It would depend on which vehicle and I would think that a 12 valve would be easier more options no steer you away from idi's but if you can find one let me kno how ur conversion goes my dad bought a 94 brand new cc drw 454 has 396xxx on the clock gets 6mpg loaded or not and i went alot of places in that truck and he gave it to me an I wanna do a diesel conversion an drive it so it won't go to waste cause it cost me about 7 bucks just to get the mail right now lol I start it once a month that's it