Offshoreonly.com - Sticky?? Cam overlap degrees for marine cam selection

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You're crossing over the theories.

1. CFM through the engine at % VE.

2. CFM flow through exhaust system to ensure zero to very low restriction. 2.2cfm/hp, is a test of a flowbench, same as a cylinder head cfm test.

You need 2.2cfm of exhaust flow per HP for very low backpressure.

You want 500HP. 2.2 x 500 = 1100 cfm of exhaust system flow.

For clarity...that's very low restriction exh flow, where any more flow would not make much of a benefit. You can still run the engine with exh backpressure, but the pumping losses soon catch up.

I don't think you'll get 500hp of exhaust+ raw water through the Y pipe.

Thanks for walking me through this stuff. Looks like adding another hole thru the transom is pretty much unavoidable if I want to hit the 500hp number.

Quote:

Originally Posted by Tartilla (Post 4934208)

My last paragraph of my last.post speaks to the theory of a vacuum, includong the math. A 5" prop shaft bullet moving though the water at 60mph displaces 61 cubic feet of water in 1 min. You're pumping about 600cfm of air. No vacuum.

.

Not sure your analysis is accurate on this. I don't think there is any consideration for gas vs liquid (exhaust vs lake water) regarding compressibility. I believe there is an error in your math. That bullet would displace about 719 cfm, so indeed there would be space for the exhaust to go.

Another twiist (no pun intended) is that some of the Merc props have twisted or pitched vanes from the hub to prop barrel that also act as a 'prop' drawing exhaust gasses out.

Quote:

Originally Posted by Trash (Post 4934332)

Trash, thank you for that. I made the calc for dia vs area. Using the phone and going back and forth to the calculator...I must have missed that.

5" bullet (2.5x2.5x 3.1416) =19.6sq"

19.6sq" x 12" = 235.62cubic inches per lineal ft travel.

60mph= 2mile per min = 5280ft/min.

235.62x 5280ft= 1244073 cubic inches displacement/min.

1244073 ÷ 1728(cu Inches in cu ft) = 720cubic ft./min.

I have a stainless prop with the turbin type vanes. No hub system...just straight up.

Had a look at the Bravo 1 upper in the exhaust transfer chambers. It's pretty tight. Extremely restrictive. With the raw warer it would be very turbulent, and just choke off flow past a certain flow.