I'm curious what power difference there is if any going from the stock 10.3 compression down to 8.5. And likewise, how much rwhp would you gain by going to 11 or 11.5?

Just curious about real world rwhp differences

 

Well, the accurate answer requires a good amount of physics. You need to look at the potential energy of the air:fuel mix in a 10.3 vs. 8.5 compression. Assuming you change no other parameters (timing, injector pulse length, injector pulse volume, piston head geometry), the energy loss to heat will be the same for both systems. Since the force vectors over the course of one complete stroke cycle net out to be linear, or at 90 degrees to the crankshaft, depending on how you look at it, you are left with the *theoretical* idea that the power is linearly related to the compression ratio. However, in reality, to run a higher compression engine without the risk of detonation, you do have to change at least some of the parameters mentioned above. Also, in going to a lower compression, you can alter parameters so that the power loss is less than it might otherwise have been had the parameters stayed the same as the 10.3 cr settings.

For example, going to an 8.5 ratio would yield a theoretical hp rating of about 237 at the crank. However, this compression would also safely allow you to advance the timing, increase fuel delivery, alter valve mechanics (cams) so that your actual power loss would be less, and you could maybe run 260 crank hp. And while going to 11.5 will theoretically give you 320 crank hp, thermodynamic and detonation issues come into play that make the safe power probably in the area of 300-310 crank hp.

This, of course is all assuming a normally aspirated vehicle. Add FI into the mix and it's a whole different story altogether again. Either way, changing the compression will require a lot of ECU/piggyback tuning to do it correctly.

 

knowing what I know now, I think this info is incorrect or at least partly incorrect and want to correct it for anyone reading it in the future

going to 8.5 pistons will make you lose around 2-3% power than stock

going to 11.5 pistons - assuming you have great octane gas available so you don't knock will net you about 2% more power than stock

it isn't a straight division/multiplication of:

8.5/10.3 = .82 x 287hp = 237hp
or
11.5/10.3 = 1.11 x 287hp = 320hp

it's a more complicated formula, but ulimately comes down to somewhat minor differences in power.

Lower compression gets a little worse gas mileage

 

Assuming you have the exact same setup on two 350Z's.

One has 8.6:1 CR pistons and the other has 11.5:1 CR pistons.

The one with 11.5:1 will make more power on the same boost that 8.6:1 is using. However, 11.5:1 won't be able to run as high of boost as the 8.6:1 can. So you can reach the same power level as 11.5:1 with 8.6:1, you just have to run more boost. Lower CR lowers the detonation threshold allowing you to run more boost than a higher CR.

 

I think I remember reading that for each 1.0 increase/decrease in static compression you get 3% change.