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I can't say why that chart ends at 3200rpm, but I can talk a little about why BSFC peaks where it does.
BSFC is better at higher loads, because the ratio of work to parasitic losses are better. At a quarter load, your friction and accessory losses will be nearly the same as they are at WOT - things like friction, turning the alternator, water and oil pumps. Additionally, as you open the throttle plate, there is less vacuum generated, which is itself a loss. So, at higher loads, a larger percent of the fuel burned does something useful (move the car) because many of the losses in an engine are relatively fixed.
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.
As for RPM, there are a few factors that come mind.
Friction goes up exponentially with RPM. So, you'd think that the lowest RPM possible would result in the best BSFC.
When spark ignites the contents of a cylinder, it takes some time for flame to propagate and for cylinder pressures to build. Nearly all engines fire the spark plug and ignite the contents of the cylinder while the piston is still rising on the compression stroke, meaning that for a short time, part of the energy from combustion is actually trying to push the engine backwards and doing "negative" work. If your piston is moving very slowly, you can fire the spark plug later (more advanced) and do less negative work.
A piston moving too slowly, however, will have hot combustion gases in the chamber for a longer period of time, allowing for a greater amount of energy to be lost through the cylinder walls and piston head as heat, rather than doing useful work.
There are a lot of other factors which have to do with the geometry of the engine and rate the piston is moving relative to flame front. There are also factors of the quality of combustion - e.g. an engine with a valvetrain designed for high volumes of air and high RPM operation may have poor mixing and poor combustion quality at low RPM.
There are almost certainly other factors I'm forgetting.
The original engine in my car peaked around 2000RPM. Many seem to peak in the 2500-3000rpm range. It really depends on the size and geometry of the engine.
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