[alfa] Fuel economy and rpm

[C M Smith <cmsmith@xxxxxxxxxxxxxxx>]

When BMW introduced the 528 e (the e standing for Eta, a Greek letter 
having something to do with thermodynamics I believe) they did so while 
advertising the results of their fuel economy testing which established 
that you should drive a gasoline powered car as close to wide open throttle 
as possible, at as low rpm as could be used to do the job (i e accelerate 
the car) then cruise at the desired speed with the throttle as open as you 
can without accelerating the car. In other words, the old jack rabbit start 
so pooh poohed by the so called experts was proved to be most efficient. 
Now a diesel powered car would probably be different. In fact, a turbo 
diesel is so efficient in part because the diesel is not throttled.

I have found that the BMW results are correct in real world driving. I find 
that using WOT or close to it in the lower gears and upshifting at 
relatively low rpm gets you to cruising speed quickly and with minimum fuel 
consumption.

Another interesting tidbit comes from my owning two nearly identical SAAB 
9000 Turbos, an 86 with a 2.0 liter high pressure turbo and a 97 Aero with 
a 2.3 liter version of the same engine. Vehicle weights are within 200 lbs 
of each other. Horsepower for the older car is 160 and for the newer is 
225, at approximately the same rpm. Highway fuel economy for the larger 
engine with bigger boost is about 10 to 15% better than the smaller engined 
car while City economy goes to the smaller engined car by about 5%. I 
surmise that the smaller engine car uses closer to WOT for a higher 
percentage of the time while City driving whereas the bigger engine can run 
at lower rpm (lower final drive) on the highway. One other fact is that the 
bigger engine car is much quicker, having 258 lb ft of torque peaking at 
1900 rpm, than the smaller engined car which only has 192 lb ft at 3,500 
rpm. Vast improvements in computer software engine  and boost management 
controls are also playing a part here.

So, for maximum fun and minimum fuel consumption, drive hard and don't use 
the brakes until you must....

Cheers

PS Off to see James Allison's GTV6 project, haven't touched a real GTV6 
in.... way too long now.
Michael Smith
White 1991 164L
Original owner
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