Final Drive

[E30m42cab]

Okay, I've never really gotten into the rear axles before... As far as I know, I have a 4.27... Is it a LSD? Is a LSD good, or bad? What do the numbers mean? Is this a good differential for highway speed, or better for acceleration? Is there any way to put a better diff for highway speeds? I'm pulling around 4000rmps doing 140km/h on the highway... which kinda sucks as far as I can tell. Feel free to link me up, or just tell me the straight story... Cheers!

 

[echothreezero]

the number is the ratio 1:4.27. For every 4.27 turns of the drive shaft, your wheels do a single turn. Thus a higher number indicates more revs for the same speed.

So you want a higher number (which is a shorter ratio) for take-off performance, or smaller engines, and a lower number (taller ratio) for highway cruising and economy.

LSD is good - it helps prevent a single wheel from spinning when you lose traction which relates to better handling, more predicatable and controllable oversteer, and better doughnuts.

BMW LSD's are indicated by a tag with an 'S' stamped on it on the diff although these commonly are missing. Jack the car up with the car in neutral and turn one wheel. If the opposite wheel turns in the opposite direction you have a standard diff. If the opposite wheel turns the same direction then you have an LSD.

 

[Big Daddy]

Well put, good information and very accurate.

 

[Doughnut]

the number is the ratio 1:4.27. For every 4.27 turns of the drive shaft, your wheels do a single turn. Thus a higher number indicates more revs for the same speed.

So you want a higher number (which is a shorter ratio) for take-off performance, or smaller engines, and a lower number (taller ratio) for highway cruising and economy.

LSD is good - it helps prevent a single wheel from spinning when you lose traction which relates to better handling, more predicatable and controllable oversteer, and better doughnuts.

BMW LSD's are indicated by a tag with an 'S' stamped on it on the diff although these commonly are missing. Jack the car up with the car in neutral and turn one wheel. If the opposite wheel turns in the opposite direction you have a standard diff. If the opposite wheel turns the same direction then you have an LSD.

Also, When you remove the diff from your car it has and "S" painted in white with a box around it indication it's an LSD

As Graham said, A "standard diff" is often referred to as a "open" or "one legger diff"
And an "LSD" is called a "limo" or "double leger diff" (for the young ones out there!")

Scotty get out there put that m42 to the limiter and drop the clutch (or "D" slap, I can't remember if your running a slushy or a manual gearbox )

 

[E30m42cab]

Scotty get out there put that m42 to the limiter and drop the clutch (or "D" slap, I can't remember if your running a slushy or a manual gearbox )

Dude!! M42's never came with a slush box attached!... and if they did, I would never own one... I think my rev limit is 7200rpm's, with the torque peak around 4600rpm... These engines are a blast to wind up, but overall a very fatiqueing car to drive as you have to constantly be on the shifter to keep your revs up in the power band. (vouch for me Dan!) Joseph, I think you'd like the M42 better than the M20, honestly.

Thanks Graham, well explained.

 

[Doughnut]

Joseph, I think you'd like the M42 better than the M20, honestly.

Bah!!!
only good thing I can really think about owning an m42 powered bimmer is that there is no valve adjustment and the dohc.

Four poppers just don't float my boat !!

 

[Big Daddy]

But the M42 is very responsive! I throughly enjoyed mine and drove it to the max on many occassions. A fun car.

 

[echothreezero]

As much as I love the torque and song from a six, I think the weight distribution of a 4 in an e30 has to make up for the power difference when you start talking m42.

 

[E30m42cab]

But the M42 is very responsive! I throughly enjoyed mine and drove it to the max on many occassions. A fun car.

Really the only way you can drive that car... acceleration isn't spectacular, but the trottle response is immediate, even punching the throttle on the highway in 5th... I always found that I had to downshift to pass on the highway, but the M42 gets up and goes all on it's own. I think the dual throttle body definately helps!

As much as I love the torque and song from a six, I think the weight distribution of a 4 in an e30 has to make up for the power difference when you start talking m42.

Poor man's M3! I loved the torque and exhaust note from my old 2.7L, but this car is more of a challenge to drive in a spirited manner as you don't have the torque of the M20 to carry you around and pull you out of corners. With the M42, you have to rely on precise braking and proper gear selection when entering and exiting corners.
The M20 you just swing the tail out and aim the front tires. The front end is more planted on a 325 whereas the 318 tends to want to understeer, which means your revs have to be high to get into the torque curve in order to bring the ass-end around to correct the front end slide... which means totally balls-out driving!

 

[E30m42cab]

Bah!!!
only good thing I can really think about owning an m42 powered bimmer is that there is no valve adjustment and the dohc.

Four poppers just don't float my boat !!

The best part about the M42 is the self adjusting vavles for sure... If you've never owned an M20, it probably goes unappreciated. The DOHC is sweet, and the timing chain means you don't have that PITA job of doing the timing belt every few years. I just installed a new chain tensioner on my dad's M42... took me approximately one minute! Perhaps the best part about the 4 banger is that there is actually room to work on stuff without pulling the rad all the time. Only 2 main rad hoses. No stupid distrbutor cap/rotor to replace. The M20 just seems dated, even in it's own time.

 

[Doughnut]

The M20 just seems dated, even in it's own time.

I'd probably have to agree with that statement on behalf of my old m20.

A manual gear driven distributor with electronic injection, It's like they just got lazy or something when they where developing the motor... Crappy L-jectronic.

 

[echothreezero]

Not really - it was developed in the early '70s don't forget for the e21, and just freshened up for the e30.

The m42, s14 etc were developed much later.

 

[E30m42cab]

okay, so basically, if I want to go faster on the highway at lower rpm's, I'd wan't a lower number like 4.10 or 3.92 or something, but I'll lose out down low accelerating from a stop?

 

[echothreezero]

exactly.

 

[E30m42cab]

got it. Thanks again Graham. I'm like that kid that made it through high school without learning how to read. It's like I know a bunch of stuff about these cars, but I don't know simple stuff. Too afraid to ever ask the stupid questions.