In Reply to: 540i Chip and differential ratio change posted by Ron M on September 19, 1999 at 17:58:24:
: I have a 1994 540 auto and am considering two performance mods - a JC chip and a lower differential gear ratio. I live at high elevation in Denver and am curious if the JC chip really does much up here. Also on the lower diff ratio I have the following questions:
: 1) Is the speedometer affected? If so, can some compensation be made?
: 2) Are gears available or do you have to go with a whole new differential?
: 3) Would the auto trans be affected?
: 4) Is it worth it? Because of the power loss here I'm looking for acceleration improvement but would be open to other ideas (a $7500 supercharger is not an option right now).
: Any experience/knowledge would be greatly appreciated. Thanks.
Remapping ignition and injector timing should not be affected by air density as air at higher elevations has the same percentage of oxygen per unit weight (mass) as air at sea level. Oxygen molecules are spread out more at higher elevations than at sea level, so a larger volume of air is required to obtain the same unit weight, however. Fortunately, the BMW system uses a mass flow sensor for the DME air intake sensor and does not rely on the volume of air as an input parameter for the DME. At higher altitudes, the volume of air that can flow through the intake plumbing is the same as at sea level. Since a given unit of volume contains fewer oxygen molecules at higher altitudes, the power output is lower, as the DME meters fuel flow proportionally to the (lower) mass of oxygen molecules flowing into the engine in a given volume of air.
A lower differential ratio would be a good and less expensive method to improve low to mid RPM power at the wheels. Of course top speed would be compromised. Since the speedometer sensor is external to the differential at the output shaft, speedometer readout should remain accurate as long as you use the same size tires. There should be no effect on the transmission, although the engine would be turning at a higher RPM to achieve the same road speed. Actually, the '94 540iA, especially the earlier models had a lower differential ratio than the later 540's. I'm not sure whether the factory has ring and pinion sets available to customers for different ratios. I have seen ads for gear sets in different ratios at Dinan and other tuners (Korman(?)). It would certainly be easier to simply change out the entire differential as opposed to taking it apart and swapping gears. It is certainly possible to change the gear set in the differential housing, but it isn't something that should be done by a novice or shade tree mechanic.