Corsa captions choice noise

[Beech2000]

This is a strange one.

In final stages of engine break in and noticed flapper noise coming from corsa
exhaust tips. The noise seems to be the doors slamming open and closed during high loads (Before plane).

Not sure if result of bigger cam or what.

Reply would be greatly appreciated.

[Zackman]

Reading your post, it sounds like this is occuring while your accelerating to plane? How close to the water line are your exhaust tips? Could the tips be slightly submerged as your accelerating to plane?

Also, you mention a bigger cam. That bigger cam is hopefully rated for marine use and your not getting any type of water inversion from to much valve overlap.

[Frosty]

At 2000 RPM you have exactly 133.3 exhaust pulses per second. Or even 66.6 per bank.


I doubt if the flapper valve would be or could open and shut at that speed.


Take it off and try it. Or tie or tape them open. Or put some one on the swim platform to see where the noise is coming from.

Sure it not a misfire occuring?

[Beech2000]

My apologies for not clarifying better.

The Corsa Exhaust flappers are internal on my tips. The noise I hear is best described as a banging of flappers mainly at idle. I believe I hear the noise during high load and low speed as well but at much faster pulse as JF describes but not sure

Yes Frost, Thought already entered my mind regarding misfiring and not ruling out as separate noise but the engine runs so smooth at power.

I feel sure the idle noise is in fact the flappers rapidly opening and closing at idle speeds below 1000 rpm but not sure about the second.

I spoke to Scott at Boatstoreonline.com and he says he hears of this scenario frequently.

To much overlap in cam selection and water inversion is the inevitable.
So chalk another one down for stupidity on my part.

Fortunately Scott had me do a compression test and all cylinders have 180 to 190 psi of compression. The engine has 8.7 hours on it and it appears as if no cupping to Inconel exhaust valves or leakage has occurred.

Obviously concerned about the large amount of work required to replace cam, he suggested replacing lifters with the Rhoads OE roller type might be a better solution as they reduce the overlap at idle by 12 to 15 degrees while still retaining the power at RPM’s above 3500.

I spoke to a couple of reputable marine engine builders as well as the one I used and they all say the same thing. Stay away from Comp Cams 270 marine for Mercruiser installations.

None of them have any experience with the Rhoads lifters but say it sounds good.
Rhoads advertises the fix for this scenario and power loss is minimal if any.

We'll see right?

OH Zack, Yes the tips do get submerged for about 3 to 5 seconds before plane. All side transom exhaust boats (new sea ray boats included) do as well. Still should be blowing not sucking and blowing like mine LOL.

Will post results and as always thank you for your inputs all.


So another lesson learned. Anyway, Looks like a day job ahead of me but fortionetly no damage that I can tell.