Re: I/O Swap... Omc 800 to alpha 1.
Well, you came to the right place.
First of all, when you start talking HP in a boat, most people are talking prop HP, your existing motor is most likely rated in prop HP. So if you're going to compare you're dyno software numbers to boat's you have to subtract for friction losses of the drive (I think like 5-8% or something in that nature).
Regarding exhaust:
Yes, it will actually suck water in the exhaust valve and hydro lock you're motor.
Without getting too much into it.
Intake duration: How long you're intake valve is open (degrees)
Exhaust duration: How long you're exhaust valve is open (degrees)
Over-lap: How long both you're intake and exhaust valves are open
Strokes:
Compression: Both valves closed, piston moving up
Power: Both Valves closed, piston moving down
Exhaust: Exhaust valve open, piston moving up
Intake: Intake valve open, piston moving down
We all know this, but, here is something to think about.
At high RPMS, high duration and over-lap is good, it means the valves are open longer, which means more net time for the gasses to flow in and out of the combustion chamber
BUT (and this is a huge but)
High duration and overlap at lower RPMS is BAD, It reduces volumetric efficiency (Torque) and in fact it creates vacuum pulses through the exhaust which WILL suck water into the engine.
On a marine grade cam, the piston goes up on exhaust, and the valve closes just as the piston reaches TDC. Then the piston changes direction, and the intake valve opens, and draws in the fuel/air.
On a Hotter cam, the piston goes up on exhaust, but the exhaust valve stays open, even after the piston reaches TDC and starts to turn around. in fact BOTH valves are open (overlap) for a period of time. So as the piston goes down, it creates a vacuum. If you're exhaust valve is still open when this vacuum is created, the vacuum "pulse" will travel through you're entire exhaust system.
See. At high rpms, you're port velocities are higher, the fast moving gases suck the rest of the exhaust out of the combustion chamber, that's why hotter cams make more power.
But at lower RPMS, you're port velocity's are much much lower, the sucking and inertia effects just aren't there.
Now, I know my cam theory, but I don't have nearly as much experience as a lot of the guys in this forum. Bond-o has over 22,000 posts, I'm sure he knows a thing or two about cams. When he mentioned dry pipes, he was inferring that the cam you picked runs the risk of sucking up water, and to be safe, you might need to run dry exhaust. (it's an oxy-moron, dry exhaust is really dangerous, unless you leave you're motor un-covered like a jet boat)
So, think of it this way:
Stock marine cams are already pretty close to the limit of what they can run for duration and over-lap, there really is no wiggle room there.
There are a FEW hotter cams out there that may work, but you're always increasing the risk of water ingestion, which can kill a motor pretty quick.
that is the cam I've been talking about running and the one I used with the dyno software. 