R180 3.90 Differential

Mr Tenno

Digital Officer
Staff member
Site Administrator
I'm looking to switch out the US unit in my 240.
 
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SeanDezart

Well-Known Forum User
A 'good' bolt-in replacement would be an LSD, they're available in various ratios and I question whether or not the 3.9 is 'best' simply because it was OEM over here.
 

jonbills

Membership Secretary
Site Administrator
What ratio does the car have now? A 3.9 would certainly be better than 3.5 or taller.
 

Albrecht

Well-Known Forum User
A 'good' bolt-in replacement would be an LSD, they're available in various ratios and I question whether or not the 3.9 is 'best' simply because it was OEM over here.

An LSD in a 3.364:1 ratio rear end is probably going to need 300ps or more driving it to make it fit for purpose.

What's *best* is subjective, and subjectivity itself is subjective, but a 3.364:1 ratio diff is not a *good* choice in the context of a sports/GT car despite the fact it was stock, OEM fitment 'over there'...
 

Albrecht

Well-Known Forum User
Why is 3.9 'best/better' ?

Please qualify your statements chaps.

Because it makes a sensible pairing with the 5-speed Overdrive transmission he wants to package it with, in terms of overall gearing, driveability and reasonable 5th speed cruising RPMs.

It's also a ratio that should be relatively easy to find in the UK. Personally I'd be happy to use much lower ratios like 4.375 and 4.44 (and do...) but Mr_Tenno is probably looking for something that's still a little a little on the conservative side, which 3.9 is.

Turning the question on its head, perhaps you could offer some qualification for the suggestion of sticking with a 3.364:1 diff ratio when fitting a 5-speed O/D and 15inch wheels/tyres? We are talking about an L24 here, not a VAG turbodiesel...
 

SeanDezart

Well-Known Forum User
Because it makes a sensible pairing with the 5-speed Overdrive transmission he wants to package it with, in terms of overall gearing, driveability and reasonable 5th speed cruising RPMs.

Turning the question on its head, perhaps you could offer some qualification for the suggestion of sticking with a 3.364:1 diff ratio when fitting a 5-speed O/D and 15inch wheels/tyres? We are talking about an L24 here, not a VAG turbodiesel...

Read back - II never suggested sticking with the 3.364:1.

I merely questioned why 'only' a 3.9 !

For a road* car using 4x gears and a long 5th (almost an overdrive), why not use an intermediate diff ratio.

I had a 3.9 on my first 240Z, 3.7 on my current 240Z, I'm changing that again......maybe it's age-related :D !

*also for track days - use only the first 4x gears can be quicker unless one has a direct 5th ! Hands-up all those running such a gearbox :unsure::rolleyes:
 

Albrecht

Well-Known Forum User
Read back - II never suggested sticking with the 3.364:1.

But that's what he's got.

SeanDezart said:
I merely questioned why 'only' a 3.9 !

You never mentioned any "only". You wrote:

SeanDezart said:
Why a 3.9 ?

Nissan thought it was a good pairing for both of the 5-speeds which were fitted as standard equipment on the 240Zs, so - trusting their expertise - I would tend to go along with it as a conservative recommendation...

SeanDezart said:
For a road* car using 4x gears and a long 5th (almost an overdrive), why not use an intermediate diff ratio.

3.9:1 is an 'intermediate ratio'...
 

uk66fastback

Club Member
In the real world, what's the rpm difference at 70mph in fifth between what the US cars came with and a 3.9 (or the 4.x)
 

SeanDezart

Well-Known Forum User
But that's what he's got.



You never mentioned any "only". You wrote:



Nissan thought it was a good pairing for both of the 5-speeds which were fitted as standard equipment on the 240Zs, so - trusting their expertise - I would tend to go along with it as a conservative recommendation...



3.9:1 is an 'intermediate ratio'...

You're nit-picking Alan :D !

Alright, why 'only' a 3.9, why not another ratio when changing from a 3.364 ?

And yes, I know that's what he's got:rolleyes: but I never implied he stick with it - there are more choices out there than stock USA manual 3.364 and euro manual 3.9 !

Indeed, the 3.9:1 IS an intermediate ration when looking at the BIG picture but when simply comparing a 3.364 and a 3.9, those were the ratio limits and there are those that are intermediate between those two ratios.

Frankly, I respect Nissan's choices, decisions but I don't care - given different options today, why not deviate to suit each person and their specific needs rather than ther mass market ?

I have some files to share with you tonight (when I get home and have access to them) to answer your questions Rob and uk66.
 

Albrecht

Well-Known Forum User
In the real world, what's the rpm difference at 70mph in fifth between what the US cars came with and a 3.9 (or the 4.x)

That's fairly easy to work out, but it's not the only consideration. A rear end ratio like 3.364:1 effectively widens the gaps between gears in the transmission, giving you a bigger rpm drop. It's *better* (more sporty, certainly) to have a closer spread across gears and less rpm drop when changing up - so you can keep the engine closer to its most effective operating range.

Rear end ratios in the range of 3.9, 4.1, 4.375, 4.44 and up (to a point!) on 'our' cars work nicely with 5-speed OD transmissions because they give a nice overlap in overall gearing whilst retaining a reasonable cruising speed/rpm.

Nissan gave us charts to illustrate this in their Japanese literature. Here's an example for the HS30-H model Fairlady 240ZG, which had a 2.957:1 first gear, 0.852:1 overdrive fifth and 3.9:1 diff ratio, same as the early UK market cars:

 

Rob Gaskin

Treasurer
Staff member
Site Administrator
In my 'real world' I've driven my trackday car (2.8) with 3.7 and 3.9. It was good fun with either.

On the road I think a 3.7 would be better and that's why I can't understand the desire to bring the gearing down.

At the end of the day wheels/tyres give you some flexibility - my track tyres were smaller dia than my road tyres.
 
In the real world, what's the rpm difference at 70mph in fifth between what the US cars came with and a 3.9 (or the 4.x)

There's lots of factors

final diff ratio
wheel size
tyre profile/ratio
gearbox

Theres a few good gearing calculators on the net to play with.

As an example, 4.44 205/16/14 tyres, 'c' box, 70mph is about 3000ish rpm.
 

Albrecht

Well-Known Forum User
In my 'real world' I've driven my trackday car (2.8) with 3.7 and 3.9. It was good fun with either.

On the road I think a 3.7 would be better and that's why I can't understand the desire to bring the gearing down.

In my 'real world' I used to use 4.375 and - for a while - 4.6 ratios, combined with plate LSDs and 14-inch wheels, on my daily commute in central London as well as longer journeys, the odd track day and etc. It was a hoot.

One of my current projects - at least the one closest to completion - is fitted with a 4.44:1 ratio R192 diff with plate LSD matched to an FS5C71-A 5-speed OD trans and 200ps S20 engine. It should be pretty close in character to the stock and near-stock 432s I've driven in Japan (one of which was - head and shoulders above all others - the very nicest driving S30-series Z I've ever had the pleasure to have a go in) and they buzz along very nicely at motorway speeds whilst rowing along very nicely in 2nd, 3rd and 4th, where most fun in driving is had.

I would not be afraid to spin an L24 up a bit, but people generally seem to be scared to actually rev them. What practical use is a theoretical 150mph top speed gearing in one of these cars?
 
In my 'real world' I used to use 4.375 and - for a while - 4.6 ratios, combined with plate LSDs and 14-inch wheels, on my daily commute in central London as well as longer journeys, the odd track day and etc. It was a hoot.

One of my current projects - at least the one closest to completion - is fitted with a 4.44:1 ratio R192 diff with plate LSD matched to an FS5C71-A 5-speed OD trans and 200ps S20 engine. It should be pretty close in character to the stock and near-stock 432s I've driven in Japan (one of which was - head and shoulders above all others - the very nicest driving S30-series Z I've ever had the pleasure to have a go in) and they buzz along very nicely at motorway speeds whilst rowing along very nicely in 2nd, 3rd and 4th, where most fun in driving is had.

I would not be afraid to spin an L24 up a bit, but people generally seem to be scared to actually rev them. What practical use is a theoretical 150mph top speed gearing in one of these cars?

Its because at a 3 hour, 75mph motorway cruise the world will topple off its axis.
 
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