Rear end for E30 - S52 from a noobie

[passdanachos]

I did a search but I am still confused about upgrading my diff on my 318is. Basically I am having an S52 motor with the M3 tranmission being put in my E30. It should be done in a few more weeks. But my mechanic who is doing my engine swap is telling me that I should upgrade" my rear end because I won't get much out of the first gear with the new motor. So it will be another grand CDN to go with a bigger diff and beefy axels.

Do you know if the stock 318is rear end ratio is sufficient with the new motor? If not, what do you guys recommend?

I don't understand the ratios, I don't even know what the ratio is for my stock 318is rear end is.

Is it difficult to do myself? I am capable of doing mechanical work, and suspension but I never worked on transmissions or the rear end before.

What is the general cost of the parts?

Help!!!!

[Mike B.]

I would use either a 3.25LSD or a 3.46LSD with a E36 ZF transmission. These diffs usually run anywhere from $150 to $600 depending on their condition and if the have been rebuilt or modified (higher % of lockup) in any way.

I think a 318is comes with a small bodied 4.10, so you will definitely want to get something different. I'm not sure about the axles, but if you need a set from an E30, PM me because I have some available.

They are pretty easy to swap out. Just unbolt and bolt the new one back in it's place. Just support the driveshaft so it doesn't just hang there and be careful when you lower it - they are HEAVY. A jack is necessary.

You do know that with the E36 ZF transmission that you are going to need a drive shaft out of an E30M3 or a '98 E36. If you use the E36 one, you will also have to change out the CSB to an E30 version. The stock driveshaft won't work.

[passdanachos]

Yea, my mechanic mentioned something about modifying or replacing the driveshaft. He had to send it out and have it modified. I think. He started talking techincal drivetrain stuff and I started to glaze over. I do know that right now the 99 M3 motor and transmission is in and the driveshaft is connected to the M3 transmission to the 318 rear diff.

If it is basically a swap, bolt on, then it wouldn't be a problem. I looked at realOEM and it looks pretty simple.

I just need to know what replacement rear end to bolt on. Which from your suggestion is a 3.25LSD or a 3.46 LSD

What does 3.25, 3.46 mean?

I assume LSD is limited Slip Diff.

Thanks for the info so far!!!

[passdanachos]

Which cars (year and model) have the 3.25 or 3.46 LSD?

[MrK]

What does 3.25, 3.46 mean?

That would be the diff's gear ratio.

3.25 means that each 3.25 turns of the driveshaft would equal one turn of the wheels.

A lower number means longer gears, and vice versa.

[Mike B.]

3.25 and 3.46's can usually be found on 80's 5- and 6-series cars. They will require you to change the rear cover and output flanges for them to work in an E30.

[matt]

Definitely a 3.25.

[BigD]

Definitely a 3.25.

I'm kind of curious to see how my car will go with the 3.73. :) It's easy to change and the 3.25s are pretty cheap and common so I'm going to wait until everything is done to change it but first I want to see what it's like. :) It's a track car and the 3.73 gives me a 215 kmh top speed in 5th at 7000 which isn't that bad unless I hit a track like MoSport (where I'll top out 1/2 way down the back straight).

[passdanachos]

Could I get an E30 325 rear diff? What is that ratio?

Basically it sounds like with my 4.10 diff, I am going to have lotsa acceleration, quick shifting but no top speed.

3.25 ratio?.....Is that available in the E30 325?

I looked at the rear diff on my car at lunch and yes, it looks like a direct unbolt, bolt on setup. Man, the 318 diff looks really small!

[Brew]

The 4.10 diff with the e36 tranny will give you incredibly short gearing. You'd redline 5th gear at about 120mph, and a 80 mph cruise would put at about 4600rpm in 5th.

A 3.73 would also be very short, but if you don't do much highway driving it might be liveable. Top speed would be 133mph, 4200rpm @ 80mph.

3.25 seems to be the ideal rear end ratio if you're using a e36 tranny with the 1:1 5th.

[e30ryan///m3]

I have the 3.73 gears in my s50 e30 vert and it makes the car an animal out of the hole but for driving anywhere on the highway it is a little annoying! I would go with the 3.25 diff for the s52 swap it will still give you the fast accelleration you want but it will make everyday driving much more pleasurable!
Not to thread jack or anything but I am currently putting an s50 into my e30 m3 and I am going to be adding a turbo to the setup. Any ideas on what diff I should run with the turbo (8psi, T3/T4 unit, 36lb injectors) I was thinking 3.25 LSD, or should I use a 2.93 LSD and what cars had these rear diffs in them?

Thanks

[PeaveyBassist]

2.93LSD was 325e.

Will