My boat came with twin 615 ci talldecks. It is freshwater cooled but also has the standard water pump on the engine that you would see on a closed system. I have seen other engines on boats that were freshwater cooled and the engine water pump was replaced with a crossover pipe.
My questions are:
1. If the engine water pump is replaced with a crossver pipe, will the standard mercruiser sea pump be capable of providing all the cooling needs?

2. What are the pros and cons between closed and open systems?

Thanks a bunch....Casey

EDIT

I believe you are saying the engines are now "raw" water cooled, if a cross overs will work with the stock sea pump there is really no need for the circ pump.
I like closed cooling, it protects the block and heads from damage if you run in salt. You can run higher engine temperature which will add a little power, last but not least it will reduce the internal block pressure. Many do not take block pressure issue into consideration, on a closed cooling system the pressure is normally kept at about 14psi by isolating the sea water circuit from the engines cooling circuit via the heat exchanger. The sea water circuit pressure can rise to over 100psi at high speeds, Mercury offers a pressure relief valve to alleviate the pressure #863208A3 which keeps the pressure at 30psi which is still double the normal block pressure. Did you ever notice the rust marks at the head to block mating surfaces? I worked as a marine mech for about 30 years, if you want a simple demonstration with out hooking up a pressure gage measure the OD on the engines circulation hose then go for a ride. Once at speed measure the hose again you will be surprised at just how much it expands from the pressure. Many racers utilize over board dumps to protect their engines. I myself have and over pressure problem that came up when I switched from a Blackhawk drive to a BravoX with the dual pickups, first run out I noticed a fair amount of water in the bilge. I noticed the top of my raw water stainer was wet, I wiped it down and sure enough it was pushing water out of the cap. For the time being I have just cranked down way harder then needed to stop the leakage but I will be installing a relief valve when the cover comes of. If you do nothing else put relief valves in, but if you run in salt and also want a little bit better performance go to closed cooling. I run a full roller SB with a Lunati Pro Mod stroker kit which is nothing compared to your tall decks and I would not be without closed cooling.

Phil

Thanks Phil, maybe that explains why the guy that owned the boat before, was blowing head gaskets. The motors are currently at the machine shop and I will be assembling them in June (Im overseas now)

Do you have a line on where I can get the closed cooling system for these motors? Thanks a bunch, you've already been a BIG help....Casey

Casey:
"Fresh water cooled" and "Closed cooling system" are two names for the same thing. If you have a heat exchanger and anti-freeze coolant in the engine, then you have this system. If you have the water your boat is floating in inside your entire engine, then you have raw-water cooling.
To answer your questions:
#1. Yes. Many engine builders remove the circulating pump you are referring to with no adverse results. However, most Merc. reps I've spoken to say it's easier to control the temperature with the pump.

#2. Phil covered this one very well. Certainly it is better for the engine to have coolant in it rather than corrosive salt water. Down here in SE Fla. our warm salt water is almost as nasty as acid.

Hope this clears things up for you. Good Luck with those tall-decks!
Tommy at Chief Engines always tells me: "There's no replacement for displacement!".

Thanks Edward, I've been building engines for a little while compared to some of you guys but the whole marine engine environment certainly throws a twist in building one. I just dont want to make any stupid mistakes. Besides the heat exchanger, are there any other parts or considerations to converting to a closed system? What about the stock GM waterpump? Is there a marine version of that? thanks....Casey

If you want to dump the belt driven pump you may want to look here http://www.meziere.com/ they are suppose to give you back the HP the belt steals.
As far as exchangers you need to get some good advice from those running your type of a setup, you are making big power big power makes big heat.

Phil

Can you adjust the PSI on those pumps?

Where is the best place to get them?

The link I posted was for electric circ pumps, pressure in the system is controlled by the Tstat not the pump.

Phil

On a raw water cooled engine, water pressure is affected by the size and type of exhaust. On a closely fit tailpipe system, some of the water may dumped early, in order to minimize the amount that needs to be squeezed through the exhaust.