Rear Differential Ratios

[twright]

I'm in the midst of an experiment and would like to know what others have found:

I recently swapped the 4.27 differential ratio in my Touring for a 2.93. ( I never liked the 4000 rpm highway RPM's.)

I now have 2300 rpm at 70 mph.

It's now really nice at highway speeds, but I've lost some acceleration at lower speeds.

I'm considering putting back in the 4.27 and installing the 2.93 if I'm going on a trip. However, that's kind of inconvenient and would be nice to have one good ratio that is a compromise.

Does anyone have a good compromise?

[ForcedFirebird]

You went from one extreme to the other. 3.73 is very common, they came in the 325i/S's with manuals.

[roguetoaster]

Small case 3.46 or 3.64 from an E30 if you can find one, or 3.45/3.38 from E36s would seem to be the best fit for what you want to do with your 318i wagon.

3.15 is also a good option if you want a torsen locker. That ratio was pretty solid for my S52/G250 318ti, even with no overdrive like a G240.

[twright]

I thought the E36 didn't fit. Don't they have a different rear mounting?

[roguetoaster]

318ti, and Z3 diffs work. So E36/5, /7, and /8. Maybe E36 tourings too, but I haven't seen one.

[twright]

My diff doesn't look like this: https://www.ebay.com/i/324421691245?...iABEgIxj_D_BwE

Mine only has one rear mounting point.

[roguetoaster]

My diff doesn't look like this: https://www.ebay.com/i/324421691245?...iABEgIxj_D_BwE

Mine only has one rear mounting point.

Like above, standard E36 diffs likely won't work, but Z3s and 318ti work fine since they use the same style of rear mount, and their bolt points line up.

[808ETA]

I put a 3.45 318ti in my 84 coupe. It was a perfect fit.

[digger]

average would be about 3.64, 3.73 probably close enough and more common.

[PieterVN]

Like said above, try and find a ratio somewhere in the middle.

These are all the ratio's i found that were originally installed on all different E30's.
4.45, 4.27, 4.10, 3.91, 3.73, 3.64, 3.46, 3.25, 3.23, 3.15, 2.93, 2,79.

The trick is finding the right balance between acceleration and high rpm's on high ways. Most of the times BMW did quite great. If i were you i wouldn't want to go extreme in changes.
If your car came with the 4.27 i would try a 3.91 or 3.73 at max.

Good luck!

[twright]

Looking at this chart leads me to believe that ratios around 3.5 are going to be pretty scarce in the US:

https://www.e30zone.net/e30wiki/index.php/Differential

[AWDBOB]

3.46/3.64 is your best bet for all around, but they're rare and expensive. The only chance you have at a cheap 3.64/3.46 ratio is out of a Z3 from the junkyard.

3.73 is what I run on all of my cars and I do plenty of highway cruising, works fine for me.

Also re: the 3.91 above, they're also quite rare in the US, as they really only came in manual iXes and a very small number of other cars.

[nick496]

Yeah, I remember my old 318i sedan. That ran a 4.1 diff stock, but the touring they upped the ratio due to the weight so that it's 0-60 times were still comparable.

But the most common recommendation is 3.73.

[Vincenze]

What is the slant angle of your engine?

30°?
Then the 6-speed GS6-37BZ should work.

The GS6-17BG should be enough for a 4-cylinder car. It even has a double overdrive that will work nicely with the 4.1 differential.

[twright]

I'm not sure about the exact slant angle, but it does look like the typical E30 engine "lean".

What cars did the GS6-37BZ come in? Would it just be cheaper to do an engine swap?