the hf's manifold and tb are about 1" smaller in diameter...as for just putting an si manifold on the hf i don't know...you'd definitely be throwing codes on the hf computer b/c the si didn't come with an egr valve...the best si mod on the hf would be the tranny with the performance oriented gearing where the hf only had 1st gear for acceleration and 2 and up were so tall for gas mileage it would slow it down considerably