Are there any benefits of using spacers between the manifolds and the risers?

[mwdgolf]

I own a 1997 330 sundancer with twin 7.4 mercruisers with stern drives. I'm in the process of replacing the manifolds and risers. There is a 3" spacer used between the manifold and the riser now. I'm thinking of not putting a new spacer in because the space from the top a the riser to the bottom of the deck is tight. My question is. Is there any benefits of using the spacer compared to not using it?


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[SCORPIO]

Yes, it may even be essential. You want the riser outlet high enough above the boats waterline to prevent water from backing up and into the engine, causing hydrolock. I'm willing to bet those spacers are there to do just that, raise the outlet up three extra inches and prevent backflow of water. I'd check with Sea Ray or a good mechanic before I deleted them, you could end up costing yourself a lot of bucks.

 

[dpmulvey]

Leave them in. I have 6.2's because of the inversion mentioned above on the OEM 7.4's. As an aside, ask you mech if he recommends a high idle at the dock and if so, why.

 

[Henry Boyd]

Ditto all of the above. From the records I got from the PO of Bella, Merc/Sea Ray ran a retro fit program on the early 496s like us to install spacers to prevent water back flow.

Henry


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[kap2]

The spacers are required to meet the proper exhaust design requirements to guard against water ingestion. Google MerCruiser Service Bulletin No 2001-13. Part of that bulletin describes what the angle of the exhaust system needs to be.

Don't eliminate the spacers unless you are positive that the exhaust system angle is acquit.

 

[mwdgolf]

Thanks guys for your input. Not worth eliminated them! The spaces stay in.


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[kap2]

Leave them in. I have 6.2's because of the inversion mentioned above on the OEM 7.4's. As an aside, ask you mech if he recommends a high idle at the dock and if so, why.

Excerpt from Merc service bulletin 2001-13 regarding the high idle.
2. Extended idling before shutting the engine off.
a. This affects an engine with thru-hull exhaust outlets more that thru-prop exhaust
models. Because of ‘No Wake’ zones and other conditions, engines that idle 30+
minutes prior to being shut off, tend to be more likely to have water ingestion. Quiz
the boat operator about their idle times. One way to stop or minimize this condition
is to increase engine rpm (in neutral gear) to 1300 for 45 seconds then slowly return
throttle to idle position and shut the engine off.