Iknow the LSD won't give you more bottom end torque, what I was refering to was the difference in gear ration. If I'm not mistaken there is a difference between a stock 97 SC4 rear diff and a 98 auto TT rear diff in gear ratio's alone. Is this true?

 

So please clearify this for me... The 97 SC400 has what rear end gear ratio? I hear 3.26 and I hear 3.9X what is it really?

 

I'm sorry, WhiteGsr. I wish that I know about the gearing ratio of each car. I think the 97 SC400 might have the same ratio as the earlier model, because it has the same chassis as the earlier models. Check with ScottUrnot (moderator). He has the 97 SC400 with a monster built engine and a Getrag 6 speed tranny. Perhaps he could tell you something.

 

Thank you Steve!

 

The TT Auto differential utilizes a Torsen (Torque Sensing) LSD design that is completely gear-driven. The Torsen technology will send -if I recall correctly- four times the torque being sent to the slipping wheel to the wheel with traction when engaged. These units are fairly sturdy: capable of handling ~450whp before you are playing with fire.

The TT NA differential with the factory LSD option (only available in 97/98) utilizes a clutch-type LSD design. The clutch-type LSD technology is very common both in OEM applications and the aftermarket. The ability of the LSD to hold power depends on the amount of friction that the clutch discs can hold without slipping. The NA differential is weaker than the Torsen unit in the TT differential, but it can still hold ~300whp before you are risking failure.

These are both competitively priced. $900-$1000 is the going market price for these differentials. While neither is a steal of a deal, you might have to wait a number of months searching the forum classifieds every hour to find something significantly better.

The TT Auto LSD will decrease the amount of torque to the road in every gear from the taller gearing (not good for performance). The companion flange on the TT differential is also larger to mate to the TT driveshaft, which will require swapping your stock SC400 companion flange over (requiring the purchase of a 35mm deep well socket and additional labor).

The 4.08 NA LSD is the way to go:
(1) The gearing alone would increase the performance of your SC400 from stock (3.92)
(2) The LSD is plenty strong enough to hold n/a SC400 power for years to come
(3) It is $25 cheaper
(4) The companion flange should mate to your stock SC400 driveshaft (if memory is serving me)

Even if the companion flange on the 4.08 differential would need to be swapped, it is still the better choice for your car because of the gearing. For reference, I switched from 3.92 to 4.27 gearing on my SC400 and dropped ~0.35s on my 0-60mph and 45-65mph times.

 

Hey Blizzy,

I have the auto tt diff in my SC. I've found a gs300 punkin with 4.08s and I want to swap ring and pinions so I can get some lost torque back.

I am a competent machinist and auto mechanic.

I'm wondering if you know of a link or a resource showing how to setup the rear differentials for Supra's and SC's.

 

I used to have a copy of the Toyota service manual excerpt in my email, but I have deleted the email since it was a large file and a few years old. But, if you PM me your email address, I could get you in contact with the guy who could pass along another copy of it to you.

 

So I too have a auto TT rear diff, can I use that LSD unit (auto tt) that's in that pumpkin in another pumpkin that has a higher gear ratio to achive better bottom end torque?

 

Yes. The Auto TT or NA MKIV Supra LSD unit will swap into any SC300/SC400/GS400/GS430/LS400/MKIV rear end.

A higher numerical gear ratio will increase torque to the road in every gear at every rpm. It will not increase torque production by the motor. The term "bottom end torque" is often used to refer to a motor's torque production at the low end of the rpm band.

Swapping the differential will not alter the power band in any way, it will simply make it more easy for the motor to scroll through any given gear by giving the motor a mechanical advantage OR make it more difficult for the motor to scroll through any given gear because of a mechanical disadvantage.

Why would you ever want a lower numerical gear ratio? The lower numbered ratio can optimize engine rpm at highway speeds by lowering the engine rpm required to travel a given speed. Last year, when the price of oil was out of control, Ford went from a ~3.7 to ~3.5 ratio in their Diesel truck and made 2mpg better fuel economy on the highway.

But, to answer your question: if you wanted to put an Auto TT LSD into a 4.27 SC300 4AT pumpkin, you could do it. I have done just that for my car in fact: got the LSD for $400 and the SC300 4AT pumpkin for $150.

 

I dropped down to a 3.76 because my revs going down the highway were buzzy, and it's like my 5th gear was almost maxed out. People were passing me easily at 60mph

 

Thank you Blizzy for helping me with the education. So if I'm not mistaken the advantage I have installing the 98 Auto TT rear diff is that I now have an LSD, have a higher top speed, but ultimately I decreased the torque because I went to a lower gear ratio.

I gained one thing(LSD) but lost what I also wanted to gain which is off the line get up because I am keeping the car NA.


That helped me a lot Blizzy thanks agains!!!