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The Indy head on the bottom has noticeably larger valve bowls than a stock 906. Also notice on the Indy head the intake port to the left has been ported while the one on the right has not. Even a fully ported cast-iron 906 or 452 head is not as good as a stock Indy SR. The Indy features angled plugs (toward the exhaust valve) and a heart-shaped combustion chamber, which makes this head more efficient.