Hi,
I am buying an 88 635csi with and auto transmission and I don't want to go the standard route and chip my car because I don't want to run premium fuel. I read that 4.10 LSD swap is the biggest bang for the buck for these cars in regards to acceleration and I was wondering if that is true? I saw a few rebuilt units on eBay and one rebuilder even said that there is an BMW 4.27 LSD rear end that will fit in my car. With no other mods to a perfectly running 635 how would this feel in town? How about on the highway? I don't care if my top speed is reduced as long as it isn't annoyingly loud to cruise at 75-95mph. Is this a good idea? I want to keep this car easy to run and maintain, but I still would like better acceleration than stock. Any advice is welcome!
Thanks,
WT

I've never heard of a BMW BMW 4.27. You'll lose both top end and fuel economy with a 4.10, but you will get better off-the-line performance. I'm not sure what RPM range you'll be out there. You might want to go search the Roadfly forums or just go ask over there - pretty sure the RPM's have been mapped and posted there.

What is in your car right now? I'm guessing 3.46 - I'd consider going to 3.91. A little less over-the-top. If you really want to have fun, do the 5-speed conversion. That with the diff. would be a big transition.

I'll post over at Roadfly as well! Thanks! The current owner isn't sure what is got and I won't see the car again until Saturday so I'm not really sure what is in it now. I can see that the 3.91 would be a good conservative choice that would definitly be an improvement, but I want to be able to "light them up" with the automatic and no 5 speed at least a little bit! Can I "peel out" with the 3.91 and automatic transmission? I know I sound like I'm 12, but I'm 35 and I'm serious about this;)
Thanks again,
WT

I'm not sure if you could from a standstill or not - I've seen 635 autos peel rubber on turns or turning around (with authority!). I think you'll be happy with either the 3.73 (which is what my M635 has) or the 3.91.

Enjoy!

It sounds like the 4.10 is a little much and the noise in the cabin cruising at 80 would suck. Thanks for the all the input. I can't wait to get my 635! I'll post pics when its here!
Thanks again,
WT

Keep in mind, if you are new to the E24 scene, as i still am, there are things you have to watch for, especially with the auto trannys. They are something to watch at about 130k-150k. If your shark has that kind of mileage, you might not want to do burnouts and what not too much/if not at all because you might just blow the sucker up. If the tranny was rebuilt, then you should be alright. Just DONT do a nuetral drop, not worth the risk. Those M30B35 motors have a good bit of torque in them, im sure 640csi will chime in about what can be done the the 88+ auto sharks as he has one.

As for the differentials, if you want to just drive the car after you burnout, the 4.10 is not gonna be waht you want. Youll be very content with the 3.91. I want a lsd for wolfie, and i was hoping to find a 3.43 lsd, and at the most a 3.73 just because i want the best of performance and cruising economy.

-Jeff

That's probably a good point because the car I'm buying has 140,000 miles on the clock with the original, but well cared for transmission. My wife won't let me modify it until I acually own it for a few months anyways! I suppose I like dreaming up ways to spend money!
Thanks again
WT

As a B35 owner, I should be content that I have one up on a lot of other M30's out there and so far haven't done much to the motor besides regular maintenance although i still thirst for MORE POWER! I think the biggest mod I am preparing for at the moment is the 5 speed swap. I have a good dogleg core but it needs an input bearing, and I noticed that the rear mounting ear is broken which kindly left a pinhole in the rear of the case forcing me to purchase a 4 speed for a replacement rear case section. I think that 4.10 is pretty high for a DD and for the record, the tragimatic won't burn 'em unless i'm making a 90 degree turn in the rain!