Is it possible to modify stock pistons (to lower compression rate to 8,5:1 or 8,8:1 ?

May be it's possible on milling machine?

 

Not really... it would be too easy to mess them up. However, you can use a head gasket to lower compression if you wanted, although its more of a band-aid then pistons, its not as hard of an install and they are cheaper then getting true forged pistons.

 

it's all a matter of material thickness. Generally V8s are a little overbuilt due to the conditions they are expected to run in and the sometimes poor materials they are made out of.. Remember that most performance V8s you see at the track are older muscle cars with engines designed way back in the sixties...so the top of the piston is generally pretty thick.. this is one of the reasons that american v8s do not rev that well.. all that weight in the wrong place..

Our pistons are made a little differently. Being a four cylinder car, weight is a big issue, also, with hyundai being an economy car builder first, performance second (if at all) they pistons are built with as little material as needed to cut costs and weight.

So, if can be done... but like Tim said.. you will blow a hole through the piston in no time.. especially if you get ANY detonation

 

If you are taking them out to do that, why bother and risk to blow a hole in them when kspec sells cast turbopistons for around $350???
Those are AT LEAST as strong as your OEM pistons, if you shave a OEM piston, then it will be surely weaker.
The point is, you won't reach 250+WHP with OEM-shaved pistons, so, if your not going that far, with cast turbopistons designed to be it, you should be more than OK.

 

Yeah, I would think swapping them would be the only way to go. Time and cost of taking them out, milling each one down, then reinstalling them. Plus having the limit of hp on FULL stock pistons now bestowed onto shaved/milled ones. I personally think it's a bad idea. Get some Tigerlillies or Weiscos that are better suited for higher HP, then no worries.