt vr-4]

Quoting gvr4ever:


Try to gently break it in a bit, and try again in a month.

I thought a taller head gasket created more compression, not less. No?



He said only the head was removed. Bottom end was left intact so there's no need to break it in. Rings are already seated since the motor already has xxxx miles on it.

Thicker gasket will lower compression, not raise it.