Convert a Briggs 'mower engine' OB to underwater exhaust?

[Elkins45]

This isn't a repair question, so I hope this is the right forum. I'm seriously considering buying one of the Briggs and Stratton 5 HP air cooled four stroke outboards for very occasional use at a local lake. I'm just finally tired of pouring money into Grandpa's 1967 9.9HP Sportwin.

The regs for this particular lake require <10 HP and underwater exhaust. The B&S, being essentially a lawnmower engine with a lower unit, uses a traditional muffler and exhausts above water. If I do decide to buy one I'm giving some thought to doing a little modification with some iron plumbing pipe by removing the muffler and running the exhaust down into the water a few inches.

Anybody ever try this? Would there be big problems because of back pressure issues? Is this a stupid idea? I know I SHOULD just buy a proper engine but the low cost of the mower-based unit is really attractive.

TIA for any advice.

 

[robert graham]

Re: Convert a Briggs 'mower engine' OB to underwater exhaust?

Maybe take your old B&S lawnmower down to the dock with a piece of pipe and try it out to see! Seems like it might make some popping/splashing noises and like you say, the backpressure might not be good, but you could drill a small pressure relief hole(1/16") in the pipe above the water line to keep it from sucking water back up in the motor....give it a try!

 

[ezmobee]

Re: Convert a Briggs 'mower engine' OB to underwater exhaust?

I'd rather spend the money on a real outboard. The money you'd spent on a B&S could get you a very nice used 80's or newer Johnson/Evinrude/Mercury 9.9 that should serve you very well and provide loads more power than the B&S.

 

[Chris1956]

Re: Convert a Briggs 'mower engine' OB to underwater exhaust?

Take a look at your old OB. You will see that the exhaust exits just behind the prop, and at the tip of the prop "circle". That would be a good place to pipe the B&S exhaust, since the prop will pull the exhaust out of the motor and mix it with water. This decrease in backpressure will add to the power of the motor.

 

[ondarvr]

Re: Convert a Briggs 'mower engine' OB to underwater exhaust?

The short answer and easy solution is buy a real outboard, you'll be much happier.

 

[road kill]

Re: Convert a Briggs 'mower engine' OB to underwater exhaust?

How happy you will be with the Briggs 5hp depends on how much noise you can tolerate. My guess would be that running the exhaust into the water will lose you a lot of power. That motor just isn't designed to run with any serious back pressure.
A buddy has one on a 12' aluminum Sears V hull that runs fine but he got tired of all the noise. The boat goes fast enough but will not plane out. I'd guess it's moving about 11 to 12 mph on a calm lake with no wind with my 310 lb self in the boat and 3 gallons of gas. It seems to actually do better with some weight up front. A smaller second passenger don't seem to slow it down much. Current and wind hurt top speed a lot. It would probably plane the boat with two smaller people. It seems like it's trying to rise up a bit on place with just me in the boat but it doesn't quite make it.
He got tired of the noise and mounted a cheap car muffler and ran flexible exhaust tubing to the muffler which hangs from the back of the outboard. It looks like crap but it really quiets down that motor. I doubt if the car muffler adds any back pressure and its not heavy. He used two brackets to hold it in place, and it points down towards the water but not in the water. It was closer to the water before but it would stumble and stall when the water washed up and covered the outlet.

Running wise, the vibrate a lot compared to a normal outboard, but they start easy and are decent on gas. The simple fact that you don't need to mix oil makes it easier to use.
In comparison, I'd say it's about equal to an older Johnson 3hp. Which is what he had before. The 3hp did about the same speed but had no neutral or reverse and it ran on two stroke mix. The Briggs is heavier, noisier, has more vibration but is reliable and it does get the job done for far less than a water cooled motor, and best of all, there's no water pump to burn up or replace.
Of course, my old Johnson 3hp and my older 5.5hp have never let me down and both together, including any money I spent for them and put into them haven't come close to the $850 price tag for a Briggs outboard.

 

[mercrewser]

Re: Convert a Briggs 'mower engine' OB to underwater exhaust?

Back pressure is not the issue. It is reversion. You can experiment by running pipe into the water, but you will need to drill a pin hole somewhere above the waterline to relieve the vacuum pulling water up the pipe into the cylinder. Otherwise you will flood the engine with water by pulling the starter cord before it even fires. Air cooled engines have overlapping valve timing for additional cooling. Also, the pipe will get really hot - too hot really. Other aircooled outboards with underwater exhaust had waterpumps in the lower housing to pump water into the exhaust for proper cooling.

Buy a better designed outboard.