Gear Ratios and LSDs
 

I began a thread about a month ago requesting LSD advice as I was looking to upgrade my rear end. My 96 SC400 with 58k miles is bone stock. I have owned it for two months and love it. I have continued to learn more about my car, but I have a few further questions about gear ratios and LSDs:

(1) I hear the Torsen LSD units will last longer and require less maintenance than the TRD Clutch-based units. How much more maintenance is required with clutch-based units? What is the average life span of a clutch-based unit? **Assume that my car is driven about ten city miles per day. Weekend highway trips are common. 90% of the time spent in the car is at a normal pace. 10% of the time is spent aggressively. Also consider that I intend on installing a PI Dragon torque converter at the same time I install my LSD.**

(2) I notice that Torsen LSD's with a 3.76 gearing are relatively easy to find. How will this affect my lauching in comparison to the stock gearing (3.92 I believe)? Would the snappiness of my launch be less pronounced with a lower gearing present in my rear end? Would launching be better with the stock gearing and the TRD LSD over the Torsen with 3.76 gearing? If so, how much better? Strictly in terms of performance, what are the advantages of a higher gearing or a lower gearing?

(3) I have read that a lower gearing will decrease my rpm at a given speed. Does this mean that I will get slightly better fuel economy on the highway with the 3.76 gearing of the Torsen LSD in comparison to the 3.92 stock gearing?

(4) Would a 3.76 Torsen unit found from a major retailer online be new? For example: http://www.newtoyotaparts.com/product/?id=1754. Is this a brand new unit? Should I expect this unit to last for the life of my car? If not, what is the estimated life span of the Torsen unit given my driving habits listed above? Would the above linked unit bolt on and fit to my Lexus rear end?

In advance, thanks for any and all advice. I am loving the learning experience of discovering the intricacies of owning a performance auto. I appreciate this forum as it contains endless information in the areas that I am trying to become more competent.

****! I just noticed this should be in the engine and transmission section. I apologize for that: newbie mistake.

It has been 24 hours and no response....

Any advice on the differences in longevity and maintenance between the Torsen LSD and the TRD LSD in a daily driver?

Any insight as to performance losses and gains from moving from the stock 3.92 gearing to a 3.76 gearing in the Torsen unit?

Any experience with the newness or quality of the Torsen products offered online from major Toyota dealerships?

alright i will chime up but i only seldom know of the gear reduction question.
I will assume you dont know about the gearing at all so please i mean no offense by saying the basics then explaining how that affects your performance.
your differential is your final gear reduction. in this case your gear reduction is 3.92:1. which means for every time you input 3.92 revolutions into your diff your wheels ( i think) spin once. so a rule of thumb the bigger the gear ratio the more torque or power to rotate your tires you have. so logically your acceleration is improved with a bigger gear ratio but will lower your gas mileage because youll notice your rpms will rise easier and in the end you will have lost top speed. with the 3.76 torsen it will lower your ratio giving you less torque on the wheels meaning less acceleration but in the end a higher top speed

thats why we have transmissions which give us different gear ratios. Ie. 3.56:1 is 1st gear for a lot of torque but no top speed to get the car rolling. and an overdrive 5th gear of .57:1 meaning your car is already moving so you want more top speed. Please correct me if im wrong.. I just got out of Tech school for vehicle mechanic for the USAF, but they teach you the basics of the basics so i dont know if i'm a reliable source of info.

I have done some reading online. This is the conclusion I was coming to on my own. I do appreciate the help.

I guess my most major concerns are:
(1) How long will a clutch-based TRD LSD last with my stock gearing given my car is a daily driver (though not pushed to the limit but about 10% of the time)?

(2) How significant are the performance differences between a 3.76 and 3.92 gearing? In other words, what is the difference in performance between a TRD LSD with the stock gearing (3.92) and a Torsen unit with the 3.76 gearing?

(3) Are the Torsen units sold at major online retailers new? http://www.newtoyotaparts.com/product/?id=1754 How long should I expect a Torsen unit to last given my driving habits?

Hi! If you have an SC400, then I believe the stock gearing is 3.26. It's the SC300 that has the 3.92 gearing. That is, unless the previous owner swapped the stock diff for the 3.92.

But if you still have the OEM 3.26, then going with the 3.76 will make you acceleration better. Your launch will be more pronounced because you're closer to the engine's peak torque. That plus the PI torque convertor will make you launch MUCH better than a stock drivetrain. :)

My 96 SC400 is completely stock, 100%. The lady drove the car for 5k/year and had it serviced twie a year at the Lexus dealer only for the ten years she owned it. Is my gearing in fact 3.26?

My current uncertainties regard the longevity of the TRD and Torsen units? Which will last longer in a daily driver? I have heard the Torsen unit will last longer. How much longer?

My basic concern is value here. I am looking to factor in maintenance costs, longevity, installation costs, and unit costs to determine which unit -the TRD of the Torsen- presents the best value.

it all depends on the years also.
here is the years and gear ratio from clubna-t.com

92-97 SC300
6 cyl, AT (4.27 ratio)
6 cyl, MT (4.08 ratio)

98-00 SC300
6 cyl, AT (4.27 ratio)

92-96 SC400
8 cyl (3.92 ratio)

97-00 SC400
8 cyl (3.266 ratio)

92-96 MKIV Supra

w/o turbo; (4.27 ratio) Limited Slip
w/o turbo; (4.27 ratio)
w/turbo, MT (3.133 ratio) Limited Slip
w/turbo, AT (3.769 ratio) Limited Slip

97-98 MKIV Supra
w/o turbo; (4.08 ratio) Limited Slip
w/o turbo; (4.08 ratio)
w/turbo, MT (3.133 ratio) Limited Slip
w/turbo, AT (3.769 ratio)
w/turbo, AT (3.769 ratio) Limited Slip

I guess I have to re-ask:
3.92 stock with TRD LSD vs. Torsen 3.76

How noticeable is the difference in performance between these two scenarios?

Refer above to longevity and value concerns...
I am very curious about those questions.

trd lsd would prolly be cheaper and i doubt you'll have any problem running it as your daily
but you'd get better gas mileage with the torsen lsd with the 3.76 gearing.
the problem is finding a used one unless you are willing to pay more at the dealership.

I guess these are my final questions:

- How long does a TRD LSD unit last in a daily driver? What extra maintenance is required?
- How long does a Torsen unit last in a daily driver? What extra maintenance is required?

u should be fine with either one.

all i can add to this is that with a 4.08 and a turbo on the sc300 5spd i got itll do about 160 untill redline in 5th gear... so if you want to go faster than that get something lower haha i think im going to def lower mine to at least a 3.76 when i go high boost cause it goes through the gears pretty fast at 9 psi already

I have the SUPRA 3.76LSD on my 93 SC400 AUTO, drives like normal, but the rear wheels don't spin like before with the open diff. It's not 100% full proof, but the car goes where you want it to, especially around slick or wet corners.