Performance upgrade on Z4MC but not what you would think.

[GTWayne]

I have established that the 3.9 CWP from BMW Motorsport that is listed for the E46 M3 will fit the E86 Z4MC also and am planing to install one in the very near future as opposed to trying to tune the venerable S54 beyond its already marginally stressed original state and the diff will in effect give a performance advantage by way of working as a torque multiplier ( 8% it is claimed ).
I managed to find a virtual differential working out type chart and the figures seem quite encouraging
I have had the limiter removed and have only once seen 155+ and feel that in sacraficing the top speed maximum capacity ( which with the higher ratio diff will still be 160+ @ 8000 rpm ) to gain grunt and effectively reduce the time spent between gears to be a worth while trade off.
Does anyone have any insight pertaining to this mod.?

[Top pup]

I've no personal experience but a chap on another forum has a 4.1 diff and is very happy with it.
A quick play with a ratio calculator give a top speed of 181mph for the 3.91 and 173 for the 4.1, so I don't think the 3.91 will have any reduction on top speed and the 4.1 will only reduce it slightly.
One final thought is that first gear will become so short that it will be of very little use.

[GTWayne]

Top pup said:

...A quick play with a ratio calculator give a top speed of 181mph for the 3.91 and 173 for the 4.1...first gear will become so short that it will be of very little use...

My calcs come out @ 3.9 = 175 mph in top and 4.1 = 166 with 265/35 X 18" tyres and an 8K limit but it is the 125 mph in 4th and 154 mph in 5th using a 3.9 ratio that appeals as I am specifically trying to work on this area and with regards to the redundancy of first gear, this is no bad thing as I think that if I can train myself to pull away in second it will eliminate CDV induced bunny hop.

[physprof]

GTWayne said:

Top pup said:

I think that if I can train myself to pull away in second it will eliminate CDV induced bunny hop.

I've found that is best way or a quick dab in first then 3rd... 1 to 2 to 3 is tricky ... in many respects it acts as a deterant to needless traffic light races at 1/2 effort; either do it full bore or mature languid starts and watch the 318/320's & 1.9dti a4's blast away on their own.