8V92 Backpressure and Overboard Raw Water Divert Valve

[dailo]

Hi

I am having some issues with back pressure on my 8V92TI's (9215 injectors).

We are measuring around 4-5" hg.

I have read here extensively some boats came with a raw water divert valve.

First, I was wondering, how much power is being lost with this much back pressure from the engines.
I do not have any smoke that is visible under way. My WOT is around 2100RPM. I posted earlier about fuel delivery and injector sizes, but I am starting to think more along the lines of back pressure affecting the engine performance.

If I were to add a raw water divert valve, does anyone have any pictures of what their set up looks like? What size should the overboard hose be? I will then test to see how much water we can divert without increasing the exhaust lift muffler temperature.

Thank you

[Cricket]

Is your exhaust on the side or straight out the transom?
Lots of side exhaust boats have been modified with a "Y" to an additional smaller hole through the transom.
As an example, you probably have 10" exhaust for 892TI, add a 4" or 5" tube before the current side outlet ending with a 4" or 5" hole cut into the transom which will lower the back pressure.
Or, just moving the outlet from the side straight through the transom.
I don't like removing water from the exhaust stream because it could raise the temps in the tubing, etc.

[willin]

I have a 83' 46c with 892's. the raw water discharge has a tee in it half way before exhaust. there is a valve just before the side discharge from the tee. the original owners manual shows this set up as it was factory installed. it clearly says and warns that the valve should be open 3-1/4 turns. to prevent back pressure from prolonged idle. that's about all I can add to the situation other than the engines have run for years without issue.

[Cricket]

Also, have heard about boats developing back pressure issues because of broken baffles in the mufflers. Doubt both mufflers would go bad at the same exact time though.

[dailo]

My mufflers were remade into lift type mufflers in the engine rooms, so this is where the issues started.

Does anyone happen know the ID size of the diverted water hose?

Thanks!

[KPTIM]

I think you will find two stroke Detroits DO NOT work well with lift type mufflers.
Diverting some water is not going to solve the whole problem.
Every time the muffler fills with water it will need a burst of pressure to evacuate it, you will get a big back pressure surge at that point.

They like to be loud and the like to breath.

I hope I am wrong but been running two stroke Detroits for 30 years.

[Cricket]

You could be correct regarding lift mufflers and 2 stroke diesels. Sort of remember Viking having a lot of trouble with 45's that had 6v92's where people would have to drain them to be able to start the motor if it didn't fire up right away and the muffler got to much water during cranking.