How to keep cast iron headers cool in tight engine Bay?
 

I'm in middle of restoring a marquise Freeport with a Chevy 250 in line 6 in it ( Mercruiser165). I am trying to get away from the one piece intake and exhaust manifold. What does everybody do to get the heat out of the engine Bay and keep the fiber glass from lighting on fire? I was thinking of Having all the exhaust ceramic coated and then wrapping it but I don't know if that would be enough.

I use water.
It runs thru the headers.
It works well, cold water does

Unless your engine is totally exposed and in the open, you are required to run water-jacketed exhaust manifold/header. An enclosed engine compartment already has trouble getting rid of fuel vapors, and exhaust heat must be kept below unsafe levels. If you plan to get insurance on your boat, you'd better make sure they never find out about your fire-hazard exhaust. Also, if you want to direct your exhaust thru the outdrive (like it would normally be) then you have to cool the exhaust stream itself, also with water.

I'm sure you can find a header manufacturer who is willing to fabricate you a set of water jacketed headers but I would expect them to be pricey. You may also have to supply them a bare cylinder head, the mercruiser manifold and riser, as well as the exact intake manifold you plan to run in order to prevent any interference issues and to make sure the exhaust exits at the same height and angle.

So all in all im stuck with the 1 piece design that keeps cracking . I've replaced 3 since I owned the boat. And the starting date hard to get ahold of And the starting to get hard to get ahold of quality replacements.

You are seriously running non-water jacketed cast iron exhaust? That will literally glow bright red hot when running hard. You can't do that!

What is to be accomplished from a dry manifold or manufacturing a header to work on a 165... Cheaper/ easier/ safer to stay with the wet manifold system for what that power plant actually is.

All manifolds rot from the salt. Get a good marine manifold, even would recommend a new one, so you are starting with a non-leaking set-up and take care of it (flushing completely if in the salt - totally assumed from the NH state)

At the end of the day it is a small inline 6cyl, but the safety factor working against you when messing with a dry exhaust is HUGE.

So you guys are saying I should buy a tubular header and then cut it up jacket it and reassemble it?

I think everybody is sayin', just buy a stock Merc manifold for it,.....

I've never seen one crack or break, unless frost busted from not winterizin' the motor,.....

Thank would be impossible.

Exactly. If it's a marine engine then it came with a water cooled exhaust manifold.

There are only a few reasons that the 1-piece Mercruiser style manifold will crack on you.

1) the manifold flange is warped
2) the flange surface on the head is not flat and needs resurfaced.
3) you are torquing the bolts in the wrong sequence
4) water is not flowing properly through the manifold (obstructed passages in the head or block, or in the riser elbow, or ANYWHERE else in the flow path of the raw water).

Outside of those 4 things, manifold cracking is pretty much a non issue.

Cooling water comes from the outdrive lower up thru the water pump (located in the outdrive lower) and up thru a flexible hose from the outdrive upper across the gimbal into the transom plate. Any obstruction or malfunction (pump housing or impeller or even rotten impeller drive key) can cause low water flow.

Once thru the transom plate, the water flows thru some or all of the following:
oil cooler, power steering cooler, and into the engine. Water flows into the water neck and into the block, up thru the head, and back to the thermostat housing where it exits and goes into the exhaust manifold thru a dedicated hose. That water flows all the way thru the manifold and THEN thru the exhaust elbow and finally into the exhaust stream where it is carried with the exhaust down into the outdrive and out of the prop.

Since you are dealing with a used and potentially improperly assembled/plumbed setup - find a detailed cooling system diagram for your specific motor and check and verify proper assembly/plumbing/function of each component.

Pieces of broken rubber impeller blades can go upstream and lodge pretty much anywhere in the cooling loop. Old boats operated in a sandy environment can get sand/dirt accumulation in the engine block. Etc.

Bottom line, the correct Merc style combo manifold is what you want, but you will need to do a thorough piece by piece inspection/cleaning/repairing of every component in your cooling loop as well as checking/refurbishing of the mating surfaces of the head/manifold.