Well thus far I have officially done two b20 head and IM's of course since I can't compete with Toofast's prices on TB's I just let these guys go thier own route, anyways I had recenlty started on a b21 head and IM and have come across these nice little revelations. For example after disassembling the b21 manifold I can tell you that from the factory it flows far better than the b20 especially if you're looking for a motor that will give you good midrange and top end, it just has a smoother casting and plus the plenum has a far better flowing design than the b20, the b20 has a more direct and tossed flow, however if you're looking for overall which one is the best to mod I'd have to put my money on the b20 mani, just becuase you can get rid of the secondary plate and make it a pure mid to top-end manifold, which is what I have made for my car, now I haven't tried this on a customers car becuase I want to make sure I get good gains out of it. and as for the valve dimensions on the b20-b21 they are both the same, however if you're a b21 owner, you're in serious luck becuase the valve reliefs are cut deeper which mean you can actually put a pretty radical set of cams and probably still clear your pistons, however if I were you I would seriously get a spare b20 head and just mill the shit out of it till you got an oval shaped combustion chamber like on the gsr, sure this would make the head a one time use piece only but think about it you get this from the junkyard for what 30-50 bucks a head gasket, shoot after about a 100 bucks you could bump your compression up considerably, I would have to cc the chambers but it's a good jump, also the combustion chamber diameters for the b21 are 82.7mm and the b20 is 81.8, now if you're a b20 owner and want a good upgrade with factory internals then get b21 pistons with 83mm bore, and toss them in. There is a lot of other information I am leaving out. But as usual I have a hidden agenda, I am going to be finishing the P&P on the head, and it'll be reassembled, the im is already done, I have the measurements on how much larger I went on the ports and the throttle-body inlent and all, but when I do sell this I am aiming to sell this to someone with a b21 with some considerable mods to take advantage of it. I am going to be perfectly honest with you on this I have put so much time and concentrated effort on this package that I cannot emphasize to you how proud I am with my work on this, I trully will hate to sell this, becuase this is some of the best work I have done, and the most extensive I have gone, so if anyone is interested toss me a pm and I'll keep you in the loop.