time to start a huge build (well huge to me)

[Rockinlittle325i]

;)I may have a chance to build a e30 from the ground up....
I'm so exited right now,
I just trade him my 92 gti 16v for the car that I build at his shop
he has a 91 325is rolling shell that was an auto, two m20 motors to choose from (one from 87 with low miles and a 91 with slightly higher miles)
a 5-speed from a 5 car
and the shell has a 3.79 LSD in it, and a 4.11 if I want to swap it

my main question is what kind of gas milage should I expect from either diff? and what would the performance differences be between the normal 5 speed open diff, the 3.79 LSD, and 4.11 LSD?

also what do I need to expect to do when I put this 5-speed in this auto shell?

[yert315]

I think you are referring to a 3.73LSD and a 4.10lsd? I don't recall a 3.79 gear ration.

I got roughly 25mpg in my 325is with a 3.73lsd. If you are going with an m20 motor, I would not get a 4.10 diff. The revs at 70mph in a 3.73 are 3400, a 4.10 would be worse. LSD's are desirable because both wheels spin, rather than one wheel spinning with an open diff, obviously good for performance reasons.


I believe there is a auto->manual swap thread on the forums that can answer your questions.

[Rockinlittle325i]

I dont remember what the diff in a stock 5 speed is, but what is the acceleration difference between that and a 3.73?

[Kerrath]

You're asking about getting better acceleration with the 4.1 diff, so I'm guessing you're wanting fast. In that case, you don't want the 87 because its probably an eta and the 91 is more powerful. The 91 is also less efficient.

If it were me, I'd pick the 91. If they were maintained fairly similarly, they'll be in a similar state even after a hundred thousand miles or whatever you consider 'low'.

If you do mostly city driving, then the 4.10 might be for you. If you do more highway, then you'll want the 3.73 diff. Also, if you're going to switch to a manual, with the 4.10 diff, your shifts will be very short which can be a nuisance. A lot of it comes down to personal preference.

[Hallen]

You are aware that the 87 and 91 engines are different right?
... you don't want the 87 because its an eta and the 91 is more powerful. The 91 is also less efficient.

Not true. The i engine was introduced in 87. The engine might be an e or it might be an i. If if it is an i, then it is mostly like the 91 with a few minor differences. In this case, then it is just down to which engine was better maintained.

[2002maniac]

I dont remember what the diff in a stock 5 speed is, but what is the acceleration difference between that and a 3.73?

5-speed cars came with the 3.73

[blunttech]

if you dont know what youre talking about dont give advice. it just makes you look like a douche

[Rockinlittle325i]

Your right hallen, and yes they are both I engines. well I'll choose when I take a look at the motors

I have heard that the 4.1 will make the shifts very short, and I hear that 1st gear will be useless with the change
yes I'm on the highway all the time, so I'll go with the 3.73

[blunttech]

who are you talking to blunt?

the guys giving bad info. kerrath and 3400 at 70mph

[Rockinlittle325i]

I had a stock 325i manual and I thought I remember lower than that
but I havent had that car for 6 months or so..... R.I.P

[nando]

3500 is about 75mph with a 4.10.. so I don't see how you'd be at 3400 with a 3.73 at 70mph.

[Hallen]

I dont remember what the diff in a stock 5 speed is, but what is the acceleration difference between that and a 3.73?

The 3.73 was the stock diff ratio. The only question was if it had LSD or not. The 325is had the LSD stock where I believe it was an option on the 325i.

The reason to go to a 4.10 diff is if you want to keep the engine in the higher RPM range most of the time. There is more power available in the higher RPM range. But, you will be shifting sooner. So you can't compare 0-60mph times for example because with the 4.10 dif, you will be shifting to 3rd before you hit 60mph. You would probably get slower 0-60 times with the 4.10 than you would with the 3.73.

My opinion is that unless you are going to use this as a track car, stick with the 3.73 dif. I would consider buying the 4.10 dif it if it were in good shape and a fair price for my track car. :)

[nando]

you are still going to have to shift before 60 with a 3.73, unless you're running a really high redline.

also I like that the 4.10 puts you right in the middle of the torque curve, it makes freeway driving quite enjoyable. the difference in fuel economy is pretty much nill.

[Rockinlittle325i]

well this will be my daily driver (one day track car)
would the 4.1 really kill my MPG?

[blunttech]

well this will be my daily driver (one day track car)
would the 4.1 really kill my MPG?

its not that big a deal, really. either diff is fine for a daily

edit.. actually if it were me i would leave whatever diff came with the car and invest your time and money on suspension and brakes.. if you have a stock m20 the car is going to be slow no matter what you do to it so make it handle and stop well. so much more satisfying. unless you start swapping or turboing just enjoy the car for what it is. a slow great handling car