I got a 2006 WRX Sti. How much can the stock sti pistons take?

 

That would depend on the state of tune (engine management) and gas quality (octane). In CA and many other parts of the country only 91 octane premium is widely available at the pumps, while other states may have 93 octane commonly available. Unleaded race gasoline of 100 octane or higher will allow even more boost and a more aggressive ignition advance.

Using a boost controller without engine management can lead to lean mixtures when you need enrichment the most.

Are you using any form of aftermarket engine management?

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so u can boost any amount on regular pistons? and still not blow them up?

 

It is not just the pistons that you need to be weary of.

Rods bend and break, head gaskets leak from increased heat from increased boost. Rings can also crack. State of the tune is also a correct statement, if you are too lean in your fuel mixture, hte explosion is so hot it will melt, and even vaporize a piston.

So the proper question is not how much boost can I put on my 06 STI. You need to look at the engine and electronics as a whole. With that said the stock block with proper tuning and cooling mods will be good up to about 30psi on a decent sized turbo.