Differential swap

[gr8scott72]

I have a 93 J30t and would like to swap in a higher final drive differential to help with the acceleration some.

I know many people have done the swap from a J30 to a 240sx but what about going the other way? 240sx to J30? What all would I need? I don't think I would mind giving up the LSD in order to get the higher gearing from the 240sx. TIA.

[SeanC]

higher final drive = lower drive numbers = more top speed, less accelaration.lower final drive = higher drive numbers = increased accelaration, less top speed

the only different one i have read about is non-turboed 300zx's.

[Z U L8R]

If memory serves ( 50% probability ^_^ ) I beleive the 300zx TT has a .363 or something close rear end gear ratio ( top speed low accel ) where as the j30 & 240sx i believe are .411's ( high accel lower speed ) gear ratios.

IF I'm not completely retarded, and this is true, putting in an open diff or lsd from a 240 ( .411 ) into your j30 ( .411 ) won't change a thing. Your other option would be a 300zx TT rear end ( .363 ) which would give you a higher top speed but a slower acceleration. HOWEVER. The other factor to consider would be the gearing ratios in the transmissions that corespond to these differentials. To that I don't know.

Another side of the coin is make sure your rear cv axles will bolt to the flanges on whichever differential you go with, and make sure the front two bolt holes aren't too wide and/or the rear diff cover studs mount the same way.

If you're looking for faster acceleration. 1) Get more power, 2) You could get a lighter flywheel ( I forget if yours was automatic or manual ).

aaaiiiiiiiiiiiiiiight ^_^

[Sil40_Mayhem]

While I'm probably the last guy that needs to be giving mechanical advice, I'll take a shot at it. Unless you don't wish to disassemble your diff, why don't you just swap in a ring gear with a shorter ratio? I believe the final drive ratio in CA18 equipped S13s is 4.11. I know that in NA 300ZXs and 240SXs, the ratio is 4.083. It looks like the stock ratio for the J is 3.92, which would mean that the other aforementioned models should give you better acceleration (at the cost of top speed).

As far as diff swapping, the reason I enquired about changing the ring gear is that all Js come with vlsd. Since it seems that the majority of 240s come with open diffs, I don't see the benefit of slightly improved acceleration at the expense of "peglegging" (which, trust me, sucks the big one). Of course, the 300ZX diffs are of the vlsd variety, so you should be good there. Also, you wouldn't have to worry about matching the output shafts to your axles, driveshaft length, ABS sensor, etc...

Other things to consider:- your diff's ouput flanges are a 5 bolt pattern; you will either need to find a replacement that has the same pattern, or you might need to replace your axles with those whose pattern matches your diff- your car has ABS; you may want to look for a diff that has an ABS sensor (if that's important to you); also, this might affect the connection to your driveshaft since diffs with the ABS sensor are a different length from those without, which may require repositioning or even replacing the driveshaft

This is what I've picked up while researching the diff swap from the other way around (J30 diff -> 240SX). I may be wrong about a bit of it (hence why I said that I'm probably not the best suited for giving advice). Anyhow, take a look at the diff swap articles we have here (and maybe other sources on the net), and reverse the information where it applies (particularly those pertaining to the J30 -> 240SX swap). Anyway, I hope that at least helps some.

By the way, unless I missed something, all J30s are autos from the factory.

[gr8scott72]

Ok, then what does it take to swap in a different ring gear? How hard of a job is it? I'd love to be able to do just that without having to swap everything out.

[180fan]

pretty easy. make sure you seat the diff properly though.

[SeanC]

NOOOO! the ring and pinion are a matched set. they wear to each other. you need to replace them both at once.

[gr8scott72]

So where can I get the ring AND pinion and what does it take to put both in?

[180fan]

Grab the diff and pumpkin from the car you're looking to grab the parts from then do it.

Sean thanks for catching that one. Midterms really got me fried. I think I'll take a day off for some much needed R&R. ha ha ha

[tyrannix]

zenki s13 silvia diff ratio is 4.363:1 (48:11) 48 teeth on the ring, 11 on teh pinion you need special tools to build a diff (to change ring+pinion) more than you need to build an engine, to include a press

your really best bet is to get a pumpkin from a CA S13 (nonturbo Js and Qs had an even higher ratio, somewhere around 4.5:1) then toss your LSD in that, you can probably get away with only having to adjust the side shims if that even (might not even need it)

read teh FSM and make sure you check evrything twice if you go that route

i stripped a pumpkin and had a shop reassemble it (all new bearings and seals with 4.363s and an s15 HLSD with nismo 12-13mm adapter bolts necessary for the gears) ... thats the only thing that i didnt do myself on my car sofar for the entire build.. 2 hrs of labor at a shop later, i got back a pumpkin exactly what i wanted with warrentied labor

So where can I get the ring AND pinion and what does it take to put both in?

[Chingon]

So where can I get the ring AND pinion and what does it take to put both in?

I have a ring and pinion set from a 240... make an offer.

[gr8scott72]

I have a ring and pinion set from a 240... make an offer.

I guess I need to see if I can find a shop around here that can rebuild my diff with your ring and pinion set first.

[tyrannix]

you will want to check what your ratio actually is, and the one he is offering you before you buy anything

(and if youre just going with a USDM 240sx gearset, you will save big bucks by just getting the whole pumpkin and using that)

edit>also remember, increasing diff ratio will give you better acceleration, but you will have to rev higher to go the same speeds (like highway cruising will be revving higher)
Modified by tyrannix at 11:40 PM 4/22/2006

[gr8scott72]

you will want to check what your ratio actually is, and the one he is offering you before you buy anything

(and if youre just going with a USDM 240sx gearset, you will save big bucks by just getting the whole pumpkin and using that)

edit>also remember, increasing diff ratio will give you better acceleration, but you will have to rev higher to go the same speeds (like highway cruising will be revving higher)

I understand the whole acceleration-is-more-but-top-end-is-less thing and I'm willing to give up the top end and be at a higher rpm on the highway in order to have a car that doesn't feel like an overweight slug when pulling away from a stop light.

So you are saying it will be easier to find a pumpkin from a 240 and then put in the vlsd from my diff and then put it on the car?

What's the going rate for a used 240 differential? and what is the final drive on the 240?

[tyrannix]

i dont remember off the top of my head what the USDM diff ratios are, its either the 4.1xx or 3.9xx one

i picked up a 240 pumpkin for $40 that would have worked fine, oil seals and bearings all looked servicable (but i was rebuilding it and replacing everything anyway, so it didnt matter)

now, that was also a local pickup deal from a guy that wasnt trying to make loot off it.