running w/out exhaust bellows

[dhammann]

I am getting ready to replace the bellows on my 3.0 litre alpha I. I am thinking seriosly of leaving the exhaust bellows off...has anyone tried this? and if so, how did it affect performance? My thinking is it will relieve some back pressure allowing the engine to start easier. I was also wondering about the sound....will it make that pleasant "thru the hull" sound? Are there any benifits at all of running exhaust and hot water through the lower unit and out the prop?

 

[b.gagnon]

Re: running w/out exhaust bellows

I have been in a boat that had a rotted exhaust bellows, and when in the water it sounds no different because it is under water. Also water can splash up into your manifold and cause it to rot, and I think it is also possible for water to enter the exhaust valves and fill the head. Either way I don’t think you will gain any performance….

 

[dhammann]

Re: running w/out exhaust bellows

Thanks B Gagnon, I Don,t believe water intrusion would be any different and the riser should keep water out.

 

[dhammann]

Re: running w/out exhaust bellows

From an engineering standpoint, what is the reason for running exhaust thru the hub?

 

[MrBill]

Re: running w/out exhaust bellows

There may be more at issue than water intrusion. I would suspect that the exhaust bellow significantly reduces the possibility of water intrusion because it prohibits the direct flow of water into the exhaust riser assembly. But also, the bellow directs and disperses the exhaust to minimize interuption of water flow to the propeller. Couldn't this be an issue? Air (exhaust) introduced behind the propeller (if that happens without the bellow in place) would reduce efficiency and could cause cavitation.

 

[horndog]

Re: running w/out exhaust bellows

you need some restriction in the exhaust system..<br /><br />Example...<br /><br />Lets say on a car you decided to run open headers, what happens is that when you accelerate you have no restrictions which means full power, but when you de-accelerate, you create a vacuum in the headers which draws air up into the motor through the exhaust and could do some major damage.<br /><br />I have seen people do this and ended up burning valves and pistions, not good.<br /><br />You need a low restriction exhaust to get the best of both worlds for everyday use.

 

[rogerwa]

Re: running w/out exhaust bellows

It seems to me that there is an option ot the bellows that is easier to fit and eliminate the thru hub. They call it an exhaust tube. I don't believe water ingestion is an issue. On my Alpha one, there are two ports that run out of the transome ring that, when the engine is started, the exhaust comes out these holes. Only when I get the throttle up does it push the exhaust through the hub.<br /><br />At first I thought I had a busted bellows, but I examined it real close and found this was the design.<br /><br />On a 3.0l, like mine, you won't get a pleasant thru hull sound. A four popper without a muffler does not sound cool in a boat or a car. <br /><br />Here is a link to the MC exhaust hubs:<br /> http://www.boatfix.com/bykeyword.ASP?keyword=exhaust+tube&B18=Search

 

[dhammann]

Re: running w/out exhaust bellows

thanks roger, the exhaust tube looks like a straight thru tube...is that correct? If so why is it any different than no tube at all? I know a "four popper" does not sound cool, but I like the gurgling sound coming from the transom end at idle. I know some Volvo Penta drives run w/o prop exhaust....I want to know what the advantage is. I realize that it would take more vacuum to lift a column of water, but water seeks it's own level so static water level should be the same with or without the tube, right?

 

[rogerwa]

Re: running w/out exhaust bellows

If I am not mistaken, it is the riser and the flapper that protect against water intrusion. The through hub was simply for noise abatement.<br /><br />YOu can run without the bellows, the tube is just an easier, simpler install.<br /><br />Also, what is the year of your Alpha I. Mine is an 87 and when starting the engine with the lu in the water and the rest out, it sounds like its got thru hull exhaust. These are the exhaust relief holes. Don't you have these??

 

[dhammann]

Re: running w/out exhaust bellows

yes I have the relief holes (1989) but they are about 12" under the waterline. I have found out that the purpose of thru hub exhaust is to keep the top of the drive from getting dirty with carbon. Now a new question, what is the advantage of the tube vs no tube? is it just to divert the exhaust away from the upper unit.

 

[rogerwa]

Re: running w/out exhaust bellows

Your exhaust bellows is going to be underwater as well when your starting. The exhaust relief holes minimize the backpressure already. Like I said, the exhaust only goes through the prop when at higher RPM.<br /><br />From all accounts I have heard (no firsthand experience), you should be ok without the bellows. <br /><br />Honestly, I do not know of the benefits of having the tube VS going with nothing at all.

 

[dhammann]

Re: running w/out exhaust bellows

Thanks to all, I am going to leave it off

 

[hoot]

Re: running w/out exhaust bellows

here is my 0.02. i had the same thoughts about leaving the bellows off. there is a tube that you can get that only connects when the drive is down, but it is higher in price than the bellows. my thinking is that when the boat is at idle or slow speed, the hot exhaust rises around the drive bellows, possibly shortening the life of that bellows. imho, i would run either the tube or bellows.

 

[rogerwa]

Re: running w/out exhaust bellows

Good point Hoot. I think you hit the nail on the head..