D15B6 is what is in the HF // D16A6 is what is in the Si































the Si is .1 liter bigger (about 100cc's)















































D16A6 is 108hp, where as the D15B6 is 72hp (36 difference)















































Theroitically you could put a HF engine in a Si shell but then its still not going to be as fuel efficient because the Si shell is still heavier and















you will be moving this heavier car with 36 less hp.































its more common to see the other way around D16A6 swaps into the HF.































plus the final drive ratio is lower (which is better for economy) in the HF transmission ( 2.954 vs4.250(si) )































how much does final drive effect mpg,































this site will calculate this































http://crx.honda-perf.org/math/index.html































@ 55 mph (common US speed) the HF will be revving @ 1666 rpm, where as the Si would be @ 2663rpm, 1k rpm more for a long drive is a waste if your going for economy, ok now what to do?































since you have the Si (i am assuming)































you can keep your tranny and lower the final drive ratio to 2.954 (hf's final drive ratio) which would get you from 2663 rpm to 1851rpm [using si tranny with hf final drive gear ratio] (still @ 55mph like the above example) or if you still want to lower it get a HF transmission and you will be @ 1666 rpm just like the HF (but your car is still 200lbs more than the HF so that will compensate into your new mileage, basically HF will still get better mileage even if you swapped transmissions/engines)