3.73 LSD Question

[e3091318is]

I noticed the previous owner replaced the stock diff with a 3.73 LSD.  I am almost finished my engine rebuild and was wondering what differences this 3.73 is going to make.  I have did some reading online and got mixed review.  Some say its all around better with the 0-60 in 2nd gear, better MPG on the highway and lower RPM on the highway.  On the other hand I have some people saying it makes the car a complete slug.  Does anyone actually have personal EXPERIENCE with the diffs on the M42?  This is my first manual transmission car and I am not the best in the world.  So I am hoping the 3.73 will help me a bit considering I won't have to shit to 3rd just to get to 60.  Sorry for the ignorance.  Appreciate any helpful answers.

[keflaman]

I'm looking for one to install on my car. Most of my driving is rural with a little highway. I took a 400 mile round trip last week and averaged 30MPG @ 74 MPH and was quite happy with that, but I feel first gear is too low and I'd like a lower RPM on the highway.

Desktop Dave is running a 3.73 and has commented quite favorably several times on its performance.

[DesktopDave]

Yep, that's exactly what I'm running.  The car is slightly slower at times, but there is much less shifting.  Around town, I can still start in 2nd gear even on slight uphill grades.  The 4.10 makes 1st gear useless for my driving style.  That makes the car feel a bit handier IMHO.  The car is at its bets on the highway though.  Given that I prefer to average about 75MPH, the motor is turning about 3K RPM (just under the motor's torque peak).  It's a great setup for quick passes.

But I'm no speed freak by any definition.  The best part is the excellent fuel mileage with this setup - 31MPG overall average during my ownership.  I always figured I'd get a 328i or M3 if I wanted to go fast.  I am also running OEM BMW 14" rims with cheap 195/70 tires, ~25" circumference IIRC - don't forget to factor that into your desired final drive ratio.

But if you're an acceleration junkie...you'll be far happier with a 4.10, or even the seek the exotic 4.27 & 4.45 LSDs I've heard are out there. 

[e3091318is]

Yep, that's exactly what I'm running.  The car is slightly slower at times, but there is much less shifting.  Around town, I can still start in 2nd gear even on slight uphill grades.  The 4.10 makes 1st gear useless for my driving style.  That makes the car feel a bit handier IMHO.  The car is at its bets on the highway though.  Given that I prefer to average about 75MPH, the motor is turning about 3K RPM (just under the motor's torque peak).  It's a great setup for quick passes.

But I'm no speed freak by any definition.  The best part is the excellent fuel mileage with this setup - 31MPG overall average during my ownership.  I always figured I'd get a 328i or M3 if I wanted to go fast.  I am also running OEM BMW 14" rims with cheap 195/70 tires, ~25" circumference IIRC - don't forget to factor that into your desired final drive ratio.

But if you're an acceleration junkie...you'll be far happier with a 4.10, or even the seek the exotic 4.27 & 4.45 LSDs I've heard are out there. 

I appreciate the good answers!  I am not too much of a speed freak either.  I would rather have easier shifting by a gear then half a second 0-60.  I won't be punching it very often.  This may be a dumb question but how fast can you expect to hit in 2nd gear?  I too also have 14 inch rims but I think mine are 205/65 if I am not mistaken.

[DesktopDave]

Funnily, the car is faster to 60mph as I don't lose time with that 3rd gear shift!  The car will do almost 70 in 2nd IIRC.  First is good up to about 35.  If I take the car out I'll post some data.  Alternatively, I could do it the easy way with a calculator...

Found this very interesting site with transmission ratios (unfortunately not the E30  ), and this one with the E30 'box ratios.     Found this online speed calculator.  I used 195/65 R14 OEM sizes with the 3.73.

Given three engine RPM (1000 RPM idle - 3500 RPM torque peak - 6500 RPM red line), a 3.73 results in the following speeds:

1st gear (3.72) 5-23-33mph
2nd gear (2.02) 9-43-62mph
3rd gear (1.61) 12-53-77mph
4th gear (1.00) 19-86-124mph
5th gear (0.80) 24-108-155mph

[e3091318is]

Very nice.  I will definitely keep my 3.73 if I can hit 60 in 2nd.  The 3.73 seems much more practical for all around driving.