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TRACK REPORT
Re the above. I don't think keeping the diff in would help much. The subframe is much larger and unwieldy than your picture, it's also moderately heavy depending on how much stuff you leave on (I took brakes and rotors off and it's just a big clunky piece). It's not a bad job, but I think if you do anything with the diff bushings just leave alone or replace with stock if they are worn.
Did one day event at Road Atlanta today with JZilla. Good guys, have run many times with them, maybe not the most organized, but they run a good program and they are flexible. For example the driver groups are on the car stickers rather than the armbands and I wanted to have my friend drive my car and I drive his. Was no sweat to get extra stickers for the switch, to put on each car.
So, the question that everyone is wondering (are you?) is what's the difference on the track with solid sub-frame bushings. First a bit about me so that you'll understand where the opinion is coming from. I have been tracking here in the Southeast for the past 10 years. Have run tracks from Road America to Roebling and a whole bunch in between. I've never raced, just DE, no time trials either. At events where I'm timed I'm never the fastest guy (okay, once at Ron Fellows Corvette Driving school in NV), but I'd say my strong point is I'm really consistent and able to read/feel what a car is doing really well. So, bottom line I'm not an "expert" or racer but think I have a pretty good feel for cars.
So, the "Do the subframe bushings make a difference on the track" question. in my car, NO. Now my baseline was bushings with 144,000 miles on them and many thousands of track miles, but they weren't really worn at all, and the rubber was still in good shape (mild climate down here). Some people say the car feels more planted, I can't detect any difference. Keep in mind the rest of the suspension in back is still on rubber bushings, so that's why it's not any noisier or more "planted." However, that might be why it also doesn't feel like a go-kart. I'm not disappointed, the car still is fantastic.
Gabe (Dogbone) and I were exchanging a few texts today and what other car can you drive to the track, flog like maniac all day long, then peel all the stickers, numbers and other stuff off, put the suspension to something more streetable and then set the cruise control, AC and turn on the stereo on the way home? As an all-around car it's unparalleled. I'm at 144K miles and no oil consumption, engine still makes good power and the handling for a 4-door sedan is pretty amazing.
What would I do differently. Unless you are having some sort of diff issue related to the stock bushings, I would suggest you leave those alone and be happy. If they need replacing and you still want a quiet street car, then replace the diff bushings with OEM BMW bushings.
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