5.7 Mercruiser Exhaust Riser/Elbow Question

[Jersey13]

Hi all. Hope everyone is doing well.

I'm running a 1990 Mercruiser 5.7 & went to replace the upper rubber boot that connects the starboard side exhaust riser to the exhaust elbow and ran into a bit of a situation... as usual. lol.

I've replaced this riser and manifold about 2 years ago, probably ran the boat under 10 times and have always flushed after each run. The type of riser that is used in my application is the type that extends a few inches into the exhaust elbow. After removing the upper and lower boot, I could not for the life of me separate the riser from the elbow. 2 years old, hardly used and It was completely rust welded together. UG.

I decided to take a sawzall and cut right between where the 4" elbow and 4" riser meet. This left the smaller diameter piece of riser that extends inside the elbow, inside the elbow. I had to chisel and pound away at that piece for nearly 1/2 hour to get removed. Rust is not my friend.

I was wondering if it would be alright to run like this, without that extension of riser that goes into the elbow? Not sure of the exact purpose other than to possibly have the water mix with the exhaust further down the elbow? The water port coming out on top of the riser looks clear and I've done a few other riser jobs that the elbow and riser just butt close to each other, and they're perfectly fine. I'll try to post a pick or 2 to help with visual. Appreciate any advice.

Thanks in advance,
Jersey

 

[Bt Doctur]

cutting that extention off will result in exaust reversion , it is needed

 

[Jersey13]

Thanks for the quick reply Bt Doctur!

I love to learn new things so forgive me... if I understand exhaust reversion correctly as it relates to marine applications, without this extension it would suck exhaust water back down into the engine? How come this doesn't happen to the motors that the riser and elbow just butt up against each other and connected by the boot? I've heard that happen on vehicles with long duration cams, but had no idea this could happen on a stock 350. Please don't get me wrong, I'm not questioning your answer, I just welcome the knowledge.

Thanks,
Jersey

 

[alldodge]

Every motor can cause reversion, its just the more valve over lap there is the greater the chance of it. By removing the extension you have increased the possibility for water to enter the exhaust. The water doesn't have to do much more then almost pour straight in. Here is a good read dealing with standard and after market exhaust on Merc motors.

 

[Scott Danforth]

without that little extension, all that you need on a stock motor for it to hydrolock is a bit of run-on or one good back-fire while running at idle. As AllDodge indicated, your pretty much almost pouring water into the manifold without it.

 

[Bondo]

Ayuh,..... If you'd have loosened the joint below that one as well, ya mighta gotten it apart easier,....

You could also inspect the shutters that live right at that joint,....

 

[Jersey13]

Thanks ALLDodge, Scott & Bondo for the tech info. No reason to take a chance of hydrolock or worse, might as well replace them both since I have no idea what the port side looks like.

The link wont work for some reason but thanks anyway ALLDodge.

Bondo, I had removed the lower boot and was "lightly" tapping the elbow with the baby sledge and she still wasn't moving one bit. Flappers long gone.

Again, thanks for the replies and knowledge.

And so it goes.

- Jersey

 

[alldodge]

The link wont work for some reason but thanks anyway ALLDodge.

You need adobe reader and when its clicked look for something asking you do you want to download or view. Many times its hidden. If your doing this on a phone then I don't know

 

[Scott Danforth]

Flappers long gone.

You may want to fix that too