Best Differential Swap

[bootz]

I Did a search on this one but couldn't find details of what I want to do.
I have an NB8B 2002 (six speed) and I wish to go to a lower/higher(?) ratio diff to increase drivability (torque) on the road. If you want the full story you can read my thread - How easy is an SP replica? But essentially I am looking for more torque at low revs.

Sorry as far as diffs are concerned I am a real newbie.

My question is what effect will installing the following diffs have on the car?
1. SE
2. NB8A

Would either one of the above be a good solution for better torque? There has been mention in the forum of 4.3 diffs - that might be the go, but where would I get one at a reasonable price?

I am aware that this would affect the speedo calibration. Is there a solution for this?
I am sure the drivetrain experts can help out. Thanks for any comments.

[timk]

The SE diff will be the same final ratio as what you have now. The NB8A final ratio will be 4.1, which is pretty sweet combined with a 6 speed. The 4.3 is even better!

Just swap the speed sender on the gearbox to fix the speedo. It's a straight swap.

MX-5 Plus have all the bits you need, speak to Richard there in a couple of weeks when he's back from Japan.

[mx52nv]

Agree with Saboteur but just wanted to add that unless you actually want a LSD, you only need to change the crown and pinion.

I think Miataroadster.com used to sell the crown and pinion set ups new but I can't seem to remember what they have anymore. Ian Lewis got one for 'MX5T' but again my memory fails as to where he got his. Of course, having a complete diff assembly including LSD and crown & pinion will make the swap a lot easier but these are usually used items.

Finally, have a read of this document from Mazdaspeed Motorsports to help with your understanding and also what is available for your MX5.

Hope this helps you make an informed decision.
Les @ MX5 Shop

[pcmx5]

Depends what you really want,NB8A had a 4.1 diff as standard as did my NA8A;later 6 speeds have a 6th that is lower than 5th in a 5 sp so a change to a 6 sp will give a higher revs but better use of power without a diff change in early cars.NB8B's ran 3.7's and always struck me as being overgeared,with little available power in 5th and 6th.

I went to a 3.9 lsd with the 6sp in my NA8A and find it ideal for a fast road car,a higher cruising 6th than with the 5 sp 4.1 but with a great spread of useable gears in between. I would have thought that a runing numerically higher diff than what I have would have resulted in a very short 1st and less fuel ecnomy.A 4.1 may be ok but I seriously doubt a 4.3 with 6 sp would be good overall.It would accelerate like stink but may be a bit short in the gears and as a cruiser.

Chrish(you know the guy with the Clio Sport) is seriously considering doing the 3.9 diff with his NB8B which has 86RWKW compared to a standard 72 on a Dyno Dymanic Dyno.3.9 complete diffs with Lsd's have been advertised at around $1000 and in my opinion that is dollars very well spent!I am sure chris's car that is down a bit on power to my modified NA8A has a better range of torque and will absolutely fly with the change.

I have changed the 3.7 in my NC to a 4.1 and it has transormed the car I think a 3.9 will do the same for your for your car.

Changing ratios in the diff is a really inexpensive way of unlocking performance as Mazda seems obsessed with high ratio diffs in Australia after the NA series..It is a relatively inexpensive way of utilising the power that you already have and is what Mazda did with the original NA 1.6 with its 4.3 diff.Don't understand their change in philosophy.ie why go so far away fronm the original concept,4.3 worked in an NA6A, 4.1 worked in an NA8A/NB 8A why go to 3,7 in the NB8B? Why do most markets with higher speed limits get a 4.1 in an NC when we Aussies with some of the lowest limits get a 3.7 in NB8B's and NC's?

Be interested to hear how you get on.Good luck.

Peter.

[bootz]

I am really liking the answers you guys are giving. Changing the diff ratio is a lot easier than installing a turbo setup, provided you are not a speed freak. Heck I never go above 110km anyway

[bootz]

People tend to say putting an LSD in the mx5 is the best thing since sliced bread, so what does everyone think about this, looks good to me.
I don't believe it is from an Aust model, prob Jap? Unfortunately sold.

SE Diff conversion 4.1 ratio

New postby XSSIVE » Mon Nov 08, 2010 10:22 am
SE Diff conversion 4.1 ratio....it's a SE set up so it has the Bosch Torsen, heavy duty axles and tail shaft.

Will fit any model NA or NB MX5....it's all the bits you neeed..

bought it a few months back on here but wont be using it as i have to abandon the project

Dont miss out these dont come up often ..

$1500

[Guran]

Keep in mind that changing from a 3.636 diff to a 3.909 or 4.100 or 4.300 will improve in-gear acceleration, but it will also mean you top-out sooner in each gear. Of course that could be a good thing in the right circumstances, like trying to match gearing to certain tracks. Here are some calculations for your reference:

NB8B with 3.636 diff at 7000rpm
1st 57km/hr
2nd 95km/hr
3rd 130km/hr

NB8B with 3.909 diff at 7000rpm
1st 53km/hr
2nd 88km/hr
3rd 121km/hr

NB8B with 4.100 diff at 7000rpm
1st 51km/hr
2nd 84km/hr
3rd 116km/hr

NB8B with 4.300 diff at 7000rpm
1st 48km/hr
2nd 80km/hr
3rd 110km/hr

Also, when cruising at 100km/hr in 6th gear on the freeway, you'll be pulling more revs and the car will be noticeably noisier in the cabin.

NB8B with 3.636 diff - 2752rpm
NB8B with 3.909 diff - 2959rpm
NB8B with 4.100 diff - 3103rpm
NB8B with 4.300 diff - 3254rpm

Calculated using the methodology here, assuming 205/45R16 tyres and stock gear ratios.

[pcmx5]

Great work Guran

Mods can we save this somewhere please?

Effectively you are also bringing the klmph gap down between gears.ie.with 3.6 the gap is 38lkmph at 7000 between 1st and 2nd and 35 between 2nd and 3rd. with a 3.9 it is 35,33; 4.1 ,33,32;4.3,32,30,

Peter.

[Ollygt]

What would be the best final gear for a NA road vehicle, 5speed?
I ask because I plan to do the NA8 torsen lsd swap into the NA6

[pcmx5]

What would be the best final gear for a NA road vehicle, 5speed?
I ask because I plan to do the NA8 torsen lsd swap into the NA6

I think you should stay with a 4.3,loved my old 1.6.Johnd has a 90LE BBR turbo with 126 RWkw and is still at 4.3 .He could possibly gone 4.1 but his car is an amazing drive.So unless you are going to add considerably to your(from my dyno records 60-63RWKW) I would stay at 4.3.If you intend to fit a later 6sp at any stage I would suggest a 4.1 with a relatively stock engine and a 3.9 if you found another 25 RWKW.

So basically i think Mazda got it right with the NA and NB8A series but went too high with later models.

Hope this is useful.

Peter.

[bootz]

Keep in mind that changing from a 3.636 diff to a 3.909 or 4.100 or 4.300 will improve in-gear acceleration, but it will also mean you top-out sooner in each gear. Of course that could be a good thing in the right circumstances, like trying to match gearing to certain tracks. Here are some calculations for your reference:

Also, when cruising at 100km/hr in 6th gear on the freeway, you'll be pulling more revs and the car will be noticeably noisier in the cabin.

NB8B with 3.636 diff - 2752rpm
NB8B with 3.909 diff - 2959rpm
NB8B with 4.100 diff - 3103rpm
NB8B with 4.300 diff - 3254rpm

Calculated using the methodology here, assuming 205/45R16 tyres and stock gear ratios.

Thanks Guran, this is incredibly useful information for me.

I am wondering what year/model the 3.9 diff came from? As far as I could find out "10AE and Heritage are 3.9 as they are limited edition 6 speeders" A wrecked one of those is not likely.

I guess you would have to purchase the crown and pinion wheels to do this swap, or maybe the JDM cars might have them.

[pcmx5]

MX5 Plus has had them reasonably consistently over the last couple of years.(Edit Just checked and they have them for $800, best $$$ you will spend on an open diff 3.6 car !!)Not sure about the 10AE having a 3.9,there were claims and counter claims re that on this forum over the years. DriftFury on this forum may also be worth tracking down as he has a range of diffs.

They do come up from time to time but if you can get a complete 3.9 lsd it is much easier.

Good luck,

Peter.

[bootz]

MX5 Plus has had them reasonably consistently over the last couple of years.(Edit Just checked and they have them for $800, best $$$ you will spend on an open diff 3.6 car !!)Not sure about the 10AE having a 3.9,there were claims and counter claims re that on this forum over the years. DriftFury on this forum may also be worth tracking down as he has a range of diffs.

They do come up from time to time but if you can get a complete 3.9 lsd it is much easier.

Good luck,

Peter.

Thanks Peter. Sorry but I am a bit confused though by the term "3.9 Torsen 2 Diff Centre" .

Is the 'Torsen 2' significantly different to the Torsen 1 ????

The 'Diff Centre'. Is that just the diff minus the housing or some other combination???

So that might make a complete diff - the centre + housing. Am I saving a lot of extra installation cost by getting a complete diff???

Sorry, coming from Front WD cars the whole diff thing is new.

[bootz]

OK Back to Remedial Differentials 101:

Diff centre = dríven gears assembled
Front Housing = Housing containing bearings and outer connection to tailshaft
Rear Housing = Housing containing bearings and outer connections to axle shafts

So the two housings with the centre make up the complete diff. Theoretically buying a complete diff could be a DIY job, if you can handle reconnecting the tailshaft and axle shafts. Otherwise expert assembly is required.

Why didn't they teach this in school?

[Hellmun]

If you get the entire pumpkin(Front + Rear housing) then replacing the whole lot only takes a few hours. Personally I just took the whole pumpkin out, then dropped it into a diff specialist to get a 4.1:1 crown wheel /pinion and clutch pack LSD installed on my NB8B. Was $280 if I brought in the diff or $500+ if I brought in the car itself. I think i paid $190 extra to have all the seals/bearings replaced on top at the time as well.

From memory all I did was remove the lower control arm long bolt and pop out the drive shafts. Then rear subframe brace, 3 bolts on the PPF, pry the diff off the spacer in the PPF and 2 bolts to the subframe had it. That was over 3 years ago now but it's one of the easier jobs I've done on the car.