2.0 SPI vs. 2.0 zetec

My buddy has a 2003 focus with the SPI sohc motor and would like to get some more performance out of it. Lots of parts available for the zetec motor, but not for the SPI. My question: what is part interchangeability
between the two engines like? also, are there differences in the intake manifold/throttle body setup between the two engines and could a dohc header be made to fit a sohc cylinder head? Thanks in advance for your advice!
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The blocks are the same but there are many differences in the heads etc. You can get a lot of information on such specialist sites as Focus Fanatics at http://www.focusfanatics.com /
My guess is that unless your buddy is especially attached to his 2003, his best option is to be content with keeping it in good tune i.e. plugs, fuel filter, injector cleaner, clean the intake plenum etc. Also, a lower-restriction exhaust and air filter may help. Beyond that, I think he would be better off trading it for a Zetec. It would be much less trouble and expense than attempting to mod an engine designed for economy and durability, not speed.
Me, I'm really happy with the pep of my 5-speed SPI. But then I'm an old man and drive like one... uh, mostly :<)
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