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Choosing a 5 Speed and Diff Combination


Muzez

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Hi Guys, 

As far as I am concerned, gear ratios are black magic so I was hoping to lean on you guys for advice on how to pair a transmission with a diff.

I am replacing the 4 speed in my 280z next year. I find myself in the enviable position of having two rebuilt 5 speed transmissions (1977 280z and 1980 280ZX) and 2 R200s (3.545 & 3.9).

  • Is there any advantage to mixing and matching transmission and diffs?
    (e.g. a 77 open ratio 5-speed with 3.9 or a 80 close ratio 5-speed with 3.54) 
  • And are there different applications for the close ratio versus the open?
    (e.g. one is better for cruising and hills versus one is better for acceleration) 

I mostly drive the car to cruise or corner with a little SCCA next year. The car is mostly stock and the plan is to keep it that way (for now 🙂 ). I tried to look through some of the old posts on this topic, but some of it is contradictory and a lot of it was stored on GeoSites which is offline. Thanks in advance for the help!

     77-79    1980              

1    3.321     3.062            

2    2.077     1.858          

3    1.308     1.308            

4    1.000     1.000           

5    0.864     0.773       

 

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I have a 5 speed "wide B" swapped in my 240. Original in my '77. 

Easy to work with, shifter stays the same but I did change the speedo cog from the 4 speed to a 5 on the 240 upgrade. Not sure the color but with 15" wheels in pretty dead on with the MPH on my GPS as the speedometer gauge reads. 

About 2,000 to 2,500 RPM at 75 MPH through 80 MPH highway. That's my best info as far as constant speed, you see I have an aluminum flywheel so red light to red light and traffic driving is all over the place on my tachometer.

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2 hours ago, siteunseen said:

I have a 5 speed "wide B" swapped in my 240. Original in my '77. 

Easy to work with, shifter stays the same but I did change the speedo cog from the 4 speed to a 5 on the 240 upgrade. Not sure the color but with 15" wheels in pretty dead on with the MPH on my GPS as the speedometer gauge reads. 

About 2,000 to 2,500 RPM at 75 MPH through 80 MPH highway. That's my best info as far as constant speed, you see I have an aluminum flywheel so red light to red light and traffic driving is all over the place on my tachometer.

You have the stock diff installed for your 240z? 
 

And do you like the lightened flywheel?

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8 hours ago, Muzez said:

You have the stock diff installed for your 240z? 
 

And do you like the lightened flywheel?

Yes, 180 differential with the 5 speed. No problems ripping around town or 2 hour drive on the interstate.

Aluminum flywheel is my best modification so far. I'm a rev freak I guess, grew up riding dirt bikes with power bands. It's in my blood. LOL

 

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12 hours ago, Muzez said:

This is just what I was looking for! Thanks so much! What did you end up going with?

I decided on the 3.9 ration rear diff and the wide ration tranny ('77-'79).  I think I mentioned that in my last post in that thread, if not I intended to and completely forgot.

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2 hours ago, texasz said:

I decided on the 3.9 ration rear diff and the wide ration tranny ('77-'79).  I think I mentioned that in my last post in that thread, if not I intended to and completely forgot.

I think that is the direction I am heading as well.

I like the idea of a bit more top end because I drive the thing out to my parents house an hour+ away and usually dial in at 80mph. 4.5k rpm the whole way might get a little annoying after a while. 

Also one clarification because I was curious, you mention in your post “The sweet spot for 'stock' like acceleration” what did you mean by this? I assume your goal when you set off on this whole evaluation was some degree of performance improvement over your current setup but did I misunderstand?

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1 hour ago, Dave WM said:

the later zx  trans with the .75 od and a 3.54 stock r200 diff will get you 80mph right at 3k

Any idea what the performance is like compared to the wide ratio with 3.9? Also, I have an early ZX trans. How different is the .77 to the .75 od?

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First gear is where you'll notice the changes.  4th gear is always 1:1.  But between those two choices you have a 3.321 1st, or a 3.062 1st.  If you live in a hilly area or do a lot of city driving, the lower 12.95 overall might feel right.  If you live on flat ground and don't do a lot of stop and go, you'll probably find that 1st is pretty "short". 

Also, the other notable difference between the two choices is that the early 5 speed has a noticeable gap from 2nd to 3rd.  If you're climbing or trying to accelerate you need to stay in 2nd longer than you'd think to get a good shift.  Waa...waaaaaa...waaaa...waaaa...waaaaaa  versus waa..waa..waaa..waaaa...waaaaa

Don't forget to calculate the effect of tire size.

3.321 x 3.54 = 11.75

3.062 x 3.9 = 11.94

3.321 x 3.9 = 12.95

 

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Just a heads up on the wide 5 in my 240. It would pop out of gear. Finally figured out the shifter was hitting the tunnel and going completely into some gears.

Post #16 shows the nicks on the shift rod I found. Some light grinding fixed me up, no more issues.

Screenshot_20211201-182406_Samsung Internet.jpg

 

Edited by siteunseen
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21 hours ago, Muzez said:

I think that is the direction I am heading as well.

I like the idea of a bit more top end because I drive the thing out to my parents house an hour+ away and usually dial in at 80mph. 4.5k rpm the whole way might get a little annoying after a while. 

Also one clarification because I was curious, you mention in your post “

The sweet spot for 'stock' like acceleration” what did you mean by this? I assume your goal when you set off on this whole evaluation was some degree of performance improvement over your current setup but did I misunderstand?

In the thread I posted the link to read the first section (just down to the first set of screen shots) of post #11 and pay special attention to Zed Head's post that I quoted.  This is where he describes the over all drive ratio for 1st gear and the impact that has on acceleration.  The lower the number the slower the acceleration, the higher the number the faster the acceleration...so depending upon the type of driving you will be doing, the type of driver you are (i.e. how you want your ride to "feel") will start to guide you to what is the right combination for you.  I hope this helps answer your question.

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22 hours ago, Dave WM said:

the later zx  trans with the .75 od and a 3.54 stock r200 diff will get you 80mph right at 3k

This is the configuration that I’m planning on for my ‘75 280Z. Well, once I free up some time to put my close ratio ZX 5-speed back together.  I’m still trying to decide if I want to spring for a LSD upgrade to my stock diff.

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On 11/30/2021 at 5:25 PM, Muzez said:

I mostly drive the car to cruise or corner with a little SCCA next year.

 

If you are talking Autocross here, the 3.90 diff and the trans with the 1.858 2nd gear is the way to go.

Good off the line and 60+ mph in 2nd.

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Hey, I wanted to piggy back my question on this recent thread.   These transmissions can get confusing and especially telling exactly which one you have.  I pulled a 5 speed out of my car (wasn’t original - had yellow marker with 280Z 5 speed written on it).  I assumed it was a wide ration 5 speed from an early ZX or late Z.   This transmission had two electrical switches near the filler plug.  I assume one was reverse light and the other a neutral safety switch.   The transmission I got off a 82 or 83 ZX, can’t recall, only has one switch and I assume it’s the reverse light switch.   When I looked at the 82 FSM it shows four switches on that transmission?  Other 2 say  “top gear” and O.D switch.  Wondering Is it possible I don’t have a close ratio ZX tranny since it only has the one switch?   My car wiring harness only has one connection too, which. I Image again is the reverse light  switch.   So I have two 5 speeds and I’m not 100% sure which ones they are. 
My propeller shaft and transmission were not original it appears noted by the yellow marker.  No idea what Easter egg I’ll find with the differential because I bought an 82 ZX diff too…..it’s possible there is one already on my car?  II want to get the right combo of transmission and diffs as well so ID’ing everything I have properly is critical.   Time to crack open diff cases I guess.  Too bad I leave for work for 11 days on Monday.  Doesn’t my job know I’ve got work to do on the Z?  

This image below is from the 82 FSM

 

A705D0F3-317D-4FE1-91B2-F6675FE1BB91.jpeg

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I don't think that I've ever seen all four switches on a ZX transmission.  I think that two is more common.  But it's easy to tell close and far apart by the exhaust hangers on the back and the reverse lockout plate on the side, among other clues.

On the diff, it's easier to determine ratio than it sometimes seems.  If you can get the back end of the car up, one easy way is to lay on your back under the diff, reach out and turn both tires one revolution at the same time.  Count the number of times the pinion flange turns.  You don't have to worry about the decimals.  3.54 = ~3 1/2, 3.9 = ~4, 3.7 = ~ 3 3/4.

Mark the tires and the flange and just make sure that both tires end up at the same spot.  If they get off a bit in the middle it doesn't matter.  

You can also check ratios on your transmission by marking the input and output shafts and counting/estimating.  3.321 is quite a bit different than 3.062.  If you use a piece of tape on the output you don't have to be right over top of it to see when it's made a full turn.

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