Originally Posted by jamesqf
Have I missed something here? All the BSFC graphs I see seem to show that the engine is less efficient at low RPM & higher load.
High load = good. Low load = bad.
You're rarely going to be working in the optimal range/load -- you don't need the 50+ hp being generated to cruise along at normal speeds -- so the next best thing you can do is to run at the lowest RPM that will give you a decent load (~80%, above which most engines tend to run rich).
The more energy the engine can create per stroke (without running rich), the more efficient it is going to be. If you're making 10hp @ 1500 rpm, you're a lot more efficient than making it at 3000.