Lets talk about Competition Diffs

[Scooby South]

As I am venturing into getting all my Diffs done...I would like some input on what people think about certain kinds of diffs...not name brands...but Styles...I guess it would depend on what kind of car your building...a Tarmac (Autocross) car...or a Rally (Rallyx) Car...

I am looking into the options for specifically for a Street Prepared Subaru

What options are out there?
Guess it depends on what diff you want to replace....
for the front...there are Helical, Suretrac, 1.0, 1.5 and 2.0 LSD's out there...

What are the advantages of either???

As for the Center diff..I Know there are about 3 different choices for the 5 Speed and couple of more for the 6 spd...

there is the factory Viscous 5kg Center diff, there is a Viscous Gymkana 12Kg center and a OMG Viscous (rally) 20KG Center....there are also options for the DCCD... with different lock up points....

The Rear also has alot of options....along with the factory viscous has a 1.0, 1.5 and 2 way option also....

What would be the ultimate setup for tarmac....
how about AutoX....

What would be the ultimate front/rear bias????

thanks...I hope this stimulates some good discussion......

right now...my SP Setup is:

-Helical Front diff
-Gymkana 12KG Center
-Factory Viscous Rear

Plan is to swap out the rear with a mechanical 1.5 way LSD...then I should be pretty close to where I want to be..

Bill

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[Homemade WRX]

you forgot the tarmac center diff that acts pretty much like an open diff when tires slip...

I'd personally choose a helical front (always smooth transition unlike a clutch styled diff...think of turn in/corner entry), a DCCD (smooth power transition and adjustable bias) and a nice clutched rear (I say nice as no one wants a jerky power transition to disturb weight transfer...1.5way personally)...I also do like things a bit on the edge of tail happy/loose.

[Splash]

Heh, I have always found it ironic that I can strand my AWD car simply by lifting a single wheel off the ground... <grin>

To contribute, your suspension will have a lot to do with your choices. If you ever get far enough that you lift a front wheel, the front helical will not like it one bit.

The stock STi clutch-type rear diff isn't strong enough to keep the car moving forward if you lift an inside rear and spin it. This makes the clutch ring re-ordering necessary, if not the pressure ring kit as well.

Suretracs act like helicals. Lift a wheel, it spins.

I'm not impressed with DCCD. Really, it's just a clutch. The fact that it tries to change things in mid corner for me doesn't help. The fact that the brake light deactivates it and leaves it fully open doesn't help either. In fact, I can't figure out why, when you do lift an inside rear wheel and it spins up wildly, why that doesn't trigger the DCCD to SEND MORE TO THE FRONT! When the car does that, it feels like a Mustang with too much rear bar.

Personally, I'd find a center that had some rear bias (but not much) and also didn't do anything automatically. Then I'd use a 1.0-way front clutch type and a 1.5-way rear clutch type diffs. I keep the front a bit on the loose side, wind the rear pretty tight.

[neilschelly]

Originally Posted by Splash

To contribute, your suspension will have a lot to do with your choices. If you ever get far enough that you lift a front wheel, the front helical will not like it one bit.

An FSP Impreza will never make enough power to lift a front wheel.

I'm quite interested in this topic too and have been trying to find helpful advice. People really seem to like the helicals up front. I've got a Phantom Grip in back that used to be quite fun, but has lately been succumbing to the power in my new engine. If I lift a rear, it spins annoyingly.

I got some advice from Patrick L (in the DSP Thread) that he felt the locked tarmac diffs were a bad idea for autocross and more intended for track use, but he didn't expand on it much. He just said he thought the stability of the car in slaloms and tighter elements would be compromised.

If I see things pop up this winter for sale, I'll probably hop on a helical front diff, a stronger clutched rear diff, and a stronger center diff. I don't think I'm going to actively seek out any of them though since I'm trying not to spend money this winter.
-N

[Storm]

Last week I talked a bit with Steve (zzyzx) about diffs and he had some interesting ideas. He didn't elaborate enough to completely convince me about keeping an open center diff that had a 50/50 bias. To me, eliminating the viscous fluid from an oem type center diff would prevent it from transferring any torque to the rear. Maybe I wasn't understanding his thoughts completely though.....I hope he chimes in with a little more explanation on the subject. I'd sacrifice a stock center to try it.

I would think that 35F/65R bias would be most helpful in our AWD applications.

Jay

[neilschelly]

An open center diff doesn't sound right to me either, but we really don't have it too much different in stock form. I had the axles pop out of my rear diff on me a year or two back. I was able to move, but it was as if my clutch was toasted all of a sudden. It took quite a bit of time to get rolling and once I was rolling, I could keep momentum reasonable well. I'm sure everyone behind me at every red light on the way to work that morning appreciated my slow starts. The point is that the stock center diff just spun the driveshaft with so much less load on it.

That said, if you had an open center and LSDs front and rear, that could be interesting. The center would never act like an open diff because neither side of it would ever be unloaded (assuming the LSDs were doing their jobs well).
-N

[joey1313]

My factory rear diff needs to be replaced in my 05 STi. Are there any issues with just replacing the rear diff with a 1.5 LSD without replacing the front and\or center diffs at the same time? I use the car mostly for autox and a very few trackdays. It seems most tend to swap multiple diffs at the same time.

[Scooby South]

Great comments....I would like to know exactly what the torque split is between each Center diff....

so for the 5 KG its 50:50

wonder what the split is for the Gymkana and the OMG rally GrpN...

I know someone thats put the GrpN on there car and they say its not very streetable...Instant lockup...and excessive wear lead to a premature failure...so I am hoping that the 12kG Gymkana will fair better..It should have improved lockup over the stocker...and have more give than the GrpN...thats my thinking anyways...

There is probably alot of uptapped time(on course) with properly setup diffs...and yes I agree that suspension will play a integral part of all this...

I do know that Open diffs are definitely NOT the way to go..

I even heard of some people welding the diffs up...(Old trick for domestic's to get a locker effect)....not sure if thats the right answer either..

Keep um coming...I really think there is some time out there if the car is setup correctly with the diffs..


Bill

[neilschelly]

Originally Posted by joey1313

My factory rear diff needs to be replaced in my 05 STi. Are there any issues with just replacing the rear diff with a 1.5 LSD without replacing the front and\or center diffs at the same time? I use the car mostly for autox and a very few trackdays. It seems most tend to swap multiple diffs at the same time.

Yes, you should be able to just replace one without much concern for the others. If you're replacing the front or cente, it often makes sense to replace both just because they're both in the same place, roughly, but the rear is very independently accessed, so replacing only it can make more sense.
-N

[solo-x]

I can see merit in an open center diff. If it were me, the first thing I would try would be a torsen front diff, aggressive lockup clutch type rear diff, and an open center diff with a 50/50 torque split. This is my reasoning.

1) The torsen front diff will avoid generating an understeer moment on corner entry. basically, off throttle it will behave like an open diff. A clutch type diff will tend to increase entry understeer.

2) An open center diff will minimize mid-corner push. The rear tires track inside the front tires, ie, they go a shorter distance. Any kind of limited slip action in the center diff will affect mid-corner balance, generally in the understeer direction as the laden outside rear will "under-run" and generate an understeer moment. Also, with the clutch type rear diff, any wheel spin will generally spin up the outside rear this way, which should free the car on corner exit.

3) Do I need to explain the reasoning for a clutch type rear diff? Super light/airborn inside rear tire and all that.

3 diffs is a ton of stuff to worry about, and my conclusions a suppositions may be way off. I only present the theory as another possible idea. Some big time testing with a quality logger will have to be employed to know for certain, and varying driving styles may play a part as well. I don't think you can go wrong with the torsen front diff and clutch type rear diff though. The center diff? Best to get to some quality testing! I'd leave that diff for last as well, as it won't matter what you do with that diff if the others are crap.

[Mechie3]

I definitely need diffs. I was lifting the rear wheel all day Sunday making the engine rev out and not push me forward. What diffs would benefit me the most for ESP keeping in mind that my car is my DD (and my only car) so I don't want anything overly annoying or that can easily get me stuck in the snow.

[neilschelly]

Originally Posted by Scooby South

Great comments....I would like to know exactly what the torque split is between each Center diff....

so for the 5 KG its 50:50

wonder what the split is for the Gymkana and the OMG rally GrpN...

I've only ever seen the viscous diffs being 50:50. I don't think it would be possible to make a viscous that weighted one side more than the other, or at least I can't imagine the mechanics that would allow it. Disclaimer: I'm not a mechanical engineer.

Originally Posted by Scooby South

I know someone thats put the GrpN on there car and they say its not very streetable...Instant lockup...and excessive wear lead to a premature failure...

That's pretty interesting. I guess that's why it was a rally-ready one, with the assumption that a little slipping is a good thing and pretty unavoidable. In rally, making that slipping more predictable and making sure the diff will push for it probably helps.

Originally Posted by Scooby South

so I am hoping that the 12kG Gymkana will fair better..It should have improved lockup over the stocker...and have more give than the GrpN...thats my thinking anyways...

What you said about the 20kg unit makes me think that any additional viscous diff would be a problem on tarmac. Perhaps the lockup that your friend felt could have achieved a better handling balance between over- and under-steer? If not, it doesn't seem like going half-way to it is a good idea either.

Originally Posted by Scooby South

There is probably a lot of uptapped time(on course) with properly setup diffs...and yes I agree that suspension will play a integral part of all this...

When I talk to race car builders and road racer engineers, I find that they obvious answer is to put a mechanical diff (probably a Quaife) in all positions. Then, the suspension just has to keep all the wheels planted. For the relatively steady-state turning on a road course in sports cars with proper sports car drivetrain layouts, power/weight ratios, and suspension mechanics, this makes sense to me. We have much lower power/weight ratios so that only the highly modified and very powerful Imprezas can get the rear planted and front tires unweighted (which is I think why helicals are popular up front). We also have grocery-getter suspension designs with MacStruts all around, so the control over tire movement isn't as strong.

If you can get the suspension to handle like a sports car though, it seems like the typical advice of a helical all around is the way to go at least. Does anyone think it's feasible to design an Impreza suspension that can keep all 4 wheels planted in tight corners while still being able to rotate around that corner? I kinda don't, so I think most advice will lean toward a helical in front and a clutch-based in back. The center is a mystery to me still, but open doesn't sound too bad to me if the front and rear axles will each control their own balance and keep spin under control.
-N

[Homemade WRX]

Originally Posted by Splash

I'm not impressed with DCCD. Really, it's just a clutch. The fact that it tries to change things in mid corner for me doesn't help. The fact that the brake light deactivates it and leaves it fully open doesn't help either. In fact, I can't figure out why, when you do lift an inside rear wheel and it spins up wildly, why that doesn't trigger the DCCD to SEND MORE TO THE FRONT! When the car does that, it feels like a Mustang with too much rear bar.

Personally, I'd find a center that had some rear bias (but not much) and also didn't do anything automatically. Then I'd use a 1.0-way front clutch type and a 1.5-way rear clutch type diffs. I keep the front a bit on the loose side, wind the rear pretty tight.

stop using the factory controller. that will solve your issues with it. Like I said it nice to be able to adjust it for what is needed. Mine lives at 65 rear.

for the open center diff comments, that is essentially an cusco tarmac. Open diff with a 35F/65R split. the tires that slip get all the power


As for Neil's comment about roadrace engineers saying to run clutches all around, that does make sense for a larger course with less angle and less drastic (in relation to time and distance) dynamic changes to the vehicle. think of how far a road course car transitions from braking to turning to being mid corner to exiting in comparison to an auto-x car.

[neilschelly]

Originally Posted by Homemade WRX

for the open center diff comments, that is essentially an cusco tarmac. Open diff with a 35F/65R split. the tires that slip get all the power

But if you get a decent diff in the front and rear, then I don't think it's fair to say that the tires that slip get all the power. The tire opposite the tire that slips will get more power, presumably because that end of the car is weighted on one side more than the other.
-N

[sjs0433]

welded diff is not really the way to go. My brother has one on his 240 and it is really annoying. You can always hear the inside tire chirping away and I'm sure it is killing the tires 10x faster than it needs to.

[CamaroFS34]

I'm really interested in what people think would be best diff-wise for an ESP WRX. I already know it's going to cost an arm and a leg compared to my Camaro, but, right now, I am pretty committed to the idea of running a Subaru (not necessarily mine ) in ESP next year.

Karen

[neilschelly]

Originally Posted by CamaroFS34

I'm really interested in what people think would be best diff-wise for an ESP WRX. I already know it's going to cost an arm and a leg compared to my Camaro, but, right now, I am pretty committed to the idea of running a Subaru (not necessarily mine ) in ESP next year.

Karen

I imagine the WRX would be in the same boat as the NA Imprezas mostly. I doubt you can overcome the suspension limitations of lifting the inside rear wheel on cornering (unless maybe an '08 with the double-wishbone?). How much power can an ESP WRX make? Do you think you could transfer enough weight on corner exit to the rear axle in order to lift a front wheel?
-N

[solo-x]

Neil, you can easily get the inside rear to stay down.

Karen, I'd say the same thing for an ESP WRX as I said earlier.

[KC]

Originally Posted by solo-x

Neil, you can easily get the inside rear to stay down.

Yep.

--kC

[neilschelly]

Originally Posted by solo-x

Neil, you can easily get the inside rear to stay down.

Originally Posted by neilschelly

Does anyone think it's feasible to design an Impreza suspension that can keep all 4 wheels planted in tight corners while still being able to rotate around that corner?

I'm sure you can get the suspension to keep the inside rear loaded, but didn't ask that. Having the inside rear unload a bit though helps in getting rotated around the corners.

Originally Posted by Homemade WRX

As for Neil's comment about roadrace engineers saying to run clutches all around, that does make sense for a larger course with less angle and less drastic (in relation to time and distance) dynamic changes to the vehicle. think of how far a road course car transitions from braking to turning to being mid corner to exiting in comparison to an auto-x car.

I didn't say clutches all around, I said Road Racers would tend to go with helicals all around. But yes, the reasoning is the same.
-N

[KC]

There's a difference between getting a car to rotate, and getting a car to rotate that suits your comfort level.

One will yeild slower times than the other.

--kC

[Homemade WRX]

Originally Posted by neilschelly

But if you get a decent diff in the front and rear, then I don't think it's fair to say that the tires that slip get all the power. The tire opposite the tire that slips will get more power, presumably because that end of the car is weighted on one side more than the other.
-N

ok, so you have uber nice quiafes up front and out back...and you break traction in the rear, you'll spin the rears only until you regain traction.

I know this from driving dom's 450lb-ft RS...he had a cusco rear stacked to practically locked.

[Homemade WRX]

Originally Posted by neilschelly

I didn't say clutches all around, I said Road Racers would tend to go with helicals all around. But yes, the reasoning is the same.
-N

ah, sorry but the reasoning is indeed still the same. IF you can always keep the 4 tires planted and have enough traction that lessening weight on a corner won't allow spin, helical's all the way around would be best...but we don't live in a perfect world.

[neilschelly]

I feel I'm being misquoted. I said the road racers I've talked to have always said quaifes all around and keep the tires on the ground. I also said that I don't think that's appropriate for autocross as we're not driving proper sports cars with proper sports suspension and drivetrains and power/weight ratios and we're not driving on tracks with long steady-state turns.

Then I expanded on that to say that I think that's why the helicals are so popular on the front of Imprezas with a clutch diff in the rear because of the inevitable lifting inside rear tire. I said I think my ideal would be the helical up front, a strong clutch-type diff in the back and then the center would be a mystery to me. I don't think that an open center is so bad so long as the front and rear are doing a good job at locally keeping spin to a minimum.

Please re-read my post if you don't believe me - I haven't edited it.

And to KC's comment...
As for the right way and the wrong way to get rotation around a corner, I'm just missing something. From a skills standpoint, I'm missing a lot. From a mechanical standpoint, it's all about traction. Mostly, I've been modifying that with tire pressures this year, but in Finger Lakes, I dropped the rear bar because I found it so slippery out there in general (as my co-driver can attest to with his several scary-as-all-hell off-course excursions even with all 4 tires planted.).

Having the rear bar down kept 4 wheels planted enough that I could exit corners with 4 tires on the ground. At the beginning of this year, I had the bar up at its stiffest and couldn't lift a tire for the life of me though (and I've turned it up again for the last few events at Devens too). I suspect that at the beginning of the year, I was throwing the car into turns and finishing them much sooner, so that the suspension never had time to really get a tire in the air. As I've gotten used to the 245s I've been running though, I've gotten to have longer, wider turns where the suspension is leaning more (due to higher speed and longer time). It's a difference of line that has made me faster I suspect, but now I'm lifting a rear tire again at least occasionally. Next year, I'll be on wider rubber again, and I imagine that problem will exascerbate itself some more. I wish I did know more about proper rotation I guess.
-N

[Pacobeagle]

Cusco Tarmac 35/65 split f/r respectively