Diff ratio for a close-ratio transmission

[Ian]

Can anyone running a close-ratio transmission recommend a good gear ratio for the diff?  For years I have used a Getrag245 and an e21 3.91 LSD with my S14 engine.  I'd like to try a close-ratio box, and am going to swap out my Getrag 245 (final drive of 0.81) for a ZF S5D from an e36 M3 with a final drive of 1.0.  I've used a 3.91  e21 LSD with the 245 box, but that looks to short on the gear charts with that transmission.  IThe e36 M3s came with a 3.23 diff, but that might be too long for the S14. It looks like I might I might want something closer to a 3.45 diff, which would give close shifst but could still cruise at 80mph @4000 rpm.

[RogerRamJet77]

Ian,

 

Here is an excel spreadsheet that you can use to compare engine RPM, tire size, transmission ratios and final ratio.  Table and graphs output is MPH.

 

Regards,

Roger

BMW 2002 gearing.xlsx

Edited November 25, 2023 by rockyford77
clarification

[TobyB]

Ian, I forget what housing you use,

but the 3.46 out of the manual 318's is a good match

to the M42.  And it's 'easy' to put one into an E21 housing, which then bolts up.

That's the tallest easily- available ratio I've found- the 3.38's pretty uncommon.

 

I would think you'd want to go lower, though, for the longer- stroke

S14, depending on what you use it for.  The 2.93 in the manual

E46 is tall- 1st gear's good to 40- and it makes each gearchange a big step.

BUT it'll cruise at 150 mph in 5th at 6k all day.

 

t

 

[Ian]

Thanks for the info, Toby.  This is a track car, so initially I was going to leave the 3.90 diff in place, but after looking at some rpm/speed charts, it might make sense for me to pick something between a 3..64 and 3.23 with the ZF transmission. 

 

The tranny gear comparison looks like this:

Getrag 245.                   ZF S5D 320ZZF

1- 3.68                                     4.21

2- 2.00                                    2.49

3- 1.33                                    1.67

4- 1.00                                    1.24

5- 0.81                                    1.00

[conkitchen]

FWIW, on the street, I run a CR dogleg 245 with a 3:45 diff and I'm in 4th gear by 40MPH. 

[Tommy]

3,46:1 would give you the same 1st gear as now but others are more closely together . I think you will like it. Speeds below are with random 600mm tyre diameter, in km/h, but it seems to match so I think you've done your math 👍.

[Tommy]

With 3,91:1 5th would be same as 4 today and 1st would be considerably lower. That doesn't sound so good.

[Tommy]

...But then again when looking at the changes more closely it seems that the boxes are actually about same further from 1st. Meaning if you set the differential ratio to match second gears you would only notice difference in change from 1st to 2nd so I doubt that the benefit will be great. Surely depends on track but the effect is made by the differential ratio you choose, except from 1st to 2nd.

[Tommy]

If someone is more experienced with the theory, please correct me. I'm trying to figure out which of the Samsonas dogbox gear sets would be the greatest for me but I'm still not that sure

[M3M3]

Since it is for a track car,  the ratio should be defined by what track you drive on most.   You want to be close to redline at the end of the fastest straight.    Then adjust if that puts a shift in a bad place somewhere on track.   

 

There is no right answer.  It depends on what track. 

 

Eg... a 4.41 diff was the best ratio behind an s14 and a 245 at summit point.  Over 125 top speed and kept the engine away from the bad ratio gap between 2 and 3 in the 245.  The zf you are considering has an even worse gap between 2&3.  1st and 2cnd gears are useless on track.    Maximize use of 3/4/5.

 

Rpm at 80mph irrelevant.  Its a track car.   

 

Edited November 27, 2023 by M3M3M3

[TobyB]

...thus, the Muncie close- ratio Rockcrusher 4 speed, which has 4 gears where BMW transmissions all have only 3...

 

heh

 

t

WHAT?  I CAN'T HEAR YOU OVER ALL THE GEARBOX NOISE!

[conkitchen]

6 minutes ago, TobyB said:

...thus, the Muncie close- ratio Rockcrusher 4 speed, which has 4 gears where BMW transmissions all have only 3...

 

heh

 

t

WHAT?  I CAN'T HEAR YOU OVER ALL THE GEARBOX NOISE!

Can't find it? Grind it! 

[Ian]

Thanks for all the suggestions.  I did check with some local hot-shoe e36 M3 drivers, and they have stuck with the stock 3.23 ratio.  Figuring they have 2 more cylinders and a bit more torque, it's looking like a 3.45 might be the best compromise between track and road rpm.  I like the car too much to only drive it on the track, so the 60-80mph rpm is important for driving on the weekends as well as to-and-from the track.  But I'll keep the 3.91 just in case the 3.45 feels too lazy.

Edited November 26, 2023 by Ian

[uai]

I'd say get one each 3.45 and 3.64
But I'd just calculate how fast you're gonna get on the track with regards to your engine rev limit and power.
With my car for Nordschleife 3.64 is already close or at the limiter at Schwedenkreuz and Foxhole at close to 210 kph
For Spa 3.64 is a bit log geared 3.9 might be more suitable for my car.
 

Edited November 26, 2023 by uai

[kbmb02]

3 hours ago, uai said:

I'd say get one each 3.45 and 3.64

 

I've got 5 different gear-ratio diffs for racing; usually bring 2 options to an event, and select for top end RPM or sometimes best trans/diff gear-match for strong pull out of corner at best passing zone. Wheels up/down diff swap in 35 minutes or so (longer when the diff is hot as heck). -KB