5.7 Merc Ex Manifold Mods

[SneakyPete]

I was thinking about extrude honing and jet hot coating my exhaust manifolds this winter. FYI extrude honing is when an ultra-coarse media is forced through manifolds in order to open them up and let them flow more freely (kinda like porting). I know lots about headers and car manifolds, but not too much about marine liquid cooled manifolds. Do I risk breaking open a coolant passage or anything of that nature by doing this. I just dont want to spend 1400 on a hipo exhaust.

[SneakyPete]

I was thinking about extrude honing and jet hot coating my exhaust manifolds this winter. FYI extrude honing is when an ultra-coarse media is forced through manifolds in order to open them up and let them flow more freely (kinda like porting). I know lots about headers and car manifolds, but not too much about marine liquid cooled manifolds. Do I risk breaking open a coolant passage or anything of that nature by doing this. I just dont want to spend 1400 on a hipo exhaust.

[happy]

I've never heard of this being done..(marine wise) i would suspect that it would have alot to do with the age of the manifolds, as well as, condition of them.. notably, mercruiser has exhasut manifolds available that are ceramic coated.. and the price is pretty good.. for a stock manifold they flow darn good. if you do it yourself you'd probably be around 800 bucks.

some other ideas on flowing more air... install thru hull exhaust (silent choice or such) and run free flowing tips..(flappers are a must) and install a k&n filter.. this will help you more so in the mid-range.. at top end you may see a smidgen more..

anytime you start modifing exhaust.. you stand the chance of starting a major problem called reversion.. so be careful.. hope this helped.

[vinny p.]

Don't waist your money on extrude honing Merc. manifolds. They will not flow no matter what you do to them. Worse yet than their flow, is the fact that the gasket between the manifold and riser is prone to leaking, which will trash a motor in no time. The next problem is the short risers can not handle any kind of cam profiles, other than factory. Save your $$ and get aftermarket manifolds. Too bad you don't have a big block, I have a set of Gil WetOffshore manifolds I can't use anymore.