Piston Selection: Can lower compression run in stock setup for CA?
 

Just got my NA8 and tore it down to find that it needed a complete rebuild. In comes the dreams of power and the 'hey, while i'm here why not' comes up and now instead of looking for OE replacements i'm trying to calculate how far i can go and still be able to register the vehicle.

I've done some (ok, a lot) of reading about dropping down compression on the pistons to give me more headroom on a DIY turbo build. I still don't know what my machine shop is going to say about the cylinders and if they need boring or what not. I obviously want to stay as close to OE size as i can. I think they will be fine but.....

The question i have is if i go to 8.8:1 or the like will i still be able to drop to all the engine externals to stock and still be able to drive without running too rich or too lean? Would the lower compression cause it to be unable to "perform"?

(For context, i was looking at ~220hp when looking at stock internals, now that she's open, i think 300 might be a achievable target).

Compression ratio on a stock NA8 is 8.8:1 to begin with. Did you mean going down to 8:1 in compression?

Also, you shouldn’t have to drop compression any lower than that to safely make 300whp, even on 91 octane.

So i guess that's a question,

My head & crank is off 94, but the block is off a 99.

This post says they are different compression based on year https://forum.miata.net/vb/showthread.php?t=205061.

I need new pistons and the car itself is a '94 which means my emissions are based off 8.8:1 then....

If you've got a 9.5:1 NB1 shortblock in there and are using an NB8 stock ECU, then rebuilding it with pistons that match the original compression ratio should be moving things back towards stock, not away from it.

More generally though, half a point in compression isn't gonna make much difference. The engine can vary more than that just from rings that are a bit worn.

--Ian