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The mkiv Supra Owners Club

NA diff ratio change?


pedrosixfour
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Hey folks,

 

Still on the quest for lower gearing in my SZ-R I have now turned my attention to the gear ratios in the diff.

 

While doing a bit of a search I came across this link:

http://www.turbosupras.com/pages/pages/specifications/JZA80/jza80.htm

 

In it the final drive ratio for the NA diff is given as 4.083 as is the value given in Homer's gear calculator yet when you divide the number of teeth on the crown wheel (47) by the number of teeth on the pinion (11) you get a ratio of 4.273.

 

According to my calculations to get the ratio of 4.083 the crown wheel would need to have 49 teeth and the pinion would need to have 12 teeth.

 

Has anyone else noticed this discrepancy or is it just me that was sad enough to be investigating NA diff ratios?:D

Edited by pedrosixfour (see edit history)
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The EPC says 49/12=4.083 for SZ-R up to May '96, and 49/13=3.769 after that.

 

Lower gearing? So you want a 49/11? Not sure if there is one... some Chaser's had 41/10 and some 43/10, but I've no idea if they'd fit :search:

 

Does anyone know if this 10 tooth pinion would be the correct size and pitch to mate to my NA 49 tooth crownwheel?

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Ha ha, it does sound a little extreme doesn't it?

 

I would agree that 4.455 (49/11) would be a little less radical. Probably kinder to the diff too during down changes.

 

But I did a check on the gear calculator (great tool, thanks Homer!) and I think I could live with it (4.9) around Mondello and as I said in a recent thread concerning a 6 speed conversion Mondello will be the car's main racing circuit.

 

The car is a track use only toy by the way.

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Just been reading that thread... I guess since an NA can't max out 5th anyway, maybe it's not as extreme as it first appears!

 

But I don't know where you'd find one like that - maybe on a pickup or a truck?!

 

The US 4.272 might be as far as you can easily go :search:

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Just been reading that thread... I guess since an NA can't max out 5th anyway, maybe it's not as extreme as it first appears!

 

But I don't know where you'd find one like that - maybe on a pickup or a truck?!

 

The US 4.272 might be as far as you can easily go :search:

 

Cheers for the feedback Hermit.

 

Would you or anyone else for that matter know whether any Mkiii pinion might be a suitable candidate for this application or was there a complete redesign for the Mkiv.

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On mature reflection I think altering the diff ratio by too big a degree will just move the problem I have up a gear!

 

If I view my W58 box as a 4 speed instead of a 5 speed with fifth gear being redundant for the purposes of racing at Mondello a 4.27:1 diff ratio would suffice. I also think I am running rear tyres that are slightly too wide for my rims, 255/40 instead of 245/40 on 9" rims, so when its time for a change I'll effectively gain a few percent on the final drive ratio there too.

 

Plus it'll be a damn sight cheaper as I spoke to one of our Oz members yesterday who reckons grafting a JZX90 diff into place was the only method he could use which would then allow him to change the internals to a suitable ratio.

 

Sod that!

 

So with all that in mind could anyone shed any light on where I might find the toyota part no. for a 4.27 crown wheel and pinion gear set which appears to have been used in the US in certain supra models.

 

And can anyone confirm that this item will fit directly into my NA A01B Torsen I LSD diff?

 

Thank you very much you courteous, knowledgeable bunch you!

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Well it would move it up a gear, maybe, but you might be more in the power band?

 

Anyway, that's the important thing - get the gears closer so you've got a bit more freedom in gear choice, and it won't hurt your time as much when you stay in the higher gear round that corner. :)

 

Part number here, seems they come as a set... I've no idea about compatibility, but I can't see any obvious reason why not. As far as I can tell from the EPC, the same gears are used on the open and torsen diffs. I have heard of a similar swap being done to Soarer diff (it might even have been a Supra NA diff being fitted to a Soarer, since they're compatible). So... hopeful, but I can't confirm it.

 

supra_na_final_gear_pic.png supra_na_final_gear_partno.png

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