Silent Choice - is reversion more likely at idle or at cruising speed?
 

My old 461 engine was running the following Comp Cams Xtreme Marine 270H-12 flat tappet cam with stock manifolds and Silent Choice with no signs of reversion:

226/236 112 LSA, .540/.547,
at .006 INT opens 25* BTDC, closes 65* ABDC,
EXH opens 77* BBDC, closes 29* ATDC

As I stated before, I ran this cam with the divertors closed at idle, and even at slow cruising speeds up to around 3200 RPMs with no problems whatsoever.

New engine will be a 489 stroker, with rectangle port heads and flat tops, probably around 8.7:1 CR. Still mulling my cam choices, but I will be retaining the silent choice. Probably going to move to a more efficient exhaust like Gil or even Lightning headers if I can get them to work with the divertors. I know it's a compromise, but I want the option of running quiet or loud, and my budget won't allow for a set of the switchable tips like the Teagues.

My question: is anyone running a cam with specs close to the Crane 731 cam or the Merc 420 cam with switchable exhaust?

Am I going to have more likelyhood of seeing reversion with the exhaust running through the prop at idle, or at low cruising speeds between 2800-3200 RPM? My thoughts are that idle is going to be the most likely to produce reversion, because at low engine speeds the intake and exhaust velocity will be slow and lazy. However, I am also aware that there is only about 3"x2.5" of opening through the Y-pipe to the prop, so we are limited at higher speeds too. If I was running the stock 4 inch stroke I would feel pretty safe because of my prior experience, but I am a little bit concerned that the extra .25" stroke will result in higher piston velocity and a stronger "pull" on the downward stroke against the exhaust. This is why my thoughts were to stay with the wider 114* LSA of the Merc 420 cam, and perhaps a slight bit less exhaust duration.

I would love to hear some thoughts from the cam experts on this one - I'm sure I'm not the only one with this question.


Thanks!

Anyone???

Follow this link to a short but some very good information on reversion. According to them it takes place at idel.

http://www.boatheaders.com/reversion.htm

Reversion is a problem at idle. At higher rpm the water is blown out too fast to get down to the valves. That’s why I bring my engine up to 2000rpm before killing the ignition.

Good article, Cliff. So if reversion is an idle problem, am I OK to cruise at 2800 - 3200 RPM's with the exhaust running through the prop?

Good article, thanks.

I'm no expert but there should not be be problem at those rpm's.

That's what I thought. Sometimes it's nice to cruise along in quiet mode with the stereo on, or just being able to carry on a conversation without yelling.

If you insist on running short risers with silent choice you are limiting your HP and Cam choices. Just call Bob RMbuilder and he will get you the correct cam. Or you can roll the dice and hope for the best. I would get the long risers and put mufflers on it, if I were you. I however run it loud all the time, just get a better stereo if that is what you are woried about.

Jeff, I'm buying a set of Lightnings to run on it, but I am going to modify the silent choice diverters to move them back as far as possible and have them act as collectors for the headers. I'm not going for all out performance on this boat - the hull design on the older Baja's is a limiting factor compared to the newer, more efficient hulls. I like having the choice of loud or quiet at the flip of a switch, and I don't mind sacrificing a few MPH for it. Different strokes, as they say. Guess I'm getting old - that, and I have so many other things that I have to spread my $$$ around on.