I have been searching the web looking for information on Mercruisers silent choice. I have this system on my boat and was wondering at what rpm is the limit I can run them in the silent mode ( sending the exhaust thru the prop) or do they only recommend that for idle only.

Thanks

Basically idle only I've read. Like 1500rpm max I think

I have a mild crane cam and i'm also dumping excess water out of my exhaust manifolds before it gets to the elbows so no chance of reversion here. Seems senseless to only be able to go to 1500 rpms.

I have a 7.4 mpi with Silent Choice -- I can't find anywhere where is says that you can't run WOT without activating the exhaust. Silent Choice is an option -- not a need.

The larger horsepower engines that don't offer thru prop exhaust are not Silent Choice -- they need to run - for lack of better - straight pipe !

I will be the first to admit that it is better for the engine to run the Silent Choice - letting the engine breathe. I typically get about 3 mph more using Silent Choice -- and for sure sound a lot better!

Safe Boating

3pointstar

Depends on the engine - more specifically the exhaust / intake overlap of the cam. If it is just the stock Merc magnum cam in a 454 or 502, you can probably run it thru the prop to your heart's content. There are a lot of boats running around with these engines installed with full time thru-prop exhaust, and they are doing just fine with no reversion. The mag cam is very mild - something on the order of 115* LSA angle. If you have the 7.4L 330 HP engine, you have even less to worry about.

^^ what he said because merc wouldn't warrant it if you could phuk it up that easily, just mho.

I was told up to 3500 rpm ? I switched to Corsa electric way back when?

My cam is a crane 530 / 550 lift duration 224 + 236 and a 114 lobe. I thought reversion happens at idle only with big overlap cams? I have water dumping out of my exhaust before the ss elbows to stop reversion if any. I installed custom tee fittings out of my EMI manifolds just before it enters the short ss risers. I was told if you don't want reversion problems dump the water. You only need a little water to cool any exhaust hose at idle., What would be the first signs of a restriction of exhaust while on plane ? Will it start to buck die out, ??

I ran a 454 with a similar flat tappet cam thru stock Merc exhaust and silent choice for years without issues. I used to run it thru the prop at an easy cruise up to around 3500 RPM or so without issues. I think it would have been OK to run it at higher RPM's than that thru the prop, but backpressure becomes and issue then. You are trying to force all that exhaust (and water) through an opening that is about 2" by 3". You can probably tell if you are having backpressure by noting your speed at a given RPM with the exhaust open versus the speed you see with them closed. I was fine, but as they say, your mileage may vary. You are running more cubic inches than I was, which can increase the amount of reversion because a larger engine sucks back harder when the piston is on its down stroke.

Is it a flat tappet cam? Roller profiles get the valve up off the seat more quickly and can be more prone to reversion than a flat tappet with similar specs.

If you are dumping off excess water before the diverter, you have probably minimized your risk of sucking water back into the engine, but you might want to shoot a temp gun on the hoses and Y-pipe to make sure you are not overheating those parts.

My cam is a flat tappet and my exhaust pipes are cool at idle. There is still plenty of water coming out of the pipes even with the exhaust in the silent mode. I ran 1/2" hose off of a custom tee fitting made for me by EMI. 3/4 x 3/4 x 1/2 tee, then i ran 2 dumps on each side of the boat.