exhaust bellows from a 7.4l gen V to a Bravo 3

[LPmrt]

Good Day Everyone,

Appreciative of all the helpful wisdom on this site, I'm now in a dilemma:

Since January 2020 I've had a '94 Sea Ray 250 Express Cruiser with a reportedly remanufactured 454 Gen V 330hp carbureted Merc paired with a newer looking Bravo 3 outdrive. All has been reliable so far after approx. 80 hours of run time.

With the proper exhaust tube venting fumes mostly out the transom at idle and low speeds( I see no prop exhaust until higher rpms), the "station wagon effect" pulls large amounts of fumes into the cockpit and cabin. I keep the center windshield and all 3 cabin hatches open to maximize air circulation, but with the large bow, high windshield, and relatively very low transom/rear seat bench height, I fail to keep the exhaust out of the cockpit.

Here is a link with pictures of the shape of this boat:

1994 sea ray 250ec - Google Search

www.google.com

My question to the community is:

Can I install exhaust bellows to force the exhaust to the small ports and tight prop hub space in the lower unit, sending the fumes under water? (Even though Merc says not to with engines above 330hp)

I've read about the issue of the exhaust bellows ballooning and expanding into the u-joint bellows at higher rpms with >300hp engines. What if I kept my rpms below say 3,400rpm at all times while underway?

My horsepower at best at 3,500 feet elevation (Lake Powell AZ/UT) is technically under 300 according to the hp/altitude formula I found online.
HP Loss = (elevation x 0.03 x horsepower @ sea level)/1000

Is there any way I can use exhaust bellows with this application without busting my U-Joint bellows?

Thank you all very much

 

[tpenfield]

I would check for a leak somewhere along the path for the exhaust, because it all should be making it outside of the boat. At idle speeds it will exit the side ports. Bravo outdrives are best fitted with the exhaust 'tube' rather than the bellows. The tube does not create a sealed path, but more so directs the exhaust down through the lower unit of the outdrive.

 

[LPmrt]

I would check for a leak somewhere along the path for the exhaust, because it all should be making it outside of the boat. At idle speeds it will exit the side ports. Bravo outdrives are best fitted with the exhaust 'tube' rather than the bellows. The tube does not create a sealed path, but more so directs the exhaust down through the lower unit of the outdrive.

Thanks for the reply Ted. I checked under the rear seat while running and there are no fumes in the engine compartment. I should have known about the side ports, glad you mentioned them. The odor problem is at its worst when there's no wind, a tailwind matching our speed, or a strong headwind creating a major low pressure vacuum behind the driver windshield and transom. It's as if the fumes pull right in over the windshield-protected parts of the transom and stay in calm parts of the cockpit.