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Hi All,
Nova_nz, the diameter of the runner ideally should vary with the speed of the engine. A smaller diameter for lower speeds avoids the turbulence you speak of. For ecomodding, very long smaller diameter intake pipes are better.
ConnClark, while diesels do not have throttles, many hypermilers take advantage of tall gearing and run nearly full throttle at low speeds. So reduction in intake air drag on gasoline engines can be helpful. Some of the members on this board have changed out the final drive gears to do this.
The Prius with its very wide transmission ratio range, and computer controlled throttle can run at very low RPM's at a contiuous cruise, with the throttle nearly wide open. My experience is the Prius will run 1280 rpm at 53 mph, on level ground and no wind, and temps above 70 F. 4 speed automatic transmission cars typically have to run at twice this RPM at that speed.
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