Max rpm for using thru-prop exhaust?
#11
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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.
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.
#12
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Every Merc BBC I've owned had the tube instead of the bellows.
#13
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Thread Starter
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!!
#14
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Thread Starter
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