Water reversion camshaft question
 

Another cam question
I'm having second thoughts about adding the captains call exhaust to my boat. Previous I had no water reversion problems with 2 years on the boat. Will adding captains call increase the possibility of getting water reversion while running the exhaust out of the prop? Or would it be more likely that info had no reversion problems before I will not have them now?

Details on the boat
.40 over 502
Ross 8.8:1 pistons
GM performance aluminum heads
Procharger
Gil/Hardin Marine headers and risers
Custom Hydraulic roller cam. .230 intake .242 exhaust @.50 114 lobe separation installed at 113 degrees
4 inch corsa captains call exhaust

I will never run the captains call above 2500 Rpms and will very rarely run it at idle.

I had some hyd. roller's spec'd by Bob M. for my blown 540's... Mine are slightly larger and the same LSA. We discussed in depth reversion issues as I am also running CMI's and CC. Bob said no issue but to keep rev's above 1000 to be safe.

My boat idles high anyways. So anything between 1000-2500 should be safe?

Well I'm here to go on the record that with your camshaft and silent choice you will have reversion GUARANTEED. So be careful or like the other guy said just never ever dle. I don't think a 1000 rpms will do it more like 1250. Your duration at .050 spec's are well beyond a silent choice application. Maybe look at switchable mufflers as an option.

Best regards

Ok if I never use it at idle and only use it in the 1500-2500 range cruising across the lake I should be ok? What's the highest rpm everyone recommends using it? Would I be safe to 3000?

My understanding of reversion is that it is more of an issue at idle, particularly low speed idle. Basically, the intake charge and exhaust charge do not have enough velocity and momentum to overcome the piston heading downward. Seems to me as the engine speed increases, airflow velocity will increase and there will be less reversion. This is basically why you will hear the lope disappear as you increase engine speed. Now at some point if you are running the exhaust flow down a Y-pipe to a single exit point back pressure will become an issue and I guess you could once again encounter some exhaust reversion, but it's hard to say where that point will be. I have heard that there is actually a negative pressure venturi effect produced thru the prop when the boat is in motion, but I have no way to verify that.

FWIW, I was running a 454 with a Comp Came Xtreme Marine flat tappet that ran 224* / 236* @ 0.050, 112 LSA with stock Merc manifolds and Silent Choice. I ran it with the exhaust closed at idle, as well as at cruise all the way up to about 3500 RPM with no sign of reversion. However, it is somewhat of an apples to oranges comparison to your setup. You have 50 more cubes, and longer duration, although at a 114* LSA.

Might be a good idea to talk to one of the camshaft pro's, like Bob M. at Marine Kinetics.

duplicate post - seems to be some flakiness with the new format...

Hey Budman,

That cam you mentioned is the same one I was talking about in my post. So, you were able to run it okay? How did you like it?

Sorry for the hi-jack! However, your answer might be beneficial to the OP too.

Seemed to run fine - no reversion at least. Of course, as they say, YMMV. Don't know exactly how much power it made - boat was seeing about 61 MPH on a good day (88 Baja Force 235) with Performer RPM, Q-jet, GM 820 casting oval port heads with bowl porting and 2.19 intake valves, etc. Might have run better with a Holley, but then again, might not have.

How can you tell if you're getting reversion?

If it's bad enough, you will start seeing milky oil. Sometimes it can be a little bit more "sneaky", and just get enough to start pitting exhaust valves, seats and cylinder walls. Or you can end up with the worst possible scenario and have cold lake water getting sucked back onto a hot exhaust valve and shatter the valve head. That is one reason they recommend inconel valves in a performance marine application.

Sometimes you can remove a manifold or riser and see water accumulating there. I have heard people say that they can look up an exhaust pipe and actually see drops of water migrating back up the pipes!