I am going to have 425-475hp this year and I still have the stock mercruiser manifolds and have 4 inch stainless imco risers. Do I NEED to upgrade my manifolds for this kind of HP? I know someone who is selling a pair of aluminum GLM manifolds for 300 bucks... Motor is a 406 with aluminum heads and a xm270 cam.

Thanks

For that much HP out of a NA smallblock, you should upgrade to headers or at least performance manifolds and risers. GLM's are pretty much the same as stock merc stuff as far as performance.

Get a set of Stainless Marine manifolds/risers


Eric