Quote:
Originally Posted by hat_man
I'm going to assume that the given range of 16.4-19.8 ft/sec is plotted as a bell curve and will have the midpoint (18.1 ft/sec) as ideal and giving diminishing returns down both sides. Please correct me if I'm wrong.
The stroke on my Ranger is 94mm (3.7 inches)
My RPM's at 55 MPH are just over 2K (I'll call it 2050)
Putting those numbers through the PS formula, I get right at 21.0 ft/sec. According to the linked thread this is too high. If 18.1 ft/sec is ideal, then I would need to drop my cruising RPM's down to the 1760ish range. 16.4 ft/sec is around 1600 rpm and 19.8 ft/sec is around 1940 rpm.
If all my assumptions and math are accurate, it looks like I would need to drop to a rear axle gear of 3.55 and keep my slightly oversized tires to run 55 MPH at 1790 rpm.
Any insight or advice would be greatly appreciated.
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I don't believe that it's a purely Gaussian distribution. Some engine parameters vary linearly with rpm, while some geometrically.
So the curve would be 'skewed', just as in aerodynamics / hydrodynamics.
You need the BSFC map. That will tell you where the island of highest efficiency is, and the parameters associated with that island. If you can get the engine to that 'load', at that rpm, your 'ideal' surface-feet-per-minute with the piston rings will be achieved by default.
My grandad's 1961 Dodge D-100 got 11-mpg at 50-mph,and a top-speed of 50-mph, with bias-ply tires, three-on-tree 1:1 transmission, and 4.56:1 rear axle.
By arbitrarily swapping in a 1977 Dodge D-100, 4-speed overdrive, and 3.50 rear axle, plus all-season steel radials, she happened to go to 16-mpg, at 65-mph.
Then the aerodynamics pushed her to 21.5-mpg at 65-mph. Nearly a 'doubling' of her original mileage. And an indicated 100-mph, between White Sands, New Mexico and the missile range.
There was no 'science' involved. It happened to work out as HOT ROD Magazine implied it might. With a lot of $$$$$$$$$$$$$$ I could probably 'optimize' her performance, but that's outside the scale of the project.