Why a 3.9 ?
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.
....What's *best* is subjective, and subjectivity itself is subjective..
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.
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.
SeanDezart said:I merely questioned why 'only' a 3.9 !
SeanDezart said:Why a 3.9 ?
SeanDezart said:For a road* car using 4x gears and a long 5th (almost an overdrive), why not use an intermediate diff ratio.
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'...
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)
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)
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?