above water exhaust outlet mod

[Mark75H]

You can not choke a 2 stroke on the exhaust side without burning pistons.

There is no production outboard being made that will go faster by relieving the exhaust and doing no other mods. They just are not made that way because of burning pistons ...

[JohnsonM50]

"You can not choke a 2 stroke on the exhaust side without burning pistons" Agreed, what I meant by that is if you cut open an above water passage it will likely relieve too much pressure so closing the opening a little to get back some of what may be released is all, not to close it in till it burns.

[Mark75H]

Also agreed.

The above water exhaust on the newer race motors is just a styling throwback from when it was assumed no back pressure is better than some.

[Powerabout]

what you can do is burn a piston due to the lean out usually accompanied by less back pressure

[fs5]

i dunno guys,i reckon if your turning big rpms on a stock motor it gives you a gain.i've got a stock merc seapro 55 ,i modded the gearbox to let most of the exhaust exit above the prop.i made a little pate up to bolt on when i was fishing to shut it up.
i found running big pitch props where the motor only revs around 5500/5800 it made no noticable difference,but when i bolted on a smaller prop and the motor reved over 6100 it made a definate difference to the topend revs ,i reckon it picked up an easy 100+ rpms.
just my 2cents worth.

[JohnsonM50]

i dunno guys,i reckon if your turning big rpms on a stock motor it gives you a gain.i've got a stock merc seapro 55 ,i modded the gearbox to let most of the exhaust exit above the prop.i made a little pate up to bolt on when i was fishing to shut it up.
i found running big pitch props where the motor only revs around 5500/5800 it made no noticable difference,but when i bolted on a smaller prop and the motor reved over 6100 it made a definate difference to the topend revs ,i reckon it picked up an easy 100+ rpms.
just my 2cents worth.

What might work to some degree on 1 motor might not on another. 100R's might seem alot on the tach & if it helps ya pass your pal/rival even better but realistically 100R's is about 1.6 - 2% of 6000 or 5500 equaling about 1mph. [unless the prop is faster anyway]. If you net the same percent on a smaller motor it becomes less yet. So for the effort & risk you get little if any.

[Mark75H]

If you did not go back and block the hole and retest you don't know if the rpm came from that or something else.

Both results have to be repeated or its not scientific

[hydroplay]

As a kid I used to play with Mark 20 Mercury motors before I knew much about the race motors. I cut the exhaust snout below the cavitation plate off and then lopped off the end of the main tower housing to make my version of a quicksilver tower. Nope, it wasn't much faster if at all but people heard me. Next was a project to bore large holes in the tower housing with a hole saw. Lighter is faster and noisier, right? Neighbors hated me because the best water is always early morning. All were happy when a friend of my dad's gave me an old D marathon boat with a KG9-H. Now I was faster and quieter. No substitute for a bigger motor.

[sean181]

try finding some bath tub racers from canada on the forum they still race the dt 8. I used to race against the dt 8 with merc 8 and the suzukis where allways the quicker of the 2

[microbream]

thanks for the replies guys,
i ended up leaving the exhaust tube outlet below the cav plate. I just opened up the three holes on the midsection slightly from 4-6mm. Should make it slightly louder if nothing else, may help holeshot, we will see.

Further on that engine , i have just resprayed it and was very impressed by the exhaust profile, even has a diffuser built into the mid section.
Although the port matching between the cylinder sleeve and the case needs some work.