diff ratios vs. 110kmh rpm?

metris · Postby **metris** » Thu Aug 30, 2007 6:20 pm

hey hey,
im just wondering if anyone could shed some light on the difference in 110kmh rpms with the standard 3.63 diff and the 3.9# LSD that i am hoping to get...?

I want the extra bit of acceleration, but i drive hwys occasionally...
anyone? any clues? :mrgreen:

JBT · Postby **JBT** » Thu Aug 30, 2007 6:25 pm

There is about 7% difference in diff ratio, so I'd be guessing around 7% higher revs.

blackster · Postby **blackster** » Thu Aug 30, 2007 8:04 pm

Using a 22.7\" tire diameter. Top speed calculations where done using mazda's (SE) posted .38 drag coeficent and a 22.3 sq.ft frontal area, and assuming stock curb weight with a 70kg driver

Final drive 3.90

I: 3.76: 48 km/h @ 6500 rpm
II: 2.27: 80 km/h @ 6500 rpm
III: 1.65: 109 km/h @ 6500 rpm
IV: 1.26: 143 km/h @ 6500 rpm
V: 1.00: 180 km/h @ 6500 rpm
VI: 0.84: 215 mph @ 6500 rpm

Final drive 3.636

I: 3.76: 51 km/h @ 6500 rpm
II: 2.27: 85 kh/h @ 6500 rpm
III: 1.65: 117 km/h @ 6500 rpm
IV: 1.26: 154 km/h @ 6500 rpm
V: 1.00: 194 km/h @ 6500 rpm
VI: 0.84: 231 km/h @ 6500 rpm

Requires 135rwkw

For those guys contemplating hitting 200mph in a NB a 2.60 rear gear ratio is required as well as 361rwkw.

2.60 final drive

I: 3.76: 72 km/h @ 6500 rpm
II: 2.27: 119 km/h @ 6500 rpm
III: 1.65: 164 km/h @ 6500 rpm
IV: 1.26: 215 km/h @ 6500 rpm
V: 1.00: 271 km/h @ 6500 rpm
VI: 0.84: 321 km/h @ 6500 rpm

If you could reduce the drag coeficent of the NB to .30 it would only require 290rwkw...........

From mazdaspeed, I converted everything over to the metric system for convenience.

JBT · Postby **JBT** » Thu Aug 30, 2007 11:13 pm

blackster wrote:
Final drive 3.90
VI: 0.84: 215 mph @ 6500 rpm

Final drive 3.636
VI: 0.84: 231 km/h @ 6500 rpm

Seems to confirm about 7%. I reckon from those figures 110km/hr would be:

3.9 - 3325RPM
3.636 - 3095RPM

metris · Postby **metris** » Fri Aug 31, 2007 7:24 pm

wow, yeah im cool with that!
thanks for your knowledge guys... now whoooos got a 3.9 they wana sell me...? :mrgreen:

Techno · Postby **Techno** » Sat Sep 01, 2007 9:18 pm

metris wrote:wow, yeah im cool with that!
thanks for your knowledge guys... now whoooos got a 3.9 they wana sell me...?

If you have a standard NB8A then these are the rpm values for 100kph.

They are calculated using 5 speed, with 195,50,15 wheels (564.32mm circumference) giving 1773mm linear travel per revolution and 564.063 revolutions per kilometer.

4.3 3,274
4.1 3,122 This is what came standard
3.909 2,977
3.636 2,769

Here are the speeds in gears at 6,500 rpm

4.3 diff NA series
Gear ratios KPH
1st 3.14 51.2
2nd 1.89 85.1
3rd 1.33 120.9
4th 1 160.8
5th 0.81 198.5
Rev.
Diff Ratio 4.3
RPM 6,500

4.1
Gear ratios KPH
1st 3.14 53.7
2nd 1.89 89.2
3rd 1.33 126.8
4th 1 168.6
5th 0.81 208.2
Rev.
Diff Ratio 4.1
RPM 6,500

3.909
Gear ratios KPH
1st 3.14 56.3
2nd 1.89 93.6
3rd 1.33 133.0
4th 1 176.9
5th 0.81 218.4
Rev.
Diff Ratio 3.909
RPM 6,500

3.636
Gear ratios KPH
1st 3.14 60.6
2nd 1.89 100.6
3rd 1.33 143.0
4th 1 190.2
5th 0.81 234.8
Rev.
Diff Ratio 3.636
RPM 6,500

If I were going to change the diff ratio then I would first plot out the current and proposed torque bands to see what the optimum change points would be ....... and the relative speeds at the change point and what rpm you will be at when engaging the next gear.

If you want the model to take account of the changes in wheels and tyres then send me an email to robspargo@aapt.net.au.

metris · Postby **metris** » Mon Sep 03, 2007 3:12 pm

hmmm, now im confused :frown:

... i read that the NB8A had the 3.63 ratio diff as standard...
are you saying that standard is a 4.1 ratio? :?:

my mx5 is the 1998 NB8A with the 16\" wheel upgrade and the factory bodykit...
plotting current and proposed torque bands is way over my head right now... :mrgreen:

Techno · Postby **Techno** » Mon Sep 03, 2007 6:48 pm

metris wrote:hmmm, now im confused ... i read that the NB8A had the 3.63 ratio diff as standard...
are you saying that standard is a 4.1 ratio?

my mx5 is the 1998 NB8A with the 16" wheel upgrade and the factory bodykit...
plotting current and proposed torque bands is way over my head right now...

Well, it would have to come with a LSD to be 3.636

Here is the data for 205/45/16 (NB8B wheels) for 4.1, 3.909 and 3.636 ... which one best fits the current situation?

Gear ratios KPH
1st 3.14 25.8
2nd 1.89 42.9
3rd 1.33 60.9
4th 1 81.0
5th 0.81 100.0
Rev.
Diff Ratio 4.1 4.1
RPM 3,056

Gear ratios KPH
1st 3.14 29.1
2nd 1.89 48.3
3rd 1.33 68.7
4th 1 91.3
5th 0.81 112.8
Rev.
Diff Ratio 3.636 4.1
RPM 3,056

Note how the 3.636 gives 112KPH at 3056rpm whereas the 4.1 gives 100 kph for 3056 rpm.

The 3.909 will give 104.9 kph for 3056rpm.

Gear ratios KPH
1st 3.14 27.1
2nd 1.89 45.0
3rd 1.33 63.9
4th 1 85.0
5th 0.81 104.9
Rev.
Diff Ratio 3.909 4.1
RPM 3,056

What RPM are you doing at 100 kph???
Regards

Rob

metris · Postby **metris** » Tue Sep 04, 2007 10:44 am

im rather sure mine isnt an LSD as I can annoying inner wheel spin exiting corners, nothing is limiting that slip...

so to increase acceleration then, (and therefore lower the top speed), would I need to get a 4.3 ratio?
can you get an LSD centre to fit that?

does anyone have a 4.3 diff that can tell me the pro's & con's theyve found..?