Track only, welded vs open diff (LSD not an option for this exercise)

Short version: Non-M E46, if the only two choices for road course use for a season of track days were open or welded (same ratio), which way is the better choice?

Longer version: E46 330i budget limited build of my buddy's. Currently has a 2.93 diff and I happen to own (3) 3.38 E46 diffs. Told him we should pop one of those in for a little more ZIP before an event at VIR in March and he suggested I weld the spiders for him since "who cares?" about the diff itself. Just wanted to weigh the cost/benefit in terms of handling, and potential negative impacts on other components.

Appreciate any thoughts.

I'll caveat this by saying that I've never driven a car with a welded diff. 

OK- that said, I can't possibly imagine a welded diff putting down decent lap times compared to an open diff. My evidence for this is that there are hundreds of race cara with open diffs, and zero to my knowledge that have welded diffs. Forcing drift mode in every corner sounds very slow to me. 

Except for the spool in the Porsche 935. 

We have a friend of a friend of a friend (a track acquaintance, if you will) who runs a welded diff in a 325i. But given the degrees of separation in our "acquaintanceship" I wouldn't know how to solicit his feedback beyond looking for his car in the paddock next time I'm at a track.

So, it's not ZERO, but agreed...that's what gives me pause. Just isn't common. Or at least not that I'm aware of. 

Depends if you intend to drive sideways...

Ran one in an ITC class 73 Corolla SR-5.  The car was very stable at constant throttle, and you could easily steer with the throttle.  If you get in too deep, your chances of a good exit are pretty low.  Very stable under braking.  If you push the car by hand, and the steering wheel is anything but straight ahead, bring friends.  I'm not sure it was really needed on a car that made a tad over 100hp at the crank, but we had the track records at Moroso and Sebring Club.  The guys that ran oxygenated fuel had the records at Daytona and Sebring full.

I ran a viscous LSD on my Spec Miata, and after three or four laps at speed, it was basically an open diff.  Motor made 112.8hp at the wheels, and I never noticed that the lack of a diff was hurting my exits.

In short, I think it has a lot more to do with driver comfort in lower powered cars.  It will feel more stable in the corners with the Lincoln Locker, but it probably steals a little hp compared to an open diff.

We ran a mini spool in the back of an S10 Lemons car for years.  We also tried a gov lock, some other terrible one I forget, and finally a zexel Torsen from an F body.   In the end, we put the axle with the mini spool back in, it was the easiest to drive and fastest, especially on tracks with uphill corners.   Even after we sold it the next two owners also thought the limited slip would be better so they swapped axles to the torsen, and both of them have gone back to the mini spool, which it has to this day as it is still active.  This has all been over a period of 10 years.   The only time it gave any problems is parking in the paddock.  It would be no good for a street car.   
 

Don't listen to the haters who have never tried it.  For a race only car it is a fine option. 

I've driven cars with welded diffs and they don't do anything you want a car to do on a road course.

Formula 600 cars have a solid axle with no diff. They just require a different setup and driving style

The varying opinions are fascinating (and I mean that sincerely). Looking up the 935 Porsche got me to looking at other spool rear race cars... Apparently the V8 Supercars in Australia have been running spool rears since forever and ever (and at a quick glance, appear to still).

Now, the power level of either of the above compared to my buddy's 250k mile m54b30 makes the comparison moot, but seems at least some track cars do well with a locked rear. 

In reply to Sonic :

I wonder if it's a stick axle vs IRS situation with the spool?

I can see the body roll on a stick axle car, especially with leaf springs, help limit traction on the inside tire. That would possibly allow for some slip at the inside tires on corners to avoid having it fight the driver.

I wonder if an IRS car would see a different effect due to the inside rear tire maintaining better contact and not allowing as much slip when cornering?
 

Since we are talking about your stuff, not mine: weld one up and report. Then swap for science. 

It's somewhat common in lemons. I think part of the reason you don't see it more is that folks tend to run a factory Limited slip in a stockish application.

Let me start out softly: welded diffs on road race cars suck. The only reason people run them is because on many cars they are faster than the open diff and their budget doesn't stretch to a limited slip.

Proper limited slip diffs are worth every penny. 

There are anomalies; F600s are one of them and Karts as well. These vehicles have a very narrow track so they more easily unload the inside rear tire. If there was one thing I could have changed on my F600 it would be having a limited slip but the class rules mandate a kart axle.

I've driven an A-sedan Camaro with one as well; chucking the car around to compensate is hard on tires.

Not a welded diff but in my CAM T Mustang a Detroit locker had much better turn in than the torsen I replaced it with,but don't dare touch the throttle until the exit is in sight. No breathing the throttle and don't even light throttle will balance the car mid turn.

My experience with such things is limited, and is primarily with RWD Toyotas with solid axles, so no promises on how much also applies to IRS BMWs.

Took an open-diff car/setup I had experience with, and put a spool into the same car. On track, still using my accustomed open-diff driving style, turn-in performance was worse, and the car tended to understeer on corner entry. The solution to the turn-in issue was to grab it by the scruff of the neck and be more aggressive; drop throttle suddenly while shifting weight forward, and steer it with the throttle into and through the corner. Fun when I got it right, but slow. Harder to control. Not always viable in high-speed sweepers. Not popular with all the safety stewards.

It might work with the right suspension setup and driving style. I didn't put the time in to figure it out. My takeaway was that for track use LSD>Open diff>Welded/Spool. For rallycross, drifting, or fun hooning, by all means.

I drove IT and GT Mazdas for years with welded diffs. I will say that it all depends on the chassis, suspension set-up and driver ability. They can be driven well and driven fast, but if you're timid on corner entry and can't commit to getting the car "loose" mid corner and driving it hard on-throttle to get the car rotated, they push very badly. If you're late on corner entry, you're kind of screwed. 
 

'In the end, it all comes down to a mindset and commitment. They are fun to slide around with a car with enough power to haze the tires. On high HP cars it more difficult to control. 
 

If properly set-up(suspension), I be;I've an open diff would be faster. Both types of rears will require different set-ups(as-ring rates, sway bars and shock comp/rebound).

Just my 0.02¢

I think the 956 ran a spool as well, and they did okay :) I know it's the thing to do in the Miata drift wannabe crowd. We did it to a high power Miata in 2002 because we couldn't get the car to hook up out of corners, but knowing now what I did then I'd probably do something different. You'll definitely have to watch for corner entry under steer. 

My opinion: Welded differentials are great for rock climbing and drifting. I've driven vehicles set up for both. I've never driven a car with one on the track. I suspect it'd be a perfect way for me to quickly exit the track sideways. 

I'd go with the lower gearing, open diff and learn how the car handles with that in there for several events before welding anything up.

Since it was posted before, Porsche used spools extensively on the 935 and 956/962 cars.  IIRC, they used it on the 917/30 as well.  It was entirely based on it being the most reliable option.  

I drove a FSAE car with a torsen and then with a spool.  Lap times were similar, but there was a steep learning curve to become proficient with the spool.

I cannot recommend welding the diff unless your friend is committed to spending a big part of the weekend learning how to drive it.  

In reply to pushrod36 :

With the FSAE, I wonder how much that had to do with the tighter courses those run on? I don't feel it was that steep of a learning curve with no differential on a full road course

my only welded diff experience is with FWD.  we ran welded diffs in the Chump Saturns, and they were significantly better exiting corners vs an open.  i suspect if there was a drop-in LSD or Torsen, we would have used that.

I had a friend that worked for Roush back when they were running cars in GTO. I think they won the Rolex24 a bunch of years. Those cars had a spool in them for the 24hr, I'm not sure what they ran the rest of the season. Part of it was to get back to the pits if an axle got broken, part was simplicity and heat generation.

On the 330 the car will apply the brakes on a spinning wheel through the DSC system. I'm considering welding up a diff that I have to see how it feels before drop the $1200 for an lsd for my Spec E46. I know I'll end up with an lsd in the end because in a spec class the last few 1/10th of a second a lap are the hardest to get.

People who have never driven a welded diff car are always the first ones to tell you how bad it is . While a welded diff is way worse than a nice lsd, at least it puts power down. Our lemons car did faster laptimes with a welded diff than with open diff, and that is the goal. It should cost $0 to try it, so if it's faster great. If not get a real diff.