when a exhaust manuf. claims say 10%hp gain from exhaust does this mean a gain of hp at the flywheel or from the prop shaft ?

it means they are full of it!

YUP

It means that stock manifolds flow crap, and 10% better than crap is still crap. Save the money and buy headers.

Don't even waste your money on headers...unless you are changing heads, intakes, carbs and cams at the same time. I learned that valuable lesson. Headers might give you a mile or two per hour for the $3000 to $4,000 you spend per motor. Unless you are ramming a lot more fuel and air into the top of the motor, you don't need to push it out the back any faster. It just doesn't payoff. Once you add the other items however, headers are a necesity.

Well put Curt!!!

Exactly the advice I was given , but a set of Stainless marine manifolds couldn't hurt !!

1 mph was'nt worth it but the sound and looks were

Louder is better.
Randy

Loud pipes save lives!