Actually a gain will be seen without A/F tunning. First off of all...if that was true...why would a new intake or exhaust add gains??? Better pumping efficiency on top of more air being taken in. In the case of higher compression you get much better thermal efficiency in the engine. That is what leads to power gains. I am sure A/F tuning would help out...but you will see gains no matter what.
I'm not 100% sure, but I dont think you can't just "change a piston" and expect to get any type of gain. You have to increase the bore size also.
Assuming you increase your bore, then you have use a larger diameter piston which will increase your compression ratio (the ration of the Bottom Dead Center Volume to the Top Dead Center Volume) and in turn increase your thermal efficiency even without changing the air/fuel mixture. If you are able to modify your air/fuel mixture that will even further enhance what you are able to get out of your engine.
You should see gains over stock even without a/f tuning present. The only issue you would have to worry about is knock using the stock setup because the ECU can only add so much more fuel to the system before it maxes out. I have no idea how much room there is with the stock setup but you might even be fine just running that way with higher comp pistons. You would gain more though in conjunction will fuel tuning and it might be required depending upon how much fuel the stock ECU can provide. Best bet would be to monitor with a wideband O2 and see if the stock ECU is providing the proper a/f ratio and of course you'd need high octane fuel as well.
you would need to tune your car after a compression change to see some benifits. Most likely just thowing pistons in and running the car will lead to some predetonation and maybe some knocking and pinging but you will not see much of a difference untill you drive the car to the shop and get the car tuned for 93 octane gas and 100 of course for those good old racing days. And no you wouldnt need to increase bore. Although increasing the bore is a good idea because the block will be available for that kind of work if you are gonna be taking pistons and rods out in the first place.
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