Classification System Questions (Diffs)

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    Ryan Rich

    Hey everyone,

    I have a question about the classification system and how to handle an engine swapped car.

    My e30 has an S5x in it and the differential that came with that motor/trans from the factory was a 3.23. My car original diff was a 3.73 and my current diff is a 3.25.

    Here is where my lack of understanding stems from. When classifying is the differential tied to the motor/trans or to the car? If it should be tied to the motor, is a 3.23 close enough to a 3.25 to call it the same thing?

    It seems weird that once the motor/trans comes from another car, we still use parts from the cars original drivetrain to calculate points. In the end the ~80HP I gain from the motor swap costs me about 12 points.

    Here is a break down of the 3.25 vs the 3.23.

    • This topic was modified 7 years, 5 months ago by Ryan Rich.
    • This topic was modified 7 years, 5 months ago by Ryan Rich.
    Rob Powers

    On its face, 3.25 versus 3.23 is less than half a percent difference, so based on diff alone those two should be ignorable.


    When you did your motor swap, you also swapped the transmission. The gearing on the E36 transmission is very different from the E30’s transmission. So to be accurate for comparisons, you’d need to take both into consideration.

    E30 325is with diff = 3.73
    (assuming Getrag 260/5 transmission)
    First = 3.35 (*3.73 = 12.50)
    Second = 2.03 (*3.73 = 7.57)
    Third = 1.36 (*3.73 = 5.07)

    E36 M3 with diff = 3.23
    (assuming ZF 5 speed transmission)
    First = 4.20 (*3.23 = 13.57) ==> 8.6% increase
    Second = 2.49 (*3.23 = 8.04) ==> 6.2% increase
    Third = 1.66 (*3.23 = 5.36) ==> 5.7% increase

    So you are getting way more bang for your buck with the 3.23 diff and your shorter gearing on the E36 transmission than you had for the E30 one with the shorter diff.

    So only looking at the diff ratio in isolation you get a completely bogus view of what is happening from the flywheel back.

    Whenever there is a driveline change like this to put a bigger motor in a lighter car (and associated gearing stuff), I have to wonder whether it makes more sense to treat the car as the chassis it started as with a power and gearing upgrade, or to take the bigger car as the baseline and then add points to it for the “effective” weight reduction. Neither one seems like an accurate model.

    We would be well served by looking at actual run histories for cars with certain characteristics and determine the real-world equivalency for things. Our current points model is fairly arbitrary (I know Hal would strongly agree).

    Ryan Rich

    Fantastic post Rob! I think the classification system might be flawed in the way it looks at gear changes. Maybe the best option is to change the “Differential gearing modifications” field to “Driveline gearing modifications”.

    Hal Dorton

    Hal wants to start over.

    Rob Powers

    Ryan: driveline gearing mods is an interesting idea. For our events, it would probably make the most sense to look at 2nd gear ratio * diff ratio for the comparison, and for non-transmission swaps, it will end up looking just like the diff change we track today.

    Of course it still begs the question of just how many points such a change should be worth.

    Hal: how would you propose starting over?

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