350!! flat top Pistons. new dart heads 64cc. 5.7 rods 3.48 Stroke.
and my compression  is to low its 90 psi ???????????????????????  

Correction, your cylinder pressure is too low and it could be the result of a few things that, together, will cause a low reading.

Too much cam is the usual suspect with a fresh engine. You could have retarded the cam by mistake but it's not likely to drop 90psi of pressure from that and still run. 10-15 degrees too much seat to seat duration will do that very thing though.

I have a few questions still.

Did you remember to crack open the throttle while you spun the engine during the test? Forgetting this will cause a low reading on an otherwise fine engine, again not the 70-90 psi you're missing there. It could be a contributing factor though.

What was the pressure reading on the other cylinders? Maybe you got one ring on one cylinder off a bit and the gaps lined up, could be that.

Did you have the block decked? If not your pistons are likely 0.025" in the hole, 0.045" if the compression height on the pistons is 1.54". This is a big deal with advertised compression ratios. It's further compounded by 4valve reliefs that average 10cc in volume and thick head gaskets.

Did you check the volume of the combustion chambers yourself before you built the engine? I've measured more than one set of aftermarket heads and found them to be as much as 4cc off their advertised volume. This and all of the above

Those 10-1 flat tops are usually 9-1 if you're lucky. All those possible variances in your actual compression ratio and a big cam that, might be retarded, can make a good build go belly up if you're not careful and MEASURE EVERYTHING.



Larry