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Originally Posted by Ecky
I can't say why that chart ends at 3200rpm, but I can talk a little about why BSFC peaks where it does. ...
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Great stuff Ecky -- thank you for sharing! Some of this is new to me and I'm slowly absorbing it. But I'm getting the gist and I appreciate the level of detail you shared.
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In some engines, BSFC tends to drop off above ~75-85% load. I imagine this is commonly because of either 1) many engines tend to run rich at high load, because the extra fuel cools the combustion chamber which promotes longevity and reduces the chance of knock, and 2) many engines run more retarded (later) ignition timing, to reduce cylinder pressure and prevent knock, at the expense of sending some of the combustion energy unused out through the exhaust.
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This is particularly of interest to me. I was reading a bit about the M20A-FKS engine in my car, and I came across an ECU-tuning site that did some analysis on it. In their words:
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Engine runs a paltry... 10 Degrees of ignition advance at peak horsepower and 5* at peak torque, and manages 168HP. Not to mention runs rich (like most toyota engines) at full throttle.
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So if running rich and retarding timing under heavy load tends to cause a drop-off in BSFC, that doesn't seem to mesh with the chart above that shows the highest efficiency between ~76%-95% load. Could be the chart is misleading, could be the ECU-tuning people are mistaken, or could be something else entirely going on. I'm a little out of my element here, just trying to connect the dots.