Do I have an LSD? and diff gear question?

Guran · Postby **Guran** » Thu Mar 13, 2014 4:15 pm

bustafrik wrote:If I put coilovers on, better diff gears and decent semi's, am I going to break 1.15 in Wakefield?

Based on track records for the MX-5 Club of NSW, a stock NB8A is capable of 1:15s on normal road tyres. With R-specs, it is capable of 1:14s - I've done 1:14.9 in a stock NA6 with R-specs (and perfect conditions), and NBs are quicker. Add in coilovers and an NB might see 1:13s. Whether or not you can achieve these times is another matter. :wink:

It takes a LOT of practice.

If you have a 10th Anniversary Edition NB from 1999 or Heritage Edition from 2000, it is likely to have a Torsen LSD, but all other NB8As in Australia had open diffs. To test your diff, park with one rear tyre on gravel and the other on tarmac, then accelerate fairly hard. Open diff will spin on the gravel and you won't get very far. LSD will allow you to pull away fairly smoothly as you'll get some grip on the tarmac wheel. Of course, you should only do this test where it is safe and legal to do so.

Another diff test is to jack the car so both rear wheels are raised, then with the car out of gear and handbrake released, turn one wheel by hand. With a Torsen LSD, the other wheel should rotate in the same direction. With an open diff, the other wheel should turn in the opposite direction (or not at all).

As far as I'm aware, 16" wheels were not standard on any 1999 MX-5s. Pretty sure all NB8As came with 15" wheels. The 16" wheels arrived with the NB8B in late 2000. Post a photo of your car, and it will quickly give us an idea of what you actually have.

As for diff ratios, my advice would be not to rush into it. Changing the diff is only really advisable if you want to match your ratios to a certain track layout in order to avoid an unnecessary up changes, or give a little more punch out of other corners. I suspect you're a very long way from that point at the moment. I believe that changing diff ratio will change the speedo calibration on an NB.

hks_kansei · Postby **hks_kansei** » Thu Mar 13, 2014 5:27 pm

Guran wrote: I believe that changing diff ratio will change the speedo calibration on an NB.

Correct, you will also need the corresponding speedo sensor to correct the speedometer (or one of the Jaycar speedo corrector kits, but not sure how well they work)

Getting a shorter diff ratio in a NB8A however can be a bit costly, since they had a pretty short diff from factory.

Since you sound new to MX5's I'll first let you know that there are two different diffs in MX5s, the ones from the ones from the NA6 (6inch Crown) and the ones from the NA8, and all the NBs (regardless of 1.6 or 1.8, these all had a 7inch crown)
The larger NA8/NB diff is the stronger of the two (bigger crown, bigger oil capacity)

The ratios available (stock, aussie cars), and the diffs in them are:

NA6 - 4.3 - open
NA8 - 4.1 - open
NA8 Clubman - 4.1 - Torsen T1
NB8A - 4.1 - open
NB8A 10th Anniversary - 3.? - Torsen T2
NB8A Heritage - 3.? - Torsen T2 (assumed same as 10ae)
NB8B - 3.6 - open
NB8B SE - 3.6? - Torsen T2 (centre has different spline count to other MX5 diffs IIRC)

There have also been available crown and pinion sets in 4.7 and 4.9 ratios to suit the NB diffs. I think these were made my Mazda Motorsport for the Spec miata cars possibly (not really sure where they came from)
These usually cost more than the factory ratios.

The other option to shorten the gears, without changing the diff, is to swap to the 6 speed gearbox, which from memory has shorter gearing than the 5 speed.

edit: the above is directed at the OP, not Guran.... Guran knows a lot more about these cars than I do.

Guran · Postby **Guran** » Thu Mar 13, 2014 6:14 pm

You joined the ARDC and not the MX-5 Club of NSW???? Shame on you!!!! :evil:

ARDC is our "sworn mortal enemy" ... with respect to the CAMS NSW Supersprint Championship (read this). In case you didn't know, I'm President of the MX-5 Club of NSW so I have a bit of bias here! :lol:

Your car looks to me like an NB8A with aftermarket wheels and bodykit (although I'm no expert on either). This link shows the stock wheels that were available with NAs and NBs. If you have original 16" wheels, they will have come from an NB8B, not your 1999 NB8A. The side skirt and front lip look like Mazdaspeed.

You won't want to use "slicks" for some time. Better to start with regular road tyres, and then migrate to S-specs (road spec rubber with semi-slick tread pattern), and then R-specs (race spec rubber with semi-slick tread pattern). That way, the damage you will do when you flat spot them is less expensive, and you're also learning how to get the most from them as cost increases in parallel with experience.

You can definitely drive an open diff MX-5 quickly without traction issues. It requires quite a lot of patience and practice to learn how to avoid unnecessary spinning of the unloaded wheel on tight corners. Learning that skill will make you a better driver of ANY car.

Guran · Postby **Guran** » Thu Mar 13, 2014 6:32 pm

hks_kansei wrote:NB8A 10th Anniversary - 3.? - Torsen T2

In Australia, the 10AE had 3.636:1 final drive ratio (Torsen) in place of the 4.10:1 (open) used with five-speeds. The 10AE had 3.909:1 diff ratio (Torsen) in other markets.

Reference

hks_kansei · Postby **hks_kansei** » Thu Mar 13, 2014 6:43 pm

The bodykit was a factory option from Mazda, so that's normal (it's just that it wasn't a particularly common option chosen)

The wheels on there certainly aren;t factory, they're a fairly common "wheel any tyre package" thing that was available from tyre shops.

sailaholic · Postby **sailaholic** » Thu Mar 13, 2014 6:47 pm

Few clarifications

A Torsen will not do the opposite spin thing. They are based on a torque bias not a speed bias. So when both wheel are in the air they act like an open diff. You could try it with the handbrake on slightly and see if it works.
Diff identification guide is here -> http://www.miata.net/garage/diffguide/index.html. Apart from the dirt trick, your probably down to taking the cover off.

How many gears do you have? 10AE was the start of the 6 speed but with a 3.9 for memory and should have an LSD. 5 Speed is likely to be a 4.1 open diff (unless someone changed it).

Best bet for a factory diff is a 4.3 torsen.
Available here - http://mx5cartalk.com/forum/viewtopic.php?f=44&t=61771/

Aussie Stig · Postby **Aussie Stig** » Thu Mar 13, 2014 8:19 pm

dude put smaller wheels on, 15s or if you want to be a devil try a nice 14. :lol:

and your diff ratios solved, it will be a much nicer car. Also try a few doughies to see if lsd fitted, it prob is not on your car.

sailaholic · Postby **sailaholic** » Thu Mar 13, 2014 9:14 pm

Huh? How will diff ratio be solved? The tires look about the same as the SE ones which means rolling diameter will be about the same.

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Aussie Stig · Postby **Aussie Stig** » Fri Mar 14, 2014 12:08 am

sailaholic wrote:Huh? How will diff ratio be solved? The tires look about the same as the SE ones which means rolling diameter will be about the same.

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You are only guessing about his overall diameter, but hey we don't really know.

The rolling diameter of the SE 17s is larger to the oem 15s, about 5% diff, 14s would be even shorter about 10%. (rpf1 in 14s look good on an nb. )

Depends how much change in diff ratio he wants but a 4.1 to 4.3 is only a 5% change.

However, why you would want to change from a 4.1 to 4.3 is beyond me. I find 4.1 short enough for the 1.8

sailaholic · Postby **sailaholic** » Fri Mar 14, 2014 7:22 am

205/50 is right for a 15 (well a touch larger then factory actually)

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emily_mx5 · Postby **emily_mx5** » Fri Mar 14, 2014 8:10 am

Assuming not silly widths/offsets/fitment
For a 15" rim you want 195/50
For a 16" rim you want 205/45

OP be aware that 99% of this forum is happy with nothing but 15" NB8A/6UL/RPF1 wheels regardless of what MX5 you have lol.

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Dweezle · Postby **Dweezle** » Fri Mar 14, 2014 9:22 am

Assuming the same width why would you put a 205 on a 16' compared to a 195 on a 15"??

sailaholic · Postby **sailaholic** » Fri Mar 14, 2014 9:25 am

well a 205/50/15 is going to be better on the track in terms of grip (assuming comparable tire types). Plenty also run 225/50/15 race rubber.
195's are cheaper though and will give you the gearing change.

hks_kansei · Postby **hks_kansei** » Fri Mar 14, 2014 10:01 am

emily_mx5 wrote:OP be aware that 99% of this forum is happy with nothing but 15" NB8A/6UL/RPF1 wheels regardless of what MX5 you have lol.

I only use 15s because they're cheaper. Cheaper to buy, and cheaper to get tyres.

OP, standard sizing for a 99 MX5 is 195/50/15, which is a total rolling diameter of 576mm

If you've got something larger than that (rolling) it will have the effect of slightly taller gearing. (and I mean slightly)

Alos bear in mind that some states have rules about rolling diameter, in Vic it was (I think still is, but I haven't checked lately) that you could increase rolling diameter by up to 15mm I think.

Guran · Postby **Guran** » Fri Mar 14, 2014 1:35 pm

sailaholic wrote:A Torsen will not do the opposite spin thing. They are based on a torque bias not a speed bias. So when both wheel are in the air they act like an open diff.

You're correct! I recently bought a 10AE (standard Torsen LSD) and just tried this test. With both rear wheels raised, turning one wheel causes the other to slowly rotate in the opposite direction (at reduced speed).