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      12-15-2011, 02:14 PM   #30
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Quote:
Originally Posted by bruce.augenstein@comcast. View Post
This is mostly incorrect. In point of fact higher gearing (therefore lowering rpm at cruise) nets you better mileage, because there is less overall friction with reduced rpm, and more importantly, pumping losses are reduced. Pumping losses are associated with manifold vacuum (meaning when a cylinder is firing, power is lost because it must drive another piston down on the intake stroke against a vaccuum), and these losses increase when an engine is making more power, because you use less throttle to maintain a given speed, resulting in higher intake vacuum. When you use a taller gear, rpm is reduced, power is reduced, and you have to use more throttle to maintain a given speed. Therefore pumping losses are reduced.

An M3 with Vette gearing (say, 1500-1600 rpm at cruise) would be a pig out on the highway - but it would get a good deal better mileage. BMW has been saying since the eighties that to drive for best mileage, use max throttle and minimum rpm through the gears. The M3 would get best highway mileage if you had to use full throttle just to keep it going - but of course it would be more or less undriveable because you'd have to downshift for every hill or any puff of headwind.



They said the hell with mileage, and went for a strong and responsive engine at cruise.

Bruce
I wouldn't say its a strong and responsive engine at cruise. You can hit the throttle in 7th at 75 mph on hte freeway and you barely accelerate and thus have to downshift for any signifigant forward move, therefore their choice in ratio for 7 leaves much to desired. Kind of the worst of both worlds. Not tall enough for decent gas savings but not short enough to be useable
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