bravo II exhaust bellows vs exhaust tube

[demaris]

if this has been addressed previously, sorry for the post. new here and haven't figured out how to search a particular forum.

i have seen an article that using an exhaust tube is actually beneficial over a bellows arrangement due to possible swelling or increased back pressure with the bellows.

is this an alpha thing? i have a 7..4l 454/ bravo II installation.

if true the tube is certainly easier to install.

thanks,
stephen

 

[JASinIL2006]

I always thought the big advantage was the ease of installation over regular exhaust bellows. I can't imagine the tube decreases back pressure that much.

 

[Bondo]

is this an alpha thing?

Naw,.... Originally, it was a Bravo thing, that people put on Alphas,....

 

[alldodge]

The bellows to the tube started when the B3 came about, there is a lot less room for exhaust to exit thru the prop. The B1 and B2 are not that much of an issue but I like the tube over the bellows

 

[tpenfield]

The bellows is nearly impossible to get to stay on the outdrive flange . . . so the tube is better in that regard.

 

[muc]

The bellows to the tube started when the B3 came about, there is a lot less room for exhaust to exit thru the prop. The B1 and B2 are not that much of an issue but I like the tube over the bellows

This is correct. Shortly after the Bravo3 came there were reports of the exhaust bellows “ballooning” and contacting the unjoint bellows when the Bravo3 was used behind engines with more than 300hp. Part of the problem is the smaller exhaust passages and part is that Bravo3 props provide almost no exhaust scavenging.

The exhaust tube is a bit louder, some people like that some don’t.

 

[muc]

The bellows is nearly impossible to get to stay on the outdrive flange . . . so the tube is better in that regard.

This wasn’t a problem if the exhaust bellows were properly installed until about 2002 when they started coming of under warranty. MerCruiser tech support determined that the new formulation of paint was too slick and hard. Their recommendation is to use the gel style super glue instead of bellows adhesive.

 

[demaris]

thanks for the input. with the bravo II i think i'll go with the tube. less installation issue and though it never said not a cooling issue with the lower end. though is the lower end cooled enough?
stephen

 

[GA_Boater]

Using an exhaust tube, no exhaust is routed through the lower unit. So what's your question?

 

[demaris]

sorry, i would have thought that some of the exhaust would be directed through the lower drive unit when trimmed down using the tube? why a tube at all if not? maybe the exhaust with raw water from the riser was never a cooling function?

 

[alldodge]

No worries cooling with either tube or bellows

 

[demaris]

No worries cooling with either tube or bellows

thanks

 

[muc]

Using an exhaust tube, no exhaust is routed through the lower unit.

This statement is not correct. Depending on trim angle, boat speed and propeller design, most of the exhaust will go thru the lower unit when a exhaust tube is used.

 

[muc]

sorry, i would have thought that some of the exhaust would be directed through the lower drive unit when trimmed down using the tube? why a tube at all if not? maybe the exhaust with raw water from the riser was never a cooling function?

The water from riser and the exhaust gasses heat the drive up. A drive that uses “thru hull” exhaust will run a bit cooler.

 

[alldodge]

Muc, IMO your getting way to picky.

What's the difference in heat soak between a tube and a bellows? Does the heat impact actually make a difference between either one?

Thru hull was not part of the question, but then again what's the difference?

How much exhaust goes thru the prop at idle to WOT with a tube?

 

[muc]

Muc, IMO your getting way to picky.

What's the difference in heat soak between a tube and a bellows? Does the heat impact actually make a difference between either one?

Thru hull was not part of the question, but then again what's the difference?

How much exhaust goes thru the prop at idle to WOT with a tube?

Yes I can get picky at times. But I don’t think I’m being picky with my quote of GA_Boater. The difference in how much exhaust goes thru the drive, tube vs bellows is very small. I agree that there is really no heat difference between the two as I posted earlier, about the only difference is noise.

My bringing up thru hull exhaust was a (apparently misguided) attempt to assure the O.P. That reduced exhaust thru the drive would result in decreased heat, not increased. But if you didn’t get it. I’m guessing they didn’t either. Seems I’m too wordy or too brief.

You ask how much exhaust goes thru vs RPM. I don’t think RPM matters much tube vs bellows. As I posted earlier it’s more about trim, boat speed and prop design. If the drive is tilted all the way up, in neutral running at any RPM you want. Most of the exhaust will go thru the drive with a bellows and almost none with a tube. But if your trimmed in, going thru the water at higher speeds and have a prop that provides scavenging almost all exhaust will go thru the drive with a tube or bellows.

 

[achris]

Hey muc I like picky. I am very often being told I'm too picky. But then the other side of that coin is people commenting about how everything in my house/car/boat just works, nothing 'broken down'... In fact my son-in-law hates me because the daughter keeps making comments about how things in their house are broken (and never fixed) and how my wife never has to put up with 'work-arounds'....

As for bellows vs tube. I HATE the noise, so it would be bellows every time for me. I have 2006 drive, and the bellows have never slipped off. I would also only recommend the tube for B3 drives behind 300+ hp engines...Other than that, bellows. And for the people who say 'they're too hard to put on'... I say, buy (or make) the right damn tool!

And yeah, exhaust gasses going through the drive or not is a non-issue.

Chris....

 

[muc]

Hey muc I like picky. I am very often being told I'm too picky. But then the other side of that coin is people commenting about how everything in my house/car/boat just works, nothing 'broken down'... In fact my son-in-law hates me because the daughter keeps making comments about how things in their house are broken (and never fixed) and how my wife never has to put up with 'work-arounds'....

As for bellows vs tube. I HATE the noise, so it would be bellows every time for me. I have 2006 drive, and the bellows have never slipped off. I would also only recommend the tube for B3 drives behind 300+ hp engines...Other than that, bellows. And for the people who say 'they're too hard to put on'... I say, buy (or make) the right damn tool!

And yeah, exhaust gasses going through the drive or not is a non-issue.

Chris....

Ha Ha
thanks for the kind words. I’ve had the younger techs call me anal (and worse) behind my back. But the marina owners seemed to like that I’m meticulous.

At home it’s a bit different. My bride of 27 years is constantly asking why I’m taking something apart, because “it was working just fine”. My answer is always—— but honey, it will work soo much better when I’m done.

Agree with you you on the noise. But when I would give customers the choice, they would almost always pick the boot. And that’s fine because they are easier.