Low compression after rebuild

[350z_drifter]

my brother just fully rebuilt his motor a few months ago and install 9:0:1 compression piston but a few days ago he did a compression test 125 psi across the board ,stock sr runs about 155 psi across the board would the high compression pistion decrease or increase the compression PSI? also we are have boost problems install 1 bar wastegate spring but can't hold boost the turbo is a gt3076r. we check the intercooler piping and vacuum line .... and can't find out problems can you help

[Hijacker]

What headgasket did he use? If you say cometic, I will hunt you down

[exhsturbine]

What headgasket did he use? If you say cometic, I will hunt you down

hahah hijacker, im starting to like you (in a hetero sort of way of course )

but my question exactly, what sort of head gasket? any oil and water mixing?has a leak down test been performed? did the loss of performance happen gradualy? or was there never a difference? the rings could have been broken in wrong (dont suppose there is excessive blow by is there?). or you could have a bad valve seat. how many miles ago was the rebuild done, and what sort of work, other than pistons was performed?the loss in boost to me, seems like where the compression is going.

[Hijacker]

If he's been driving this motor a decent amount since the rebuild, which was a while ago, according to the OP, I would hope the rings would have seated by now. And even if they hadn't, 125 is still too low.

My first thought when it comes to low comp like that, is to look at the headgasket. Another issue also might be that the headgasket was never retorqued.

Metal headgaskets tend to seal best if the head bolts/studs are retorqued. A friend of mine has noticed that on every MLS rebuild he's done, if he doesn't retorque after the initial break in time, the gasket leaks. We retorqued my studs after my break in and got an extra 1/4 turn out of each nut.

It's an easy process. Just start in the middle and break one nut/bolt loose, and then retorque. Proceed to the next one and repeat. Then work your way out. You never want to loosen more than one at a time. It prevents fluid seepage.

My pistons are supposed to be 8.5:1 (although when I ran the numbers, it was more like 8.9:1), and I was making 125 PSI cold crank before the rings even sat in, so a motor that's been running this long should have better compression.

[rustest86]

to me it sounds like it either, it wasnt broken in properly, the cylinders got too hot and baked the rings, or theres improper clearances with the piston and cylinder wall. ive never seen a motor have the same compression across the board and have a blow/leaking head gasket unless the head wasnt torqued right at all. and in that case it would be leaking water in the oil or cylinders, blowing coolant hoses off, and leaking along the block head mating sufaces. but thats not to say the gasket wasnt damaged before install. ive be running a cometic gasket with arp studs for a year now with no retorque and havent had the first sign of trouble. "even though you cant see me, i knocking on wood right now." i also installed my gasket about six months after my rebuild, but i checked the deck and head before i installed it.going from 8.5-1 to 9.0-1 would maybe add 10-15psi maybe. a stock KA has 9.5-1 compression and good on it is 175psi, but i have seen fresh motors as high as 205psi.but really it sound like somthing wasnt done right on the rebuild or it wasnt broken in properly. and low compression would be why that gt3076r cant hold boost.

[93 Chuki FB]

125 across is not good, BUT! the number doesn't matter so much as long as the numbers are close (less than 10% difference between each cylinder). I remember on my KA, my compression was 139 across, but when i used a different tester i got 190 across. That KA was the strongest KA i have ever seen and it ran extremely well. And u could have an exhaust leak at the headers, that would cause boosting issues.

[PyR0NiAk]

Chuki brings in a valid point. I always tell people that complain of low compression to check with a 2nd gauge. There's always a chance the gauge reads low.

[the converted]

Between the low compression and the boost problems, I'd be looking straight at the cam timing.