So I am putting a 540 I have had laying around for awhile into a scarab I picked up, the engine makes 620hp/635tq, doing some research people are saying you can lose up to 50HP dropping it into a boat because of manifolds and all!? I have a set of the GLM 4 inch manifolds to get the boat running figure ill go to eddie marine set up in a year or so... how much do you think this is going to affect my jetting and power?

my big concern is the heat that the manifolds are going to build up, i understand thats why they are water cooled, but I am worried if the exhaust gasses are not scavenging well then i dont want to be melting pistons down either.

If the engine has any kind of decent cam in it, it will suck water back in with those chitty manifolds/risers and hydrolock the engine.

620 hp 540? that ain't no mild cam. and you are gonna put 330 hp manifolds on it? you would be better off taking off engine hatch and mounting car headers upside down hanging over the transom...

Answer to your question....all of it! They are right, the stock exhaust for Merc BBC dumps water into the exhuast tube very close to the manifold. The high performance exhaust such as EMI, GIL, SM, dumps close to the transom, far away avoiding reversion for the most part. Bolt on some CMI, or other true marine headers and I don't think you will see much of a loss at all. Go with dry tips and dump the water out the transom and you will have the most power and no worries about the came....however dry exhaust is loud to say the least.

If you are running wet exhaust I would still post your cam specs on here, too agressive and you will have reversion issues even with the right exhaust.

Did you seriously give that advise? Red hot car headers on a boat is not a good thing.

I think that was SARCASM Sheldon

this guys posts are always rediclious,i hope nobody takes him serious,i thing he likes to hear himself talk.car headers on a boat,ARE YOU KIDDING.

I don't know, I have read many of his posts and most are just like this.
People come to these boards asking questions and are looking for guidance not bs and sarcasm.

Any how Smitty, How are things going? The Baja all together and ready for another summer?

Jason

It's not a question of how much power you are going to loose, its a question of how much engine are you going to loose!

We don't have enough information here to barely sell you a 47 cent postage stamp.

Please give us some input on the camshaft spec., and how the engine is equipped with regards, to remote oil filter, oil cooler, cooling system pumps , etc. , fuel sytem pumps, etc., carb, efi?.
Automotive dry long tube headers will always make more power than marine water cooled short runner manifolds, but the difference will be affected by engine specs and camshaft specs.

What was the rpm peak power point of this 620HP engine?

Does it have brass or steel freeze plugs? does it have aluminum heads? Do you plan to boat in fresh water only or salt water?

You've got a lot of things to consider and possibly change or modify before you go plopping an automotive type performance engine in a boat besides just exhaust manifolds.

You may be a banginggears kinda car guy but you will be a bangingwallet kinda guy if you do it wrong!

Best Regards,
Ray @ Raylar

it was a joke, but really, an engine that aggressive with stock merc manifolds is going to drown itself.