d series sleeves are good for 300-350 hp on stock sleeves. granted some have done more, i wouldnt. eagle h beams and pistons are what you would need to hit 300 whp. the full d16 would be a better choice. parts are more abundant, and you do get a lil more displacement, which is always good to force more air into. dont build the head, its not necessary. you should be able to hit 300whp on a stock head. if you do build a z6 head- ill recommend either the crower turbo cam or the zex 59300 and matching valvesprings. if you do do a mini me, you can either get lower comp. pistons, doublestack stock headgaskets, or keep the compression and get a ignition timing controller of some sort and retard that shit to prevent detonation. definately w/o a doubt get arp headstuds, d series heads are beyond notorious for lifiting under modertate or more boost condtions. for 300+ whp, you will also need urethane mounts, as the stock ones will break in no time, a badass clutch, perhaps a lsd, and good fuel management. tuning is key! good luck
