Quote:
Originally Posted by JRMichler
I got another data point. This time I went up the hill in 4th gear, setting the throttle so as to bleed off the same amount speed as in the 5th gear test. Results:
Gear RPM MAP MPG OAT
4th 2470 10.7 15.1 32 deg F
I estimate the best efficiency RPM at about 2800. Fourth gear puts the engine closer to its best efficiency point than 5th gear, but at an inefficient MAP. It appears that those two effects cancel each other out at this power setting and speed.
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That's pretty interesting! Not what I would've expected.
Though, from the typical BSFC chart we can see that at lower rpms and maximum load the BSFC is quite high, presumably because of high cooling losses. I think it might be interesting to try going up that hill a bit faster if your engine has more torque at a higher speed, the maximum load BSFC might improve quite a bit and you might see better results despite going faster. For example I can't climb some "mountain roads" in 5th at 1400rpm but if I downshift, accelerate, and then floor the pedal at 1600-1700rpm then the engine purrs along nicely and doesn't lose speed. I imagine the mpgs are better that way.