So in the end analysis, it would be riskier to remove them
from a boat that is slipped for the summer? As opposed to a trailered boat...in terms of the potential for rusting the valves and seats? Isn't this potential for rust exhaust stream moisture almost instantaneous on shutdown wether it's on a trailer or in a boathouse?

I'll be pulling mine off in the next couple days to facilitate the changing of exhaust tips, and have watched for your response.

10-15 HP on a dyno above 4000. Any negative effect on low RPM torque?

Mercruiser does make a "Exhaust Resonator Kit" which they "strongly" recommend for all 454, 502, and 496cid engines, and can be used on other model gas engines.

The exceptions would be sterndrive models with Silent Choice, below swim platform exhaust, S-type exhaust, or mufflers that mount between the exhaust elbow and exhaust tip.

Unlike the turbulators, the purpose of the resonators is to reduce "the tuning effect" in the exhaust, which can contribute to water intrusion. And unlike the turbulators that mount between the manifold and elbow, the resonators mount in the wet exhaust hose at least 13" downstream from the elbow. The following is a direct quote from the resonator kit instruction sheet:

"The resonators help to break up the reverse pulsations, yet do not affect engine performance. In some cases, a performance increase is actually realized by using the resonators."

Dave

No negative effects on rpms or torque since torque is the work of horsepower measured over time. Remember , engines are air pumps with fuel and spark, help them breathe better and they will return both torque and horsepower.The slipped boat would be more prone to moisture (condensation) than a trailer boats thats pulled up high and dry. This is the use thats probably the hardest on stock heads with stock valves. The mercury exhaust riser for the 496 has a tuned merge bell type resonater at the end under the hose connection that does a great job of reducing reversion, tuning down the reverse exhaust pulses and helpiing to extract exhaust gases in the exhust system, The Gil and Merc engineers did a damn good job with these exhausts!, could have done better, but heck they never ask me!

Ray @ Raylar

As I said, they are to prevent condensation from entering the cylinders. Merc WILL NOT cover any damage cuased by reversion if you remove them though. Their purpose has nothing to do with reversion but they will void your warranty if your remove them.

So is it not recommended to remove them if you have silent choice? If I do remove them and I have silent choice, will I not be able to use the silent choice at all or can it be used in moderation?

My exhaust has a pretty good downward slope...

On Hotboat.net, a fellow boater running a 22 foot Donzi Classic claims to have gained 3 mph gps through the removal of the plates.

How about extrude honing these stock manifolds?

I removed the plates in my Carrera 257, with the 496HO, and gained approx 150-200rpm. It translated to maybe 1 mph. Nothing dramatic, but what the heck. Very easy removal, with no other modifications. I can put them back just as easy if evidence comes out that it isn't good for the motor in the long run. To remove both sides took about 1/2 hour or less.

In my opinion it's a no-brainer. The risks (voided warranty/rusted valves) FAR outweigh the benefits. I would guess that removimg them might gain a little RPM/speed the first season but by the next season you would see a loss.

Dave

Since these plates are for condensation boil off only then I will remove them and report the results. Think about the design of these plates. All they do is give a small shelf to catch any water dripping down the side of the exhaust risers during condensation. If you have a bunch of water come back up your exhaust (reversion) there is no way this little shelf will stop it from entering your motor. Do any motors that Merc sells with factory headers have them? I don't believe the older 454's and 502's had them and those motors seemed to have no long term effects of this condensation. To me most condensation will occur when the motor begins to heat up, all of this should be burned off quickly as the engine heats up and should not cause any corrosion problems.