Does Jon Lauter have any good dyno data on BSFC for his marine
13Bs?

BSFC is between .51 and .52 from 3000 rpm to 6250 rpm, all WOT. We don't have a full bsfc map at this time. Carbureted engine, optimized for performance, not economy.
My .02:

1. BSFC numbers by themselves are meaningless. A full map (BSFC vs. speed and load) is what you really need to determine the operating economy in a particular application. Then, there is also the influence of the engine's physical attributes on the overall vehicle.

2. If an automotive derived piston engine can really achieve a bsfc of .42 (which I doubt), I'd bet its at 2000 RPM, nearly WOT. When BSFC numbers are quoted "by themselves," they are ALWAYS at the best operating point. I've no idea about a Lycoming, but an engine designed for low speed use will probably do better than one designed for high speed, quantity of valves notwithstanding.

3. Without a lot of data to back me up, I'd guess that the 13B BSFC map is "flatter" than a piston engine map. So, at best efficiency, the rotary may be 10 or 15% behind a piston engine at its best efficiency, but elsewhere on the map the gap will narrow considerably. This is because the rotary has lower friction (which increases with the square of RPM, and because the rotary has lower pumping losses.

4. The influence of the engine's other physical attributes on the vehicle also makes direct comparisons difficult. The low weight of the rotary makes boats very much more efficient to run. We have one customer with a twin engine Chris Craft 25. He replaced a pair of 175hp piston engines with our 175hp rotaries, and gained 8 kts (!) and perhaps 20% in fuel economy. He also added a full tuna tower at the same time, so the aerodynamic drag increased a bit, and the full weight savings wasn't realized. In a car, the weight influence is more significant for acceleration. In a plane, I'd guess that the cooling system difficulties offset a lot of the weight advantage.

5. Class 8 truck diesel engines are pushing below the .30 bsfc line now. I wonder how the new direct injected auto diesels are doing. I drove one (Citroen) for 3000 miles a few weeks ago, very impressed with the performance and fuel economy (at 140km/hr).

Jon

Well there you have it Bill. 0.52 BSFC at Full power no less.
I have no doubt Tracy's is seeing .47 at low power  and altitude.
When we get this variable pitch prop we will do even better.

Paul Lamar

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