WTD: 3.63 diff in an NA8?

[manga_blue]

Does what I'm thinking about here make sense? I'd appreciate some opinions and experience.

The car has been retired from trackwork and it's gradually being transformed into a country tourer. I need a car to cruise around all these winding coast and mountain roads here with the odd long haul to Sydney or Melbourne.

The first stage of de-tuning was to build a torquey motor. It now has a ported BP4W head, 7000 redline, NA8 inlet manifold, stock cams, full exhaust, CAI and modded ECU mated to a stock NA8 5 speed gearbox. It's making a fair bit more power than an NB8B (feels like 100rwkw?), with a healthy bundle of low end grunt. For an n/a motor it pulls pretty hard from 2200rpm up.

The trouble is that there's an ominous rumbling starting in the old 4.1 torsen diff. I need to pull that and see what can be done. It may take a while, if it is repairable at all.

My initial thought is to pick up a reasonable NB8B 3.63 open diff to keep the car on the road in the meantime. In the long run it could stay there. I strongly oppose "spirited" driving on public roads, so to me a torsen is only needed for track work.

The impact of the taller diff ratio is a drop from 3350 to 3000in 5th at 110kph on the Hume Highway and a drop from 3000 to 2700 at 100kph on most of my local roads. An 80kph zone would probably take me back to 4th, which is pretty much what I do already. 2nd and 3rd would get extended into what looks like a more useful spectrum for mountains, twisties and passing.

[beavis]

While it makes complete sense, i did the opposite on my street car. I went from a 3.6 to a 4.4. (6spd gbox) Because a 3.6 is just too tall for my liking and makes the car feel slow. If you're no longer chasing times, then it's all up to your tastes really.

Either way, you have done enough of this to know what you're doing.

[manga_blue]

Thanks Beavis. I guess I'm sort of looking for affirmation. In terms of this forum's standards, this feels like putting 17s on an NA6.

[hks_kansei]

I've been in an NA8 with about 100rwkw and a 3.6 diff. (Granted, with bigger cams and headwork, so probably a far different powerband to yours)

To be honest, there was nothing wrong with it other than the car feeling like a stock one in terms of power.


So yeah, if you find standard power ok, then the tall diff on a modified engine should end up similar.

[sailaholic]

I'd say fit it and find out. I think very few people will have the combination of roads and car that you have.

I've been more thinking of your other diff thought - 4.4 or 4.7 with 225/50/15 tires.


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[manga_blue]

That was partly what led me to this thinking. I ran 225/50R15 A050s (which looked huge) on the GP circuit at Eastern Creek and the lap times were quite a bit better with those extra legs compared to 205s. Fast open track is the key, they were pretty slow on the tighter Wakefield Park track.

[hks_kansei]

By my maths, assuming 5 speed in 5th gear.

195/50/15 with a 4.1 is 3074rpm@100kph.
205/50 is 3021rpm
225/50 is 2922rpm

195/50/15 with a 3.6 is 2699rpm


Manga, send me a PM if you want me to email you my big gear/diff/tyre spreadsheet.

[manga_blue]

Manga, send me a PM if you want me to email you my big gear/diff/tyre spreadsheet.

Thanks, I've got one too. I've just corrected my calcs against yours and now they match up.

In theory I can calculate all this down to the finest detail but in practice it doesn't really tell me what it would feel like in the driver's seat. So your comments about a 100rwkw NA8 feeling stock worried me a fair bit. Or at least it did worry me until I thought a bit more about your comparisons on tyres. I was going to run 205/50R15s on classic JDM Sprint Hart rims weighing 7.0Kg but I think you're on the right path - 195/50 would make much more sense with a 3.63.

195s are lighter and cheaper with a better range anyway. I have a few wheel sets ranging from 3.85Kg per wheel up to 7.1Kg in the shed. Maybe I could offset some of the effective power loss by going for the lightest combination to get the lowest moment of inertia.

[beavis]

one consideration is, given the 3.6 should be obtainable for under $300, to just swap a diff in and see. if you dont like it, change it back.
Your out of pocket expense is pretty low.

[slomo]

Interesting comments so far. I just converted my na6 with na8 engine swap from the standard 4.3 diff to the 4.1 and found I rarely get into 5th gear except at highway speed +90kmh. I think you are giving way to much thought to highway speed revs and not enough to town or B road speed limits. A 3.6 ratio may result in a laboured engine or more lower gear use to maintain higher revs, which may defeat the purpose you are after. I would stick with a 4.1 replacement for your failing diff as cruising revs for highway use is not excessive, maybe even ideal, or at worst go to 3.9..
I found the change from 4.3 to 4.1 quite noticeable and it took a little while to get use to the change, particularly at low speed.

[manga_blue]

one consideration is, given the 3.6 should be obtainable for under $300, to just swap a diff in and see. if you dont like it, change it back.
Your out of pocket expense is pretty low.

That clinches it, thanks Brendan.

[97 MXV]

One thing we can all agree on, there is always two sides to every coin. Another way the coin falls limits use of top gear on our roads with a 3.63 diff.
Obviously that side of the coin finds 3.63 too high for a 100km speed limit and a 7000 rev limit.
The other side of the coin says a 3.6 diff in top gear on our roads turns that rev loving twin cam engine build, spirited engine experience into a vintage engine experience that struggles with changing load so much it demands extended periods of proactive downshifting.

The other side of the coin

[pcmx5]

My NA had over 100RWKW and it was great with a 3.9 and the 6 speed which is lower in top than your 5 speed.Really long hills challenged with that set up so if you stay with the 5 speed i reckon it will be a slug with a 3,6. and you would be changing down to go over the white line!

Actually i think the 4.1 5speed is the best and cheapest gearing setup,but you can obviously make that decision with what you have now.Given my time over I would have left that set up,the 3.9 6 speed was marginally better with a slightly better spread of ratios but not worth the $$ in my opinion.

Peter.

[manga_blue]

I never saw the value in the 6 speed either, Peter, especially since I broke so many gearboxes on the track.

Ultimately it all depends on what your needs are. My main drive is 28 kms into town on smooth blacktop over moderately winding and moderately steep roads. Most of it is in 4th because 5th is too high but 4th is too short. On longer trips I always start with a run up Clyde Mountain which is mostly 3rd (too short) and 4th with a bit of 2nd. My thinking about taller gearing actually started with a return trip to Port Macquarie and back, 1500kms in 3 days, when once I'd got past the Clyde I was always in 5th and often reaching for a longer gear (and that was when I had a much peakier motor than this one now).



It sort of makes sense to me but it's probably not for everybody. At any rate it's cheap and reversible. The chart shows I'll get a 4th which is like the NB8B 5th and halfway between the old 4th and 5th, which is pretty much what I want as my day to day touring gear. 5th just becomes a Hume Highway gear. 3rd will be longer but better for passing - the existing 3rd is a bit frantic. 1st and 2nd don't matter much, there's more acceleration than I could ever need there now.

[pcmx5]

Great, you have obviously got it sorted for your use, just keep on enjoying!!

Peter.