91 ls400 diff question
Thread Starter
Driver School Candidate
Joined: Jul 2014
Posts: 48
Likes: 0
From: California
91 ls400 diff question
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
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?
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?
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.
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.
Lead Lap
Joined: Jul 2010
Posts: 4,525
Likes: 98
From: ca
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.
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.
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).
Lead Lap
Joined: Jul 2010
Posts: 4,525
Likes: 98
From: ca
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.
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.
How Torque Converters Work! (Animation) - YouTube
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.
Torque Converters Explained -EricTheCarGuy - YouTube
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.
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.
Torque Converters Explained -EricTheCarGuy - YouTube
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.
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.
Trending Topics
ClubLexus Stories
Celebrating Lexus & Toyota from Around the Globe
8 Tips for Improving Your Hybrid or Plug-in Hybrid's Efficiency!
Michael S. Palmer
10 Best Lexus Models No One Remembers
Joe Kucinski
TRD Off-Road Premium: Best 2026 4Runner, Except This One Thing
Michael S. Palmer
Top 10 Lexus & Toyotas to Drive Before You Die!
Joe Kucinski
Top 10 Lexus/Toyotas With The LEAST 5-Year Depreciation
Joe Kucinski
Lexus LC500 Convertible Auction: A Preview of Rising Values?
Brett Foote
GX 550 vs TX 550: Best 3-Row Luxury Lexus Family Hauler
Michael S. Palmer
9 Best Lexus Models You Can Buy for Half Price (And 1 You Shouldn't!)
Joe Kucinski
2026 Lexus NX Buyer's Guide: Models, Features, Prices & More!
Brett Foote
Thread Starter
Driver School Candidate
Joined: Jul 2014
Posts: 48
Likes: 0
From: California
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?
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?
Thread Starter
Driver School Candidate
Joined: Jul 2014
Posts: 48
Likes: 0
From: California
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.
Thread
Thread Starter
Forum
Replies
Last Post
recoil
LS - 1st and 2nd Gen (1990-2000)
8
Jul 8, 2008 10:04 AM
