Im just looking for a second opinion, Ive been thinking 8.5:1 for compression, I was looking at some Wiseco's K598M995, I worked this out with these figures, I guessed the head gasket bore, I have no idea what a 1.6mm gasket would be compressed, I have used 49cc for the sti5 chamber volume.

The cars a classic running a Z32 maf at the moment on Apexi, its a road car running a 20g, I may fit a 321T at a later date and I would say 500 of both is my limit should I swap to a larger turbo at any point.

http://www.wiseco.com/Catalogs/SportCompact/NissanSubaruToyotaVW.pdf (http://www.wiseco.com/Catalogs/SportCompact/NissanSubaruToyotaVW.pdf)

http://www.csgnetwork.com/compcalc.html (http://www.csgnetwork.com/compcalc.html)

Bore 99.5mm
Stroke 79mm
Head gasket bore 100mm
Compressed head gasket 1.6mm?
Combustion chamber volume 49cc STI5 heads?
Dome volume -19cc
Deck clearance 0.3mm

Cr on 1.3mm gasket 8.63:1

Cr on 1.5mm gasket 8.48:1

Cr on 1.6mm gasket 8.41:1

Regards

Gary

who's headgasket are you using?

If building from scratch I would suggest you dont vary your compression ratio by altering the head gasket thickness.
Run a head gasket that gives you the ideal quench distance between piston and head this is around the 1.0mm typically.
Then factor in all the volume variables you cannot change like the head chamber, head gasket volume, quench distance volume, area around the bore above the top ring.Treat these volumes as fixed.
Then determine the compression ratio you WANT and alter the piston dish to give you this ratio.

Using a 1mm gasket I get 8.86:1 cr, would this be suitable for my goals? I havnt decided on gaskets but Cometic is a possibility, I also still need to cc my heads which are still on the car.

So to get the ideal quench distance volume I need a gasket of 0.7mm, this then gives me a cr of 9.1:1 which is to high, I would need to find 6cc to get down to 8.5:1. I dont want to go touching the heads so would need to find it in the piston if I used these, could I take 6cc out of these pistons or would that weaken them to much.

Ideally I need to find a piston with a bowl of -25cc that will give me my desired cr with my 49cc heads, anybody know of any possibles?

I use CP pistons in my engines.
I simply tell them what dish volume i require and they do the rest. They will cover all aspects of piston strength etc.

Although I use CP's I would expect other piston suppliers could do the same.

any company can make custom pistons for you. I wouldn't take 6cc's out of any companies off the shelf piston.

Also what "ideal" quench distance are you trying to achieve? what's you're current machined deck height, and piston compression height (pin height)?
using standard 130.5mm rods?

now for the biggest question...why do you so badly want to have an 8.5:1 compression ratio?

I was hoping for an off the shelf solution to limit the amount of time the car is off the road, theres a couple of options for me here in the UK but will cost I would think. Ideally a quench of 1mm, I dont know what my current deck height is or piston compression height, its an Axis stage 2 block with CP pistons and forged rods.

Im future proofing to be able to run 500 of both so 8.5:1 or less will make life easier and I wont be running on the ragged edge.

well you'd have to have some 7-8" micrometers to measure your deck height...compression height will be a piston spec for which ever ones you are looking at. then its easy to calc...
deck height - [ (1/2 X stroke) + rod length + compression height + compressed HG thickness] = piston distance @ TDC in relation to the the head...if its negative, your piston is "in" the head ;)

Ive came to the conclusion that theres no off the shelf piston that will give me a cr of 8.5:1 with my sti5 49cc heads, correct me if Im wrong, if anyone knows of an 8:1 piston for the 57cc head let me know because this will get me close to 8.5:1 on my heads.

Ok no flames I just want to air all options, is it possible to take 2cc out of the heads without any detrimental effect?

yes, it is easy to take 2cc out of the CC

don't choose a crappy piston because the dish is the volume you want.