Backpressure would be your issue at speeds above idle, and if it gets excessive, it could possibly result in water / moisture getting back into the cylinders. Reversion itself is going to be most marked at idle, particularly a slow (600 RPM) idle. As the charge velocity picks up with RPM, charge inertia overcomes the valve overlap. I guess if there was a way to measure exhaust flow at different RPM's, you could determine what point the backpressure becomes an issue, but I think you would probably be fine up to around 3000 RPM. There are a lot of issues at play here that will determine if you will have a problem.

In regards to the exhaust tube, Wouldn't the prop create a low pressure area behind it as the boat moves forward and actually create a vacuum effect? I would think this would help matters, but ultimately, the constriction is in the passages that go through the Y-pipe and out the drive.

Every Merc BBC I've owned had the tube instead of the bellows.

Thanks.... I try keep the rpm,s about 800 or so in the no wake area.... When I approach the dock I open the exhaust and bring it to idle around 700rpm.... Just in case any of any reversion.... I don't think I will ever run this engine more than 1,500rpm thru the prop.... This engine breathes very good and I am afraid of reversion.... This thing is really making some power and easily cruises at 3,600 rpm half throttle at 50mph.... It sounds like a race boat for sure!!

I will try and get something soon.... It's a little choppy at idle but not radical. It still has a stable idle and does not stall out..... When I pushed the throttle up to about 3,500 the first time I was shocked! It almost sounds like I have twin big blocks? The roar was awesome! It smoothed out and with no mufflers it does sound like a race boat! The Dogs crawled up in the bow to get as far away from the noise as possible! TBF

What are the specs on the cam? Is it one of Bob's grinds?