Automatic transmission question

[Arde]

A naive question on automatic transmissions, does the transmission needs to be reconfigured when the differential ratio is changed? For example a higher ratio may be desired to get better acceleration but if the transmission is not aware of the car speed because the speedo pickup is at the transmission then the shift points will occur at lower speeds kind of negating the benefit of the higher ratio. The car will shift quicker to a lower rpm region where the engine delivers less power. What gives?

 

[lloyd]

A naive question on automatic transmissions, does the transmission needs to be reconfigured when the differential ratio is changed? For example a higher ratio may be desired to get better acceleration but if the transmission is not aware of the car speed because the speedo pickup is at the transmission then the shift points will occur at lower speeds kind of negating the benefit of the higher ratio. The car will shift quicker to a lower rpm region where the engine delivers less power.

The short answer is it depends on the ratio change. In addition to weight, this is a primary reason for limited interest in hot rodding automatic transmission equipped E9s. (Changing engine performance with wilder camshafts and higher operating ranges and/or turbo charging could also justify significant auto transmission changes, including torque converters with different stall speeds and beefier internal components.)

Until the advent of electronically (computer) controlled automatic transmissions, a combination of valve body design, regulator and throttle position typically controlled shift points. The aftermarket offered limited help with “shift kits.” A common but hardly foolproof “work-around" was adjusting the throttle position cable. Other more involve modifications included valve body tweaks.

Since there is a certain amount of slippage in the standard E9 auto transmissions, “slight” axle ratio deviations, tire heights and moderate engine mods are generally not an insurmountable problem. ("Insurmountable" and "problem" are subjective opinions.")


http://www.e9-driven.com/Public/Library/BMW-E9-Manual/pages/en/24000000.html#refertoc