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Old 02-06-2018, 10:29   #1
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Talking engine bog/efficiency

I'm gonna throw this one out there and hopefully not start too much of a fray, Capt Chains and I are considering the pros/cons of diesel v gasoline power. Has anyone who is running gasoline noticed whether or not there is a sweet spot in relation to engine speed and fuel efficiency? Let me clarify the question, an engine likes to run where an engine likes to run.....have you noticed that a mercruiser likes to run at 12-14 more than at 8-10?

Thanks for your knowledge!
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Old 02-06-2018, 11:25   #2
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engine bog/efficiency

I have put on a spread sheet and graphed both gasoline twin 454 Mercs, a single Verado outboard and twin Yanmar Diesels, all on planing fishing boats.
Without exception the more RPM, the more the fuel burn, actually the less mileage. I had Fuel flow meters and graphed fuel flow vs speed.
There were a couple of steep slopes on the graph, like you donít want to run right just before when she gets onto plane, bow high and rolling a huge wake or full throttle, lots more fuel, little additional speed.
But surprisingly there was no sweet spot, nowhere where fuel mileage actually improved over the lower throttle setting, I was sure that right where she breaks onto plane would be a sweet spot, but it wasnít.
As I said there were places where the graph steepened where increased RPM gained very little speed, but at no time did it ever crest and come down.
However in all three cases speed just as she broke onto plane was the most pleasant, least noise, felt like you were getting somewhere without pulling the guts out of the engines.
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