Surprising results on Merc exhaust
 

Engine is a 383 ci/350HP/Carbed with stock Merc thru prop exhaust, except that it has the exhaust tube (not bellow) that is only fixed at the transom end.

Prior to installing the rebuilt engine, I removed the Y-pipe so that I could drill and tap a fitting into the rectangular section immediately adjacent the transom assembly mounting. I used a 90 degree pipe thread-by-compression fitting at the y-pipe and ran about 4 feet of 3/8" copper tubing out of the bilge and into a gunwale. I put a 0-15psi fuel gage on the other end.

The purpose of all this was to get some "data" on how restrictive the stock Merc thru hub exhaust is with a 383 engine. Everybody claims an increase with thru hulls, so I wanted to see how much back pressure there was with the stock system. So, what's the bottom line? 0 psi at all times, from idle up thru 4600!!! The needle never moves. When I remove the gage with the engine running, I get a spitting mixture of water and exhaust gas, but you can seal it with your finger. There's not enough pressure to move the gage or my finger.

Am I too far remeved from the source with the 4 foot of 3/8 tubing? Or, is this an accurate reading of the pressure in the exhaust system? If accurate, then why do thru hulls report an increase? Reflected waves, etc...?

I had expected to see 3-5psi of back pressure at WOT. Thoughts/comments

I agree with your findings! I have tested a few small block equipped boats with silent choice and found no performance increases with the exhaust running through the transom. Mercruiser has always stated that the prop thrust pulls the exhaust out of the engine.

I have always wanted to do that test but never got around to it. I believe that the Y pipe acts like a 180 degree header collector and smooths out the exhaust pulses. The collector temporarily stores (collects) the exhaust gases, this allows the gases to exit in an orderly manner, reducing the requirement for a large exhaust pipe in the transom plate.

Theoretically, if the exhaust gases are totally free to flow out of the boat (no restrictive muffler), the final exhaust pipe diameter needs to be only as large as an individual exhaust port. Only one cylinder fires at a time.

Cool test!
Dennis Moore

That sound good...BUT, I had a single 540 in my last boat that tended to blow out exhuast gaskets at the head when kicked in the a$$ with the Corsa Captains Call closed:(
Needless to say , I made sure it was open before mashing on it after the second time:p After all , 540 is a lot more than 383:rolleyes:

I had a 350 mag in a 96 Baja, I installed captians call so I could have through hulls. There was no discernable difference in speed or rpm when switching from prop to through hulls.

I found the same with my stock 330's no differance in speeed or rpm's...

very cool test...

I will tell you though my engines will stall at complete idle if I switch on my captian's call before they are warmed up.

A poster on another forum said I need to use an inches of water or mercury gage to read it. I will give that a try some weekend. Who knows, I may record a vacuum at 5200 rpm!!

So, a stock Merc Y-pipe exhaust on a SBC is less restrictive than a steet legal car (1-3 psi). Who woulda thunk!

The pressure guage should be at the exhaust riser not at the exit point. The closer you get to the exit point of the exhaust system the lower the pressure will be. The highest pressure will be at the exhaust port by the head.
If you really want to see some change cut back on the amount of water that is injected into the exhaust.

Measuring the pressure in the riser would be easy enough to do, although I disagree with your conclusion. I'll give it a try and let you know.

Interesting point about the amount of water. I have a open (RWC) system with the 4 hose thermostat housing (ie no hoses to the risers). Merc's service bulletin says to run the 4 slot riser gasket (which I am), but I have always wanted to run the 2slot/2pin hole gasket. That would keep more pressure in the block and reduce the water in the exhaust. What do you think?

Interesting topic. It hits close to home for me. I have a '81 260/350 that I believe is stock internally. It has an aluminum Edlebrock performer intake and a holley 650 carb. Sorry I don't know the exact number, they where on the boat.

I currently have trough prop exhaust although it doesn't have a bellow as described by 220BR. I've been toying with the idea of putting in though hull. I'm rally just looking for sound more than a performance gain. But hay if it happens great. The problem is I have the old style exhaust manifolds that have the riser on the end instead of the middle. This makes it difficult to install trough hull, and impossible if I wanted to run switchable. I've been thinking of swapping to '82 and up manifolds or maybe even a pair of Dennis Moores(wink wink). Now that I know that there won't be a performance gain from the through hull. Will there be with a manifold swap?

Brian

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

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.

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.

Limiting water in exhaust stream...
 

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?

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?

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.

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

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.

Re: Surprising results on Merc exhaust
 

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