Silent Choice exhaust question

[Dreamer Dave]

When I was test driving my boat just before I bought it (18C (http://mail.aol.com/38109-111/aol-6/en-us/suite.aspx#) w (http://mail.aol.com/38109-111/aol-6/en-us/suite.aspx#)/350), the guy selling the boat stressed that you never switch modes (either from wet to dry or from dry to wet) at anything other than at idle. Even though this didn't sound right, I accepted it as fact. I definitely don't see anything wrong with switching from wet to dry at high RPM's but what about the other way. Does the tiny extra back pressure to blow the wet leg out cause any adverse wear to the valves?

[Morgan's Cloud]

My guess is that the guy once heard something and then got it all mixed up .

Big blocks prefer to run straight through the transom . Not so much of a critical issue with 350's IMO.

The fit on the flappers on a Corsa system when shut are not so air tight that every last bit of pressure is going through the leg. There's still a fair bit going out the tail pipes.
So in the long and short it's not really a back pressure worry. Reversion is a different kettle of fish all together and if you have that problem it wont make any difference if the diverters are open or closed.

I wouldn't be concerned about what engine speed you actuate them at to be honest. I've inadvertently hit the switch on my 502 at various engine speeds and nothing's happened yet .

[osur866]

IMO, and I do this with my engine (a little more than a stock motor) 3,000 and below exhaust closed 3,000 above open and I switch on the go all the time and have never had an issue.

[Pismo]

Switch anytime, not an issue

[Ghost]

Suspect there are at least two cases to consider: the steady state at any given rpm and the action of making the switch at any given rpm.

Some motors (425 hp 496 merc for one) tell you not to install with thru-hub-only exhaust. And they may give an rpm, I can't recall, above which they should not be run through the hub on a switchable system (somehow 2500 or 3000 comes to mind, not sure why). This is all the steady-state case listed above. I'd see if I could find the merc installation guide for your motor. They have some of these on their website, under "repower" I believe.

As for the case of making the switch at a given rpm, I thought they were made to be able to switch OPEN (going the easy way) on demand, essentially at any time or speed. In addition to not thinking that'd damage the motor, it also seems like it wouldn't harm the switching mechanism either. It's basically just de-powering a solenoid and letting the flap open and hit the stop with most systems, right? Going the other way seems like it could be an issue for the switch mechanism itself at high enough rpm (I'd call Corsa and ask), as well as being a potential issue for the motor. (Certainly an issue for the motor if there's a steady-state rpm you shouldn't exceed in thru-hub mode.)

All that said, my gut guess tells me the only real issue is harming the motor at steady-state operation above some fairly high rpm, depending on whether your 350 is down near 260 hp or is tuned up (perhaps way up) above 300 hp. If, say, it's 400 hp, there's probably some threshold rpm you should never exceed if only going thru-hub.

[Carl C]

It depends on the engine. The Mercury manual for my 525 says not to run in quiet mode over 3,500 rpm. (I could have that number wrong but it's close). I only use it at idle. On a production engine it should be fine. Yes, the added back pressure can cause reversion in big cammed engines.

[Dreamer Dave]

Thanks everyone. Forgot to say it's a bone stock 300hp (http://mail.aol.com/38109-111/aol-6/en-us/Suite.aspx#) MPI (http://mail.aol.com/38109-111/aol-6/en-us/Suite.aspx#) 350. Thinking about it, I'm never flat out and usually slow to under 3000RPM (http://mail.aol.com/38109-111/aol-6/en-us/Suite.aspx#) anyway when I get to where to I prefer to quiet things a bit, due to the shorefront houses. Think I'll use that as a "most case" threshold for switching from stack to thru hub just to play it safe but knowing I can occasionally hit the switch at higher speeds. It was the dropping off plane and climbing back up that was such a royal pain.

The guy who told me this, I'm guessing, thought it was a 3-way valve that completely blocked exhaust flow during transition. Oh well. Complaining about the previous owner is a tradition as old as boats.

[BLACKBOX]

Corsa told me last year 3,500 RPM is the limit for switching either way.

[Ghost]

Corsa told me last year 3,500 RPM is the limit for switching either way.. That's interesting, thanks. Did they say that based on your particular motor, and if so, what displacement and hp is it? (If not, based on what?)



It depends on the engine. The Mercury manual for my 525 says not to run in quiet mode over 3,500 rpm. (I could have that number wrong but it's close). I only use it at idle. On a production engine it should be fine. Yes, the added back pressure can cause reversion in big cammed engines.. The 3500 rpm or so spec for the 502-cube 525 motor sounds like a useful data point. I'm thinking with a 300 hp 350, thru-hub is probably enough flow even at the top of the recommended rpm range (unlike with the production 496 HO, where Merc says not to install a thru-hub only setup, due to the limit on flow).

[BLACKBOX]

Corsa custom built mine, since the activators had to be placed on the side instead of the top so the engine hatch could close - They just told me not to engage or disengage the diverters over 3,500 rpm, as it could cause damage to them. I have the 220 HP, 4.3 MPI, which is the smallest engine they make their system for...

[woobs]

Corsa custom built mine, since the activators had to be placed on the side instead of the top so the engine hatch could close - They just told me not to engage or disengage the diverters over 3,500 rpm, as it could cause damage to them. I have the 220 HP, 4.3 MPI, which is the smallest engine they make their system for...
I'd like to see any pictures you might have of that installation.... I'm looking at a set for my 4.3

[BLACKBOX]

Pics attached...

[woobs]

Thanks for the pix!

[BLACKBOX]

If you need any more info, PM me!

[wwahl]

Why would it not be a major deal for reversion? If the Silent Choice is on the exhaust is diverted in a totally new direction. I don't understand.


My guess is that the guy once heard something and then got it all mixed up .

Big blocks prefer to run straight through the transom . Not so much of a critical issue with 350's IMO.

The fit on the flappers on a Corsa system when shut are not so air tight that every last bit of pressure is going through the leg. There's still a fair bit going out the tail pipes.
So in the long and short it's not really a back pressure worry. Reversion is a different kettle of fish all together and if you have that problem it wont make any difference if the diverters are open or closed.

I wouldn't be concerned about what engine speed you actuate them at to be honest. I've inadvertently hit the switch on my 502 at various engine speeds and nothing's happened yet .

[Morgan's Cloud]

I might have been misinterpreted there . Reversion is a serious issue but it's not caused by the silent choice . It's a cam issue (if I'm not mistaken) and will be there all the time whether the flaps are open or not.

The original question posed here was about opening the flaps at given speeds based on something that the prev owner said without clarifying .

As has been mentioned by at least two folks above , the rules for big blocks are something like 'ok to be shut or open under 3500 but always open above 3500'

Because the engine in question is a stock 350 and back pressure is not an issue , nor is reversion , I found the seller's remarks curious .

[Ghost]

I'd pile on further in saying we have another (conflicting with the rule of thumb cited by MC above) data point from Corsa on the 4.3.

I suspect part of this apparent conflict is a function of folks outlining behaviors to address different problems. The are different types of reversion, with different causes (back pressure, pure geometry of exhaust, cam, resonance, etc.), as well as a wholly separate question about what is safe vs harmful for the switching mechanism itself.

Suspect the real answer is an intersection of various rules of thumb. Sometimes some don't apply at all to a given boat, other times several may apply. Toward understanding that, have cited this study before but it's worth including here too.

http://www.michel-christen.com/ExhaustElbows.pdf