above water exhaust outlet mod

[microbream]

Hi guys,
just ran the 10ci Suzuki on my microbass boat and my god did she fly!
Topped 30 easy @ 7000RPM. Yeh while i get my prop repitched i am thinking of doing some exhaust mods. ATM the exhaust exits below the cav plate, plus their are three small exhaust holes further up which exit to the atmosphere. The main outlet kinda exits directly behind the prop from a tube that protudes 2 inches below the cav plate. I was thinking of moving the outlet above the cav plate or even above the water pump. Now i now noise may be an issue, but surely this will improve mid range response and result in less drag once i have removed the protruding exhaust tube.
should i aim for same outlet area around 30mm or smaller considering it is above water
cheers
Microbream

[Mark75H]

It will not make any performance increase

[Ron Hill]

I used to drill two 3/8 holes, just so the motor made enough noise that I would know where the boat (s) were....But speed advantage??.....NO....

[JohnsonM50]

It could be that the below water outlet improves power a bit. I agree that no performance gain can be expected & think it might even slow ya. Your turning alot of R's .. whats it rated for?

[microbream]

It could be that the below water outlet improves power a bit. I agree that no performance gain can be expected & think it might even slow ya. Your turning alot of R's .. whats it rated for?

i realy can not find any info on this engine?
its an 81 dt8 and yes 7000 does seem a bit high, but the way it pulled from 5-7k was awesome! I will look to go to a 2" higher pitch to bring it down to 6 ish.
Interestingly the mid section is almost a perfect diffuser. Then it hits the water pump and runs out thu a straight 25 mm tube at 30 deg exiting below the prop.

[JohnsonM50]

i realy can not find any info on this engine?
its an 81 dt8 and yes 7000 does seem a bit high, but the way it pulled from 5-7k was awesome! I will look to go to a 2" higher pitch to bring it down to 6 ish.
Interestingly the mid section is almost a perfect diffuser. Then it hits the water pump and runs out thu a straight 25 mm tube at 30 deg exiting below the prop.

How much pitch change has to do with gearing so don't know about 2p more or less but targeting 6 sounds good, better to over rev a little than to lug it under stress.

[microbream]

It could be that the below water outlet improves power a bit. I agree that no performance gain can be expected & think it might even slow ya. Your turning alot of R's .. whats it rated for?

i am curious about say a merc 25xs v standard 25. The xs has the racing lower unit and above cav exhaust outlet. where as the stock 25 has thu prop exhaust. Would seem like the different exhaust configs would have an effect on running good or bad, any ideas.

[JohnsonM50]

Its possible that it does, [perform best above water] but not likely because of it in itself. Im not that familiar with the 25XS but would guess whatever it uses for a tuner has more to do with it than the position of the outlet. The stock motor has noise to contend with & ais probably the best it can be as is. Improving on the tuner would be more likely a place to succeed but even there its probably optimal. If you go ahead & do it, let us know if it proves to work.

[microbream]

i guess when you look at it above water exhaust has constant backpressure regards of boat speed. where as thru prop exhaust backpressure changes with boat speed. As no or low speed backpressure = water pressue. But as speed increases the moving water will actually reduce this back pressue. I beleive this is best illustrated by gunning the engine from a standing start, the motor output definitly improves as speed increases.
interesting may put on a pressure gauge and do some testing

[JohnsonM50]

i guess when you look at it above water exhaust has constant backpressure regards of boat speed. where as thru prop exhaust backpressure changes with boat speed. As no or low speed backpressure = water pressue. But as speed increases the moving water will actually reduce this back pressue. I beleive this is best illustrated by gunning the engine from a standing start, the motor output definitly improves as speed increases.
interesting may put on a pressure gauge and do some testing.

A small race motor is a different animal in several aspects, Exhaust being one of them. They still suffer the same semi submerged start out above or below tho. Race motors generally use locked time so don't have the full range time calibration as a stock motor. Then the race props are of smaller dia. than their stock counterpart. There's a host of other possible differences too like different ports, carb changes, padding, reeds & last but not least exhaust tuning. 1 change to be more like a racer is for those possible reasons alone unlikely to make a difference. Not to say don't try anything, just not to expect a dramatic boost of power, its likely not there. [Radical change is best enjoyed with spare parts-n-donor motors ] Good Luck