Max rpm for using thru-prop exhaust?
 

I don't want to get any reversion problems but wondering if there is a max rpm for using the corsa thru-prop vs. transom? This 509 sounds pretty damn rowdy at idle with 10.2-1 C/R and a Marine Kinetics .600" lift cam....... I have about a 15 minute run past at least 100 waterfront homes at idle power to get to the open bay....... This thing sounds downright angry with no mufflers!
My marine guy did a great job moving the transom exhaust holes up about 5" as the 496 exhaust with the tall risers was way higher than the stock 502 setup.... Plus I like the idea of the flappers sitting above the waterline...... TBF

The hole through the gimbal is actually quite small like 1.5"-2" diameter and from there it is a long uphill stretch to the headers, I don't think you would have any reversion problems, but I wouldn't use the through prop at much above your minimum planing speed.

I read on here somewhere that 2500 RPM was max

Do you have the accordion exhaust bellows connected to the drive? If so might investigate replacing that with the tube type so the exhaust isn't forced to go thru the prop reducing back pressure

I also heard 2500 r.p.m. Very considerate for you to be thinking about the noise level.
Always a good idea to look out for the performance boat image before it becomes a problem.

Well, it was not too bad.... Idle speed is not too loud but when I put a load on it holy crap! I came in at sunset and anytime I come in late I will keep it thru the prop.... I do have the accordion style tube so maybe I will change it over....... Thanks.....

Yep, 2,500 is what the Mercury 525 manual says. I only use it at idle when I need to.

Video clip of boat running on the hose so we can hear the cam?

For reference...There are cruisers I run with that are straight thru prop exhaust with BBC's. I can't see those being much different than a HP motor being ran with closed exhaust at a lower RPM for the range you are referring too. Not like you are wanting to run WOT with exhaust closed, while these cruisers do it w/o issue.

i was thinking along US1's line of reasoning. 260 thru 330 hp uses thru prop as only option. if your secondaries aren't opening, you are probably not into too much back pressure.

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?