E46 330i 3.38 Diff Swap - An Objective Review

[flyitlikearental]

Great review:clapper:

I do feel that the ‘poor man’s supercharger’ moniker is an exaggeration. While the difference is noticeable and a lot more exciting when driven hard, the car is not significantly faster.

On this matter.....
We have tested 2 different types of cars before and after changing diff ratios, and in both cases, the shorter diff made absolutely NO DIFFERENCE in all out acceleration.

What would happen is that the car with the short diff would edge ahead by a meter or 2 initially, but then have to shift earlier. As soon as the car with the short ratio shifts, the other car which is still in the same gear catches up or would sometimes even pass. Then the tall ratio car would have to change gear, and falls back a little by the same amount as initially. But on a rolling run from 20 - 220 there is nothing in it.

It only feels faster, because it has the effect of being in a higher gear all the time.
Because it "feels" faster, the car is more fun to drive.

The only benefit with the shorter gearing was when both cars are at 60 in 4th gear for instance..... Accelerating from there in the same gear, the car with the shorter diff would out accelerate the one with the taller ratio.

And there is our resident mad scientist's scientific approach to problems raise:

As with any car mod, you need to measure before and after, to get the whole picture. This is very interesting Chris, as I actually thought that the lower gearing should pip the other car to top speed, but seeing as you are swopping cogs more often that does make sense. I suppose where this might give the shorter gearing the edge will be on something like a de-limited E46 M3, where there isn't enough power available to overcome drag in 6th gear? Revving the motor higher in 6th might make slightly better use of the torque available in that scenario, and could possibly result in a slightly higher top speed as you are effectively putting down more torque at the wheels due to the shorter gearing?

I also suppose this will be better for track day use, just be careful that you don't end up having to change gears mid corner, as that can also cause issues of its own.

 

[Fordkoppie]

A few rare occasions there might be a small increase in acceleration where the engine sports a very narrow torque band.
Think old N/A F1 engines, or 2 stroke race engines, or something like a Vtec Honda, Rotary, etc..... A peaky engine in other words, and usually a low torque engine.

But on an engine with such a linear torque curve as the M54 (the other test mule was an M60b40), going up or down in ratio by less than 15% have almost 0 effect on all out acceleration.

Again - I want to stipulate that this is when going all out (meaning both cars are in their optimum gear, all the time). But if you are sitting in top gear at low RPM, and simply put foot without downshifting, the car with the short ratio will always win