What is the difference between a mercruiser 357 mag 4v alpha and mercruiser 357 4v bravo?

-From what I have seen, Mercruiser makes two models of the 357 mag. The alpha, rated at 275hp and the bravo rated at 325hp. Other than the hp and water pump for the bravo, whats the difference that makes the power?I'm pretty sure that the engines are the same, but the outdrive (sterndrives) are different. I don't know why one has more hp, must have something to do witht he gear ratio and one may be rated for 2 props or higher rpms.
Alpha one has one prop. Bravo has two counter rotating props.
The Alpha drive has one prop and the bravo has two counter rotating props.