Through Transom Exhausts for Mercruiser

[joliette]

I've decided to fit through transom exhausts on my twin V6 installation. I've familiarised myself with the contents of the Merc Manuals - block off plate installation, height of riser above waterline, angle of exhaust pipe etc. Merc do not say much on the subject and don't seem to provide much in the way of a product solution, apart from flaps? I think Merc suggest installing flaps on the pipe ends and water shutters in pipe, which I guess is a necessary precaution to protect the engine from potential water ingress. I guess they'll have to be 4" i/d as the risers are 4".

If anyone can offer any advice on through transom solutions and point me in the direction of any recommended product that would be a great help. Thanks


The Project: http://www.facebook.com/group.php?gid=179427293999

 

[Dom81]

make sure you get crossovers as straight throughs can deafen you after a while

Dom

 

[volvopaul]

ID say they will be nice at the outset and become a drain for you passengers after a while.

 

[Firefly625]

ID say they will be nice at the outset and become a drain for you passengers after a while.

yup, I had a car like that once, got a full Stainless Steel exhaust made, had it fitted. When I first heard it I was as excited as a small boy could be.... I swapped it back after 2 weeks to standard... an expensive mistake. Sounds very similar to this project... just IMO...

 

[303hound]

I've fitted a few exhaust systems, I've always used corsa performane in the states. Brilliant to deal with and a pretty quick delivery.

What boat are you fitting them to?

 

[volvopaul]

yup, I had a car like that once, got a full Stainless Steel exhaust made, had it fitted. When I first heard it I was as excited as a small boy could be.... I swapped it back after 2 weeks to standard... an expensive mistake. Sounds very similar to this project... just IMO...

Yes thought of a prevous post, Escort mexicos, twin cams and bdas all great with twin 40s , Janspeed exhausts etc ,till you go on holiday to Cornwall!!!

 

[Firefly625]

Yes thought of a prevous post, Escort mexicos, twin cams and bdas all great with twin 40s , Janspeed exhausts etc ,till you go on holiday to Cornwall!!!

excellent.....ahhh the 80's... weren't they great

 

[volvopaul]

excellent.....ahhh the 80's... weren't they great

Yes they were and as the boats not sold, I have resigned myself to keeping it this season now ive been looking at rs2000 on ebay, just to have it sat there for a run out in the summer evenings, just love em!!

 

[MapisM]

I've decided to fit through transom exhausts on my twin V6 installation.

May I ask you why?
See, I actually disagree with the previous comments re.comfort.
I have the switchable exhaust on my boat (and on a big block!), but I keep them always open while cruising. In fact, above 40mph or so, you can hardly tell the difference from inside the boat.
Otoh, the whole point of straight exhaust is to improve the engines efficiency, get 5% or so more ponies at the prop, and squeeze the last 2mph out of the boat at WOT (if that).
Imho, that is neither worth the effort nor the cost, on a fine classic boat as yours.

But if you really want to go that route, you should better specify what you have in mind:
- switchable or straight direct?
- transom tip, thru-transom or completely dry exhaust?
- original Merc manifolds/risers, or entirely new exhaust system?
Each of the above alternatives being in increasing order of performance.
Mind, with the "full monty" (non-switchable, completely dry, s/steel manifolds), you'd better stay clear of the police...
...and also your neighbours might have some objections, if you plan to flush the engines with fresh water at home!

 

[joliette]

Why?

My original engine beds are high against the transom, almost on the engine centre line. I'd need to take out about 4", right against the transom, to fit the Merc Y pipes. I would rather not subtract structural strength by doing this, which is why I want to fit through transom exhausts (as fitted to the original boat) + my feeling is that the engines will breath better with this set up.

I'm looking to keep it as simple as possible, coming off the original Merc risers.

 

[MapisM]

I see. I didn't consider that the boat was born with the BPMs with V-drives+shafts, right?

Anyway, in this case I'd definitely go for tip style tailpipes, which are the easiest and quietest among the possible styles of thru-hull exhausts.
You might be interested to have a look at this paper from CMI, which summarises the pros and cons of each solution.
In the last pages, you can also see their build sheets, according to which they can supply you with custom built tailpipes.

Other than that, for a generic selection of tailpipes, rubber hoses, tips and mufflers from other builders, this US supplier is among the most specialised, to my knowledge.

 

[spannerman]

It won't necessarily improve performance, we had two identical Rinker 23's with 350 Magnum motors, one had the switchable exhausts and they both did 52 knots with the exhaust thro the prop, the one with the silent choice system lost 3 knts when you went to straight thro exhaust as there wasn't any back pressure in the system. And the drone got on your nerves after a few minutes.
Anyone who tried to mod their car as a teenager by removing silencer boxes knows that a standard engine runs worse without backpressure, unless specifically built to run an open exhaust i.e. race motors.
Just a warning it may not give you the performance gains you expect.

 

[joliette]

I see. I didn't consider that the boat was born with the BPMs with V-drives+shafts, right?

Anyway, in this case I'd definitely go for tip style tailpipes, which are the easiest and quietest among the possible styles of thru-hull exhausts.
You might be interested to have a look at this paper from CMI, which summarises the pros and cons of each solution.
In the last pages, you can also see their build sheets, according to which they can supply you with custom built tailpipes.

Other than that, for a generic selection of tailpipes, rubber hoses, tips and mufflers from other builders, this US supplier is among the most specialised, to my knowledge.

That's the one ... ex BPM with V drives. There are clearly quite a few options here that I was not aware of from the information provided by Mercruiser in their manuals. Thank you for the CMI paper. I will work through that. I'm not really looking to try and increase performance with a through transom exhaust. If that is a side effect then that will be great. The main reason is that I do not want to hack the boat up any more than absolutely necessary and to stay close to the original sterndrive boats, which would've been fitted with through transom exhausts.

 

[MapisM]

There are clearly quite a few options here

Absolutely.
One option which you might want to consider for your setup - unless you don't mind waking up anyone in your marina every time you turn your engines on - are the so called "turndown tips". This is one example I just googled, to give you an idea.
They keep the exhaust submerged while the boat is in displacement, and still have a bit of noise reduction also at planing speed, because the exhaust is directed towards (rather than in parallel with) the water surface.