454 Exhaust Sound
 

Hi All,

I recently purchased a 1996 Formula 252ss with a 454 / Bravo 3 combo. The yard I bought it from had decked and rebuilt the heads. All cylinders compression test well, and timing/advance checked out OK. I put a new Edlebrock on and she runs very nicely.

My question is related to exhaust note/sound; the engine has fully closed cooling with HGE cast iron manifolds and the exhaust exits the risers into a very short (~ 60cm) rubber hose connected to straight through stainless stubs which exit above the waterline. Despite this the exhaust noise is negligible at idle and it doesn’t exactly roar at speed.

I would like to hear more of that V8 burble and wonder if just replacing the risers & rubber hoses with a solid stainless setup would make a difference, or whether I need to go for the full stainless header setup as well?

Thanks
Will

Not sure I understand your goal by doing those changes... you are looking for more hp?
more sound? Less sound?

Just to be clear, you don't have any sort of 'Captain's Call/Silent Choice' exhaust on the boat that is directing the exhaust through the prop instead of out the exhaust pipes? A stock 7.4MPI in the video below...does your sound like that?

Will, you're not going to change much just by having a stainless riser system that is "wet". (Riser injected raw water)

The mild BBC cam and iron egh manifolds, is going to limit the amount of 'lump' noise from the exh.

Going to a 'dry' riser system would lose the water attenuation the raw water provides in the riser injected system you have now.

As susggested, mayne you have a dual system that diverts som flow through the leg/prop as well?

The other option is to turn the idle down, if you can, while still having good shifting manners etc...it will naturally lope a bit more.

Thank you all for the feedback. Just to confirm that yes the pipes are straight through, there is no silent choice or diverter system installed. Unfortunately I cannot post pictures yet as a newcomer.

The video that Bajaman posted is exactly what I ‘expected’ from the boat, but whilst the sound is similar it is much more heavily attenuated. I got the idle down to 450rpm (as measured by laser tach on the flywheel) and the lumpiness was there but the volume on the water is negligible. I’m wondering if the swim platform also dampens it from the cockpit perspective.

To answer the question of ‘what do you want’ - I am not chasing performance just more volume.

Thanks again
Will

Will,

This is not to say your boat isn't quieter than it could be, but it will definitely sound quieter from the cockpit than it does to anyone outside the boat, particularly behind it.

Others might chime in with counter advice, but I'd get that idle back up above 700 or so. Reversion can be a real issue with the wrong cam at idle speed that low. Especially in light of the assumption that you have a LOT of water in your exhaust, making it so quiet. You don't want chocolate milk in your oil pan.

Post spam on this thread until you have ten posts. Then you can post pics.

Thanks. Brad.

As said above, turn the idle back to 700 or so RPM's to avoid any chance of water reversion.

Most boats I have owned over the years have had mufflers inserted inline in exhaust hose, or a baffled
exhaust tip.

As a side note, I’m 62 years old and have been an avid snowmobiler, dirt biker, offshore boater, and, in recent years, an aerobatic open-cockpit airplane owner and pilot. I truly regret the impact all of these activities have had on my hearing. If I could go back, I would stick with stock (or near-stock) exhaust systems and never operate any of them without proper ear protection. Just my two cents.

That boat came with a Silent Thunder box. Does it look like this?


https://cimg4.ibsrv.net/gimg/www.off...c88a6a5ecd.jpg

If you are looking for more of an exhaust sound volume note, then swapping out your factory crude breathing exhaust risers with all the sharp turn 90° angles while water and exhaust gases mixed together are what is killing a lot of the sound that you're wanting to hear.

Swapping your factory riser system out with something like a DRY stainless steel tail pipe with more of a curved smoother transition would give you the desired sound effect ---- I am mainly talking about retaining your factory cast iron exhaust manifolds but swapping out the super crude facty design "risers" only.

Just understand when you do the swap you will have to make other ripple effect modifications along with it to make everything fit and work correctly. Just from that modification alone you may possibly pick up about 10 horsepower. Either way you will get the desired sound effect for sure even while sitting in the driver's helm station.
Make the pipes completely dry or even if you mix the water into the exhaust gases at the VERY END TIP of the tailpipe. You're going to want a ONE PIECE tailpipe that goes in bolts on top of the exhaust manifold and all the way out through the transom of your boat.
The downside and disadvantage of all of this is it may be easier for you to get watery version through the exhaust system because your tailpipes will no longer have the sharp 90° turns from the factory risers anymore and come out through the transom very low and close to the water line

.
​​​...so if you're going to do the modification I would still at least use the rubber flappers I think they call them Salisbury rubber tips on the ends of your tail pipes to help prevent water reversion.
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If you have the fiberglass Silent Thunder swim.platform, you need to know that it has baffles in it. They collect water and burble the exhaust thru it. Taking the baffles out does increase the noise. Alternately (and simpler) is to hole saw 2" diameter holes from below into the exhaust chamber very close to the transom. This DUMPS the baffling water out and substantially increases the rumble at idle.
I had a 1991 311 with twin 502's and did this. The boat ALWAYS got compliments on its mean rumble without it being a nuisance.