Piston options
 

I see that there are 10.7:1 pistons from mazdaspeed and "The Man" is using 92-93 octane gas with these pistons no problem...I believe I read that the "Sport 20" model in Japan uses 10.5:1 pistons and I would guess that 92-93 octane gas is a must for those as well but has anyone tried to get ahold of the Australian market 9.7:1 pistons? I would think that it would be possible to use those with 89 octane gas and it would give a slight increase in power and fuel economy for the US market vehicles. Are there any other piston options available that I am missing? Of course if you are willing to bore the cylinders you could probably get OEM oversize pistons up to 0.5mm overbore but that is a lot of work (which would give you a displacement of roughly 2015cc instead of 1991cc and a compression ratio of roughly 9.2:1 instead of the stock 9.1:1 unless the pistons have a slightly different top to account for the extra displacement to lower the compression back to stock).

Bumping the stock compression from 9.1:1 to 9.7:1 would yeild next to nothing. Even with proper tuning the mazdaspeed and J-spec pistons (which are 10.7:1 and 10.5:1, respectively) give about 12 to 20hp. If you don't do anything to ECU the gain is barely apperant. Camshafts are much easier to install or cheaper depending on whether or not you are doing it and will give similar gains.

I think you hit all the options. You could look into getting custom forged pistons and rods made with your choice of compression.

Yes but my wife isn't looking for ultimate power I am only hoping to slightly improve the efficiency of the engine without being counterproductive by requiring more expensive gas. There is no need to alter the engine management either since a higher compression ratio makes use of the same air and fuel as before. Of course more power is always available if you put in a standalone engine management unit but that holds true for even a completely stock engine.

Has anyone done an emissions test with the camshafts? Increasing compression will not alter the emissions negatively other than a slight increase in oxides of Nitrogen while camshafts can have a larger impact. Also I would personally prefer to install cams along with installing higher compression pistons. There is probably a good chance that if the dynamic compression at low rpm (due to the cams) is low enough even with the 9.7:1 pistons you could probably use 87 octane for everyday driving unless you decided to push the engine hard. Also anything but the most conservative cam would be useless for me since my wife's vehicle has an automatic tranny (and it isn't the "sport shift" version either).

According to Bruce Bowlings Automotive calculators a static compression ratio of 10.7:1 would yield a 6hp increase on an engine with 130hp and initial CR of 9.1:1

The 9.7:1 that I was interested in would yield a 2hp gain

Of course this is just a rough estimate given based on a generic equation.
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