sc4shelby

Ok so i did a search and i couldnt find the info i was looking for, so here goes. My neighbor is giving me his old diff from his wrecked 5 speed sc300 now to my understanding, the m/t 300 had a gear ratio of 4.08 while the a/t had a 4.27 gear ratio, i see info about sc400 diffs, but my question is, will the m/t diff bolt into my 91 ls, would i need to swap internals, or will it flat out not fit. im getting the diff for free so if i can use it that would be nice

timmy0tool

iTrader: (7)

to my limited understanding SC diffs have different outter casings so it will not bolt onto the rear subframe outright. you will need to swap casings. please confirm by searching supra rear diffs, and also with puredrifter!

wouldn't a manual gear ratio result in poorer gas mileage, but awesome acceleration? what's the benefit here other than using a part that was given for free?

PureDrifter

iTrader: (10)

The internals would need to be swapped, which means you would have to re-setup the diff backlash and shimming.

The gearing would be pretty impractical if it's the 4.27 ratio.

For example, a Stock '92 LS does 65mph at ~2200rpm.
With 4.08 rear gears you'll be doing 65mph at ~2450rpm
with 4.27 rear gears you'll be doing 65mph at a buzzy ~2600rpm.

If you plan on cruising at 70mph or higher the problem just gets worse.

dicer

From past experience auto trans set ups always had higher gearing, the TC made up for it. PureDrifter what is the ratio for a 92? I thought those ratios sc4 posted seemed very low, those are about what you would find in a pickup that is for pulling a trailer. I know that OD helps a bunch but you still need higher ratio for economy.

PureDrifter

iTrader: (10)

TC has little do with it, it is purely a gearing issue. The manual trans used in the SC has much shorter gears, requiring a taller final drive than the automatic. W58 1st gear was 3.285, A341e ('89-94 LS400s) 1st gear was 2.531.

OEM 1st gen rear end gearing is 3.62:1.

A "higher ratio" (or a taller gear, aka smaller numerical number) rear end does not alone grant economy. The engine has to be running at it's sweet spot, as too tall a gear will make the engine lug outside of powerband and reduce economy. If you stay with the stock trans, there's little economic reason to change ratios. If you want a little extra torque multiplication, go with a mild upgrade (the 4.08 would be my limit).

dicer

In essence the Torque converter is another "gear".

The whole function of the combined gear sets is torque multiplication.
That means if you have a transmission gear ratio of 3:1 and a rear end gear ratio of 4:1,
then the engine puts 100 lbs ft of torque into the transmission that you will get 1200 lbs ft. out of the rear end.

The engine spins 3 times to turn the output shaft of the trans one time, and you need to spin the input of the rear end 4 times to get 1 turn of the axle. So the combined reduction is 12:1.

He says it doubles the engine torque. So then that 100 lbs ft would turn into 200lbs ft.
Nice ha
The funny thing about torque multiplication is with hypothetical zero losses the horse power remains the same from input to output. But since there are friction losses wheel hp is always less than flywheel hp.
So yeah the TC plays a part, and is why the rear gears are most always a higher ratio for an automatic vs a manual transmissioned car.

PureDrifter

iTrader: (10)

That's great and all, but it the torque converter doesn't matter when you're comparing cruise RPM here as these are modern lockup TCs, so there's a mechanical link (aka 0 torque multiplication) at cruise. torque multiplication in lockup torque converters only applies during non-lockup periods, sch as acceleration. The ratio is variable depending on the torque converter set-up, it is not fixed to a 2:1 ratio. Also, the "hypothetical zero loss" is a bit of a misnomer, it is as far as I know physically impossible for even a theoretical torque converter (or fluid coupling) to run at perfect efficiency (0-loss).

In this instance where OP is trying to compare different gearing options on a car without switching back and forth between auto and manual trans, the TC is irrelevant. It is a gearing issue.

dicer

Agree all correct.

sc4shelby

I don't care about the gas mileage, I was looking for a boost in takeoff/acceleration but I didn't want to spend the price of a torque converter, or a manual swap yet if I can get this for free right now. So from what I'm understanding is I basically would have to swap internals?

PureDrifter

iTrader: (10)

yes, which unless you're experienced in setting up/shimming diffs, is not a great idea.

sc4shelby

Are there any write ups or sources of information you could point me towards, I would like to learn about it, its something I haven't really worked on, but I want more experience.

PureDrifter

iTrader: (10)

The factory repair manual is your best bet. Detailled instructions on disassembling/reassembling the diff is in it, as well as how to check gear mesh and backlash.

sc4shelby

Alright thank you, I will post back with my progress