zeiss150 Posted December 28, 2012 Share Posted December 28, 2012 Hi, I have a 1972 240z with some mods from the previous owner, so there are a few things that I'm not sure about. I believe that I have an R180 diff (because of the rectangular back cover). But the gear raito seems to be 3.70:1 (18 tooth blue speedo gear) and speedo is accurate as far as I can tell (my wife was in her car next to me verifying speed via cell phone... yeah... I'm high tech). The speedo was off by about 5mph, but thats because there was a 17 tooth (black) speedo gear in the trans, I switched it to the 18 tooth (blue) speedo gear and that fixed that. My tires are pretty much stock (well, stock size, zx rims) 205/7O/R14 so I don't think the speedo was off because of that. Here's the problem, I don't think Datsun/Nissan made an R180 3.70:1 For a Z car, at least, I have not been able to find any info about such a differential. I did a lot of research and I found that Nissan did make an R180 3.70:1 in the Maxima, Manual trans, but I have no idea if that's true (internet source) . I suppose it is possible that the previous owner put in a maxima rear diff. I know that there was an R200 3.70:1 in the 1979 280zx 2+2 with manual 5 speed trans, but I don't believe that I have an R200 in my car. Soooooo, I have no idea what kind of differential I have. I don't want to take the back cover off right now to look at the stamping on the gears. I would like to put a R180 4.11:1 in (that's from a 84-88 NA 200sx, right?), Or the R200 4.11:1 (85-86 turbo 200sx) Or any other 4.11:1 that is easy to swap into my 240Z. If you have any idea whats going on PLEASE LET ME KNOW!!!!!. I'm stupmed, and I don't want to pull the diff cover off if I don't have to. Thanks in advance, Matt- Quote Link to comment Share on other sites More sharing options...
NewZed Posted December 28, 2012 Share Posted December 28, 2012 Mark the input flange and the wheel's, then turn the input flange 3.7 times. The wheels will each turn exactly one revolution, if drag is equal on both sides. Or lock one wheel, and the other will turn two revolutions. Stuff like that will give a good approximation. Quote Link to comment Share on other sites More sharing options...
78zstyle Posted December 28, 2012 Share Posted December 28, 2012 Take a picture of it from underneath and show us. Its pretty obvious the difference in r180s and r200s. Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted December 28, 2012 Author Share Posted December 28, 2012 I'll take a photo tomorrow and post it up. I'm pretty sure its an R180, because of the retangular cover and smaller size of the diff. Does anyone have the width messurment of an R200 or R180? I'll also do the wheel turning trick. 3.7 turns is kind of hard to approximate I guess that would be 3 3/4 turns on the drive shaft for one revelution of the wheel. I'm guessing that if its a 3.54 then that would be 3.5 turns of the drive shaft for one turn of the wheel. Thanks guys... I'll update tomorrow. Quote Link to comment Share on other sites More sharing options...
Gollum Posted December 28, 2012 Share Posted December 28, 2012 205/70/14 is tall for a S30. You're almost an inch taller than stock, which would result in about a 3% speed difference. But it should be noted... that's almost a moot point compared to how off the speedo was STOCK. Comparing it against another car is almost useless. What's MORE reliable is to actually calculate it out. You can use a handy tool like a transmission calculator here - http://sd2cx1.webring.org/l/rd?ring=zcars;id=157;url=http%3A%2F%2Fwebspace%2Ewebring%2Ecom%2Fpeople%2Fcz%2Fz_design_studio%2F If you know what trans you have, calculating the diff is easy. If you know the diff, calculating the trans is easy. Punch in your tire size, select trans, and then get some readings using a GPS speedo. Take measurements at higher RPM ranges since this will accentuate differences. Transmissions are RATIO devices, so it's hard to tell the different from one diff to the next at low RPM. Get 4th (since it's 1:1 in almost any trans you could have) up over 4,000rpm and measure your speed. Now you can calculate the diff ratio. OR... go the much easier route. Go to the local auto store and get some diff fluid (many like 90 weight), and then pop your rear cover. There will be a tooth readout scribed into the gear who's smooth face is facing you. Rotate the wheels till you find this scribe mark. Should say something like 10/37. Divide the larger number by the smaller, you now have your ratio. Tada. Put rear cover back on, refill with bought fluid. Done. Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted December 28, 2012 Author Share Posted December 28, 2012 205/70R14 is an increase? I thought stock was 195/70R14? My tires are wider but not taller. Unless of course stock was 65. wouldn't a taller tire make the speedo slower? longer for one revlolution = slower speedo. Or is it longer for one revolution more drive shaft turns = faster speedo? At any rate I'm gonna spine the drive shaft tomorrow and see how many turns it takes to turn the wheel one time. Thanks for the info! Quote Link to comment Share on other sites More sharing options...
Gollum Posted December 28, 2012 Share Posted December 28, 2012 (edited) Stock wheel would have been a 175SR14, or more accurately 175/78/14 I believe. The wider you go, the taller for a given ratio. The second number is a RATIO of the with. So a 300/40/15 would have literally TWICE the sidewall height as a 150/40/15. For your next tire, I'd suggest going 215/60/14 if your rims can fit it. 225/60/14 would be even better, again, if your rims can fit it. You can use this calculator here to test different setups. http://www.rimsntires.com/specs.jsp Oh, and yes, a taller wheel will end up give you a larger GPS reading per your speedo reading. This would lead to making you think you have a lower ratio rear end than you actually have 3.9. Won't know until you do at least a semi-definitive test, which is sounds like you're well on your way to do. BTW, my preferred method when it comes to just rotating stuff and guessing, is to draw a line on the tire with chalk, then put a line on the driveshaft going down the length. Spin the driveshaft 1 turn as perfectly as you can, while having someone watch the wheel. If you can guess about how many degrees the wheel turned you can now do some basic maths to get you in the ballpark, or close enough to be sure which diff you have. I find guessing the chalk on the wheel easier than guessing how far around the input of the diff as spun. Edited December 28, 2012 by Gollum Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted December 28, 2012 Author Share Posted December 28, 2012 Ok I attached some photos of my Diff. I hope they are clear enough so someone can id it for me. I tried turning the drive shaft and that didn't work so good. It looks like the driver side wheel doesn't turn at the same rate of speed as the passenger side wheel. I'm going to have my wife hold the driver side wheel and try it again and see if I can get it to work out to 3.7 turns of the drive shaft to 2 turns of the tire. Any tips would be great. I used a GPS Speedometer on my Iphone and It worked awesome (free at the apps store BTW). My speedo seems to be off between 0-35mph and dead nuts from 40-70mph. Over 70MPH it is off by about 5 mph to fast, so 75 was reading 80mph. So I pretty sure that Its a 3.70:1 just going by the speedo alone. Questions???? Comments??? Thanks, Matt- Quote Link to comment Share on other sites More sharing options...
Miles Posted December 28, 2012 Share Posted December 28, 2012 Most of what you want to know can be found in the tech section of http://zhome.com/ List of datsun differential ratios Method for determining ratio of differentail Quote Link to comment Share on other sites More sharing options...
Leon Posted December 28, 2012 Share Posted December 28, 2012 Ok I attached some photos of my Diff. I hope they are clear enough so someone can id it for me. I tried turning the drive shaft and that didn't work so good. It looks like the driver side wheel doesn't turn at the same rate of speed as the passenger side wheel. I'm going to have my wife hold the driver side wheel and try it again and see if I can get it to work out to 3.7 turns of the drive shaft to 2 turns of the tire. Any tips would be great. I used a GPS Speedometer on my Iphone and It worked awesome (free at the apps store BTW). My speedo seems to be off between 0-35mph and dead nuts from 40-70mph. Over 70MPH it is off by about 5 mph to fast, so 75 was reading 80mph. So I pretty sure that Its a 3.70:1 just going by the speedo alone. Questions???? Comments??? Thanks, Matt- What you're doing is checking speedo accuracy. What Gollum is telling you, is to check the speed of the car (using GPS) versus your tach. Using the calculator that Gollum linked to earlier, 4000rpm in 4th gear (Z transmissions typically have 1:1 4th) gives you 81mph if you have a 3.7 rear diff. Your pics look like an R180 to me. Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted December 28, 2012 Author Share Posted December 28, 2012 ITS A 3.54!!!!! I feel like I just found out what sex my first child was. I followed the instructions at zhome.com (which I've been on a million times, and I love it, its just a little hard to navigate) for figuring out the diff raitios by turning on tire (while the other one is stationary) and counting the rotations of the drive shaft. I had 17.7 rotations exactly ... multiply that by 2.... BOOM. 3.54. So now I know I have a 3.54:1 R180. My Car is gonna feel awesome with a 4.11 in there. My speedo must be really off if a 3.70 gear is making the speedometer work at the right speed. Oh well, it is a 40 year old car. Thanks for all the help guys. Matt- Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted January 1, 2013 Author Share Posted January 1, 2013 I have a question... I have an R180 3.54:1, right. So, if thats the case it must have come out of a 280zx automatic. Becuase that's the only Z's that had an R180 3.54:1... right? So my question is: My half shafts are secured to the diff with a snap ring and they are 27 spline... is that correct? Also will my half shafts plug into the WRX R180 3.90:1 VLSD. My plan is to use a KA transmision (240sx) with the WRX diff. Sound like a good idea? Commments?! thanks, Matt- Quote Link to comment Share on other sites More sharing options...
NewZed Posted January 1, 2013 Share Posted January 1, 2013 (edited) The bolt-in/clip-in answers are here - http://forums.hybridz.org/topic/49194-differential-cv-lsd-hp-torque-r160-r180-r200-r230-diff-mount/ and here - http://forums.hybridz.org/topic/108917-subaru-wxr-sti-r180-side-axles-part-six/ The other parts, 1-5, of the story are out there too. You'll have to search. Edited January 1, 2013 by NewZed Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted January 1, 2013 Author Share Posted January 1, 2013 thats a great post with lots of info and I've read it about 25 times... but it doesn't specificaly mention the WRX VLSD. It just says that there is a difference between early and late modle (k casting) R180's stub shafts ... bolted and clip in. But is there a difference with the VLSD? Or... does it not matter. If its an R180 with clip stubs... you're good to go? I really just wanna know if my Stub shafts will be plug and play with the WRX R180 VLSD 3.90:1 ( remember I have the 280zx R180 3.54:1 in my car now) Thanks, Matt- Quote Link to comment Share on other sites More sharing options...
NewZed Posted January 1, 2013 Share Posted January 1, 2013 (edited) I would start a new thread with Subaru Diff in the title. John C will probably see it, and probably knows the answer. "Differenital Problem Help" doesn't really get anyone's attention. If you download an FSM for the ZX's you'll see that the ZX R180's are bolt-in, re one of your questions. www.xenons130/reference . It's also described on the first page of the Differential/CV/LSD.. link. You might find more info on the "KA" swap by searching "71C" or "240SX" swap. I thnk that the details are the same. A Z/ZX bellhousing/front case swap on the later model transmission. And the later transmissions have a 3.321 first gear, so with a 3.9 you'll have a pretty low first gear. Edited January 1, 2013 by NewZed Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted January 1, 2013 Author Share Posted January 1, 2013 I read a great KA trans swap write up at Zhome.com... so I'm covered as far as that goes. So do you think the KA trans and the WRX diff is a bad combo? I have a Hanes manual...I'll look at the diff right now Quote Link to comment Share on other sites More sharing options...
srgunz Posted January 2, 2013 Share Posted January 2, 2013 I have a Hanes manual...I'll look at the diff right now Hanes manual? Well better than nothing I guess. But not much better. Quote Link to comment Share on other sites More sharing options...
BluDestiny Posted January 2, 2013 Share Posted January 2, 2013 The Combo is fine, just know that you need beta motorsports halfshafts to use your original stub axles. Your stub shafts will not be plug and play. The KA trans has identical ratios to a late (I think) 280zx tranny. So that really won't change anything except your ability to replace the trannies easier. You can download a free factory service manual at xenons30.com Quote Link to comment Share on other sites More sharing options...
zeiss150 Posted January 3, 2013 Author Share Posted January 3, 2013 I agree the Hanes Manual is "whatever" but it'll do in a pinch. I'll down load the FSM. I did a little more research and I found out that the subaru diff takes a 27 spline stub axle (diff side not wheel side) and the Z R180 is a 25 spline so those stub axles are out of course. And I looked at the beta motorsport stub axles and they are a little pricey... $500. Its not that I'm cheap.... I just don't have that kind of money. I know they make a quality part that would never break, but can't I just get the original subaru stub axles modified? I mean, can't I just take the subaru and Z stub axles down to a good weld shop and get a nice solid tig weld job on them? I read a thread that a memeber here did that ... no report of the outcome unfortunatly. Check this out... I was driving in my Z on the freeway today and I thought to myself, "hey you should do that thing with the 4000rpm's and 81mph" So i turned on my gps iphone speedometer put my z in to 4th gear and pressed the gas until I was at 4k on the tach. to my surprise, and bothered... the speedometer said 81mph. DAMN IT!!!!! If that's right something else must be wrong. because I was 99% sure I had an R180 3.54:1. I did it 4 different times and each time... 4th gear, 4000rpm, 81mph. So, by that test I have a 3.70:1. I even came home and double checked the calculator. If I had an 3.54 it would have been 85mp at 4000rpm. Maybe my tach is off too. Or maybe I didn't do the wheel turning counting thing right.... I'm really close to taking the back cover off, but what a pain in the ass just to solve the mistery. Even if I know what my dif is, it doesn't change anything. I'm still gonna put that subaru diff in there, so who cares right? Any insight would be great. Thanks guys! Matt- Quote Link to comment Share on other sites More sharing options...
ironhelix Posted January 3, 2013 Share Posted January 3, 2013 (edited) Just a late comment. 240z's with automatic transmissions also had 3.54 R180 diffs. Edited January 3, 2013 by ironhelix Quote Link to comment Share on other sites More sharing options...
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