oooh oooh oooh, here's my already-done response for this question since it has been asked eleventy billion times:
Originally Posted by 04stang
as far as them being EXACTLY THE SAME, thats correct and incorrect.
thats realistically correct b/c all parts for a 3.8 will fit a 3.9 and there is none or very very little differences in hp/tq, and the engines look the EXACT same. the only difference is on top the fan, it would say '3.8' whereas on a 3.9 is says 'V6'. also on side of the engine, it says '3.9L RWD V6'
its technically incorrect b/c there are a couple internal differences between the two. the crankshaft is a little offset and the bore/stroke is a little different, thus giving it enough displacement for ford to round up to a 3.9, its something like 3.87xxxx for the 3.9 vs 3.81xxxx for the 3.8L. ford switched this over in mid-04 production, due to extra parts left over from the windstar that was introduced with "the new V6" which is why some 04s have a 3.8 while others have a 3.9.
if it has any extra power at all, its maybe like 1hp/1tq
if you both have coups or verts, as well as trannies. also, make sure you get the same launches. racing from like a 5-10mph roll would be best b/c it just involves flooring it. but race them and see if there is any diff, but make sure the cars are the same as far as weight goes. that will show you how much difference there is. there wont be much, if any.