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Old 01-13-2012, 03:10 PM   #1
jkershaw
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Default Rear differential ratios

I'm planning on doing a full jdm swap into my car. I know most of what this swap entails, but my question is what is the rear differential ratio in jdm stis? Would I be able to just my stock rear, buy a jdm rear, or can I rebuild my rear? I searched for awhile on forms and the internet and can't really get a straight answer. Thanks in advance for answers!
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Old 01-13-2012, 03:14 PM   #2
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Which JDM engine/transmission combo are you going with?
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Old 01-13-2012, 05:08 PM   #3
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Quote:
Originally Posted by jkershaw View Post
I'm planning on doing a full jdm swap into my car. I know most of what this swap entails, but my question is what is the rear differential ratio in jdm stis? Would I be able to just my stock rear, buy a jdm rear, or can I rebuild my rear? I searched for awhile on forms and the internet and can't really get a straight answer. Thanks in advance for answers!
'full jdm swap'........full WHAT jdm swap.....which one...there are hundreds of possibilities

into WHAT car...exactly????...there are dozens

the 'common' fd ratios are 4.11 and 4.44 and r160 or r180???

i would suggest reviewing and spending a couple months researching things before you buy anything


oh...and just because it is m4d tyte jdm y0 f00.....does not mean that it is good or you want it
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Old 01-14-2012, 05:56 PM   #4
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Its a v8 and the swap consists of compmete motor, trans, harness, and ecu. Everything came out out the same car and only has about 30k on it. Engine runs and has good compression. The swap is going into a 2004 WRX sedan manual.
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Old 01-14-2012, 06:12 PM   #5
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And uncle scotty relax, I'm not some tool that is doing the swap just because its "JDM DUDE". A friend of mine found a place near us that is selling a few of these swaps and he has bought other motors from them for different cars and never had problems. We are just trying to figure out the situation with the rears because the guy is cutting us a deal if we buy both at the same time. I will admit I didn't give all the information that I should have when I started this post, my apologies, I should have put more detail into it. There is no need to assume I'm a tool, just ask it or say normally and say I didn't give enough detail.
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Old 01-15-2012, 07:20 AM   #6
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I asked around a bit about these.
One day, I will have to upgrade to 6 speed.
One answer I've gotten from a vendor is that you should be able to build yourself hybrid rear axles, using the inner joints they provide you with. This would justify why the JDM swap places include axles, but not hubs and brakes, for a full swap.
With this, you would use the R180 they provide you.
Others have rebuilt the R160 with R180 internals.

However, the first thing you need to research is if you're prepared for the JDM 5th and 6th ratios. I don't think I like or need them.
I'd rather go with a '06 USDM 6 speed and take my chances with my R160.
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Old 01-15-2012, 09:16 AM   #7
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I got you. I've had a couple people tell me about rebuilding the axles like that. The place I might get the swap from said my axles should work but I don't know that completely since majority have said otherwise. I noticed the difference in ratios, and I agree that the usdm six speed would be better, but I can't beat the deal I'm getting everything for if I go with this company, those ratios wouldn't bother me to much.
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Old 01-15-2012, 09:51 AM   #8
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pretty sure the are 3.9 for the mostpart. may not be all but most. look up the SPDA subaru transmission chart and other various transmission charts they will give you all the info you are in need of. 3.90 can be run r160 or r180 differentials. if you run r180 diff with stock hubs spec b axles will work for fitment with r160 outers and r180 inners. r180 diff with r180 hubs any sti axles from 05-07 should work, maybe the 04 sti too but i am not sure if the are r160 or r180 outers due to the hub changes that were made from 04-05. r160 differential with r180 outers i believe you are looking at hybrid axles. best option cost wise will be r180 diff (stronger), spec b axles (stronger), and stock hubs (less cost on swaping suspension and brake components). then you can resell extra parts such as the version 8 hubs and such unless you do upgrade to the hubs, then ignore the previous suggestion and enjoy good luck!
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Old 01-15-2012, 11:52 AM   #9
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I wonder how much rotating mass the R180 adds.
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Old 01-15-2012, 01:46 PM   #10
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Quote:
Originally Posted by Vlad View Post
One answer I've gotten from a vendor is that you should be able to build yourself hybrid rear axles, using the inner joints they provide you with. This would justify why the JDM swap places include axles, but not hubs and brakes, for a full swap.
With this, you would use the R180 they provide you.
Others have rebuilt the R160 with R180 internals.
Quote:
Originally Posted by jkershaw View Post
I got you. I've had a couple people tell me about rebuilding the axles like that. The place I might get the swap from said my axles should work but I don't know that completely since majority have said otherwise.
it is not as easy as pulling your shafts apart and installing the R180 inner on the R160 shaft. the R180 shaft is 32 splines and the R160 shaft is 21 or 22 splines, depending on year.

see this post for information on building hybrid shafts. http://forums.nasioc.com/forums/show...e+missing+link

and you can not rebuild your R160 diff with R180 inturnals. they will not fit. the R180 ring gear is almost an inch larger diameter.

if its a V8 engine and trany it should be a 3.9:1 final drive ratio. i am currently running a V8 6 Speed and R180 in my car. your right the 5-6 gears are kinda low. 3300-3400 on the freeway at 65mph.

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Old 01-15-2012, 04:39 PM   #11
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Wow, thanks for the information guys, so correct if I'm wrong but, with this swap I could get away with using my r160 rear, which is weaker, or just get a r180 rear and and make hybrid rear axles and I would be good, our get the spec b axles?
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Old 01-15-2012, 06:41 PM   #12
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yeah the swap would not be all that hard. since you can use your 04 wiring harness and just plug the JDM ecu into it. you just have to do a few pin changes with your wiring to remove the TGV's and add the AVSC.

as for the diff you will have to find a 3.9:1 R160, since i beleave your rear diff will be a 3.54:1.

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Old 01-15-2012, 08:09 PM   #13
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Ok, from research I have done my car's final drive is 3.9, but like you said, I'm pretty sure I have 3.54 gears so all I really need is that 3.9 rear
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Old 01-15-2012, 08:48 PM   #14
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front is 3.9:1 but they use a 1.1:1 center so the rear is 3.54:1
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Old 01-15-2012, 10:08 PM   #15
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check on your spline count for your hubs i have spec b axles if you need info on splines.
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Old 01-16-2012, 03:38 PM   #16
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Ill check my spline count when I get home today, but I'm confused my car has different ratio rears from front to back? How do they not fight each other, or are they supppse to and the center diff just deals with it?
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Old 01-16-2012, 03:52 PM   #17
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02-05 Wrx uses a 1.1:1 transfer gear ratio after the center diff, making the rear diff ratio 3.54 and the front is still the 3.9. 3.54x1.1=3.8995 or something. I don't know why subaru did this, maybe to be special?


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Old 01-16-2012, 07:17 PM   #18
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Ah, ok makes sense, and if any manufacturer didn't do something dumb like that the world would come to an end, lol.
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Old 01-16-2012, 07:29 PM   #19
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So, with what you guys told me and what I just found I need to find a 3.9 rear diff correct?
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Old 01-17-2012, 04:10 AM   #20
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If the transmission that you are swapping in has a 1:1 center differential then yes you need a 3.9:1 rear differential to replace your 3.54:1 ring and pinion

Perhaps I missed it but you didn't say which transmission you are swapping into the car. You may also need a new driveshaft.
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Old 01-17-2012, 06:11 AM   #21
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I'm using the transmission with the v8 swap, which it does have a 1:1 center, I already figured I would need a new driveshaft since I'm going from a five to six speed trans.
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