Lofty

If you use a crossover, plumb it correctly, use a T stat and a pressure relief valve if needed you will have a better and more reliable set up. Circ pumps are nice if you run in cold water but for most people they aren't needed. Your sea pump puts out plenty of water pressure, the circ pump slows it down and restricts flow.

I've asked Merc, mechanics and dozens of people who have been involved with performance boats from the start and none of them have given me a good answer as to why you should run a circ pump. All they ever say is that Merc designed them that way. I don't think so, GM did.

I've run plenty of motors with no circ pump and never had a problem. Once you get the set-up right with a crossover they are maintainance free.

RaggedEdge

Here's my take on this................... Merc has always used the circulating pumps on the Hi Perf series motors, and they stand behind them with a warranty. They do a great deal of testing because they warranty these motors and don't use the cross over set ups. That in itself should answer the question, if the crossover set up was less problems, fewer failures, my guess is that Merc Hi Performance would have jumped on them years ago. Having watched friends dick around with all the related BS of the cross overs, temp and pressure issues, I can fully understand why Merc wouldn't touch such a set up. Always been a believer in the saying "If it ain't broke, don't fix it".

Lofty

This is about the most compelling argument for keeping a circulation pump that you'll get. No good reason other than that's how Merc builds them.

I've never actually removed one from a running set up. I just don't re-build a boat with them unless they are Merc crate motors.

open87

iTrader: (2)

not on the 750 super cat motors they didn't.. eickert thermo housing ,mercury racing crossover..back of dave scotts trailer in biloxi..

go with the stainless marine

SkiDoc

iTrader: (4)

Scientific reason.....Back two cylinders could run hotter due to the areas trapping air....crossovers are chosen on big hp motors that run on the edge... decreased head and cylinder temps prevent a melt down(predetonation.)With the problems I have mentioned above, proper water pressure, rigging, i.e. sea strainers, pop off valves, water pressure guages will cure. BTW I like my crossovers on my 575's.
Lofty, I think merc. engineers do this because they make engines for the masses. They put there babies in crates and send them off to boat companies who may or may not do what is recommended in rigging. It is fact that with the way they do the water systems there is less chance to screw it up. Their motors have to be able to run in the desert and frigid water, a crossover and thermostat give them more consistent temps for air/fuel mixture calibration. I think they are doing it right with closed cooling systems! Eric

Lofty

Eric, yeah that is what happens. We run small water bleeds off the manifold in the back to improve circulation on those cylinders.

I'm sure you're right about Mercs reasons for leaving the pumps on. My point is the motors will work just fine without them if they are rigged properly. I've seen many new boats rigged so poorly it's a wonder they run atoll.

ROTAX454

Like this?

Lofty

That's it! We've done a few with the water going everboard too. Depends on the set up.

ROTAX454

That statement is true, and for good reason. That motor has a specific design/purpose-----racing. Take for example a performance boater with an HP525efi motor. That motor see's WOT to idle. It needs a consistent to short tolerance range of water temps (oil too). That can only be accomplished properly by using a recir pump and t-stat.

Example of why using a crossover (on many, not all cases) has it's problems: Your running at WOT and the water temps are just fine. Then you pull back on the stick/sticks and bring the motor back to idle. Watch the water temp gage fall in the basement. There goes your consistency. An open invitation to the possibility of condensation. My .02

ROTAX454

I was curious on how that intake manifold mod got started. One manifold manufacturer told me that it was from the early desert race motors. As told to me, "try holding a motor at WOT for about 5-10 miles, or more". You want as much free flowing coolant as possible.