What e30 diff ratio would be best to get for a m44b19 engine and transmission?

[318istourer]

As my e30 has had a m44 swap and the revs were high in normal speeds, I recently changed over my diff from a 4:27 to a 4:10 but it didn't make much of a difference to lower the revs at all except first gear, after a bit of research it turns out it's the gear ratios of the m44 transmission, something to do with not having an overdriven 5th gear, it needs to have a much lower ratio, now, I don't want to spend too much getting a e36 diff custom fit, so what e30 medium case diff ratio would make it more liveable?

I know the standard for the e36 318is is 3:45 but it sounds too low and will eat up the acceleration, however I do plan to put a supercharger kit I have in the future so would like to hear your advice.

Just for interest:
5th gear at 50 = 2750 rpm
5th gear at 60 ~ 3.5k rpm

Thanks

[reelizmpro]

If the trans has 1:1 5th gear, it's just like 4th gear in an overdrive trans. A common E30 diff would be 3.73 from a 325i. That's the one I would try next.

[roguetoaster]

I've driven E30s with stock M42s and G240s (overdrive 5th) with anything from 3.15 to 4.44s. They all have trade offs, and I don't think a 3.15 makes it too slow. Consider the typical use case for your car, such as crusing speed, and match ratios accordingly to keep it pleasant during sustained use without giving up too much sporty feel.

With a G250, which I assume you have, I'd consider 3.73 or lower numerically to be ideal. I'd probably land on the far lower end, since to me M44s never seemed to enjoy being revved as much as M42s.

[TobyB]

Yeah, I was going to suggest a 3.64 from an M10 318-

the E36 M42 got a 1:1 5th and a 3.46, and that's not bad,
if you plan to do much 70+ highway driving.
That diff from a ti will come set up for an E30....

4.27 was auto M42 vert 90-92 only, wasn't it?

t

[roguetoaster]

4.27 probably means an M40 car for the OP

[packratbimmer]

If you have the 250 Getrag that came with the M44, I would find a 3.64 medium case diff from a Z3. The 3.64 small case from a 1984 318i would be great too, but even harder to find. 3.45 or 3.46 diffs would be fine too unless your wheels are too heavy.

[varg]

What's with the gearing questions lately? There are many gearing calculators a 10 second google search away, or you can crunch the gear ratio numbers and find your speed at a given rpm in each gear in minutes then pick whichever ratio gives results you like best as far as highway rpm and speed at shift rpm. Acceleration differences between something like a 3.45 vs 3.73 will be pretty small, it's a slow car anyway and it's not a huge ratio change. The supercharger will make it moot and some ratios are harder to find than others. It might be easier to find a 318is G240 than a 3.45 E30 diff, for example.

eng rpm * 1/transmission gearing * 1/rear end = wheel rpm
wheel rpm * tire height (in) * pi * 60 = in/hr
in/hr / 63360 = mph

Punch it into excel, google sheets, open office calc, any spreadsheet program, and you can see exactly what the differences in rpm will be.

[318istourer]

Thanks guys, a lot of very good advice, I’ll have a look on the gear ratio calculator, luckily the e36 compacts have the same diff as the e30s so I can get the 3:45 diff, or if I find a medium case 3:73/3:64 I’ll get it, I don’t mind new props too as the current ones are looking old.

Getting a small case/medium case lsd is either expensive or rare, so where would be best to get the actual internal lsd part with the clutch plates?

[packratbimmer]

I think you will like the 3.45 small case 168MM diff from the E36 318ti. Do keep in mind that everything will work great EXCEPT the inner CV joints. You have two options: Use the inner E36 smaller CV joints or use small case E30 diff output flanges with a non-stock seal (the output flanges have drastically different journals).