Low Compression ?????

[SAturboTA]

I did a compression test on my 87 T yesterday, looking for the source of why my car is not making the power I think it should be making.
Compression was 80 psi on all cylinders. Test was conducted with all spark plugs out and throttle open. I thought maybe the battery was a little weak so I hooked up the battery charger and did a second test. Same results . shouldn't compression tests on these motors yield around 120-140 psi? The car starts fine and runs great I just thought that the car should do better than 11.70 @ 117mph 1.70 60 ft. with thhis combo.

.030 over 109 block ,champ. irons,218/218 flat tappet, 72 lb. inj., pt71gtq, pte front mount, accufab 70mm plenum/tb., walboro double pumper, 3.5 THDP, 1.6 roller rockers, accufab FPR, FAST ecu.

2 months after those first E.T.'s the car still runs 11.70 but mph is down to 114!!
I will be checking camshaft soon, any other inputfrom you guys would be great!!

 

[my1stgn]

Yeah those numbers seam pretty low. I had 145-155PSI on all my cylinders , engine still stock/un opened. Maybe weak valve springs, flat cam, or leaking head gaskets (doubt it though) or even timing chain off?

 

[SPOOLN]

You are running a bigger cam 218, that cam will bleed off some compression.

 

[bison]

That doesnt sound right at all. It shouldnt bleed off that much. Its to consistent to say its a leakage problem. Maybe the cam timing is off. Mine has 180 psi with a 212-212 comp roller and 8.8-1 comp ratio.

 

[Lee Thompson]

Imo

I would try another gage--those # are too consistent for any problem I can think of.

 

[SAturboTA]

compression????

checked cam timing today it was fine. I pulled the cam and found that almost all lobes showed signs of wear , some more than others.
could this be linked to my low compression problem????

This motor is fresh (3k miles). The motor used to have a 224/224 solid flat tappet cam and still has the valve springs for this setup. Maybe too much spring pressure for the hyd. cam?? or the dreaded low zinc oil epidemic!!!!!! or BOTH!!

THANKS FOR THE INPUT GUYS!!!!!!!

 

[Turbo1dr]

I would try another gage--those # are too consistent for any problem I can think of.

I was thinking the same thing. The timing chain being off one or two teeth could also cause that. Washed down piston rings from running too rich or flooding??? Seen that happen also.

 

[boostmaster]

I would try another gage--those # are too consistent for any problem I can think of.

I second Lee, a faulty gauge hose made a 15 psi difference in compression for me. Try a new gauge and hose.

 

[bison]

checked cam timing today it was fine. I pulled the cam and found that almost all lobes showed signs of wear , some more than others.
could this be linked to my low compression problem????

This motor is fresh (3k miles). The motor used to have a 224/224 solid flat tappet cam and still has the valve springs for this setup. Maybe too much spring pressure for the hyd. cam?? or the dreaded low zinc oil epidemic!!!!!! or BOTH!!

THANKS FOR THE INPUT GUYS!!!!!!!

How much wear? Are the lifters dished from wear? Is there evidence of grindings in the oil pan? I would replace the cam and lifters at this point. Verify spring pressures before starting it. I use stock springs for my flat cam break ins. Then i switch to a good matched set thats at 100lbs at 1.75 height. No more is needed for a flat cam and may cause premature lobe wear. Add GM EOS at break in and every oil change. The solid lifters are usually lighter and require less spring pressure for the same rpm/lift. Its possible the springs are way over 100lbs at 1.75in installed. You should always verify spring pressures. The stamping on the box doesnt mean $hit. They often vary from set to set 20lbs or more.

 

[GNVAIR]

I just ran into a problem like this on my fathers car. Similar scenario as far as miles on the engine. All the cylinders were testing low (90lbs). Leak down test was less than 10% on all with one cylinder slightly higher. Checked everything and found too much lifter preload. I started backing the rocker shaft bolts out and the compression came up to the 130lb range. Needless to say this is another possibility as too much preload can hang the valves open.