carmudgeons.com - View Single Post - Speaking of torque (i.e. no wonder the E90/92 M3 is so fast...)

John V · 04-08-2009, 06:39 PM

Torque at the hub. Once you take into account the diameter of the tire, you're talking thrust (i.e. force).

Fuel economy is tied to throttle position but not nearly so much as it is tied to RPM for gasoline engines. Anyone who has ever delved into the programming for a gas FI system knows this, because injector firing rate is ONLY determined RPM and duty cycle is tied to airflow. The trim functions here are throttle position, intake air temperature, coolant temperature, and some other minor contributors. The other factor here is pumping loss - which is lowest when the throttle is wide open. The most efficient way that we know of to operate a gas engine is to have no throttle and manage the speed with valve lift and duration (aka Valvetronic for BMW)... but it's not necessarily practical for all applications (read: it's expensive to implement and not easy to build).

In diesel engines, fuel economy is tied very closely to "throttle" position because there is no throttle - you're directly controlling the pulsewidth of the injectors.

The bottom line is the BMW is damned fast - especially for how heavy it is - but once first gear is done, the BMW is going to get smoked by the vette.

04-08-2009, 06:39 PM	#13
John V No more BMWs Join Date: Apr 2005 Carmudgeonly Ride: Ram, MS3, CX-5, RX-8 Location: Glenwood, MD Posts: 14,753	Torque at the hub. Once you take into account the diameter of the tire, you're talking thrust (i.e. force). Fuel economy is tied to throttle position but not nearly so much as it is tied to RPM for gasoline engines. Anyone who has ever delved into the programming for a gas FI system knows this, because injector firing rate is ONLY determined RPM and duty cycle is tied to airflow. The trim functions here are throttle position, intake air temperature, coolant temperature, and some other minor contributors. The other factor here is pumping loss - which is lowest when the throttle is wide open. The most efficient way that we know of to operate a gas engine is to have no throttle and manage the speed with valve lift and duration (aka Valvetronic for BMW)... but it's not necessarily practical for all applications (read: it's expensive to implement and not easy to build). In diesel engines, fuel economy is tied very closely to "throttle" position because there is no throttle - you're directly controlling the pulsewidth of the injectors. The bottom line is the BMW is damned fast - especially for how heavy it is - but once first gear is done, the BMW is going to get smoked by the vette.