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Thread: 318ti build: What diff ratio to install in a Chumpcar?

  1. #1
    Join Date
    Feb 2018
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    Trenton, NJ
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    1997 318ti

    318ti build: What diff ratio to install in a Chumpcar?

    I'm lost. I feel like I've spent more time researching various differential options for the car than all other research combined. Here's the plan. I'm building for the Chumpcar (now Champcar) series. The car is a 97 318ti with a 5-speed. I bought the car with a welded open diff. So, obviously, I need to swap that out for an LSD. The other limiting factor is that I need to keep the 168MM case in order to avoid 25 penalty points for having an non-OEM diff (think, 3 laps down out of the gate).

    So, here's what I know. The 3.45 ratio is the baseline since it was the stock ratio for the 5-speed. But, I'm wondering how useful it would be to bump it up, to say the 3.9 or 4.1 ratio diffs. I love the idea of being a demon in the corners with those ratios, but I also don't want to give it all back in the straights. Of course, the tracks themselves need to be part of the decision making too. I think there are only two really long straights on the tracks we might run the car on (The Glenn and VIR).

    Any help would be greatly appreciated!
    Last edited by Olmsteez; 02-13-2018 at 10:38 AM.

  2. #2
    Join Date
    Jun 2011
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    1993 325is
    IDK if you'd consider a longer ratio, but I have a 168mm LSD in an E36 coupe housing with a 3.23 ratio for sale that is pretty much bullet proof and will last a lot longer than the OEM 168mm E30 and E36 offerings.





    I'd be prepared to remove the LSD assembly from the E36 coupe housing as I know you won't be able to fit it into your Ti.

    Jacques

  3. #3
    Join Date
    Feb 2018
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    Trenton, NJ
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    1997 318ti
    Quote Originally Posted by jakermac View Post
    IDK if you'd consider a longer ratio, but I have a 168mm LSD in an E36 coupe housing with a 3.23 ratio for sale that is pretty much bullet proof and will last a lot longer than the OEM 168mm E30 and E36 offerings.

    I'd be prepared to remove the LSD assembly from the E36 coupe housing as I know you won't be able to fit it into your Ti.
    That's awfully nice of you, man. But, I feel like your ratio is going in the opposite direction than I am thinking about. Also, I've read about some threads about how tricky it is to swap internals out of a diff case. The tolerances are super tight and if you get it wrong, its a complete melt down.

  4. #4
    Join Date
    Nov 2001
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    Holden SSV, GTS2 E36
    So, a couple of things to consider. When selecting a diff for track use, you usually want to look at several factors:

    1. what track will I run at the most?
    2. what does that track look like?
    a) short, technical
    b) lots of long straights
    3. what does the power band of the car look like

    On short / technical courses, a lower gear set (higher numerically) helps you dig out of turns which can be an advantage. That same diff on a course with long straights can have you shifting more and/or running out of gearing at the top-end.

    Basically, what I look for is what diff will produce the least amount of shifting (reduces margin of error re: missed shifts, $$ shift) and gives me ample pull out of turns without sacrificing top-end on the straights. Locally, that is the 3.64 I've got in my car. 3.73 and 3.91 resulted in too much shifting, lack of gearing on the top-end. 3.43 and lower resulted in less pull out of the turns, but excellent top-end.

    Just some things to add to your decision making process.
    GMC 2500
    1992 318is running in GTS2
    2014 Holden SSV

  5. #5
    Join Date
    Feb 2018
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    1997 318ti
    Quote Originally Posted by John in Houston View Post
    So, a couple of things to consider. When selecting a diff for track use, you usually want to look at several factors:

    1. what track will I run at the most?
    2. what does that track look like?
    a) short, technical
    b) lots of long straights
    3. what does the power band of the car look like

    On short / technical courses, a lower gear set (higher numerically) helps you dig out of turns which can be an advantage. That same diff on a course with long straights can have you shifting more and/or running out of gearing at the top-end.

    Basically, what I look for is what diff will produce the least amount of shifting (reduces margin of error re: missed shifts, $$ shift) and gives me ample pull out of turns without sacrificing top-end on the straights. Locally, that is the 3.64 I've got in my car. 3.73 and 3.91 resulted in too much shifting, lack of gearing on the top-end. 3.43 and lower resulted in less pull out of the turns, but excellent top-end.

    Just some things to add to your decision making process.
    Thanks, John this is really helpful. I guess I'm looking for a good all around setup. I think the longest straight the car might see is Watkins Glenn. But, I don't necessarily want to build it for that straight. It will probably see a lot of days at NJMSP and Summit (low-med speed). And maybe VIR (which I think might be considered high speed for this car).

  6. #6
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    Also, take a look at the gear ratio / speed calculator, link below. My Spec3 has a 3.15 LSD -- mostly because they're cheap and readily available. According to the calculator, it has a theoretical top speed (max RPM, top gear) of about 150mph. Of course, the car (basically stock M50 motor) doesn't have enough power to hit 150mph, so a lower geared diff would be better suited. With a 3.64, the theoretical top speed decreases to about 130mph -- and that top speed is achievable based on power. So for me, the 3.64 would provide better power delivery on corner exit and I'd use all of 5th gear, rather than just the lower 70% of it. Obviously, your Ti's ideal gearing will be different.

    As others have stated, you also need to consider number of shifts and where those shifts fall in the courses you run often, but the calculator gives you a simple starting point.

    http://www.endtuning.com/gearratios/
    Last edited by OCRentAPopo; 02-14-2018 at 09:30 AM.

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