S50 E30 swap - Differential Question

[Uber E30]

3.46 is an excellent ratio for either an overdriven gearbox or a direct final drive box.

That will work good for a daily driven vehicle as well. If I were looking for another gear to go behind my S52, it would be a 3.46:1.

----In my opinion.

You're crazy he is gonna be at 4k RPM going 80

[rThor432]

You're crazy he is gonna be at 4k RPM going 80

I've got a 3.38 in my E36 M3 and I could hardly imagine it being that different.. I havent really payed any attention to what I'm turning at 80, but it isnt 4k. I've done ~31 mpg in that car cruising on the highway at 70 mph. The 1:1 5th and 3.38 work fine for me, and the 3.46 wouldnt be a huge change from that at all.

Although, I have to admit that the E36 tire sidewalls are up one size from the E30 and that would probably be good for ~200-300 rpm-- my guess.

[Uber E30]

Dood really? You have never seen the gearing calculator?

[cactii29]

Wow.....the evidence is in the table. I think I'll go with the 3.25 then.

[Uber E30]

I personally went with a 2.93. I based this decision on DD a 260/4.10 and HATING it. (the 260/4.10 is about the same as the ZF/3.25)

[cactii29]

I did some online searching and the opinions on the 3.25LSD vs 3.46LSD with the ZF tranny are basically split.

It seems very subjective what each driver likes so I might as well go with the cheaper option....which is the 3.25LSD.

[BigD]

Go with 3.25, end of story.

BTW I still have a guy who wants 200 shipped for what sounds like a clean diff (probably more to Jamaica though...). So if you're interested PM me your email, I'll put you in touch with him.

[cactii29]

PM sent

[nmlss2006]

Dood really? You have never seen the gearing calculator?

Except the E30 stock doesn't have a 576mm wheel but a 609mm one (195/65-14) - so gears will be a little longer. We have:
195/65-14 wheel: 195*.65*2 + 14*25.4 = 609.1mm = 0.6091m = wdia
1000rpm = 60k rev/hr -> through 1:1 -> through 3.25:1 -> 18461 wrev/hr
wdia*PI*wrev/1610(metres in a mile) = 21.94MPH per 1000rpm in 5th with a direct drive fifth and a 3.25 rear diff -> 87.76 mph @ 4000, 3419RPM @ 75mph.
For a daily driver that never sees the track, even the 3.25 is borderline short, so the 2.93 you chose is what I would also recommend - 3081 RPM @ 75mph with stock circumference tyres.
With a 3.46, you'd get 20.61 mph/1000rpm which puts 75mph at 3639RPM.

[e30polak]

Nah, you get that in 4th. ;)

I'd rather have the torquier feel of my 1.24 in 4th. :p

But why does that matter?

Because it gets you into peak powerband quicker. ;)

3.46 is an excellent ratio for either an overdriven gearbox or a direct final drive box.

That will work good for a daily driven vehicle as well. If I were looking for another gear to go behind my S52, it would be a 3.46:1.

----In my opinion.

Definitely the PERFECT ratio for a street driven 24v car with the ZF trans. I used to have the 3.25, and it was definitely not as fun. I want to try a 2.93 one day, simply because I think the longer first and second gears would be fun.

The 3.46 definitely requires a little more shifting, but I don't mind.

Differential Gearing Worksheet










Step 1: Enter Gear Ratio
3.46





Width Aspect ratio Diameter

Step 2: Enter Tire Size: 235 40.00 17










1st 2nd 3rd 4th 5th 6th
Ratio: 4.20 2.49 1.66 1.24 1.00 0.82



1st 2nd 3rd 4th 5th 6th
750 4 6 9 13 16 19
1000 5 8 13 17 21 26
1250 6 11 16 21 26 32
1500 7 13 19 25 31 38
1750 9 15 22 30 37 45
2000 10 17 25 34 42 51
2250 11 19 28 38 47 58
2500 12 21 32 42 52 64
2750 14 23 35 47 58 70
3000 15 25 38 51 63 77
3250 16 27 41 55 68 83
3500 17 29 44 59 73 90 3750 19 32 47 63 79 96
4000 20 34 51 68 84 102
4250 21 36 54 72 89 109
4500 22 38 57 76 94 115
4750 24 40 60 80 100 122
5000 25 42 63 85 105 128
5250 26 44 66 89 110 134
5500 27 46 70 93 115 141
5750 29 48 73 97 121 147
6000 30 51 76 102 126 154
6250 31 53 79 106 131 160
6500 32 55 82 110 136 166
6750 34 57 85 114 142 173
7000 35 59 88 118 147 179
7250 36 61 92 123 152 186
7500 37 63 95 127 157 192
7750 39 65 98 131 163 198
8000 40 67 101 135 168 205
8250 41 70 104 140 173 211
8500 42 72 107 144 178 218
8750 44 74 111 148 184 224
9000 45 76 114 152 189 230
9250 46 78 117 157 194 237
9500 47 80 120 161 199 243
9750 49 82 123 165 205 250
10000 50 84 126 169 210 256

or you could have a 3.73 with the zf and walk sti's

I was able to do that with my 3.25 diff. :p

Of course since you have the m52, you need to compensate. ;)

[Uber E30]

Except the E30 stock doesn't have a 576mm wheel but a 609mm one (195/65-14) - so gears will be a little longer. We have:
195/65-14 wheel: 195*.65*2 + 14*25.4 = 609.1mm = 0.6091m = wdia
1000rpm = 60k rev/hr -> through 1:1 -> through 3.25:1 -> 18461 wrev/hr
wdia*PI*wrev/1610(metres in a mile) = 21.94MPH per 1000rpm in 5th with a direct drive fifth and a 3.25 rear diff -> 87.76 mph @ 4000, 3419RPM @ 75mph.
For a daily driver that never sees the track, even the 3.25 is borderline short, so the 2.93 you chose is what I would also recommend - 3081 RPM @ 75mph with stock circumference tyres.
With a 3.46, you'd get 20.61 mph/1000rpm which puts 75mph at 3639RPM.

Dont nitpick, no one is going to be running 14's with a 24v

[nmlss2006]

I was trying to be helpful and incidentally agreeing with you - you can't really go much above 609mm in the space you're given in an E30 anyways. And BTW - 205/55-15, 225/45-16, 215/40-17 all have the same external diametre, give or take a couple of mm. OTOH, 225/45-17 225/50-16 etc are all E36 wheel sizes, 631mmish, and they will have a much harder time fitting.

[cactii29]

Anyway guys I ordered a 3.25LSD today. Thanks for all your honest and expert opinions and facts.

Kevin

[daniel]

i just did a 2000 mile roadtrip, ZF320+3.25LSD, and i was wishing for a 2.93 or 2.79 the whole time. with that said, my normal driving habits are about 95% city...

[jrdeamicis]

I just bought a 6 speed. It made my life better. However my 6th is like most 5th :)