running hard with or without Silent Choice open
 

I have heard somewhere along the line that when running hard you need to open the silent choice, is this true?

would it affect the motor in a negative way at all via running fat/lean etc?

or just possiby pick up a few RPM/MPH?

this is on a stock 502

I would be concerned with the back pressure running them closed at WOT !

Yea not a good thing...let her breathe. Silent choice is for marinas and quiet idle areas

To All AMEN -- The silent choice is for the quiet times - funerals etc. But when you run anything over 2500 rpm's open them up -- let the engine breath!!!! If windows rattle - they were loose anyway! If it wakes up the neighbors - tell them to go to bed earlier-

I don't have a high performance boat 7.4 mpi with Captains Call- at WOT I gain about 3-4 mph opening them up- if it save fuel I sure haven't noticed.

safe boating

3pointstar

The stock Merc cams, even for the magnum engines, are very mild, on the order of 224*/224* on a 115* LSA. I think running at a decent cruise, say 3500 RPM with the diverters closed is not likely to hurt anything. However, I wouldn't advise it for WOT. That being said, there were a lot of the boats built with 454 mags that had full time prop exhaust and are still running fine, but a 502's extra displacement could be the tipping point. I wouldn't risk it at anything above 3500 RPM, but should be fine below that with the stock cam. Also might be a good idea after running it hard to come back down to idle gradually - don't chop the throttle, and let it idle at around 1000 RPM for 45 seconds or so just in case. Those hot exhaust valves are not going to like any water on them after a hard run.

I for one anytime I run WOT for an extended period ALWAYS idle it down and let just naturally cool down- I don't know if it is an old wives tale/tradition - but it sure does seem to make sense. I've carried that over to the lawn mowers, and more especially my car - E320 CDI (Turbo Diesel)- I NEVER get off the expressway and just shut it off -- I don't care how good Mobil 1 is a red hot turbo needs to cool - especially the oil in the bearings.

safe boating

3pointstar

I have heard running at high power w/exh thru the drive will make the drive run hotter and shorten it's life. No proof of that however.

Thanks for the great info, I hadn't even thought of the exra heat on the drive...

I would think with the amount of water dumping into the exhaust that would not be a problem. The drive makes more heat than that on its own. Ever looked at a silent choice or captains call setup? Even when the flappers are open, you still have exhaust and water going down the drive at the same time as thru hull.

Here is a link to a Merc service bulletin that addresses reversion and mentions silent choice and thru-hull setups. It actually states that reversion may be less likely if the SC is set to thru-prop at idle, which runs contrary to what I thought. It also mentions muffler tips and flappers as a means of cutting down on reversion by disrupting the waves that develop. I would say that this might apply for the stock Merc cams, but I don't know if I would try to apply this to a high performance marine engine with an aggressive cam.

http://www.boatfix.com/merc/Bullet/01/01_13.pdf