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assuming best-case of 7.5 compared to 10, that's a 33.3% difference between the two.... that's not a very tight range.
BSFC and stroke have no relation to each other whatsoever. good example is an engine that can be relatively tame from the manufacturer but also has a large aftermarket. small-block chevrolet engines are a great example. the chevrolet 350 has been around for ages in various forms, has a 3.48" stroke. from the factory, let's think about a more recent version like the Vortecs, built for low-RPM torque, decent fuel economy. now swap out the heads, cam and intake for something a bit more radical. stroke doesn't change at all, yet BSFC certainly moves around when the powerband does. now instead of torque peaking at probably ~3500 RPM, now it's ~4500 and fuel economy suffers everywhere.
now let's get even more ridiculous for sake of example: let's swap the block entirely for a SBC 400, but use the 350's 3.48" crank. now we're at ~377 cubic inches IIRC. now let's bore it some more and we end up at 383 cubic inches. assuming the same intake/cam/heads, the powerband will be a bit lower in the RPM compared to the 350, so economy may actually improve with more displacement. so let's up the ante even more. throw on some purpose built racing heads/cam/intake and move our torque to peak at like 5500 RPM, with a HP peak around 7000. not unheard of, just a pain to drive on the street. now BSFC in any range where you would likely drive on the street is so bad that you'll probably see single-digit MPGs on a good day.
long story short: this is not something you can correlate. so much more is so much more important than stroke.
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