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Well the 452's will have hardened valve seats,but the 906 will flow a bit more depending on how it was ported. Possibly you could get the 452 to flow just as well with some template work. Not sure if the 906 has hardened seats,could if they were installed ya? You should check to see if the guides are good as well.I think the comp will be a bit healthier with the 906 tho...Dont forget to check em for cracks before putting in the work.