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From digest.v7.n519 Fri Oct 31 06:39:36 1997
From: "Carl Buckland" <>
Date: Thu, 30 Oct 1997 22:22:31 +0000
Subject: Gearing

Corey H. asks some questions that I think may have general interest, so I will answer to the lists at large:

I will have to be very brief:

> I saw the pictures of your M3 on your new web page.
> The car looks GoOoOoD!!!. You did an excellent job.

thank you

>. What size tire did you use on your 18" BBS'?

225/40/18. The 235/35 tire will "fit" even better, but it looks ridiculous. Very low profile. The diameter of the 225/40/18 is slightly larger than the stock 235/40/17's , while the 235/35/18 is almost exactly the same. Gearing will change very little with either tire, over stock. The WIDTH of the P-Zero in the 225/40/18 seems to be about the same as the 235/40/17 MXX3.

> few other questions, from your experience. I see you have or had a 40%
> 3:38 differential on your M3. How much did it affect your top speed?

Top speed is theoretical. on the track, seldom do we see more than 130, yet I am sure that it will still do at least 145. the difference is that the car PULLS at the high end. I have 5 gears to work with.

In autocross, that can work against you. If the whole course is first and second, you will have more pull with no drawback. If the course requires a lot of shifting from second to third to second, you will lose more time due to shift lag than you will gain by gearing that shifts the torque further down the speed range.

Changing your gearing has broader ramifications than just low end pull and top speed. The most important question to ask is " what speed range do you drive at most of the time, and will the gearing change give me more or less power in a given gear at that speed?" (spoken as a driver, not as an engineer. I would welcome a better, more technical phrasing) Specifically, my current gearing gives me about 59 in second (with a 3.15, it would do 62, with a Conforti chip setting rev limiter at 7000+), and 94 in third (previous, 100). This gearing is ideal for most tracks, where third gear grunt is a real plus. However, in autocross, much of the course may be at 55 to 64 mph, and the lower gearing means having to shift into third, when running on the rev limiter in second was possible with the previous gearing.

Overall, I would say that the gearing is ideal for the track, and is ideal for Pro Solo, where launch is so important. I think that a 3.23 is ideal for a typical autocross, where a broad second gear range is needed. The only way to have the "perfect" gearing is to have a whole trailer full of pinion gears and gear sets. Gearing is always a compromise.

The 40% lock up is essential for autocross or track use. Track will allow even 95%, although that is too much for autox or street use. The wheels really hook up on exit.

> I was thinking of putting a 97 M3 3.23 or Korman 3.25 differential in
> exchange for my 3.15('94 325is).
> How much difference do you think that will make in bottom end
> acceleration (just a simple estimate)?

I would not recommend it. There is only a minutiae of change in gearing from a 3.15 to a 3.23 .

I cannot give a quantitative answer, but it "seems" that with the lower gearing , my car comes off the line like a scalded dog. Cruising at the higher speeds requires, of course, higher revs, but not higher than I find acceptable. Freeway gas mileage has not seemed to have suffered, although I did expect it to drop.

> My top speed, right now, is about 140-144mph with my modifications.
> My 0-60 speed, right now, is about 6.8 to 7.0 sec with my modifications.
> Korman recommends the 3.46, but I think that's going to affect my top
> speed too much. That reminds me, Is you 3.38 from the 96-97 M3 sedan
> automatic?

You are correct about the origin of the diff ratio. If you go from a 3.23 to a 3.46, you will see very substantial improvement in your off-the-line performance. your top end will drop, but what is the difference between a *theoretical* 144 and a *theoretical* 135? The most important factor is HOW IMPORTANT IS YOUR LAUNCH, AND WHERE DO YOU WANT TO SHIFT INTO SECOND AND THIRD GEARS?

> I was also thinking about getting a lightweight flywheel. I heard they rattle
> alot,
> making your engine sound unrefined. Is that true?

The engine rattle, which could be (an unqualified guess) the throw-

  • -out bearing, or it could be the main bearings, is soft but noticeable, and not refined in sound. The rattle occurs at 600 rpms, but goes away periodically, and completely at 900 rpms. Jim C has said that he could increase the idle with the chip; but for me, getting rid of the rattle is not as important as being able to come *completely* off the throttle when that is what is called for. I think that it is a very personal call.

> Is there any company out there that sell one without those symptoms?

I will let others answer that question. I hear that not all drivers have had the rattle "problem," if it is a problem.

> Do you have any opinion on the following aluminum flywheels:
> -Korman
> -Bekker
> -TMS
> -Metric Mechanic
> -Autothority

I am told that nearly everyone uses the same one or two suppliers for their flywheel. I bought mine from BMP, and have been completely satisfied with it. It reduces the flywheel weight from the very heavy stock, dual mass flywheel, at 25 lbs, to 11 pounds. I have heard that the lightened flywheel weighs only 8 lbs. I never put it on the scales. I put it on my desk instead, and grooved on the cool machining. A lightened flywheel will improve your acceleration just as much if not more than lower gearing.

> P.S. If I put on the 96-97 M3 diff. what else would I need to change?

I can't answer that question, as I don't know much about the 325.

As a final note.............I seldom drive over 100, and keep it at 80 most of the time. The "thrill" of going 160 on the freeway just isn't worth the ramifications, which are broad.


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