So, I go to www.seriesoneperformance.com and check out pistons and find that 92 and 93 have different pistons available. I know that the rings are different, but when I asked DrJay about it he tells me 93'* will work in a 92 but will change the compresison ratio. I haven't got around to asking why. I suppose the compression ring (top one) is at a different height. Anyone know if it'* higher or lower? And what was different about the engines to keep the compression ratio the same between the two despite the ring difference?

Ring height does not change compression.

Increasing or decreasing combustion chamber size or piston domes does.

I suspect, but can't say for sure, that the compression went up in '93. One point in compression is worth about 2% in horsepower and about 4% in fuel mileage, and GM was hurting for CAFE improvements that year.

However, compression is usually lowered to compensate for an increase in boost.

So if the boost increased in 93 (although I'm uncertain if it did) then I would liken the compression was decreased.

Neither boost nor compression changed. And top ring height DOES have a small effect on compression ratio. I know I'm not mistaken on that.

The top ring moved down in order to prevent cracking the top of the piston.

Ok. Thanks.

So if the engines had the same compression ratio, but swapping pistons changes it, what else is different? Something has to be.

Are the heads (and thus combustion chamber size) a different size?

The heads differ, but as far as I know, it'* only the rocker pedestals that are different. 92'* don't have roller rockers.

I thought the chamber might be a possiblity. So, when I put the 93 heads on, will it affect my compression? That is the real important question here. Will it go up slightly because the chamber is just a little smaller? How far did the top ring move?