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Datsun Patrol MQ 1982 RB25DE NEO
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Discussion Starter · #1 ·
Sometimes I am an idiot. This time for assuming my MQ with the auto box had the same diff ratios as the manual wagon which should be 4.625. The only other ratios I was aware off until today, was either the 4.375 for the SWB hardtop, or the 4.875 for the LWB ute.
Long story short, after trying the R34 4 speed auto box to replace the factory 3 speed auto, I swapped back to the 3 speed because the ratios are too tall and the revs are too low for the RB25. Lots of work and lots of time wasted back to squire 1.

So I was in the process looking at manual gearbox swaps, when earlier today, I started getting all my diff spares together. Then I notices one of the diff main gears looking slightly different. Checked the ratio stamped on it and then compared it to the other one on the bench. This is the result :
Tire Automotive tire Wheel Motor vehicle Tread


Right side is the 4.375 and left is the 4.625 ratio. I then decided to jack up the front wheel of the MQ and do a quick test. I have to know what the ratio is on mine. Made some chalk marks on the tire and the tail shaft flange. Start turning the tire while counting the flange rotations. Easy right?
One Two and a bit. Crap, wrong start again, same result. Cannot be. Turn it the other way around, same result. Around 2.3 flange turns for a tire rotation. Something is wrong! Jack up one of the rear tires... Same result! What?????

That may explain why I can do 80km/h plus in 1st gear!!! Double check the pumpkin on the bench with the 4.375 ration diff in the photo above, yes around 4.4 turns of flange for every full rotation.
So why am I an idiot? One for not checking the ratios 1st, but actually another reason! I sold my spare set of 4.375 complete front and rear diffs about 6 months ago cleaning the shed. Idiot!!!
Anybody with knowledge on weird diff ratios please comment. Or if you have a spare front 4.375 diff of a complete set of 4.625 diffs available.
So this is the complete 4.375 H233 rear:
Automotive tire Black Gear Tread Tire


But what is this? Same size, same bolt pattern???
Automotive tire Motor vehicle Rim Gas Automotive wheel system
 

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Rust is just natural weight reduction.
1986 SD33T SWB
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11,234 Posts
Were both hubs locked when checking ratios?
All I can think is at some points when rotating the drive flange it was "easier" for the diff to spin one side than the other.

Diff ratios in AU were 4.11, 4.375, 4.625 and 4.875. Overseas there were others up to 3.3 or so, I think the tallest for H233B was 3.9 or something.

Your complete 4.375 is either 4 pinion open or LSD, most likely LSD, the bare carrier is an early 2 pinion open centre.
 

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Datsun Patrol MQ 1982 RB25DE NEO
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Discussion Starter · #3 ·
That's why I check front and rear with one eye on the tail shaft flange rotation and the other on the wheel itself. In both cases, front and back, one wheel was on the ground. I guess on Saturday morning I am going to swap out the rear diff center with the complete center on the bench and also check the ratio stamped on the current rear diff.

I also double checked Nerok's site to confirm the ratios and the options. But maybe because of the factory auto box, they decided to change the rations, who knows?

Yes I guess the open carrier is much older, probably from a G60?

In the end, top gear 3rd is the same ratio as a manual fourth gear ratio of 1:1. So if the ratios of 3rd gear auto and 4th gear manual is the same, then why is the RPM 2000 lower at 100km/h in the auto? That is if the diff ratios are the same as the manual and the wheels the same size?
I guess Saturday is going to reveal something new.... Imagine having proper ratios with the auto. No "T" suffix needed then. The L28 ute I owned a few years back was heavily modified and did a 8.5 second 0-100. The RB25 makes about 36kw more to the crank.
 

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Rust is just natural weight reduction.
1986 SD33T SWB
Joined
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11,234 Posts
That's why I check front and rear with one eye on the tail shaft flange rotation and the other on the wheel itself. In both cases, front and back, one wheel was on the ground. I guess on Saturday morning I am going to swap out the rear diff center with the complete center on the bench and also check the ratio stamped on the current rear diff.

I also double checked Nerok's site to confirm the ratios and the options. But maybe because of the factory auto box, they decided to change the rations, who knows?

Yes I guess the open carrier is much older, probably from a G60?

In the end, top gear 3rd is the same ratio as a manual fourth gear ratio of 1:1. So if the ratios of 3rd gear auto and 4th gear manual is the same, then why is the RPM 2000 lower at 100km/h in the auto? That is if the diff ratios are the same as the manual and the wheels the same size?
I guess Saturday is going to reveal something new.... Imagine having proper ratios with the auto. No "T" suffix needed then. The L28 ute I owned a few years back was heavily modified and did a 8.5 second 0-100. The RB25 makes about 36kw more to the crank.
Auto should have the same 4.6 diffs as manual, but through the 80s and 90s Nissan seemed to think it was always 4:30 on a Friday arvo.
Some changes just aren't tracked in the service manuals, so its perhaps possible.
For that reason I tend to use parts catalogues as my main reference now.

I will be interested to know the diff ratio in yours.

The open carrier would be from MQ, G60 used H215/H260, Aus models were all H260 as far as I know though.

Heh, wish I had an MQ that could even think about 8.5s 0-100.
I hit 16 once in a stock SD. Never checked again after adding boost/fuel. Didn't want to be disappointed.
 

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Datsun Patrol MQ 1982 RB25DE NEO
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Discussion Starter · #5 ·
I am slightly disappointed... hoping for a quick easy fix. Not this time. So lets talk MQ diff ratios again. So I pulled the rear diff out, easy no dramas except it's heavy. Yes open diff as expected. Oil looks good after 335 000km. I will call it mint condition, tight no play still with factory paint on it. So as I said before, I checked both front and rear diffs rotations outside to determine the ratio. Finally outside on the bench left next to the LS with the 4.375 right, the main gears looks quite similar.
Sports equipment Helmet Automotive tire Automotive lighting Sports gear


And it is stamped 37:8 which is 4.625, double of what I got when I tried to determine the ratio outside. Not sure why, as the 1st thing what I checked is the ratio on the bench, yes it's a 4.6. Cannot explain why it behaved differently still inside the front and rear diff housings.
Automotive tire Wheel Motor vehicle Bumper Automotive design


So I decided to swap the open center for the LS center. Easy got everything back together without the need to heat op the crown wheel.

Oil level when I opened the housing. Perfect!
Automotive tire Blue Motor vehicle Tread Alloy wheel


After the clean out of the old SAE90. Did not want to contaminate the brand new Castrol LSX
Tire Automotive tire Tread Motor vehicle Synthetic rubber


Picked up this transfer pump at Aldi..... worked well for the $9 I spent.

Automotive lighting Fluid Automotive tire Asphalt Gas


So back to the rations:
Top is the 3 speed factory auto. In yellow and red the 1st gear ratio which explains the ability to go 80+ in 1st gear.
Compared to the standard manual 4 speed MQ box, the ratio in green is much much higher. Then 2nd gear on the auto is equal to 3rd gear on the manual.
Slope Font Pattern Parallel Number


So the plan is still to find a suitable donor 5 speed gearbox. The requirement is a reasonable 1st gear ratio around 3.5 as well as a 5th gear for cruising. Now the problem with the L28 with the divorced transfer case is the length of the gearbox. The standard Patrol gearbox is short, but anything else is way to tall to fit in the space. I have an idea and after some googling, the gearbox I have in mind will bolt op the the RB and is short. More later, going to hunt this unicorn down 1st.

This same factory 3 speed auto box was used in the 240z. The only difference according to the WSM is that 1st gear actually has more engine breaking than standard. This is true as when I installed the shift kit, the hole for 1st gear was already the right size and no need to re-drill.
 

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1989 GQ TD42 wagon
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The "problem" is you had a wheel on the ground ;).

This is how I did it. You will need an assistant.
  • Safety first: vehicle parked on a hard, level surface; chock both rear wheels, and one front wheel.
  • Make sure the free-wheeling hubs are locked, and the transfer case is in 2H.
  • Mark the unchocked wheel, and the driveway so you know your starting point.
  • Under the vehicle, mark the diff carrier, and the input flange - again, so you know your starting point.
  • Jack up the unchocked wheel.
  • One of you needs to be watching under the car (I lay under it), while the other turns the wheel. Both count (silently ;)) how many rotations it takes for BOTH sets of marks (wheel and diff) to line up simultaneously.
  • Divide the number of rotations of the input flange by HALF the number of rotations of the wheel.
  • E.g.: I got 37 rotations of the input flange, and 18 rotations of the wheel. 37 / (18 / 2) = 37 / 9 = 4.11 diff ratio.
 

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Datsun Patrol MQ 1982 RB25DE NEO
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Discussion Starter · #7 ·
Thanks for that. I think this is the actual bit that answered my question:

* Divide the number of rotations of the input flange by HALF

Or in my case, which comes to the same, rotate the tire 2x. Looking at how the planetary gears work, exactly right.
Now the interesting part is,as soon as jou put the diff in the bench and you count the ring gear rotation compared to the pinion rotations, you get the exact ratio because you bypass the planetary or spider gears which halves the result.

Lesson learnt, but I hope that I will remember it again in a few years time

Sent from my BISON GT using Tapatalk
 
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