Tor-what? Viscous Huh? – MX5 Differentials -An Introduction

Overview

If you’re into your MX5s the word Torsen should sound familiar, this refers to one of the types of Limited Slip differential that were available on special editions and as a dealer upgrade for new MX5s.

We’re specifically talking about 89′-2005 cars (Mk1, Mk2 & Mk2.5) in this article and we’ll go through all the available LSD options and some detail on the Torsen.

The original MX5, 89′ -> end 93 came with a 1.6l engine and a VLSD differential, the VLSD was new technology and Mazda, being the interesting company they are (see Wankel engine), decided to adopt the technology to improve the drivability of the MX5 in all road conditions.

Now, really I should do my best to make sure everyone reading this article has a good understanding of how a differential works and why you would want one at all, let alone a ‘Limited Slip’ unit.

It’s a little difficult to put into words without excellent pictures and diagrams, so I’ll draw you to a promotional video from 1937 which I honestly couldn’t recommend more. Spend 10 minutes and take it all in.

Finished? Excellent – now everything from here on out should make a little more sense.

As you will have learned, the purpose of a differential is to allow the inside wheel to move slower than the outside wheel – to ‘differentiate’ the power between the wheels.

Now this is great because it creates a consumer-friendly, quiet, comfortable and robust driving experience. It also is inherently quite safe – but kind of like when you look at a bottle of engine cleaner, the more warning labels there are the more effective its going to be. Just because something is safe, it doesn’t make it good.

What does it mean on the road?

What an open differential (one with no limited slip function) does on the road is something everyone will have experienced, say take a sweeping left hand corner, you turn the wheel to tip the car into it and everything works, but what you will feel is that the inside wheel – the one doing all the work – is slowing down and you’re (maybe only a little bit) pushing wide and having to add a little steering angle.

Grab your keys and go and test it – with an Mx5 you will notice that the inside wheel has the tendency to ‘give up’, to not push you through the corner and has the effect of slowing you down. Its not the most confidence inspiring feeling when you need that grip to pull you around a tight bend.

OK, so how does an LSD behave?

So, this is one where once you’ve tried a car with and without an LSD, it’s very easy to then tell the two apart just from driving them around a few back roads.

We’ve discussed the way the open differential will ‘give up’ on the inside wheel, forcing you to move wide and add in steering angle – this really isn’t that noticeable when driving like a responsible adult but is massively off-putting when you have a burst of the red mist.

The Limited Slip Differential does exactly what it says in the name, it limits the amount of slip, or rather the difference in wheel speed between the inside and outside wheel. As the helpful video from Chevrolet explained, we need slip to move around a corner with the least amount of resistance.

This isn’t really all  that helpful when you’re turning a corner and loading up your inside wheel, and all the power goes to the wheel without any grip – that just makes the car want to slow down. If you have an LSD you feel the inside wheel forcing you, dragging you through the corner, pushing you on.

It’s a very different feeling, very confidence inspiring and just by having one it makes your lines tighter, requiring less steering effort and you guessed it – faster.

And because the car is now putting power down even when it maybe doesn’t make sense to do so, you have more control and the car becomes more predictable – pressing the throttle will always (in a gear style ATB / Torsen differential) put power down to the road and will always be more likely to hook up.

What about the VLSD?

The VLSD was quite a new concept when Mazda took it on board for the development of the MX5, Mazda has a habit of taking quirky and new ideas (see Wankel Engine) and running with them, making them their own. In this case the VLSD’s remit was to improve the vehicles ability to get un-stuck and to give the car better drive ability all year round.

This is an excellent business strategy, in a car with so little in the way of driver aids anything to help the average middle class customer keep the car moving and not succumbing to the issues of an open diff while at the same time not fully committing to the aggressive nature of a proper locking differential.

However, the VLSD was never engineered as a Motorsport solution, or even realistically hard road or trackday use. The reason for this is simply in the design, the VLSD operates its locking action by the viscous fluid’s shear force, that is to say that it has clutch packs on either side with fluid suspended between the plates and depending on how the fluid resists the twisting force, is what causes the differential to transmit power.

The units themselves are sealed, meaning you cannot replace or renew the silicone based fluid held between these clutch plates. The misconception here is changing to a different weight or more premium brand of gear oil can help these units – with testing we can see that it really has very little effect on the unit at all in this respect.

So, as these units were made for the original 1.6 cars, they are all getting on for over 25 years old now and we can imagine that the fluid is past its best. Considering that the specification for the VLSD brand new could allow 10-20% of lock, this really doesn’t come close to a Torsen or ATB style differential at up to 50/50 or a 1.5/2 way Clutch differential.

Keep in mind that these units, being 25 years old, will have backlash issues as the preload on the bearings changes with age and wear. That’s what gives you the characteristic whine and judder on and off power.

What next?

Well, we’ve written about one solution to your LSD craving, the Ashcroft ATB insert which works in a not dissimilar way to the Quaife ATB. It’s a little further removed from a Torsen, but realistically performs comparably on the road.

For the MX5 the first thing you should be looking to do is get your hands on a 1.8 Differential, Prop and Shafts – if you already have a 1.8 then no worries – you just need to find a Torsen.

The other consideration with a differential is the gear ratio, completely separate to the LSD unit inside and completely interchangeable between the differential carriers. The early 1.6 cars came with a 4.3 differential, its good to keep this to preserve the cars power.

I’ll step back a little here and explain what the ratios are going to do to change the car. As a rule of thumb, the higher the number the more biased it is to acceleration, which means less top speed. Now personally I’d rather be coming out of corners faster than eventually hitting some silly top speed on the public highway so I opted to keep the 4.3 ratio for the 1.6.

Which, is damned hard to do. You need a 1.8 differential because the ring gear is 7″ rather than 6″ which means its stronger and inevitably there is more aftermarket support – its just a better unit if you were ever to change it and sell it on. There are very few suppliers of LSD inserts for the 1.6 differential, most likely because the people who generally slap in an insert also have their eyes on forced induction, which can push the 6″ ring a little too hard!

That’d be you then, avid reader.

The 4.3 1.8 differential was only available for a few years on a few special editions, the RS Limited being one (only 500 units made!) and the S Special option. All of these 4.3 differentials also came with a Type 1 or a Type 2 Torsen, the Type 2s coming from post 96 special edition cars.

The type 2 is a marginally stronger design, but both will hold in excess of 250ftlbs of torque. The ATB (Ashcroft, Quaife) style differentials are stronger still and are likely to outlast the rest of the drivetrain.

My advice, if you’re keeping the car naturally aspirated, would be to either keep the same differential ratio or if you can, get a higher ratio – It is possible to be silly and get a 4.7 Ring / Pinion from a Kia and I’m deliberately not going into detail here because you’re talking 4.5k cruise RPMs – if you would like to know more, drop us an email or message.

So, the holy grail for the naturally aspirated MX5 is really a 5 Speed + 4.3 or a 6 Speed + 4.1 (in that order).

MK2.5 SVT (Sport Cars) came with a 3.6 differential with a Super Fuji clutch style differential, the clutch plates within this diff fail over time and can end badly, it is highly recommended to replace the centre with an ATB or KAAZ style clutch pack inner – whichever is your preferred but get it outta there.

What about forced induction?

So, you’ve got yourself a Turbo or a supercharger, we’ll assume you’re running say 200HP Supercharged or 250HP turbocharged.

Superchargers

We will do a whole segment on this, seriously, it’s going to be our usual anorak level of detail, but right now – we’re talking drivetrains. The beauty of the supercharger is giving you linear, yet rapid acceleration so we can really start to think about this like a bigger NA engine. A good rule of thumb here is a 3.9 differential or at the minimum a 4.1 which then means 1st and second gear are just that little bit more usable.

In AutoSOLO what this will mean is you can sit at either the top end of 2nd or get a little into 3rd gear – the goal in AutoSOLO is to sit in a gear that keeps you in the power band the longest. Granted, the tracks change and that makes this whole argument irrelevant, but usually you won’t get out of 3rd for longer than half a second, so there is little benefit in massively increasing your gearing.

Turbos

It’s pretty much the same story, the only difference being the way the car makes power and how you keep in boost. There can be a lot of times where you’re just managing the throttle without putting your foot in it during an AutoSOLO, which means you’re not really getting into boost until the straights.

Having a higher ratio differential allows the car to keep in the mid to high range of 2nd gear staying in boost and lowering that ‘lag’ when you’re on / off throttle.

The holy grail for a high power build is a 3.6 ring / pinion setup, there are people running 350+ BHP on these + 6 speed boxes and while I can’t comment on the new top speed they can achieve, it’d suit the power band very well for a strong 2nd gear pull all the way round the course.

A 3.9 ratio would be the minimum recommended for something running 250+ BHP, otherwise you really start to struggle with 1st and 2nd being utterly useless, people at these power levels will generally be running a 6 speed box as it is proven to be stronger.

So what should I do – A list of well known combos.

If you’re a 1.6 owner:

  • Get a rare LSD Insert for your 6″ Ring Gear
  • Get a rare 4.3 Torsen Unit
  • Get a 4.1 Open & an ATB Insert
  • Get a 4.1 Torsen

+ Get a 1.8 Prop and 1.8 Shafts

If you’re a 1.8 owner

  • Get a 4.1 Open and an ATB Insert
  • Get a 4.1 Torsen

If you’re a 1.8 SVT Owner with A Fuji Diff

  • Get an ATB Insert
  • Get a Torsen Insert
  • Get a Quaife Insert

If you’re Supercharged

  • Get a 4.1 or 3.9 Torsen / ATB

If you’re Turbo

  • Get a 3.9 ATB / Torsen
  • Get a 3.6 and ATB/Torsen Insert

If you’re a MK2.5 SVT

  • Quick! Get a diff insert!

So, hopefully that clears up a few misconceptions and maybe has make it clearer about what direction to take your build.

If you’d like to get in touch, we’d love to hear from you. Our contact page is always open.

 

 

 

 

11 comments

  1. Kevin McCarthy

    I have a Torsen II LSD differential in a 1999 1.8L car. This car is stock. Lots of “clunk” and enough slop in the drive shaft to cause clutch chatter. On the bench, I found the ring and pinion serviceable. The R&P backlash is nominal – .0040″ but the helical gears in the Torsen unit move left-right (along their axis) with the friction disks as much as .050″ when I duplicate the clunk. I plugged the axles into the diff on the bench and placed a sand bag over each shaft to give me equal load. So the slop I see, hear and feel in the box is in the Torsen unit. I don’t find specs or parts for this Torsen-2 anywhere. Suggestions?

  2. MARK BENNETT

    Hi
    Im looking to fit a 7″ 4.6 to 1 or higher diff ratio into my mx5 which is fitted with 162 wheels and 80 profile tyres as we use it in of roar trials and autotests
    can you advise if and where and what parts are available
    cheers Mark b

  3. 4.3 type 2 from a 1.6rs for a 1.8 nb. What’s your thoughts? Thanks Callum

  4. Howdy,

    I have a NB running 330bhp, 315lbft. I recently stripped the gears in my 5 speed so I’ve replaced it with a 6 speed. My diff is a 4.1 Torsen.

    I’ve found a noisy (whines) 3.6 Fuji really cheap. I assume the clutch plates are broken.

    My question is, does the crown wheel and pinion usually get damaged when the clutches break? I’m was hoping to fit the crown wheel and pinion to my Torsen centre, along with a Bofi bearing kit.

    Thanks in advance.

    1. James Ellington

      Hi,

      We don’t usually find damage on the crown wheel or pinion when inspecting / refreshing a differential. When you remove yours, give the teeth a good inspection, but you’ll likely find them to be serviceable.

      James

  5. Hi!

    Thanks for the great post, it is really helpful.

    I have got a mk2.5 sport with a Fuji. Sadly I have also found metal bits when changing the oil diff recently.

    Is it possible to get a second hand torsen insert (even if it is from a different diff ratio) installed or should I just go for a brand new Kaaz?

    Regards

    Edgar

    1. Daniel Marshall

      Hi Edgar,

      It’s certainly possible to get a torsen insert, used. Normally they are sold as whole differentials – the Kaaz certainly has its benefits but it depends if you can stomach the extra cost.

      Cheers
      Dan

  6. Hi Guys,

    I have a Mk2.5 VVT, and there was a single metal shard in the diff oil when I change it 18 months ago

    The oil change made it quieter (it wasn’t overly noisy but you could tell the difference afterwards)

    The car is N/A at the moment, but will be turbocharging in the new year, I see Ashcroft no longer list their ATB diff, would a Kaaz be up to the job? I’m leaving the engine internals stock so won’t be going crazy with power

    Keep up the good work

    Wayne

    1. Daniel Marshall

      Hey Wayne,

      Certainly KaaZ would be up to the job but getting a nice torsen unit would be a great cheap alternative, or rather, cheaper. Especially if you’re keeping the internals stock.

      Thanks for the kind words it means a lot!

      Dan

  7. Bernard Deane-Armstrong

    I have a Mk1 UK car MX5 and have a noise coming from the gearbox as I lift off the gas.

    At first I thought it was the diff as the noise was coming from the NSR of the car, but I took it to my local Mazda garage and they put it on the ramp and span the engine up tracing the noise to the gearbox and not the diff.

    I now have to swap a gearbox and a propshaft (the guys spotted that the propshaft had a bit of play where it joined the diff).

    My question is: can I use the gearbox and propshaft from a Mk1 Eunos?

    Please help as I want to get the car straight before the winter..

    Thanks.

    Berne

    1. Daniel Marshall

      Hi There,

      You certainly can use an EUNOS gearbox and prop, just ensure they are for the correct diff (the props are different between 1.6 and 1.8. However consider that if the clutch has been changed its likely the PPF alignment is not correct which tends to be the root cause of the issue.

      For reference, generally everything is interchangeable with a Eunos on the Mk1 cars.

      Hope you get it sorted.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Over 300 Brands From

UK, Europe, USA & Japan

85% of Orders

Shipped Within 24 Hours

Worldwide Delivery

to 200+ Countries

Honest Expert Advice

From Real MX-5 Owners