Jersey Posted June 27, 2003 Share Posted June 27, 2003 So, at work playing around with the Trans/Rear ratio calculator at http://www.geocities.com/z_design_studio yesterday and figured with my T5 and the 3.54 rear that came out of the same '82ZXT, that i shouldn't have been really topping off 4th gear at the end of the strip Wednesday night. I was crossing the line in 4th at about 6100rpm @ about 106mph. So on the site, i chose the BW-T5 and the 3.54 (i'm running 215/50/15) and it says i should be at about 120mph at 6100rpm in 4th! So, last night i take her out and do some runs with my GPS while wathing the tach in 4rth - 3k rpm..52mph 3.5k rpm...61mph 4k rpm...70mph Now, the tach could be off by a hundered or so but i did check it against my meter at different rpms and it was pretty much right there. I go home, jump back on the Z studio site and select the BW-T5, my size tire and the 3.9 R200 and it's A LOT closer #'s as to what i'm running. I also punched in a 4.11 which turns out to be close as well. Now this has me stumped because i know i checked the ratio by spinning and counting when i did the swap. No, i didn't take the cover off like an idiot and honestly, i'm being lazy and really don't want to this weekend. I really shouldn't have to pull it if i do the turning wheel/couting method correctly. So i get back in the garage, jack her up on one side, other wheel touching the ground and check it again..3.54. Now, i may have lost my brain somewhere along the line and might be checking it wrong but this is how i always did it...with an open diff, lift one wheel off the ground or have someone hold the other stationary, rotate the opposite wheel 2 revolutions while counting the amount of times the DS spins...which i got a little over 3.5 turns....figuring a 3.54 again. What the heck am i doing wrong here? Note to self - losing mind @ 35 yrs old! AHHHhh. Thanks in advance for anyone that clears this brain fart of mine! Quote Link to comment Share on other sites More sharing options...
Drax240z Posted June 27, 2003 Share Posted June 27, 2003 Well my first bit of advice is to spin it a lot more than 2 times to check! That could be a cause of great error. Try spinning it 10 times and counting and you'll be a lot closer. Also, just because your tires are 215/50-15's doesn't mean they have the diameter the program says they do. It would depend on pressure, temperature and the make of the tire. I'd measure your actual tire diameter, and use that to find what your pseudo tire size should be. Also as you say, the tachs are off by a few hundred RPM. Add all these errors together and I can see the problem allready. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted June 27, 2003 Share Posted June 27, 2003 the way your car gets out of hole, you might have the 3.90's. Really, only way to check it is to take the diff. cover off and read the stamping on the gear. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted June 27, 2003 Share Posted June 27, 2003 You know.... wish I had access to a GPS, because I checked that Z design studio thing too, and it showed my speedo being off by like 5mph. But I know what diff I have and I know the little colored speedo gear is the right one for my diff/tranny combo. And I was kinda thinking what Drax is thinking about tire sizes. Another funny thing, I've used different programs that do that same thing, you tell it gearing, tire size, rear end, and it tells you speed in RPM and gear, but I get different results from different ones, only about 2mph difference but hey that's 2 mph... Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 27, 2003 Author Share Posted June 27, 2003 Drax - the reason why you spin the tire only 2 times because when you count the DS turns, that should equal your ratio. Example - 1 turn of the tire would make the DS spin about 1.75 turns...2 rotations of the tire should equal your rear ratio - make the DS spin 3.5 (plus a little) turns equaling 3.54. If you spin it 10 times, then you would have to divide the # of turns the DS made by 5 and then you would get your ratio, if you wanted to spin it that many times, try keeping count of how many full revolutions the DS made and then do more math. If you only rotate the tire twice and count, there should no cause of great error. The difference of tire diameter could be a factor but wouldn't throw my ratio off by as far as i am. The tach was reading within a 100 rpm difference at about 5k rpm than my meter and i figured maybe a little more at 6krpm but again, it would have to be off by 800rpm to throw me off as far as i am. I'm not trying to knock all your suggestions and appreciate all the suggestions, i just think something else is screwy. It has got to be that rear and i'm calculating on how to read an open diff or something. Yo - That's what i was thinking..then again, i'll be happy if it's a 3.9 'cause i'd ditch it and go for like a 3.54 or 3.7 which i think would help me boost off the line. Right now i'm through the first 3 gears so fast, i don't think it has the time to spool up. I wish i could post the video somewhere so you guys could see it. I have nothing to compare it to because i've never seen a Z with a BW-T5 mated with a 3.54 or 3.70, 3.90 take off at the strip. Dropping the diff and reading the gear is really the way to be 100% sure but...i PROMISED the wife to due the wainscoting she bought over a week ago and it will haunt me all weekend not knowing!!! Does anyone know if i'm doing the "spin the tire, count the DS revolutions" correct? I swear this was the way i did it on open rears for the past amost 20 years! I have to be doing it wrong.... Quote Link to comment Share on other sites More sharing options...
Guest zrl Posted June 29, 2003 Share Posted June 29, 2003 for 3.9:1 --> turn the wheel 10 times even, you should get 39 rotations even on the shaft; for 4.11 --> 9 times on wheel should give you 37 (to be exact: 36.99....) counts on the shaft. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted June 29, 2003 Share Posted June 29, 2003 My point was are you 100% sure you can tell the difference between 3.54 turns, and 3.36 turns, or 3.70 turns? I'd use the method zrl describes, as it is a bit more failsafe. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted June 29, 2003 Share Posted June 29, 2003 You know, I asked this question a while ago, but this was before I knew I was getting my turbo, so now I want to ask it again, but with a turbo setup in mind. Would you launch better if you launched in 2nd gear? Seeing as this would give you much more time off the line to build up boost, to get thru the whole of 2nd gear before shifting, as opposed to taking off in first where you only get like 2 seconds. Also, launching in second would allow you to rev the engine higher before either dumping or slipping the clutch, getting the turbo spooling faster, and allow you to launch without losing traction or at least not nearly as badly as you do in first. Just an idea.... I wont have my turbo for a month yet, so I have no way to try it myself, when I tried it with the engine I have now I got mixed results, dumping the clutch was no good but with a good slip launch it seemed to help. Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 29, 2003 Author Share Posted June 29, 2003 ZRL - i've never heard it done that way. I've always used the "spin the wheel 2 full rotations , while holding the other stationary, and count the # of DS full revoltions and you will come up with your ratio" method. Your way, which i will try, is telling me that if i spin my wheel one full revoltion, the DS should spin 3.54 times for a 3.54 ratio, correct? Because, using your example to detemine 3.9 gears comes out like this - 39 divided by 10 = 3.9 1 wheel revoltion should spin the DS 3.9 turns If i spin my wheel one full relolution, the DS turns 1.75 turns. If I turn it 2 revolutions, i get 3.5 and a tiny bit more. Why i considered it 3.54. If i turn the wheel 10 times, the DS would spin 17.50 full revolutions - 1.75x10. Try it when you ever have one of your rear wheels off the ground i bet you'll be surprised. Drax - a 3.54 rear will spin just about exactly 3 1/2 turns. A 3.70 rear will spin the DS just about 3 3/4 turns and 4.11's will spin it just over 4 turns. You can tell the difference easily if you know where you started on both the wheel and DS. I don't think DRL's method works with an open rear like the R200 i have, or i have something very unusual. I wish it did so i wouldn't have to drop this rear! Thanks again guys for the replies. Havent had the time, or will, this weekend to drop the rear and read the ring. And it's been bothering me all weekend! I will drop it soon and figure it all out. Thanks again and appreciate it much. Quote Link to comment Share on other sites More sharing options...
fl327 Posted June 29, 2003 Share Posted June 29, 2003 i hope you have a 4.375, that would even us all up! thanx again jersey-for making 280s feel real fat.... Quote Link to comment Share on other sites More sharing options...
Jersey Posted June 30, 2003 Author Share Posted June 30, 2003 4.375! ha. Yes, i've got to be doing something wrong, unless they are 3.54's and i'm reading the tach wrong or something at the end of the track...or something. At this point, i think i'm just confusing myself and the info i write even more! I said to spin the tire 2 revoltions in my previous post and i meant only one wheel turn and count the DS turns. I get a smidge over 3 1/2 turns on the DS, which would mean 3.54's. 13 wheel turns should be 46 (46.02) revs of the DS. Got it Drax. I just really need an hour to pull the damn thing and read the gear! Thanks guys. Appreciate it all. Quote Link to comment Share on other sites More sharing options...
mobythevan Posted June 30, 2003 Share Posted June 30, 2003 I am suppose to have 3.54 R200 sitting on the garage floor. I tried rotating the "tire" one turn and got about 1.75 on the driveshaft. I will just pull the diff cover and see what the gears are, but don't feel too weird mine seems to be the same. Quote Link to comment Share on other sites More sharing options...
Mike C Posted July 1, 2003 Share Posted July 1, 2003 Depending on the design of the diff, the reason you leave one tire on the ground and spin it twice is because of the spider and side gears. Holding one in place causes the spiders to rotate which effectively changes the number of times the driveshaft rotates. BUT this is only accurate if you have a true 2:1 spider to side gear ratio, and I hondestly have no idea what the Nissan spider to side ratio is. Get someone with an accurate speedo to drive by you on the highway at 60, and mark your rpm. If your tranny was setup for 3.54 and you have 3.90, your speedo will be off 10% which would skew your assumptions significantly. Other than that, suck it up and pull the cover! Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted July 1, 2003 Share Posted July 1, 2003 I think this is the problem with my car..... my mechanis says he used the right speedo gear, but it is off by an aweful lot..... tested it with a GPS the other day, doing 40mph my speedo showed 45, doing 53 it showed 60..... the faster it goes the more off it gets..... grrr gonna try to get him to fix it when hes putting the turbo motor in.... if I can get him to admit he messed up Quote Link to comment Share on other sites More sharing options...
Jersey Posted July 1, 2003 Author Share Posted July 1, 2003 Thanks on the spider info mike. I have no idea what the ratio is either. I did another test this morning on the way to work. With my GPS reading 60mph in 4th gear, my tach was reading 3350rpm. On the trans calculator, with my size tires - 215-50-15 - that equals exactly a 3.9 rear. The tach should have been around 3rpm if it were a 3.54. Yes, i just need to pull the diff. Thanks for the words of motivation! haha. Quote Link to comment Share on other sites More sharing options...
Guest the_dj Posted July 1, 2003 Share Posted July 1, 2003 How hard would it be for me to send you the guts of my 3.54, you send me the guts of your 3.90, and install them in our respective housings? Would you consider that an even trade? Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted July 1, 2003 Share Posted July 1, 2003 How hard would it be for me to send you the guts of my 3.54' date=' you send me the guts of your 3.90, and install them in our respective housings? Would you consider that an even trade?[/quote'] Sounds a lot more difficult than just swapping diffs, considering you have to set backlash and otherwise torque everything properly. Quote Link to comment Share on other sites More sharing options...
Guest the_dj Posted July 1, 2003 Share Posted July 1, 2003 I wonder if he'd trade diffs with me. Quote Link to comment Share on other sites More sharing options...
yo2001 Posted July 1, 2003 Share Posted July 1, 2003 I wouldn't mess with diff, 12.8 iis stout wit that set up. I bet it'll slow you down with 3.54. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted July 1, 2003 Share Posted July 1, 2003 I wouldn't mess with diff, 12.8 iis stout wit that set up. I bet it'll slow you down with 3.54. I'll take that bet. Every turbo car I know of that has compared has found the 3.54 to be faster in the 1/4. You just don't have time to build boost with a 3.90 in first gear. (or possibly even second) Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.