mx5cartalk.com

Its a 5-6% difference.

http://www.apexgarage.com/tech/gear_ratios.shtml Input the required information and change the final drive ratio.
https://tiresize.com/tyre-size-calculator/ Calculate tyre diameter

6 Speed Ratios (http://www.miata.net/garage/KnowYourCar/S8_Gears.html)
1st 3.760
2nd 2.269
3rd 1.646
4th 1.257
5th 1.000
6th 0.843

Assuming 205/50/15 wheels with a 4.1 in 4th gear at 100kmh you will use 4700 RPM where as with a 4.3 100kmh will be at 4,900 rpm. A 4.08% in 4th.

Same situation in 6th gear for 110kmh 3,470/3,640 RPM. A 4.90% in 6th.

Use this:
https://www.flyinmiata.com/tech/gearing.php

It's in MPH, but you can just plug in 68mph (~110kph)

From a 4.1 to a 4.3 final drive torque will increase RW torque by 4.87% whereas going from a 4.1 to a 3.636 will see torque at the RW decrease by 11.31%

cherrymx5 wrote:Are there any reliability related issues with the extra torque on the gearbox/engine?


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Absolutely not.

No Australian nb8b came with a torsen.
The only 3.63 torsen diff came in the SE but had a different spline count to all other diffs.

Not all 3.9s are torsen centered either. 3.9 was the standard diff ratio of the 6-speed American and Japanese (1. nb8bs, but the torsen was optional.

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All Australian nb8bs were 3.63 with no exceptions

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Give it a go and see if you like it, if not, switch back.

The 3.6 diff was only an lsd in the SE model, for which you'd also need matching axles.

Making a few assumptions
CD 0.31
Frontal area 22.6 sq ft
Weight 2,300 lbs
Speed 158 mph

You need approx 201 RWHP to achieve 158 mph. For 132 mph you need around 120 RWHP.

Exactly the same within 1/10th of a second.

Here is a file I made a while ago to calculate RW torque, acceleration and speed based on gear ratios, final drive, car mass, wheel radius and torque every 250 RPM from a dyno plot.

You will need to add torque every 250 RPM, wheel radius and car mass to get 'sensible' information. It is set to calculate only above 4,250 RPM and allows 3 final drive ratios to be used for comparison.

You can use it to determine shift points as it allows you to see the acceleration in the gear you are changing to.

Feel free to have it calculate lower RPM's. As previously posted there is aerodynamic resistance to factor in to see if you could attain the calculated max speed. Happy to provide calcs for this, but you will need to determine cd and frontal surface area at a minimum.