Alright, ive done alot of reading on this but theres still a couple questions in my new set up
First off. my stock current open rear diff ratio is 4.06.
My New VLSD is 4:3:1 final drive.
Theres 49 teeth on the ring gear but i didnt count the smaller gear.
**QUESTIONS (modified for JClark :
)
If theres any missing info, lemme know and ill find you the answer.
Wait, what’s the question again?
I pulled this off of KA24DEvelopment:
49/12 = 4.083636 (stock ratio)
47/12 = 3.91666 (J30 ration)
48/11 (‘JDM’ 180SX diff, 4.36:1 ratio, 5-bolt output flanges)
IIRC: The higher the ratio the faster acceleration, lower top speed; lower the ratio the slower acceleration, higher top speed. Correct me if I’m wrong. People also claim that the lower ratio is better for turbo cars.
You can also swap the ring gears if you don’t want to change the gear ratio.
Correct and incorrect at the same time. Shorter gear ratios provide better acceleration characteristics and yes bring overal speeds down. Taller gear ratios give the car “leg room” as it’s said and is recommended for high horsepower cars to make use of the power and the gears in the transmission. Keeps you from breaking the tires free all the time as well 
Shorter gears tends to be easier on the transmission because there is less side loading of the helicals and torque is more easily transmitted through the drivetrain.
However the commnet about just replacing ring gears is incorrect. Ring gears and pinions come in matched sets and should NEVER be mismatched. Also once any part of the final drive is replaced, the gears must be setup for proper mesh or you’ll eventually make an expensive paperweight out of the transmission, or rear end in this case…
What I meant about the ring gear is that you can take the stock open differential, remove the ring gear and install it on the VLSD and then install the VLSD (with the stock ring gear) back into your stock diff housing, that way the gear set will still be matched.
When you go to a numerically higher final drive (for example, 4.1:1 to 4.3:1), you are making it easier for the engine to turn the wheels. You are improving the mechanical advantage of the engine itself by a factor of 4.3 / 4.1 and therefore torque to the wheels improves by the same factor, assuming you are at the same speed.
Here’s a good write up I found for ya:
One way to improve acceleration is to put a higher (numerically) ratio final drive in the vehicle. The torque available at the drive wheels is the engine torque multiplied by the transmission ratio multiplied by the final drive ratio. Let’s just use some hypothetical numbers to make the math easy.
Let’s say for an example that the engine has 200 lb-ft of torque all across the usable speed range (a gross oversimplification); let’s have a final drive ratio of 3:1, a rolling radius of 1.25’ (15" radius, 30" diameter tire); and let’s say that the transmission ratios are 3:1 (1st), 2:1 (2nd), 1.5:1 (3rd), 1:1 (4th), and .75:1 (5th).
So, the torque available at drive wheels in first gear is 200 (engine torque) x 3 (first gear ratio) x 3 (final drive ratio) = 1800 lb-ft. (The force at the point where the drive wheels contact the road is a total of 1800 lb-ft / 1.25 ft (rolling radius) = 1440 lb.)
If we change the final drive ratio to 4:1, the first gear torque will be 200 x 3 (first gear ratio) x 4 (final drive ratio) = 2400 lb-ft. This is a 33% increase in drive wheel torque, and it will result in dramatically improved acceleration. (It will also increase engine speed on the highway, resulting in lower fuel mileage, increased noise & engine wear, and a lowered top speed of the vehicle.)
The final drive ratio has no effect on the horsepower of the vehicle, but it does affect the torque available to the drive wheels. However, the drive wheels will be turning slower than they would normally be at the same engine RPM (assuming no slipping of wheels or tranny).
Also as a comment on longer gearing VS shorter gearing for different types of cars.
Rule of thumb :
If you have a broad torque range, use longer gears to take advantage of it : IE high displacement cars or well tuned NA cars.
If you have a narrow power band, use should use the shortest gearing that will allow you to stay in the power range without dropping out : IE most turbo cars.
Thank you all, this is probably the only car-related thing im a noob at. ive been taking in so much info about it.
:bowdown :bowdown THANK YOU. also if you guys have anything else PLEASE ADD!! ;D
Sweet! good tech info for a change on this site… ;D
Turbo cars usually like a little taller gear to get the engine under more load to spool the turbo faster. NA like the short gears to stay in their narrower powerband.
NA has a narrower power band than turbo? :confused
On the KA24(d)e the powerband is from 2700-5600. (relines around 7). Turbo or not it dosnt change. anyways, im hoping for ALITTLE more accel, and def, to break the rears loose. lol.
Sweet! good tech info for a change on this site…
Ive got a couple right ups, just finalizing them. ;D
Theres a couple things concerning my new (slightly larger shell) VLSD. since the old one fits i WAS going to switch all guts over. But On the S13 (240sx 89-94) my new Vlsd wont fit by 1 thousanths of an inch. (ill make it fit) they said to take the stock back cover and put it on my new diff, for cleaernce. BULL. They dont fit and different bolt pattern.
1-i didnt do anything except drain old oil. clean it up a bit, replaced the gasket from nissan dealer, and cleaned the outside. so im goooood!!
Nice Jeff, lemme know how it works out…
It’s hard to generalize. There are NA cars with a big fat powerband, they usually have at least eight big ol’ cylinders though.
There are also turbo cars with a small powerband, probably the result of under or oversizing the turbo with respect to engine size?
But if the displacements are the same, it’s almost a given that the turbo would have a wider powerband…
Should be as simple as un bolting and bolting back up.
I replaced the diff gasket, with a OEM one from nissan straight from japan.
cleaned the outside as best as i could with at a car wash and engine degreser (that bish was dirty/ covered in dif oil)
Changed Oil to Motul.
so hopefully it takes 3 hours at most with rusty bolts.
But i dont have any soo… ;D
You’ll want a longer rear-end for a turbo car so you can build boost in lower gears quicker amirite?
…among other reasons, I’m just saying that’s another advantage.
jesus christ. its all unbolted. just not coming out. :headbang
i need a bigger jack. o well ill mess with it in the morning.
alright. got it in, and now it acts like its a welded diff. which its NOT.
i looked inside, and cleary a VLSD, no welding anywhere.
Here’s the problem,
Around sharp turn (parking spaces, sharp turn) basically it sounds like the inside tire skips/drags going around a corner w/o accel. With accel, it just kinda breaks loose somewhat.
I drove it to HIN, in boston all was highway miles, but ~2 miles were turns and i dont want to drive it more than i have to and have something happen. Any suggestions?
First - get AAA.
Second, lift the rear end and make sure everything is installed correctly and spin one tire and see what the other does.
Thats a start :ponder