Jump to content

M20 And A One Tire Fire (Purists Need Not Apply)


13
 Share

Recommended Posts

Hey guys,

 

As a few of you know I am getting closer to having the M20 swap in my 2002 finished and pretty much being stored for winter. Anywho i've been building the car and its real only purpose is to be a fun street car to toss around from time to time on the local drift/auto cross days.

 

Lately I've been tossing around the idea about welding the rear diff up, just so I can use the power of the M20 to shred both tires at will. Almost every post i've read on the FAQ about welded diffs go on about how awesome it will hook up in a straight line but absolutely be useless and crazy squirly in the corners (obviously it will try to kick out if you over step the throttle or carry any real speed). This is somewhat worrying to hear especially if that happens with mild M10's.

 

So what should one do (finding a local lsd rear end for a 2002 is a no go)? I do plan on taking the car on spirited drives on back roads but the last thing I want to do is have it kick out with an over zealous right foot on a public road (i'd imagine that m20 will toss that little 2002 around pretty easily). Is it worth welding the diff and living with the pains involved, or is there any other retrofit parts I can use? I've heard ramblings of small case e30 innards but no clue if that is true or if any are lsd.

 

I've driven my buddies 396 chevelle with 4.11 and lockers numerous times even in winter and I never really had much trouble in slippery or dry conditions. He uses the Chevelle as a daily driver year round and besides being a total pain at slow speeds he has never really had any issues with a locked rear end...

 

any advice or input?

Link to comment
Share on other sites

If you plan on auto-xing with a welded diff it is not a good idea. In fact, I would only weld the diff. if just drifting the car and taking it out on weekends for slow cruising was going to be its purpose. 

 

I guess I should defend my reasons. Putting down any power while turning the car might and probably will induce over steer into your line in auto-x. And it will kill your tire wear on routine driving. I know in a 240sx with a welded diff. even taking a turn at a slower place and the wheels will squeal. 

Edited by ejbluth
Link to comment
Share on other sites

my miata has a welded diff (needed a replacement quick and got a weldy from a friend). it chews the tires heaps. i drive it on the street only and while it's ridiculous, predictable fun in the wet, it's also hard to drive responsibly and very uncomfortable with grippy tires, as well as making the steering much heavier (which would be even worse with an M20 i'm guessing)

 

i plan on changing it eventually. i say get an LSD however you can.

Link to comment
Share on other sites

Yeah, some form of LSD.  

 

For racing, a locked diff isn't too bad.  But the few times I ran them on the street, they were just uncomfortable.

The car binds, balks, doesn't turn in well on easy corners, and is a bear to  maneuver in parking lots and such.

 

And for an M20 swap, going to the E30 diff (do a search, it looks like it's down to a pretty easy job) makes sense.

Then you could run the lower- ratio mid- case gears (3.23 and such) which, with the extra torque of the M20,

would make the 2002 into a real high- speed long- range cruiser without hurting acceleration too much.

 

And you can get an E30 LSD on ePay for just a couple hundred bucks.

 

What I'd do, if I had your 'problem'  (and I wish I did!)

 

t

Link to comment
Share on other sites

Installing an E30 diff on a 2002 is not a simple bolt on.  It's not even an easy weld on either.  2002haus makes a rear subframe for the 2002 that will take an E30 diff.  Of course, you also need to make a driveshaft and axle shafts to fit.

Welded diffs are difficult to drive on the street.  Turning becomes a chore especially in a car with no power steering.  Welded diffs work well on the track.

The bigger problem with welded diffs is the stress it creates on the axle shafts.  Those are prone to fail on track cars.  In fact, racers get them regularly magnafluxed to check for cracks since a failure can have disastrous consequences.

Get an LSD.  Build it to a higher lockup setting.  There are many gear ratios available from E30 small case diffs (and E36 ti and Z3) that will fit the E21 LSD case that will bolt up to a 2002.  I have a 4.44 diff off my 96 318ti automatic that was converted to an S52.

Link to comment
Share on other sites

Guys .... we need to be clear on the size of the diff in the first place.  As we know (but the OP might not know and get confused about) was that the E30 used both the small 168mm and medium 188mm differentials depending on models (318's got the smaller).  We also know that an E30 318 LSD center will fit in an 02 case and this is a budget friendly option IF the person is capable.  But this is also not a beginner job.

 

OP - This may help you .... http://www.bmw2002faq.com/_/technical-articles/engine-and-drivetrain/strongest-2002-limited-slip-differential-build-r74

Link to comment
Share on other sites

... I did say mid- case... but yeah, Andrew, you're right, I was talking about changing to a 188mm ring gear, the 'Medium' sized BMW diff,

with the subframe that's been talked about within the last year.

 

No, it's not a trivial job, I agree, Pierre- but given that the 3.64 LSD has gotten hard to find in 168 mm size, the extra effort

to go to the mid case will be offset somewhat by the availability of the 'lower' (3.64, 3.23, and even 2.93)  E30 diffs. 

And a nice, long- legged diff is a good match to an M20 in a light 2002.

 

Never had halfshaft problems with a welded racing diff.  Nor even stubshaft problems, really, beyond the usual.

DID have several 'un- weld' themselves before I learned to weld the pi$$ out of them.

 

Jus' sayin'

 

t

Link to comment
Share on other sites

We tried a welded 3.64 diff in the A/X car once. Car launched like a rocketship but wouldn't rotate or turn worth a damn. Basically had to push like crazy to get it to turn and when it did more than likely broke loose and came around. Net result was to take a neutral handling car and trade better straight line acceleration for an evil handling pig that fought you at every change of direction. Reminded me  of one of these all wheel drive turbo cars that want to go straight and can be forced to turn but only with suspension tricks and huge tires. The A/X courses we have here in Atlanta are generally pretty rough so suspension compliance is generally more valuable than ultimate grip. If the car is in the air or won't turn having great traction doesn't really matter.  

Link to comment
Share on other sites

Okay thanks guys, I think I will have to bite the bullet and build the frankenstein e30/02 LSD AceAndrew talked about.

 

The car will be pretty much finished by the time it gets cold out again so it will keep me busy over winter and I know where I can source a ton of those parts for a decent price so I guess its worth a try. Building the rest of the car to handle abuse so I might aswell build a decent diff too. Should compliment the new powerplant, and refreshed suspension and brakes nicely...

 

Biggest reason is indeed autocross. We usually have pretty tight courses and if the thing is going to be that big of a pain I might aswell just go LSD and try to keep a decent lap time. Besides, with big sticky 205's (Still might go a bit bigger) I could imagine how much it would want to go straight with a welded diff.

 

What ratio should I try and look for for a M20 power plant. I dont want it to be so low that 2nd gear is just screaming, but not so high that it will bog (if thats even possible with the car lol)

Link to comment
Share on other sites

www.r3vlimited.com for your gearing questions- then shoot for the lower end of the recommended ratios,

because you'll be lighter.

 

The M20 pulls strong from 3500 to 6200 with a stock chip.  Unless you port it and open up

the air pathway (flapper door especially) you really don't get much over 6300.

 

Took me a LONG time to get used to shifting so early- the M10 was making good power to the

7500 limit, the M20's doing nothing at 6500....

 

t

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

  • Upcoming Events

×
×
  • Create New...