Category Archives: Maintenance

Customer Questions, Maintenance

Ask Torsen: Oil Choice Questions

Brian D wrote to us inquiring about oil options in a high-performance application:

Hello Torsen!

I have a 2017 Mustang GT Performance Pack car with Torsen 3.73s. I have added a Paxton Supercharger and will be occasionally taking it to the drag strip.

Due to the vast amount of opinions on the internet I figured best to ask you.

Two questions.

  1. What weight oil would be suggested

75/90 or 75/140?

  1. Should I add a fiction modifier to the oil as well?

I appreciate any feedback back

Thank you

-Brian

Brian,

Thanks for contacting Torsen.  Normally, I’d tell people to stick with the OEM recommendation, since the differential is not the only (or most important) component dependent on that lube.  In this case, the application is a little removed from a stock situation.  Ford uses their own 75W85 oil with friction modifier from the factory.  They mainly do this to reduce oil windage drag load in the axle, though it comes with a slight penalty of the quality of lubrication it provides.  Previously, they used 75W140 for anything that was high torque or severe duty.

Typically, heavier oil weights provide better protection since they leave a heaver film of oil on parts.  This provides better lubrication, it also can provide better shock protection.  It is worth noting, however, that the heavier the oil film is, the more it will reduce locking effect (or torque bias ratio) of the differential.  Essentially, anything that reduces friction in the diff reduces its locking characteristic.  Between 75W90 and 75W140, you probably won’t know the difference.  But in applications (like FWD cars) that run in ATF, the difference is notable.  Also, heavier weight oils typically absorb and handle high heat better before breaking down.  With all of that in mind, I would personally use the 75W140.

With regards to friction modifier, it is there for NVH reasons.  Limited slip differentials – even helical gear designs like Torsen – can make assorted noises in tight turns with low to moderate loads.  Clutch diffs chatter, gear diffs squeak or moan.  Modifier usually fixes these noises because it helps manage the frictional transition between static and dynamic modes.  But in doing this, it does reduce the overall friction in the differential.  As I said above, this has bearing on the differential’s performance as a result.  In some cases, it can reduce the locking effect as much as 10%.  For normal street cars, this is worth the tradeoff.  For you, it may not be.  However, you always have the option of adding modifier to the lube later on if needed.

I hope this helps.

JTEKT Torsen North America, Inc.

Tech Info, Maintenance

More about TBR, Lube, Friction Modifier & You

Torque Bias Ratio (TBR) – also referred to as Locking Effect – is a term used to quantify how much resistance to wheel spin a differential has. This is true of any type of LSD, plate or a helical design like Torsen®. These devices work by creating internal friction; friction in the diff is what allows it resist wheel spin.

By the same token, that resistance is also what allows a differential of the non-open variety to distribute more torque to the side that can best use it. The higher the TBR setting is, the greater the resistance will be to the start of slippage. That means more torque can be sent to whichever tire has better traction. In essence, by building resistance to spin, the differential can hold back the wheel with lower traction from spinning, allowing the higher traction wheel make use of more torque.

The actual amount of friction that the differential develops will be different depending on if the internal gears are static (not moving in relation to the diff case) or if differentiation is taking place. Think of trying to push a crate across a floor – you need more force to get it to move initially than you need to keep it moving. The coefficient of friction is higher when things are not moving compared when to when they’re sliding.  The same is true for the gearing in the differential.

So, there is more resistance to wheelspin when the diff gearing is not moving inside the case, which is the case when driving in a straight line. But when you turn, and one wheel speeds up while the other slows down, the differential gears want to move. Once there is enough force is on the axle shafts to overcome the friction, the gearing will start to move. In doing that, the amount of friction generated is reduced. However, this is true for clutches as well as for helical gears.

Now, that transition from static to dynamic friction is often kind of abrupt; torque gets wound into the gearing until the friction is overcome, then it pops free and moves. But when it does that, the movement slows and the coefficient goes up, and so it stops moving. At least, until the friction is again overcome and it pops free – again. In that transition, sometimes you find that stick/slip/stick/slip behavior occurs a lot. This stick/slip characteristic is also what causes clutch plates to chatter in a tradition limited slip device.

Bringing this all back around to friction modifier – the actual function of modifier (the “modification” it makes to the friction) is that it brings the static coefficient of friction down to a similar level as the dynamic coefficient of friction. This helps smooth out the transition from static to dynamic friction, getting rid of the tendency for abrupt stick/slip. This, as a result, largely eliminates the chatter or squeak . However, it does this at a price – the price of lowering the coefficient of friction for the whole system.

So, if we return to the notion that the differential is a friction device, it then stands to reason that the friction the differential generates is directly influenced by the lubricity of the oil blend that is used in the system. If the oil’s friction properties are reduced, the amount of friction that the diff produces is proportionally reduced.  That, in turn, has the same affect on TBR. Usually, it is a small but measurable amount, maybe up to a 10% loss of TBR.

This begs the question of whether or not friction modifier is actually necessary in a Torsen.  Like I said earlier, the modifier is beneficial in smoothing out the operations of friction devices.  However, because our friction properties are different than that of a clutch plate differential, you might not see the same symptoms of stick-slip behavior that those would.  As noted, clutch plate differentials have a distinct tendency to chatter without friction modifier in the oil, and that chattering can become very objectionable.   So, the modifier is essentially a requirement.

In a helical gear diff, however, the stick-slip tends to show up as an audible noise, described by some as a “rusty screw” noise, or as a squeak or moan.  You tend to hear it in very low-speed turns, when the steering is at or near full-lock.  So, you might hear it while maneuvering in a parking lot, or turning the car around in your driveway.  This noise is, as I said, just a characteristic of the friction properties, and is normal.  But, if you hear it a lot and it annoys you, adding a few ounces of friction modifier to the axle lubricant will almost always eliminate it.  So, the bottom line is it’s up to you.

Customer Questions, Tech Info, Maintenance

Ask Torsen: Differential Contribution to Axle Temp

This question comes by way of www.mustang6g.com forum user Brent Dalton.  This came from a discussion regarding managing axle temperature on the S550 Mustang and preventing overheating of the axle during track use.  For reference, 2016 and newer Mustang GT models with the Performance Pack (PP) option (which includes a Torsen® differential) have a temperature sensor on the rear axle cover to monitor axle temperature and warn the driver if temps become high.

Quote:

Originally Posted by Brent Dalton

…So it’s only really on 3.73 torsen diffs… so it makes some of us wonder… is it overheating due to the torsen and the regular type diff’s don’t suffer the same? A few guys … are working on the answer.

 

This is a worthwhile question, especially as more people take these cars on track day events.  A couple of comments – all limited slip differentials (LSD) are friction devices. The higher the TBR/locking effect is, the more friction it generates.  So, the Torsen® in this car – with a TBR of around 2.8:1 (versus 1.8:1 for base clutch plate LSD) – does create more friction than the base differential does.  However, with all of that said, friction only creates heat when you force slippage of the friction surfaces under high (torque) load.  I think that if you could actually watch wheel speeds during track events, you’d be surprised by how little differentiation occurs, at least under load.  At that TBR level, the Torsen® isn’t allowing the wheels to change speed much, even if it continues to bias torque side to side as warranted by traction conditions.  In general, in track use under high torque loads, the TBR level is sufficient to prevent most differentiation.  If you have a plug-in device that’s interacting with the vehicle CAN by way of the OBD2 port, see if you can monitor the rear wheel speed sensors and see what actually goes on during a hot lap.  You might find it interesting.

Anyway, the differentiation level is low, and even when it does, the differential rate (referred to as delta-N) is also quite low – on the order of 15-20 RPM difference from side to side.  So the frictional heating component that the differential contributes to the axle oil is pretty low, in relation to the system as a whole.  You get a lot more heat generated by the ring and pinion due to the hypoid mesh, which is running constantly, and at much higher speeds than the differential gears operate at.  Past that, most of the heat probably comes from external sources – like the proximity of the exhaust, which passes quite close to the axle carrier on this car.  Combine that with the fairly small volume of oil available to absorb heat, and you get the situation you’re in.

In my opinion, likely the biggest reason that Ford monitors axle temp on Performance Pack cars isn’t because the Torsen® creates more heat, but rather because they see a greater need to pay attention to axle temp on those cars, due to the perceived (track) usage.  I expect that in their minds, the non-PP GT models are much less likely to be on the track, so they (Ford) doesn’t need to spend the extra couple of bucks to fit those cars with the sensor.  It isn’t that the non-PP axle is likely to run cooler, but instead they are less likely to be in a situation where excessive heat is generated.

Tech Info, Maintenance

Lubricant & You: Part 2

Part Two: Previously, we talked about some of the basics of choosing the correct oil for your Torsen differential. Now, can get into a little bit more depth. You may recall that that the Torsen® will operate in whatever oil makes your ring gear happy. However, that is not to say that all oils are equivalent in the eyes of your differential – they’re not. There are a lot of quality aftermarket lubricants out there with a lot of fans that will swear by using them. And that’s great. But be wary of off brand products that you’ve never heard of, or don’t meet some sort of SAE standard. So, again, that’s great, so what would we recommend?

Unfortunately, we aren’t in a position to effectively compare multiple brands of lubricant. It would certainly be interesting to study. Imagine conducting a secret, blind taste test of all the popular brands (Brand-RL, Brand-RP, Brand-Am, Brand-M1, etc) in our lab, and comparing the results. The results would probably be fascinating.  But a test like that has a lot of variables and takes a lot of time.  In the end, it would still only offer a slice of what factors are important.

Such a comparison would only tell us what offers the “best” differential performance. It would say nothing about secondary effects on the rest of the axle or transmission system. So, it isn’t a real practical endeavor. Most of the testing that we conduct is with oil provided by our OEM customer, for use with their product. I should point out that OE manufacturers conduct extensive testing, using their preferred lube. They validate the entire system with that oil. This should not be overlooked, as the testing is typically quite rigorous.  It is, however, a point that a lot of people miss.

For our own R&D work, we use Chevron Supreme LS 80W90 as a “house” oil. That leads me to another point. While a differential like the Torsen Type-2 is happy in whatever good quality oil that you may choose, oil weight (viscosity) comes into play with regards to performance. As I stated earlier, Torsen works by generating internal friction. So, logically, if different oil blends have different frictional properties, that mean they have different influences on the differential’s behavior.

Essentially, the heavier, thicker, or more viscous that the oil is, the better it lubricates (generally).  It leaves a heavier film adhered to the surfaces being lubricated. This reduces friction. But, that means that a differential operating in 75W-140 oil will have fractionally less locking effect than the same unit operating in 75W-90.  This difference is not night and day – the changes are subtle. But if you’re fine-tuning a race car, it does offer one more knob that you can turn. By the same token, lubricant formulation and additives used have a similar bearing on the matter.

Generally, synthetic lubricants have lower friction properties than conventional, non-synthetic oils do. Synthetic lube may still be preferable if the duty is severe or high temperatures are a concern. Synthetics typically offer superior performance in those areas due to a more durable molecular structure. The overall friction properties of the lubricant are also influenced by additives and modifiers blended in. Clutch type differentials typically need these modifiers. They help to even out the transition from static to dynamic coefficients of friction between the clutch plates. This, in turn, smooths out their operation and reduces chatter. However, it also means that they may slightly lower the overall differential locking performance as well.

Lubricants sold as “limited slip oils” have these additives premixed in. Be aware of this when making a selection. For OEM applications, the validation testing that I referred to above includes extensive vehicle handling and chassis dynamic tests. Consequently, the OEM oil (and differential performance) is matched to the intended handling behavior for that car. So, where does that  leave us? Quite simply, here: use a good quality product, one that meets the needs of your ring & pinion gearing. Beyond that, think about what your specific needs are and choose accordingly.

Tech Info, Maintenance

Lubricant & You: Part 1

Part One: Perhaps the most common question we hear, here at Torsen® Intergalactic Central, is “what oil do you recommend?” It seems simple enough, right? Well, not so fast there, pal – it’s not a black and white matter. You may have heard that oil is your engine’s life blood; for all the geared systems within an automobile that is also true. Typically, the differential shares its lubricant with other components. In a rear axle, that includes the ring & pinion set as well as the axle bearings.  For a front wheel drive transaxle, however, that also includes the entire transmission system.  This entails the gears, the bearings, the syncros, everything.  The catch is that a limited slip differential – like Torsen – works by creating friction. On purpose. So, you have to balance proper lubrication with allowing the differential to do its job. That means a trade-off.

In reality, a limited slip differential is the only (oil-lubricated) component within a vehicle that intentionally produces friction. Friction within the differential is what gives it resistance to wheel slip.  And, as previously noted, the ability to transfer torque in an advantageous way. So, choosing a type of oil that has minimal reduction of friction is the obvious answer, right? Yet, the differential is spinning on bearings within the axle.  That whole system uses the same oil, including the final drive gearing.  It all needs proper lubrication to survive. Gear life directly depends on proper lubrication.  Lubrication also has a direct impact on other factors, like fuel economy. This is why lube choice is tricky – and why vehicle manufacturers spend a lot time testing, both for performance and for durability.

In truth, a Torsen differential typically isn’t that fussy about which lubricant you choose – within reason. A Type-2 or T-2R model will operate in really any common lubricant, ranging from ATF to heavy weight gear oil. On the other hand, if you happen to be using a Type-1 differential, you need to a little more choosy.  The crossed-axis gear mesh of Invex™ gearing requires the use of oil with high-pressure additives. This means they need gear oil with a GL-5 rating. But even that isn’t too difficult.  Hypoid ring & pinion gears have the same need, so almost all normal axle lubricants are up to the challenge. But be sure to look for the rating on the bottle anyway.

Because the ring & pinion final drive gearing are constantly running, they are a lot fussier regarding proper lubrication. So, your best bet is to seek out the recommendation of the ring & pinion manufacturer. If your car came with a Torsen from the factory, then you can find that information in your owner’s manual. Or, if you’ve installed a different gear set or changed ratios, go back to the supplier or manufacturer of the gearing you used. Most are very specific about what fluids should be used during break-in and normal operation of their gears. But either way, if the lube is suitable for your final drive gearing, the Torsen will be happy as well.