Dennis Moore

There will be a nice performance gain, going from the old, rear dump, log style manifolds to center riser style manifolds. You will also need a new style thermostat housing to go with the new manifolds. The water inlet hose from the transom to the thermostat housing will need to be changed too. All good modifications though.
Dennis Moore

mcollinstn

I've got a quad set of mercury gauges for synching bike carbs. I use them for everything.

On my cruiser, I attached hoses and put them several places in the engine compartment to check for adequate air openings. The diesels pulled 8 inches of vacuum at WOT and full load in the engine compartment with the carpet over all of the hatch seams. Pulled 3 with no carpet. Modified some of the blower vents on the rear of the hull and ended up with 2 inches with carpet. Picked up a few rpm in the process as well.

Your length of hose will not play a factor on your test. Gauges should read the same with short hose or long hose.

Tinkerer

Cain-- Yes change to the center rise exhaust manifolds.

220 -- What I did was run the hoses that went to the risers ( it is only slightly warmer than the lake ) and ran a water dump out the transom that sprays on the sides of the drives. The only problem with doing this is that untill the thermostat opens you will not get any water going out the exhaust to cool it.
I also have the four hose thermostat housing so what I did was drill two holes that bypass water arround the thermostat drill the holes behind the plastic thing that holds the thermostat down. The holes go into the area of the thermostat housing where the inlet water comes in from the seawater pump. Try two 3/8 inch holes.
I have 1/4 inch holes but I also lose exhaust hoses due to too much heat.

Budman

I have also been interested in trying this. I was thinking that some type of adjustable bypass valve might be the way to go. It could be installed in the hose that goes to the manifold, and the excess could be routed out the transom and onto the drives as Tinkerboater described. I'm looking for someone who has actually tried this to see if any real gains can be made before I go to the time and expense of doing this. I also have silent choice exhaust, so there is some concern on my part that I could toast my divertors or Y-pipe flappers if I do this.

Has anyone tried installing a bypass valve?

220BR

What about installing blocking plates between manifold and riser and putting a "Y" in the manifold feed and running it into the riser with an adjustable valve.

My thought is to increase pressure a little in the block and manifolds to insure adequate cooling, yet control the amount of water to the riser that is mixed with the exhaust?

Budman

I agree with your idea of the blocking plate. That is another thing that I would like to try, to eliminate the water passage in the flange between manifold and riser. In my opinion, that would be one of the best mods that Merc could make to their exhaust systems. Seems like it would eliminate a lot of problems with leaky riser gaskets. Maybe it's a case of planned obsolecence.

I would be leery of increasing the block pressure too much, especially without a H2O pressure gauge. Much more than 20 lbs could cause intake or head gasket leaks.

BAD-HABIT

On my stock 700hour 260/5.7 I gained just under 2mph(per GPS) going from stock center risers manifolds with y-pipe and thru-prop. to using straight 4" thru hull stainless marine exhaust. Couldn't tell ya about my new motor but It really helped the stock motor... Explain that to me????

BH

220BR

Yeah, that's the issue, isn't it? My WAG is that its due more to the"tuned" stainless marine manifolds rather than the thru-hulls. Merc stock manifolds are un-tuned or semi-tuned at best, and switching to a tuned manifold(separate or quais-separate runners) helps with reflected waves.

offshore121

You need to use a monometer to measure back presure and underway on a boat you most likley would not get a good reading. They DO make a differance in most of the 100 or so I have put on I have seen a 50-150 rpm change.

Scott Dooling
ownwer offshore performance & the boat store