Engine Sound "management"
 

I have a set of EMI Thunder aluminum exhaust manifolds and SS risers with the extra 3" height on my 383. It's 9.8:1. I wouldn't call it exotic but it has a somewhat aggressive cam, by no means radical. 218/224 @ .050 and .565/.57 lift with 109 LSA. (ed. 113 lsa, 109 icl)

It's freakin loud inside the boat, to the point of requiring sound reduction.

I know most want the loud, but it requires shouting to communicate. So I'm thinking of ways to keep as much power as possible while mitigating the decibels some.

I have the Formula Silent Thunder baffled platform.

Thoughts,
Dyna mat? Is there any sound absorbent that's better in a marine environment? Is it OK to use, any drawbacks?
A rubber gasket along the leading edge or all the edges of the engine cover/sunpad?
Exhaust Baffles?
Less cam?
Way less cam and wimpier risers?
Way, way less cam and factory exhaust?

Quit crying and drive faster to get ahead of the sound? It IS way faster.

Maybe just try turn downs to direct the sound somewhere else? That would be my first try...

I have same comp cam, but 113LSA, in a 350, with same Eddie exhaust. In fact,boat on the same lake as you. I would get pulled over alot for exhaust noise (even with stock motor) before installing the old style (corkscrew) GGB inserts. Never got pulled over for noise after that, even with the engine mods.

Can't help with the Silent Thunder box, but figured I'd relay my experience on the lake. I would quiet it down a little, even though they don't seem to be as tough now as they where not too many years ago.

Also, being too loud out here doesn't do the rest of us any favors. Non performance boaters equate loud exhaust with being too fast.

Edit in: sounds like you are having fun with your new motor. Congrats !

It should be pretty quiet through the Silent Thunder. Have you modded the platform in any way? Even with the baffles knocked out it should be manageable.

The Silent Thunder box USUALLY has "dams" that hold water in it for the exhaust to gurgle thru. Pretty effective. But if you have knocked out the "dams" or drilled relief holes large enough to let too much water out, then there is zero baffling. Also, when using aftermarket exhaust, some of them have "reduced water" thru the tailpipes. If that's the case, then make certain that your water "dumps" still dump INTO the Silent Thunder platform or.. no baffling. Additionally, if the dams ARE in place, and you are dumping ALL engine water INTO the platform (into both sides), then you can install expanding rubber (wingnut) plugs into any relief holes in the platform to "tune" your baffling water.

A lot of guys knock out the dams FIRST THING.. Unless you purchased the boat new, be sure to double check that.
There should be a dam that holds water about 3" deep, and then farther towards the exhaust should be a "top down" divider that forces the exhaust to go under it thru the water and then out over the "dam".

SB I might have called it 109 lsa when that's the intake centerline. I ripped off your cam idea from a post way back somewhere.
And it's not too loud outside the boat according to my brother who video'd it for a couple a moments.
I still haven't passed by a patrol under power though.
I got a chance to run it wide open the other day, reached 5000 on a 21 stainless pretty effortlessly before I let off. It felt like that was near the top though. I don't think it would turn a 23. I was giggling like a schoolgirl.

Is the box loose or rattley? If so that can cause quite a bit of noise inside the boat. Both my boats with silent thunder with baffles taken out were pretty quiet in the boat, but the exhaust was also further away from the helm since the boats were longer. My 206 has no platform on it, and it's way louder in the boat, too loud to talk at more than an idle.

I have not had the platform off, it's not loose or rattley. It takes a bit to drain when I take it out of the water so the baffles are hopefully in there.

Is it the exhaust noise or the engine noise that's the problem?

Great question. The engine itself should not be noisey.

Cool !

Mine (5.7/350) turned the boat's favorite 21" to 6200 (ooops) last summer with only about 12% slip I think it worked out too. Short blasts too. LOL.
My 23's didn't knock down the rpm enough....so I am trying to figure out what to do as most 25's jump up in diameter, which this boat does not like.

Glad your having fun.

Yes, it's the exhaust that is loud.
I don't have a good enough bottom end to go 6000 rpm.. Cast crank & pm rods. I'll have to run it till it stops accelerating at some point. But I don't think the Mercruiser ignition will go past 5200, supposedly the spark quits at 5200 til the engine falls off to 4700.
My boat is pretty fat, so you can pass me by with yours! But now it's at least respectable.
I gotta sell my 350 Mag now.

TBIV has no rev limiter, That's what mine has and yours being an 87 should have too.

The engine that I bought it with is a 97 350 mag carb with TB V. I read somewhere that they have a limiter. Is that true?

Read http://www.boatfix.com/merc/Bullet/94/94_02.pdf

It looks like the GGB baffle is the one for my situation.
My pipes are 4" OD and get pretty close to the transom. The inner diameters are 3.5" to my memory, but I'm going to have to measure them when I go back to the boat. I'm thinking about whether to use 3.5" OD baffles to fit inside the ends of the exhaust risers. . My boat is up north and I'm home working.
I think the normal way these inserts are done is in the metal tips but my risers go back so far that I may need to put the baffles in the aft ends of the risers.

I have the newer GGB inserts, they help quite a bit but still have a good sound. Didn't affect the performance at all.

Also, don't discount the engine noise. After I put the ggb in, was still pretty loud. Working the sunpad some to adjust it to seat better helped. Now we can talk in a pretty normal voice until about 4500 rpm. After that wind and exhaust noise require raising your voice.

Also, I have a bimini and when it is up it reflects the exhaust noise and it is much louder.

I'm going to put a foam rubber strip along the top of the rear seat back where the sunpad hits as a first step, Then might do insulation inside the motor box on the rear of the seat back.
I'm tired of buying parts for my engine right now, but I suppose I can pony up for the inserts if it's still too loud. They are under $200. I just don't have my boat at home where my tools are and when I went to the lake yesterday I didn't even think to measure for the inserts.
My riser tails are so long that I have very little distance between the ends of the risers and the transom fittings. My transom outlets have flappers in them that I want to keep, so the inserts will have to be in front of there. I'm thinking to put them inside my riser tails.
I see they are manufactured with attachment points. I'd think you have to bolt them down so they don't rattle. apollard did you bolt yours in to your risers?

If you put them in the riser tails, do not put them closer than 18" from riser to manifold connection. This is for hp reasons, lol.

You do have them pointed out the back of the boat right. Just kidding.
My 598 at 800 + HP IS LOUD but I don't have to raise my voice that much to make my self heard.

Yes, I bolted mine in the tails. Bolt is actually a couple of inches outside the transom, inserts run from there forward. Last part of the insert is actually in the hose connecting risers to tails.

I will say this about the inserts - they need water to work best. Before the water hits the tails, it is much louder. Then drops to a nice mellow tone. If I ever loose water flow, I know it way before the temp goes high by the exhaust tone.