What ratio diff?

sticky nicky · Postby **sticky nicky** » Mon Aug 24, 2015 6:05 pm

I am currently looking at upgrading my 1.6 LSD, ratio 4.3 to a 1.8 set up for more strength. I have a turbo and around 180hp. Would people recommend I go with a 4.3 ratio again or would I be better with 4.1 for track work?

Thanks

hks_kansei · Postby **hks_kansei** » Tue Aug 25, 2015 10:15 am

The difference between a 4.1 and a 4.3 is minimal.

The 4.1 is only 5% taller than a 4.3, so at 100kph with a 4.1 the engine RPM will be 150rpm lower.

The 4.3 ring and pinion from the 1.6 diff won't fit on a 1.8 diff.
And finding a 4.3 to fit a 1.8 is somewhat difficult (Mazda factory ratios were 4.1, 3.9 and 3.6)

TrackAttack · Postby **TrackAttack** » Tue Aug 25, 2015 10:34 am

Japanese NB6's had a 4.3 T2 in the large size diff. This would be the ideal diff for you.

spikey · Postby **spikey** » Tue Aug 25, 2015 11:31 am

[quote="hks_kansei"]The difference between a 4.1 and a 4.3 is minimal.

The 4.1 is only 5% taller than a 4.3, so at 100kph with a 4.1 the engine RPM will be 150rpm lower.

The 4.3 ring and pinion from the 1.6 diff won't fit on a 1.8 diff.
And finding a 4.3 to fit a 1.8 is somewhat difficult (Mazda factory ratios were 4.1, 3.9 and 3.6)[/quote]

Nicky does a bit of the short track autokhana as well, would that help in certain situations that you have to change to 3rd because 2nd hits the limiter just before you get to a cone? I'm not sure so asking people that have these experiences with their setup, I can supply her 4.1 or 4.3 Torsen, just wanting her to get the best diff for here needs.

Postby **beavis** » Tue Aug 25, 2015 11:48 am

Assuming a 5 speed. I'd go 3.9 or 4.1.

spikey · Postby **spikey** » Tue Aug 25, 2015 1:15 pm

it's a 5 speed.

hks_kansei · Postby **hks_kansei** » Tue Aug 25, 2015 1:33 pm

spikey wrote:Nicky does a bit of the short track autokhana as well, would that help in certain situations that you have to change to 3rd because 2nd hits the limiter just before you get to a cone? I'm not sure so asking people that have these experiences with their setup, I can supply her 4.1 or 4.3 Torsen, just wanting her to get the best diff for here needs.

Road speed at 7,000rpm in 2nd (on a 5 speed) is approximately:

4.3 - 93.61kph
4.1 - 98.18kph

So the 4.1 would only buy another 5kph give or take.

StanTheMan · Postby **StanTheMan** » Tue Aug 25, 2015 2:46 pm

If the focus of your driving is road....I'd go 4.1 especially with 180 hp

if your focus is on tracking your car.....4.3

sticky nicky · Postby **sticky nicky** » Wed Aug 26, 2015 7:52 pm

Hi guys, yes my focus is on tracking the car. I think I might stick with the 4.3 if there is minimal difference. I dont drive on massive tracks with huge straights. Appreciate your help

hks_kansei · Postby **hks_kansei** » Thu Aug 27, 2015 11:02 am

The basic way to look at it.

With the current 4.3 have you ever wished for more speed? if not, keep it.
If yes, look at a different ratio (bearing in mind the 4.3 to 4.1 difference is only 5%)

Ordinarily i'd just say use the 4.1 since it's so close to the 4.3, and a lot easier to find for the 1.8 diff.
BUT, since you've got a 4.3 available already, use that. It will be easier, and also means your speedo will still be accurate.

Dweezle · Postby **Dweezle** » Thu Aug 27, 2015 11:22 am

Being Turbo, would you not want to lengthen the ratios so that you can ride the midrange turbo torque for longer??

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Dan · Postby **Dan** » Thu Aug 27, 2015 1:10 pm

For a turbo, generally speaking you want the lowest final drive you can have which allows you to have the turbo at full boost coming out of the corners as that reduces shifting on the straights, avoids you having difficult late shifts into corners and reduces problems with shifts shortly after corners.

On NSW tracks (SMSP and Wakefield) the lower speed corners are around 70km/h. My car has a 4.1 with 235/45/17 tyres and with that set up 70km is around 4000RPM in third which is good enough to get drive out of the corners and the higher speed corners I don't have trouble keeping over 4000RPM in 4th/5th.

If my car was a turbo that hits full boost at at 4200RPM I'd want a higher final drive to have more revs and full boost exiting the corner but if my car hit full boost at 4000RPM I'd be happy with the 4.1. In both situations I'd be unhappy with the stock 3.7 as I would either need to downshift to second which creates a more difficult shift into the corner and an extra shift after the corner or be out of boost while the turbo spools.

If you can't make a decision based on track speeds then I'd say bigger is probably better.

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sticky nicky · Postby **sticky nicky** » Thu Sep 03, 2015 9:33 pm

OK, so Ive gone for the 4.3 Torsen LSD 1.8 litre upgrade thanks to Spikey. He supplied me with the diff, drive shafts and prop shaft and did a great job. Thanks for you input guys