Final Drive and SIR

[mr. black]

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.

[JoshS]

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.

[JeffYoung]

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?

[JoshS]

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

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.

[mr. black]

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.

[JeffYoung]

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.