Best Differential Swap

[slowpoke]

Hello bootz,
As 93C'man reported, my '10' now runs a 4.1CWP BUT it also has a 5 speed g'box!
Lowering the Diff/FD ratio from 3.6 to 3.9 would improve the 'driveability' of 5th and 6th gears at lower RPM BUT(in my view) at the expense of exacerbating the already TOO SHORT intermediate ratios (ie.1-2-3-4) of the NB 6 speed g'box!
Neither the 3.9 nor the 4.1 should be particularly rare, secondhand.
Early RX7 s' were fitted with a 3.9 CWP.
Generally, 4.1 's were fitted to all manual and automatic NA8s' and to manual and automatic NB8As' (excluding the 10AE),as well as to all AUTOMATIC NB8B and C models.
PS. Norman H.Garrett 3's book "Mazda Miata-Performance Handbook", on page 55, "The Easiest Way To Get More Power".. is worth reading....

[slowpoke]

PPS. Please cross out "at lower RPM" !!
Cheers from slowpoke.

[bootz]

Hello bootz,
As 93C'man reported, my '10' now runs a 4.1CWP BUT it also has a 5 speed g'box!
Lowering the Diff/FD ratio from 3.6 to 3.9 would improve the 'driveability' of 5th and 6th gears at lower RPM BUT(in my view) at the expense of exacerbating the already TOO SHORT intermediate ratios (ie.1-2-3-4) of the NB 6 speed g'box!
Neither the 3.9 nor the 4.1 should be particularly rare, secondhand.
Early RX7 s' were fitted with a 3.9 CWP.
Generally, 4.1 's were fitted to all manual and automatic NA8s' and to manual and automatic NB8As' (excluding the 10AE),as well as to all AUTOMATIC NB8B and C models.
PS. Norman H.Garrett 3's book "Mazda Miata-Performance Handbook", on page 55, "The Easiest Way To Get More Power".. is worth reading....

Thanks, a 5 speed in a 10, did you install and why?
I think in the six speed first to second is way too short. You can almost shift straight first to third. Didn't notice the others as particularly short.

Any comments re the torsen lsd on the street would be appreciated.

[pcmx5]

I am not troubled by the closeness of the ratios with my 6 speed/3.9 combo in my NA, just love it in the hills as it allows me to keep the car in the right rev range as it has tighe cams etc, may be different in a standard car.In the town I sometimes skip shift. I guess everyone has different goals and expectations.

Idon't think anyone who already has a 4.1 5sp ( as I did ) can justify the benefits of going 3.9 6sp for the cost and would be only marginally disadvantaged with their current combo.

Bootz, pity you cannot drive a few of these different set ups,you would have a few dollars in your pocket if you sold the 6speed and bought a 5sp!!

You would also get better change quality in the 5 sp,although a short shifter and Voodoo knob seem to have made a huge difference to my 6 sp,never find the nothing plane that I used to get going 4 to 3,usually just when you needed the power on a corner!!

My NC has much better acceleration with the 4.1 than 3.7 and again i skip shift around town but the closeness of the ratios is a delight inthe hills.My car has 225 instead of 205 tyres but if it didn't I think a 3.9 would be perfect(not certain there is a 3.9 available for the NC).

Peter.

[bootz]

For me the best diff ratio is 4.3. 3,400rpm @ 100kph in 6th gear is perfect (5th gear on a 5-speed was 3,300rpm @ 100kph)... right in the middle of the torque curve. The car is geared to give a top speed around 200kph. Perfect! If you want performance, not improved fuel economy on the highway then 4.3 or higher is the answer. Fuel economy around town does not suffer because you should be one gear higher with the 6-speed than the 5-speed. ie. 5th is the new fourth.

From Fitting a 6 speed box to a NA8. I would have thought the NA8 came with a 4.1 maybe is he saying he changed it to the 4.3 So if we take the above comments and look at Gurans calculations the 4.1 and 4.3 are best:

6 spd with 3.636 diff - 2752rpm
6 spd with 3.909 diff - 2959rpm
6 spd with 4.100 diff - 3103rpm
6 spd with 4.300 diff - 3254rpm

By the way the UK NB models had 5spd with 4.3 and the 6spd with 3.9 in the ratios. Also thanks Peter your comments are extremely helpful as well.

[orx626]

For me the best diff ratio is 4.3. 3,400rpm @ 100kph in 6th gear is perfect (5th gear on a 5-speed was 3,300rpm @ 100kph)... right in the middle of the torque curve. The car is geared to give a top speed around 200kph. Perfect! If you want performance, not improved fuel economy on the highway then 4.3 or higher is the answer. Fuel economy around town does not suffer because you should be one gear higher with the 6-speed than the 5-speed. ie. 5th is the new fourth.

From Fitting a 6 speed box to a NA8. I would have thought the NA8 came with a 4.1 maybe is he saying he changed it to the 4.3 So if we take the above comments and look at Gurans calculations the 4.1 and 4.3 are best:

6 spd with 3.636 diff - 2752rpm
6 spd with 3.909 diff - 2959rpm
6 spd with 4.100 diff - 3103rpm
6 spd with 4.300 diff - 3254rpm

By the way the UK NB models had 5spd with 4.3 and the 6spd with 3.9 in the ratios. Also thanks Peter your comments are extremely helpful as well.

Bootz, the running gear in my NA is from a Jap spec 1996 VR Limited model which has the 4.3 raio coupled with a Torsen LSD. I changed out the 5-speed for the NB8B 6-speed.

Cheers,
Danny

[bootz]

Bootz, the running gear in my NA is from a Jap spec 1996 VR Limited model which has the 4.3 raio coupled with a Torsen LSD. I changed out the 5-speed for the NB8B 6-speed.

Cheers,
Danny

Thanks, Danny, that clears one question up. I guess your original comments still stand and in a light NA it would be a cracker of an accelerator.

This is very close to my ideal setup, except I would go a 4.1 final ratio. I know the 6spd has been unfairly compared to the excellent 5spd but apart from being a tad quirky it still has that MX5 magic.

Now to figure out gear/rpm for the critical cruise speeds of 60, 80 and Guran has already done 100 km/hr.

[bootz]

USA Rear end ratios:
1994-97 5-speed: 4.10
1999-05 5-speed: 4:30 (I'm pretty sure the 04-05 has that ratio)
1999-03 6-speed: 3.909
2004-05 6-speed: 4.10 (terrible combo)

I found this thread from US disturbing, but I am pretty sure they are referring to the US Mazdaspeed turbo so the terrible diff combo should not apply to me. The normally aspirated US miata appears to be a 5spd. US Mazdaspeed appear to have been given the 4.1 while we in OZ got the SE 3.6 and everywhere else got the 3.9. I think if I had the turbo I would want the 3.6 ratio. I honestly don't know why Mazda changed it radically for so many markets.

Being still keen on the 4.1, have been driving around imagining an extra 500 rpm on the tacho. Doesn't worry me. Using a gear calculator with the smallest (195/50) tyres, sixth becomes usable at 80km/hr, instead of 90km/hr. Conversely 2nd gear only pulls to 80 instead of 90. I don't think either worries me unduly. At 110km/hr the car is pulling 3500 in 6th which may be a tad high for cruising but it would discourage speeding. If I did a lot of highway driving I would probably think differently but for me occasionally cruising the freeway, it is fine.

The diff change is like making the 6spd into a 5spd car, ie. 5th in the 6sp now equates with 4th in 5spd and 6th-6spd = 5th-5spd. This may sound a bit retrograde but I am hoping it suits what I want with the car. If I had FI would not consider it.

Other Mazdaspeed comments for your interest: http://forum.miata.net/vb/showthread.php?t=102506

[StanTheMan]

for $200 if you can find one.....would have to be the one most sensational mod you can do to your 5. I mean, thats the bargain of the decade.

yea the 3,9 diff discussion.....that was an eye opener LOL.

[StanTheMan]

as far as a 4,1 diff goes in a 1.6.....slight difference. its not earth shattering. Its just a little less lively. But that didn't stop me keeping up with the SP's along the mountain roads.
when it was modded with a considerable more amount of hp the 4.1 felt awesome.....but did take a little while to get going as i had sweet F. A. power below 3000 rpm.

[StanTheMan]

Any comments re the torsen lsd on the street would be appreciated.

the torsen is probably the best compromise for a street car for street driving. Its not agressive as some others. In fact, normal driving you will not notice any difference. It will also help you if you get to the occasional track day. If you really serioulsy want to race more often....get something more agressive.

It is perfect for every day driving. Its awesome on a crisp mountain road run.

[wazman]

My nb8a is turbo and makes a truck load of torque nice and early.......i have the 6 speed and on the highway sitting on 110 kph it is on about 3700 rpm !!!

around town 1st gear is useless..... your in it for 1 second before you have to change if you take off with any sort of zest! so annoying.....

which year model in aus has the 3.6 ?

[bootz]

IMHO, the NB8a is the 'revvy' 4.100FR diff, but unless you have the 10AE I don't see an NB8a with a six speed. Maybe you retrofitted.
The NB8b from 2001onwards is the fuel economy model 3.636FR which is the go if you want relaxed country cruising. Quite flexible around town but not as 'sporty' as the 4.1 for a norm aspirated engine. For a 200hp+ turbo/supercharged mx5 the 3.6FR would be the go, plus an LSD to mitigate against wheelspin. This is what the SE came with.

Lots of people overseas would love the 3.6 but sadly for them only Australia got it.
The NA made its reputation as a ripper sports car partly because it had a 4.3FR. No wonder it was considered 'revvy'.

Just think how much fun I have had learning about the diffs in the MX-5.

[bootz]

Wazman we should do a swap, my 3.636 for your 4.100. Sounds like it would be a perfect match!

[bootz]

Installed the diff yesterday. This is my reaction



Thanks to all forum members who helped me with the decision, esp pcmx5 and Guran. You guys are spot on.

Big Big thanks to flycasta who supplied the diff and with service 2nd to none. Best diff man in Australia IMHO.