Resent...list owner said too long
mayf
Duke, all....
The issue of better fuel mileage seems to periodically pop up, which is
not a bad thing. The Sunbeam's drag coefficient is published at 0.45,
which amounts to it being a rolling brick. At a generally constant
speed, drag, which directly translates to hp and tq needed to move the
car down the road, you can generally estimate the power needs based on
the summation of the drag forces. At least to the point where you can
convince yourselves that a change in driving habits is needed for better
mileage, lol. In this case, we'll run some numbers assuming that
getting to speed is done in a gentle fashion and not in some screaming
kamakazi banzai manner. We'll start here:
frontal area is approximately 17.9 sq ft
Cd is 0.45
We won't use the mechanical or rolling resistance losses since they will
be fairly constant is the range of highway speeds we'll look at
Drag = 1/2 * rho * V * V * A * Cd
rho = air density, in slugs.. and we will use a STP value of 0.0023718
V = car speed in ft/sec for th etwo cases, we will use 78 mph =
114.4 ft/sec and 70 mph = 102.7 ft /sec.
Plugging in Drag (78) = 0.5 * 0.0023718 * 114.4 * 114.4 * 17.9 * 0.45
= 125.02 lbs
Drag (70) = 0.5 * 0.0023718 * 102.7 * 102.7 * 17.9 *
0.45 = 100.8 lbs
HP(78) = Drag (78) * 114.4 / 550 = 26.004 hp
HP(78) = Drag (70) * 102.7 / 550 = 18.8 hp
As mentioed, mechanical losses and rolling resistance adds to the power
required. As can bee seen, the power difference is significant. And
that power requires fuel in direct proportion. It all points to the old
adage of "Speed costs, how fast do you want to go?" The best fuel
mileage can be had by simply going slower. In this case a drop from 78
mph to 70 will increase your mileage quite a bit. But, it also depends
on the other two facors as well. If you have happy feet then it is
going to get worse mileage, happy feet being the ones that are
constantly twiddling with the gas pedal: everytime you do that, it pumps
raw fuel into the carb through the accelerator pump.
So, my suggestions?
First, slow down
Second, get larger skinnier tires as fat ones have a lot more rolling
resistance. In fact maybe some old bias belted tires that can be pumped
up hard. High tire pressures reduce rolling resistance in those tires.
Also get skinnier tires on the front and keep them pumped up.
Third, get an AFR meter, not even a fancy wideband. Just one of those
that have led lights and show rich or lean. Then tune the car for as
lean or as close to stoic as you can. Then drive it with the AFR meter
lean side led lit.
Fourth, maybe get a old timey vacuum meter and put it on the dash as
well. A high vacuum means best economy.
Fifth, keep it tuned. Get an electronic dizzy or innerds like Pertronic.
I hope this may help. But, as in all things, your milage may vary..
mayf
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