how much can a stock FD differential handle?
 

I think I f#c*^@ my diff up, I was at the track today, on my 3rd run I dropped the clutch at about 3k and grinddddding sound, it would not go foward, i had to tow it home about 30miles, and cost about $200. fu*^. I figure its not the tranny becaause it shifts into all gears smoothly, just when you start to go, you hear the awfull grinding-metal-twisting noise. i dynoed in about 367rwhp, is the stock diff able to handle this?

bump

No need to bump a thread at 2AM...not going to get any faster responses. :D

Well, I guess you answered your own question. ;)

Once the power numbers go up, the longevity of the diff goes down (among other things). Dragging puts a lot of stress on the drive train and it's somewhat well known that the FD diff doesn't like to be abused like that. How much is too much? There may have been cases where close to stock power have been enough to lunch it.

I had thought that the trannys were pretty strong (other than the synchros). But I've witnessed the same thing with another putting down close to the same power you are...and at the strip too.

The stock diff should be able to handle that power easily. Something must be up with your diff, such as Did you ever experience alot of wheel hop??? Has the diff ever been taken apart? I think it is your Torsen LSD, it will need to be relpaced or your bearings are gone if the diff was taken apart and not setup properly.

If you were drag racing it is your Torsen 100%.

Some sticky tires and a drag launch at that power will tear up the stock diff as you have found. If you really plan of drag racing, you'll most likely end up running a KAAZ or TII diff with chromoly axles. The problem is, once you tighten up the diff, then something else has got to give and that will be your axles.

You could probably use some tires which aren't as sticky. That would allow you to spin the tires a little when launching and reduce the shock to the diff and axles, but then again that depends on how serious you are going to be with it.

BTW, don't bump messages when it hasn't even been on the forum for 15 minutes. Only bump messages when they drop off the first page.

sorry about the early bumps, Anyways does the T2 Diff just mount up on a FD? Is it just built stronger and better?

Yes, it's basically a bolt up change.

Stronger? For drag launches yes. Better? That depends on what you are doing with the car. For drag launches, definitely.

what about autocross? are the internals just stronger on the T2 tranny?

I have no proof, but I would imagine that the FD diff is better suited for AutoX/Road Course driving than the TII diff. The TII diff is typically a good change for the drag racers as it's almost as rigid as the KAAZ unit without the clucking that can come with the KAAZ.

You really can't build a perfect car for the twisty stuff and for the drag stuff. You'll have to make compromises somewhere. As the old saying goes, Jack of all traits, but a Master of none. Which means you'll have to make compromises to do everything 'well', but you won't do any of them 'great'.

For auto-x or full road course, the stock is plenty strong enough. It's the shock from a launch that kills them.

Constant, smooth power delivery is the diff's best friend. As far as the T2 being better than the stock FD in that regard, I can't say, but I'd bet that there's no issue there. It would probably be less of a hassle to replace it with a KAAZ unit.

thanks for your guys help and quick response, im going to go with the T2 unit.

The stock diff should be good into the high 10's. Thats when you start breaking them noramlly. Then you upgrade to a kazz or s4 tII and you should be good to low 10's. Then the axles start breaking. Upgrade to 300m axles and you should be good into the 9's.

Basicly the stock diff is good to around 450 whp. It will last quite a while like that. With a tII clutch pack installed and 300m axles you should be good to around 650 whp. Now it wont last forever but it should last a couple of years at least.

On a side note some times things just blow up because they are old or were treated poorly in the past.

There is no way in hell the stock FD diff will take that kind of power on drag radials or anything else which will allow it to hook up on a launch.

The only way would be to use some tires which would allow the car to spin on the launch as to reduce the shock to the diff. However, that's less than ideal with drag racing. People have been breaking the stock FD diff anywhere in the 340-400 rwhp range when using drag radials.

Question:

Isn't the peak load on the diff during a launch more a result of transferring the rotational momentum of the engine & flywheel into forward motion of the car?

The real determining factor for the load on the diff, it seems to me, is the torque available through the clutch to the rear wheels, which would be much the same, everything else being equal, no matter what the HP.

And, assuming it is, then engine HP, per se, is less of a factor than the flywheel/clutch/engine rotational inertia, and a lightweight flywheel would be less bad than the stock one.

It also seems to me that drag slicks and a heavier clutch would be added once the HP got so high that getting the car to "hook up" on street tires and getting the clutch to live became difficult, and that is why there is a correlation between large HP #'s and breaking diffs. IMO, it's the drag slicks and H-D clutch, not the HP that is the determining factor. Of course, big HP requires these, so maybe its only a matter of semantics. :)

Comments?

Dave

moktan, you might want to update your sig. No way in hell you made 367 rwhp with the mods listed.

I had a brand new cryo-treated S4 TII diff installed along with a 4.3 gearset not too long ago and I'm very happy with the combination.

You'll need about 100-200 in additional parts----a pair of side oil seals, a pair of bearings, and (if you decide to go with the 4.3) a crush sleeve. Some people reuse the oil seals and bearings, but to do the job right they should be replaced.

Your right, definatly not making that much power from the mods listed on my sig, that was about 11months ago, the car was built by Chuck and Sunny at Rotary Extreme, the final Fd he worked on. Im running 99spec turbos on race map, pushing 15psi, mild street port and a list of many other goodies that I havnt had time to update on my sig. will do shortly.

Differential swap?
 

The T2 differential just swap into the rear end of the Fd, My stock diif is gone on the fd.

If your asking if it fits yes it does. You will need to be able to set back lash, and check it for proper fitment. And its just diff not pinion and ring gear.

I would replace the bearings and axle seals too. Its a good idea to have some extra shims in case you need to adjust the lash. Doing differential bushings at the same time wouldnt be bad either.

MERGED threads since Rich already answered your question last week.....

Im not very clear, I know that the diff and the gears are inside the diff housing, I just wanted to know if I were to purchase a diff casing with with everything inside it from a T2, would it just bolt up to the Fd? Is it a direct swap or do I need to crack open the differential housing, swap all the guts from a t2?

Also on the other hand, I actuall dont know if my differential went out for sure..... i can shift all the gears freely, when I ingage into first and start moving real slow, and feathering the clutch (I mean really slow), the car moves foward without any problem but as soon as i realease the clutch just a just a tad bit it starts this horrible grinding and twisting noise. Is this my diff or something else?

I also wanted to add, that this occured while i was at the track for drag races. I made my mistake using two different tires on the rears, same size, similar trad pattern but different brands, maybe this is what caused it?

If you want to know what's really broken, you need to get the car up on the lift and start inspecting. If you can actually roll forward, it may only be the PPF.

Well, drag launching will toast the stock diff, but using two different tires certainly doesn't help.

As I understand it, you are just swapping out the entire pumpkin, that's why Rich and Ihor recommended you replace the seals and axle bearings (and set the lash).

It rolls foward in neutral no problem and evnen a little while its in gear while I feather the clutch and go about 1mph, but If I start to realese it then shit happens...what is ppf?

Power Plant Frame.

It is *kinda* like a torque arm (though not really, but if you didnt know better you could make the mistake of thinking it was one)

It connects the engine/tranny to the diff. It also serves to keep the driveline in line, and as the rear tranny mount, and front diff mount.