I purchased the TD head because the stock cams become restrictive at about 330bhp not only that with the ARP's and valve springs it will take abit more of a beating, it was ideal for me really, im very happy with the power i made and now i can always run a bigger turbo if i wanted to, just need a bigger inlet first but im waiting for TD to manufacture their own as in my eyes they are going to be the best on the market when they are released