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On a Lycoming IO-540, it seems there is a Max RPM difference between the -C and -D versions (and thus a max HP difference too). Does anybody know what is different in the engines that causes this, or are they similar internals with a different governer?

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Those engines have different valve configurations, which mean they "breathe" differently. Better breathing supports more RPM's before the engine runs out of puff, so the most probable cause is the better breathing in the one that revs higher.

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  • $\begingroup$ What are the different configurations? Both are parallel valve. $\endgroup$
    – cdeggz
    Commented Aug 28, 2021 at 2:25
  • $\begingroup$ I thought one of them had the angle valve heads. $\endgroup$ Commented Aug 28, 2021 at 3:05

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