Final Drive and SIR

Back again. I have been doing track days at Summit Point in my IT s/r 325is. I am running out of gear about 2/3rd of the way down the straight. 7k redline surge in 5th gear. No problems anywhere else. I have 3.73s in now and I believe my back up that came with the car is a 4.10. The car ran in the NE region and I am in the Mid-Atlantic region. I'm headed to VIR at the end of the month and have, without thinking about it, pulled the rear out already. I was told that car will be even worse at VIR. Are there any suggestions on a final drive for the Mid-Atlantic region and where I can get a relatively inexpensive one quick? I'm willing to trade one of my drives as well. I plan on running mostly Summit and VIR and then adding Beaver Run, Pocono, and perhaps eventually Mid Ohio, Road Atlanta, and the Glen. Which drive should I keep?

Also, the guy I bought the car from told me that he stop running the car in ITR, the faster class, because he had to use the SIR and it made the car uncompetitive. But I just read a thread that says that the SIR is required in ITS not ITR. I don't want to run the SIR because it's a PiTA. Is this car competitive in ITR and what other mods can I or should I make to run ITR? Right now the engine has been refreshed and decked, chip, intake, exhaust, coil-overs, brake bias, and that's about it.

It can definitely be competitive in ITR, and yes, you're right, the restrictor is required for ITS, not for ITR. Your guy probably meant that he gave up on ITS, not on ITR.

In ITR you basically run the same car with no restrictor, less weight, and if you like, bigger wheels (up to 17x8.5 instead of 16x7).

And sorry, I'm in CA, can't help you on the diff ratio. I use a 3.73 in my car (same gears) and out here I never run out of 5th.

I am surprised you are running out of gear with a 3.73 and 7k in avialable revs. Are you running 15" wheels?

I've never driven Summit, but the front straight doesn't appear to be longer than VIR's back straight. Top speed there is about 128-130 for a good R or S car, maybe a bit more.

Wheels are cheap gearing. You can run 17s" in R right? You might want to try that first, although again, very surprised you are out of gear at Summit with a 3.73. I run a 3.45 in my car, with 15" wheels and 225/45 R6s. With a .82 or something 5th I'm not even close.

Of course, I jsut remembered -- you guys have a 1:1 Fifth gear don't you?

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Originally Posted by JeffYoung

Of course, I jsut remembered -- you guys have a 1:1 Fifth gear don't you?

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You bet. With a 245/40-17 like I use, that's good for 138 mph with a 3.73. The OP must have pretty small tires.

EDIT: Perhaps the same size as you Jeff. a 225/45-15 and a 3.73 is good for only 128 mph.

So, basically I need to borrow a set of 17's and see if they fit, commit to ITR (too bad I really liked the ITS guys I've met and ITR seems slim around here), and adjust accordingly. Why am I having a problem that the no one else with identical car is having? I wonder if the owner has the diffs mixed up as well. The diff I pulled has 3.73 written on it, but I didn't do the drive shaft/wheel revolution check before I pulled it.

Not necessarily.

You can stay in S (there are more cars in S right now, at least in the SEDiv) with the 15s and the SIR. What tires are on the car? 45s? If you go to 50s you will get 200-300 rpm right there probably.

Or just call the guys at Bimmerworld and ask them what they did when they ran Summit, VIR, etc. The Z and RX7 guys change rear ends frequently -- we have it down to 30 minutes or so on Ron's 260z -- to match a particular track.

I'm still surprised that a 3.73 is too short, but that 1:1 is nice in some respects and not so nice in others.

Tires are your easiest change. Hoosier sells 225/45-15, 225/50-15, and 225/55-15. The 55 series would be the same diameter as the 245/40-17 I run, meaning you'd get another 10 mph with your 3.73. It will fit.

Well, I did the spin check and found out that the diffs are mis-labled. I had 4.10s in and my back up is the 3.73. That should solve my problems. But I learned a lot. I'm running 225/50 R16's right now. What kind of 16" tire would put me on par with ITR on 17's?

I can't imagine changing this rear in 30 minutes. The Bentley manual says to remove the A-Arms before pulling the carrier with the diff. When I tried to do that, the bolt backs out against the diff. which won't allow you to remove the arms and the diff is tucked in front of the tank so it doesn't look like you can pull the diff and leave the carrier. I pulled the whole thing, a-arms and all without fully removing the half-shafts. Let's see if I can get it back in. Any suggestions for now and the future swaps? Thanks again for all the 411.

All you should remove is the rear sway bar, drive shaft nuts, aves bolts and the three mounting bolts. No need to remove the carrier or rear control arms. I've seen them swapped in 20 minutes.

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Originally Posted by mr. black

I'm running 225/50 R16's right now. What kind of 16" tire would put me on par with ITR on 17's?

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That's about the same diameter as the 245/40-17 I use.

Thanks guyz. I will start putting it back together tonight. One last question. The book also says to have the car realined. Is that necessary? Especially if I did not remove the half shafts from the hubs? This is my first independent rear that I've taken out.

If you pulled the entire sub-frame out of the car you will need to do an alignment.

By subframe, you mean the carrier that the a-arms bolt to?

If anyone is out there, especially in the NE division. I spoke with the previous owner and it turns out that both rears had been changed and not relabled. One diff has 4.27s and the other is 3.91. I will most likely never need the 4.27s. I'm going to try the 3.91s while looking for a back up 3.73. Both my diffs have 70% lock. Hard to find, but willing to do partial trade or sell. I would need 70% lock on the 3.73s to consider even trade.

I run a 3.91 Quaiffe at the short tracks (NHMS & LRP) and a 3.64 for longer tracks (WGI & M-O). Need lower yet for Pocono, possible to the stock 3.15. I am running 225/50-16 tires. The 4.11 is certainly too much for any track.

My crew chief changes the diff in 20 minutes at the track, just pull the half shafts and drive shaft and drop it out. Since no suspension points are moved there is no need for a realignment. Hope that helps your planning.

That does help, thanks. I will be looking for a new rear. I take it no one has any problems getting the diff around the spare tire bay. That and per the manual's instructions, I pulled the entire carrier and diff with arms attached. So, I need an alignment.