Tranny/Diff Ratio and RPM @ Speed

[texasz]

I need some help here.  I've put together a lot of information (below) on possible transmission/diff combinations I may put in my Z and need a little guidance as to what this all means.  Here's what I think this is telling me...the wide ratio (77-79) is going to give me more off the line acceleration in 1st and 2nd with a long wait to $^!# to 3rd and poorer highway top end speed than the narrow ratio (81-83) tranny.  That being said, the it gets messy when you start to look at different diffs in the mix.  

 

These are the sites I have used as references for all this research.

http://www.zhome.com/ZCMnL/tech/GearRatios.html

https://tiresize.com/calculator/

http://www.vmar.com/java/gear.html

http://www.zcsd.org/tech-articles/differential-ratios.php

 

 

 

 

 

 

 

 

The numbers in orange are the MPH difference from one gear to the next.  In my mind I'm thinking this is good to look at since it will help illustrate "time" spent in each gear, the size of the jump from one gear to the next which would relate to time in the 'power band'.  Maybe I'm completely out of my mind, you tell me. 😉

 

So of course the big question...what do I install???

 

 

 

Edited March 25, 2021 by texasz

[Zed Head]

Nissan seemed to shoot for about 12 overall in first gear.  You live in Texas so might do some 80 - 90 mph cruising, and not so many hill starts to worry about.  Really depends on how you plan to use the car.

These are coupe numbers.  Things get weird in the 2+2 cars.

Early cars 3.592 x 3.364 = 12.08

280Z's  3.321 x 3.54 = 11.76

280ZX's  3.062 x 3.9 = 11.94

[texasz]

@Zed Head, so these are the over all drive ratio in first gear with those diff ratios and those trannys.  I get that though what does it mean in the real world (not in terms of revolutions and all, that part I understand)?

[siteunseen]

Here's a very good read about everything. It has details on the diff and transmission too.

https://datsunzgarage.us/engine/

 

[texasz]

12 minutes ago, siteunseen said:

Here's a very good read about everything. It has details on the diff and transmission too.

https://datsunzgarage.us/engine/

 

Yep, I've read Bryan's entire site many times over the years.  Great stuff!!  

[Zed Head]

1 hour ago, texasz said:

@Zed Head, so these are the over all drive ratio in first gear with those diff ratios and those trannys.  I get that though what does it mean in the real world (not in terms of revolutions and all, that part I understand)?

Taking off from a dead stop.  The higher the number, the easier it is.  Less clutch work.  If you use a ZX 3.062 with a Z 3.54, the 10.83 ratio (just multiply the numbers)  is noticeably "doggy".  Slow acceleration.  On the other hand, some people put a 3.9 diff in their Z's to give them more zip.  3.321 x 3.9 = 12.95.  Think of about 12 as what the Nissan engineers chose for the average driver.

So about 11 is tolerable at the stop signs and lights, and about 13 is what you want if you're a light to light racer type.

[S30Driver]

I think @Dave WM  put a 5 speed (not sure which one) in his car and retained the stock 3.54 rear end.  He might might be able to provide some insight and his driving impressions of that combination.

[Dave WM]

yes I did, I get 70mph at about 2500-2600 rpm. 80mph at 3000 rpm (vs 4000 in 4th).

Its pretty sluggish below 65 so I consider it only a highway gear. I try not to allow the rpms to drop below 2500 in 5th.

Also I live it flatlands, IF you have hills, you may have to down shift even at 70 mph. This personally fits my driving style and area, but may be too tall if you want peppy performance in ALL gears.

So its great for interstate trips, above 75-80 mph otherwise not much use. 1st gear was a bit taller than the orig 4 speed, but not so much as to be noticeable, I can still start off with no throttle, just ease the clutch and the flywheel is more than enough for it. Again hills would be different, perhaps a bit more of a challenge to start off up hill.

This is from a ZX what is commonly called the "close ratio 5 speed", the .75 OD model.

Edited March 23, 2021 by Dave WM

[texasz]

1 hour ago, Dave WM said:

yes I did, I get 70mph at about 2500-2600 rpm. 80mph at 3000 rpm (vs 4000 in 4th).

Its pretty sluggish below 65 so I consider it only a highway gear. I try not to allow the rpms to drop below 2500 in 5th.

Also I live it flatlands, IF you have hills, you may have to down shift even at 70 mph. This personally fits my driving style and area, but may be too tall if you want peppy performance in ALL gears.

So its great for interstate trips, above 75-80 mph otherwise not much use. 1st gear was a bit taller than the orig 4 speed, but not so much as to be noticeable, I can still start off with no throttle, just ease the clutch and the flywheel is more than enough for it. Again hills would be different, perhaps a bit more of a challenge to start off up hill.

This is from a ZX what is commonly called the "close ratio 5 speed", the .75 OD model.

Is there a magical combination that would give peppy performance in ALL gears?  Probably not though thought it was worth asking.

[Dave WM]

I have heard the 3.9 with the zx trans is the stock setup, should be pretty good for overall performance.

[texasz]

17 hours ago, Zed Head said:

Taking off from a dead stop.  The higher the number, the easier it is.  Less clutch work.  If you use a ZX 3.062 with a Z 3.54, the 10.83 ratio (just multiply the numbers)  is noticeably "doggy".  Slow acceleration.  On the other hand, some people put a 3.9 diff in their Z's to give them more zip.  3.321 x 3.9 = 12.95.  Think of about 12 as what the Nissan engineers chose for the average driver.

So about 11 is tolerable at the stop signs and lights, and about 13 is what you want if you're a light to light racer type.

That is perfect and just the kind of information I needed, thank you!!

 

Now with all this new information I've updated the spreadsheet I made (below).  Clearly the wide ratio tranny with the 3.364 diff is not desirable.  And the close/narrow ratio tranny with the 3.9 may be a bit much.  The sweet spot for 'stock' like acceleration looks to be the wide ratio tranny with the 3.9 diff, that combination would result in the speeds shown in the second chart below...and that would seem to indicate an estimated 84MPH @ 3500RPM in 5th.

 

                    

 

                    

 

Based upon info others have shared and the calculations the next best may be the close/narrow ratio tranny with the 3.364 diff.  This results in less responsive acceleration than the previously mentioned combination and an estimated 85MPH @ 3500RPM in 5th (chart below).  So barely any difference at highway speed so probably not a good choice given the loss in acceleration responsiveness.

 

                    

 

Just to continue on this line for those who want that supper snappy response off the line the close/narrow ratio tranny with the 3.9 diff looks great though you will be greatly sacrificing top-end since it looks like you would have an estimated 73MPH @ 3500RPM in 5th...that is a loss of over 10MPH on the highway (chart below).

 

                    

 

 

 

Edited March 23, 2021 by texasz

[texasz]

I have a few more thoughts and questions.  We have been discussing the tranny/diff in a vacuum when there may be other factors that impact how the Z performs.  What about...

• A high HP engine (stock is 150-170 depending)?
• A high torque engine such as a stroker?
• A lighter flywheel? 

What effect would each of these have on the tranny/diff discussion we have been having?