View Single Post
      06-14-2021, 11:20 PM   #73
proboner
Private First Class
40
Rep
161
Posts

Drives: 2003 Z4 3.0 6-speed
Join Date: Feb 2017
Location: Mountain View, CA

iTrader: (0)

And I finished her up today.

First thing I noticed, much more abrupt engagement when reversing. Honestly I was initially worried that maybe the lash on the diff was set wrong or something was loose, as the clutch engagement in reverse was so abrupt that it felt more like an on off switch. After backing her into the street and putting the car in 1st, I was able to feather the clutch and drive without an harsh engagement, so that was a relief! Definitely the engagement through the drivetrain is much more direct. My guess would be solid rubber diff bushing is causing this, which I would say isn't something I dislike, but more of a worry initially that something was wrong.

Second thing I noticed, no more exhaust rattle! Great success! So happy to be rid of that damn thing! Although now that I've had the exhaust all the way out and seen that horrendous necking down to like 1" ID in the mid-pipe, I may have convinced myself to go aftermarket with the whole thing.

After driving around for a bit and getting her up to temp, I decided to get on it. Did a WOT pull through part of 2nd and shifted early to 3rd and went WOT a bit more. I was really surprised with how different it felt. It feels... well... planted. The rear end just hunkered down and went. It was very different from the old diff, which almost felt like it skated and searched for grip. This rear end just grabs hold and takes the traction for it's own. It also feels surprisingly smoother. Under power there's no vibration at all, just smooth power delivery.

There is a bit of annoyance though. When maneuvering at slow speed, like when trying to creep up into my driveway or switching between 1st and reverse when changing directions, there's a bit of engagement weirdness. Almost like it's loaded up when it shouldn't be and pressing in the clutch releases the tension. I know that a lot of people talk about banging going into gear, and I'm definitely not getting that. But I do think that it could be the same potential culprit, which is driveshaft center support preload. I knew there needed to be some and I did push the center support towards the rear of the car to introduce that preload, but now I'm wondering if I should have maybe preloaded it in the opposite direction? I'll have to do some more driving and research to decide if it's worth digging back in there to change it.

Sorry no pictures this time around.

edit: How could I forget! The gearing change. It definitely feels quicker, especially at lower rpms off boost and especially compared to how it felt before the diff swap but with the new taller tires. Let's talk through the calculations really quickly shall we?

Stock rear tire- 255/35/18 aka 25" diameter- with the stock 3.07 final drive ratio this results in:
80mph cruise- 2880rpm
top speed- 194mph
2nd gear 7000rpm shift- 71mph
513M rear tire- 275/35/18 aka 25.6" diameter- with the stock 3.07 final drive ratio this results in:
80mph cruise- 2820rpm
top speed- 199mph
2nd gear 7000rpm shift- 72mph
Volk rear tire- 265/35/19 aka 26.3" diameter- with stock 3.07 final drive ratio this results in:
80mph cruise- 2750rpm
top speed- 204mph
2nd gear 7000rpm shift- 74mph

All in all not a huge difference, let's look at the 3.46 final drive

Stock rear tire- 255/35/18 aka 25" diameter- with the LSD 3.46 final drive ratio this results in:
80mph cruise- 3250rpm
top speed- 173mph
2nd gear 7000rpm shift- 63mph
513M rear tire- 275/35/18 aka 25.6" diameter- with the LSD 3.46 final drive ratio this results in:
80mph cruise- 3150rpm
top speed- 177mph
2nd gear 7000rpm shift- 64mph
Volk rear tire- 265/35/19 aka 26.3" diameter- with the LSD 3.46 final drive ratio this results in:
80mph cruise- 3090rpm
top speed- 182mph
2nd gear 7000rpm shift- 66mph

The 3.46 final drive obviously makes a much larger difference, which on the stock tire diameter really cuts the legs off. With the 1.3" taller tires on the Volks however, things start looking much closer to the stock tire diameter on stock diff, just a little bit quicker. We'll see how it actually works out in the real world, but I really like that it allows me to increase acceleration despite these much taller tires but not cut too much into my 80mph cruise rpm.

Last edited by proboner; 06-15-2021 at 12:07 AM..
Appreciate 0