Re: Help for two problems on early Z4s - BMW Z4 (E85) Message Forum

Shifting IMHO:
1. Depress clutch pedal—physically disconnects the engine from the transmission by lifting the pressure plate off the flywheel.

2. Remove foot from gas pedal—Engine speed drops based on engine compression, backpressure, computer mapping, etc. Because of 1., the transmission is taken out of the equation.

3. Disengage 1st and engage 2nd. How fast the synchros mesh relative to transmission fluid mechanics/properties can probably be measured in milliseconds. When you are in 2nd gear, you are in 2nd gear regardless of how many milliseconds it took you to get there.

4. Release clutch pedal with left foot and apply gas with right—this is where the smoothness or lack thereof occurs. The engine is at whatever rpms it wants to be at based on engine design and computer control until you release the clutch and apply gas. As the clutch friction material engages the flywheel and accelerates up to speed, if it does so smoothly there will be no bobble heading. If not, there will be.

5. Different transmission fluids can make the gear change feel different and make the synchros work a bit different, but what goes on at the other end of the stick that your hand moves can not affect the speed at which the engine drops rpms when you take your foot off the gas and disconnect the transmissison. If you're saying that the RedLine allows you to change gears faster (get out of 1st and into 2nd faster), I'd guess that could theoretically be possible when you are trying to shift as fast as humanly possible by slamming the lever from 1st to second literally as fast as you can, but again,the difference would be in fractions of a second.

6. You say that the RedLine forces the synchros to accelerate the gears up to speed more quickly than the stock fluid does. That tells me that the RedLine causes more friction/has higher shear resistance. A better lubricating oil would cause the synchros to hook up in less time (and would likely increase synchro wear.)

I don’t see how transmission fluid could have any bearing on how smooth the reengagement of the clutch is.

BMW Z4 Message Forum (E85)