the zetec engine can hold a lot of power its just how long are you going to be able to hold that power before the engine gives out. it will probably hold up to 300whp on the dyno but might go on the way home you never know. i ran 220whp on stock engine for 2 years just fine. 220 to 250whp is what most run and do not have problems. the further you push the higher your raising your chances of it letting go. its that simple. there is not an exact number like saying "250 wont break it but 260 will"
its not speculation though. doing enough searching over the years this forum has been up will show plenty of blown engines and what power level they let go at. it has been tested lol. also its not always power that makes these engines let go. no matter what power you make you have to have to tuned good.
psi does not matter. running 12psi though a straw will not be the same air flow as running 12psi through a fire hose. the larger the turbo the less boost you need to make the same power. i made nearly 100 more hp on the same boost with a larger turbo. its the torque that kills the engine not psi
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Zetec and Duratec Turbo kits, engine building