Question for the techs

[Wrenchjunkie]

Is there any real reason for the exhaust on an outboard to go thru the lower unit?

Reason I ask is that the exhaust bellows on my L drive looks a bit on the weathered side. and replacements are pretty overpriced and may not be avaliable in a few years. I was thinking of constructing a Exhaust nozzle
and removing this problem part when it degrades to uselessness. If there is no reason to be venting the exhaust thru the lower unit.


WJ

 

[bman1bpm]

Re: Question for the techs

as far as I know, the reason the exhaust goes through the lower unit is because it uses the water to act like a muffler.

 

[Wrenchjunkie]

Re: Question for the techs

if that is all it does then I can make a fitting that disperses the exhaust underwater like the lower unit but eliminates the troublesome bellows hose

if so I will post plans and a how to when I finish it.

WJ

 

[roscoe]

Re: Question for the techs

The water also provides back pressure on the exhaust side of the engine.

Make sure you route the exhaust away from the prop.

If the prop goes through the exhaust, it will ventilate and spin.

 

[CharlieB]

Re: Question for the techs

The cavity left in the water by the gearcase and prop hub create a low pressure area which lends a scavanging effect to the exhaust flow, inducing flow.

Be very careful not to restrict flow.

That exhaust hose is listed as NLA but some larger dealers still have one on the shelf. Looks a lot like a VERY large radiator hose.

 

[Wrenchjunkie]

Re: Question for the techs

I was thinking along the lines of a fitting with the same opening as the exhaust port at the rear of the lower unit. and pointing it toward the rear. this should give almost the same flow and when the boat is moving may create the low pressure area also if it seems restricted I can enlarge the opening.

Do you think this is a good idea Charlie B.

I am just thinking ahead to when I may not be able to find that part at all.
Also should I cap off the exhaust hose opening in the lower unit or leave it open
or should I make a vented type cap that allows some water flow.


WJ

 

[CharlieB]

Re: Question for the techs

One of my customers just bought that hose from a factory dealer at a substantial discount, they were glad to finally get it off their shelf.

Start making phone calls.

Hose manufacturers often have catalogs of hoses by dimension, I'd almost bet that there is a hose in production somewhere that could substitute for the Merc part.

 

[Bigprairie1]

Re: Question for the techs

The cavity left in the water by the gearcase and prop hub create a low pressure area which lends a scavanging effect to the exhaust flow, inducing flow.

Be very careful not to restrict flow.

Charlie is right about this big time. This is a very important part of the way a 2 stroke runs so be careful attempting to change it. I can't recall all of the science behind it but a quick google about 2 strokes will find it for you. If you do modifications to this you may likely change and possibly decrease the efficiency and power of your engine.
Check it out before fully before following this path.
Good luck and keep us posted. Good interesting project all around either way.
BP
Note: someone locally here was selling a complete L-drive unit on craigslist....just remembered seeing that.

 

[Frank Acampora]

Re: Question for the techs

Can't specifically state on L Drives BUT on the 2 cycle outboards, spent cooling water is injected into the exhaust and around the exhaust tube. This cools the exhaust, reducing its volume, and backpressure at the exhaust ports. It also cools the midleg and to a small degree, the lower unit (the part that is not in the water.

By not injecting cooling water into the exhaust, you will increase backpressure, reducing volumetric efficiency of the engine --thus power--and may overheat some parts of the drive line components, depending upon how you route the exhaust. This is especially important on 4 cylinder Force engines where the crankpins are 90 degrees apart and the exhaust ports open for about 120 degrees. There are no baffles between cylinders. The #2 and #3 cylinders fire 90 degrees apart with a 30 degree overlap during which exhaust gas can blow back into the cylinder. For this reason, it is recommended that you do NOT enlarge the exhaust ports for better performance (in other than full race) because this leads to increased exhaust blow back and an actual LOSS of power--to the tune of about 20 horsepower.

So unless you are real careful about the way you route the exhaust and cooling water, you may be dissatisfied with the results

2 cycle engines don't have valves to open and close and are more sensitive to backpressure than 4 cycle engines. That's why the model airplane and boat guys can successfully throttle these engines with an exhaust valve.

 

[bman1bpm]

Re: Question for the techs

I never would have thought there was so much engineering to the exhaust!

 

[Jimmy627]

Re: Question for the techs

I replaced my exhaust bellow on my L-Drive & it cost $237. It looked weathered so I changed it back in november. I put it in the water april 16 & used it about 5 times. Well I pulled it out of the water on tuesday to do some work on the boat & the new hose looks just like the old hose weathered. If I was you I would take the old one off & look at it & run your hands inside to make sure there are no holes in it. Thats what I should of done at first.Well you live & learn next time I will check first. The sticker on my hose said size 3 1/2 X 9 1/2 if thats any help. Also look in a west marine catalog I seen them in there for about half of what I paid. Good luck ! Jim

 

[Wrenchjunkie]

Re: Question for the techs

hmmmmm that's why I was looking at replacing the hose as it looks old and weathered. i'll check the interior before going any further if the inside is fine
I wonder if I could just paint it with a coat of black plasti dip this would give a nice flexible tough coating to protect the hose from further weathering. Would have to see if it would effect the rubber any if not it may be a slick way to keep the hose from deteriorating.

WJ

 

[Jimmy627]

Re: Question for the techs

I don't think anything will stop the weathering. but if you do have to change it,like I said look in a west marine catalog I gave you the size of it. Otherwise you can go to a mercruiser dealer the can order it, but like I said IT IS EXPENSIVE $237. I personnaly think it gets weathered between the heat of the exhaust & the water it sit's in. That is alot of heat.Well good luck hope I help you out a little bit. Jim