Chemicalblue Posted November 20, 2004 Share Posted November 20, 2004 I have installed a 88 3.7(?) Viscous Coupling LSD with a Finned Rear Cover in my 79zx. I believe I have a 83 trany(not sure how to tell just took the guys word for it) and installed 17" wheels with 205 40 17 tires. The car is screaming on the hwy 65mph@3100rpm (not cool). Is there a way to resolve this problem? Has someone sold me the wrong stuff? PLZ HELP Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 20, 2004 Share Posted November 20, 2004 Well.. what do you mean by 'resolve'? I turn about 2800 (in my 75 280z) in 4th, but I've got the 3.54 rear diff.. (you do have a 5 gear right?) I'd say you'll have to change the rearend ratio if you wana bring down the rpm at 65 mph.. mind you tire size might be part of the problem.. (does anyone know off hand what the overall hight is on the 205/40/17's?) If there's room, a taller tire will help a bit.. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 20, 2004 Share Posted November 20, 2004 Oh and dude.. don't double/triple post. Pick a forum, and ask your question ONCE. Don't worry, someone always replys. Quote Link to comment Share on other sites More sharing options...
auxilary Posted November 20, 2004 Share Posted November 20, 2004 your stock ZX gearing: 3.545:1 your stock ZX tire size was 195/70/14, and your overall tire height was 628.6mm or 24.7" your new differential is 3.7:1, and you have smaller overall diameter tires, 205/40/17, with overall height of 595.8 or 23.5" this means that your differential makes a whole lot more revolutions than it used to, and your speedo is also off, so you're actually doing less than 65 mph. In short, this is normal based on the items you selected. No one ripped you off, you just made a miscalculation. You get better off the line acceleration, but your top speed is now lower. Quote Link to comment Share on other sites More sharing options...
Chemicalblue Posted November 20, 2004 Author Share Posted November 20, 2004 My friend which has the same set up and thinks there is something wierd. He has a 240 w/ slightly larger tires but no issue cruising at 80 or 90. Is there a way to ck ser. #s or ratios. Can the ratios be changed? What would be a good set up for hwy cruising? Tnx for the help Sorry about triple post Quote Link to comment Share on other sites More sharing options...
datsunlover Posted November 20, 2004 Share Posted November 20, 2004 Are you sure your friend has the EXACT same setup? IE; idendical rear diff, and the SAME tranny as you? Also.. how much taller are the tires on his 240? You'd be suprised how much tire size will change things. I WAS running 185/65/14's for a while, and 65 MPH was LOUD.. around 3200 rpm or so.. I changed the rear tires to 195/70/14, and WOW what a diffrence.. Made the car more 'driveable' and droped the revs a LOT on the highway. Something else I just thought of.. the previous owner told you it's an 83 tranny? If you have his number you could call him and find out or sure WHAT the tranny is from... Quote Link to comment Share on other sites More sharing options...
auxilary Posted November 20, 2004 Share Posted November 20, 2004 My friend which has the same set up and thinks there is something wierd. He has a 240 w/ slightly larger tires but no issue cruising at 80 or 90. Is there a way to ck ser. #s or ratios. Can the ratios be changed? What would be a good set up for hwy cruising? Tnx for the help Sorry about triple post bing! larger tires. and your friend has a 3.7 lsd in his Z too? because manual Zs had 3.36 gearing stock. You can opt for a Z transmission with taller 5th gear. Otherwise your only choices are either put the stock diff back in, or bigger overall diameter of your tire Quote Link to comment Share on other sites More sharing options...
johnc Posted November 20, 2004 Share Posted November 20, 2004 The car is screaming on the hwy 65mph@3100rpm (not cool). BTW... 3,100 rpms is not screaming for the Nissan 6 cylinder. Quote Link to comment Share on other sites More sharing options...
TimZ Posted November 20, 2004 Share Posted November 20, 2004 Hold on a minute here... How are you determining that you are going 65mph? If you are using the speedo, then the rpm to indicated speed relationship will be exactly the same as before the swap, since the diff and tires are after the speedo pickup in the trans. Also, was this a manual or an automatic trans? According to this transmission calculator, 65mph should happen at about 2500rpm in 5th with the combo that you stated, assuming an '83 manual trans. Note that this is actual speed, not indicated speed. Quote Link to comment Share on other sites More sharing options...
fl327 Posted November 20, 2004 Share Posted November 20, 2004 I love my 3.7 LSD. Just had to say. Quote Link to comment Share on other sites More sharing options...
Chemicalblue Posted November 20, 2004 Author Share Posted November 20, 2004 Ive got some feedback today that nissan offered a 3.9 set up and that someone may have switched the ratio. How can u tell? also Ive herd that people often sell a truck (nissan) trany and pass it off for a 83 5 speed. Is there a way to tell what year the trans is made? The tires i have on the car are $225 a pc so a switch will be further down the road. but the 14 that I had on there were only .75" larger in dia. thanks everyone for all the advice Quote Link to comment Share on other sites More sharing options...
TimZ Posted November 21, 2004 Share Posted November 21, 2004 Ummm, okay... So how were you measuring your speed again? Quote Link to comment Share on other sites More sharing options...
Chemicalblue Posted November 21, 2004 Author Share Posted November 21, 2004 My wife sat next to me on the hwy with her neon said its some where around 65mph@3000rpm. my speedo shows 60mph@3000rpm Quote Link to comment Share on other sites More sharing options...
auxilary Posted November 21, 2004 Share Posted November 21, 2004 Uhm, neons are notorious for showing overestimated speed. I own one Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted November 21, 2004 Share Posted November 21, 2004 I'm betting he's not running an '83 tranny either. The earlier 5 speeds, the ones with the really crappy 2nd to 3rd shift (BIG drop in RPM when shifting into 3rd), had a relatively high numerical 5th gear.... there was very little difference between 4th and 5th, and freeway cruising was at a pretty high rpm (for me it was worse as I had a 3.9 diff as well). If those two 'symptoms' sound familiar to you (the 2-3 shift being way steep and the 4-5 shift being way close) then you have one of the earlier crappy trannies. Also, it'd be very easy to verify if your speedo is off if you know anyone with a GPS unit. I used to think my car was so fast hitting 0-60 in 6 seconds flat, then I tried my friends GPS and found out 60 was actually 50 Changing things as you have can throw them way off, or, it can just be off due to the fact that it's old, as well. Quote Link to comment Share on other sites More sharing options...
Guest SM00TH976 Posted November 23, 2004 Share Posted November 23, 2004 if you really want to know how fast you are going get on a freeway with mile markers and time how long it takes you to go a mile. use some simple algebra to figure it out from there. Quote Link to comment Share on other sites More sharing options...
Chemicalblue Posted November 23, 2004 Author Share Posted November 23, 2004 thats a good thought. seems a little more precise. tnx Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted November 23, 2004 Share Posted November 23, 2004 Get a Garmin handheld GPS and you will be very accurate. They aren't that expensive either. And when you're not using it for speed calculation, you won't have to stop for directions. Seriously though, they work well. 81zxturbo(My little bro's new toy) 95slobaru(Waiting for a 2.5) Quote Link to comment Share on other sites More sharing options...
Phantom Posted November 23, 2004 Share Posted November 23, 2004 76-79 5-spds had .86:1 OD's 80ZX 5-spd had .777:1 OD, and 81-83ZX 5-spds had .745:1 OD's Did you ensure you had the proper speedo gear when you installed the new trasmission? That could cause an erroneous speedometer reading at a given rpm. Find someone with what you know is an absolutely accurate speedometer, run your car beside it at 60mph and get your rpm. Then go back to the transmission calculator and try the different tranny ratios above and see what kind of differential ratios you come up with. Eventually you'll see numbers that make sense. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted November 23, 2004 Share Posted November 23, 2004 The most accurate I've seen a cars speedo from production dates newer than 1992, is 4.6% error. Just because it is a new car, unchanged from stock is no guarentee that the speedo is accurrate. Rallyists would jump all over a car with 0% speedo error. Quote Link to comment Share on other sites More sharing options...
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