Looking at a 272 Islander that has a 496 HO. However, the speed isn't were it should be for a boat the the HO. Someone suggested that maybe the owner or previous owner switch the motor cover and put an HO cover on what is in reality a Mag. A side from the motor cover is there another way to find out if it is a Mag or HO by looking at it?

should also be a sticker on the heat exchanger,,ecu should be stamped

It is only a 50HP difference which will only show a 2-3MPH difference between the two.

Here is link that calculates speed based of horsepower and hull type, it should give you an idea where the speeds should be. Make sure you use 225 for the "constant" value. Hope this helps.

http://www.go-fast.com/boat_speed_predictions.htm

Thanks for the info