I agree with most of what Curt has said. An aftermarket exhaust system may not give you much initially in the way of speed/performance especially for the $3K-$4K spent, BUT it IS the best thing a person can do BEFORE he starts adding other speed components to the engine. Induction type items such as cams, heads, carbs, intakes manifolds, etc, are NOT going to help much while retaining the STOCK exhaust manifolds. What a person tries to put into an engine through induction needs a smooth unrestrictive way out as well. This can be done through a set of aftermarket exhausts, THEN the MORE you do to an engine the MORE the aftermarket exhausts are going to work for you.Originally posted by Curt
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. 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. Once you add the other items however, headers are a necesity.
If you CAN'T afford to do all at once, here's an idea of what you can do: Buy the aftermarket exhausts FIRST and install them on your boat for the first year. They will enhance your engines, sound better and will USUALLY improve your fuel econ a bit and midrange cruising and you may also find that your RPM will go up slightly and maybe a slight increase in speed---maybe 1-2mph if that.
The next season, you can start adding performance components to the engine(s) and as you do, each time you do something your engines will be more sensitive to the changes because you already have the aftermarket exhaust manifolds in order for the new parts to work well.
Hope I haven't confused you any further. Best wishes