Over the last month, I have spent a lot of time searching the pro’s and con’s of running a locked center differential. The center differential determines how much power is applied front to rear. Locking the center differential splits the power 50/50, front to rear. This is mostly used for drag racing to achieve a more consistent launch and to disperse the power evening to all four wheels.
There are a few different ways to lock the center differential. The most common way is to weld your center differential, but it is irreversible (obviously). Another common type of a locked center differential is to install a VCE (Viscous Coupler Eliminator), which is what I will be using.
Words By: AJ Hunsinger | Images By: AJ Hunsinger
3 Types of Locking Center Differentials
A Spool is a device that locks the center diff from the inside and it can be reversed. But you usually need a good core center diff to make it work.
A Welded Center Differential is just that, you take your stock front diff and weld it. It is irreversible, but you can take a blown center differential and weld it, so you wouldn’t be ruining anything valuable.
A VCE (Viscous Coupling Eliminator) locks the center diff from the outside. Its main advantage is the fact that you can install and/or remove it in less than 25 minutes (about 1/3rd of the time it takes to remove a Spool or Welded center diff). So it is a preferable option if you are planning on going back and forth fairly often.
The VCE’s weak point is the fact that you are still relying on a stock center diff and its two spider gears, so those can still fail. For ultimate effect, you want to use 4 spider gear center diff and a VCE. But that becomes more expensive.
A VCE has the same affect as a welded center differential as it will lock the center diff. One of the biggest advantages of using a VCE over a welded center differential is the fact that not only is it reversible, but it also takes 30 minutes or less to install and remove. This piece was mainly produced to allow an AWD car to run in FWD mode to allow the car to be placed onto a 2WD dyno as not everyone has an AWD dyno nearby.
Running your AWD car in FWD mode only with a VCE is done by simply dropping the transfer case and/or driveshaft. Not only can a VCE allow you to run as FWD, but you can also run in RWD mode as well. In order to run in RWD mode, you would remove your front axles, hack up an old set of front axles, put the axle cups in the transmission, and cut each outer shaft of the axle to allow you to tighten the axle nut.
Before I elaborate more on the locking of a center differential, I will quickly explain what each differential does. Our AWD cars have three different differentials.
Front Differential: The front differential distributes torque to each front wheel, Left Front vs. Right Front. The front differential allows each front wheel to rotate at different speeds, which is important in corners.
When turning left, the front diff will allow the RF wheel will spin faster than the LF as it is traveling a longer distance than the LF wheel. While the front differential helps when cornering, it can hurt straight line acceleration as the differential will send most of the torque to the wheel with the least resistance. This is why you will see one will spinning from a launch/pull on a FWD DSM while the opposite wheel is not spinning. To remedy this, an LSD (Limited Slip Differential) is used to help with this exact thing. Most of our AWD’s come with a factory front LSD using a Viscous Coupling.
Center Differential: The center differential distributes torque, front to rear.
Rear Differential: The rear differential distributes torque to each rear wheel. Left Rear vs. Right Rear. It works very similar to how our front differentials work. Some AWD DSM’s came from the factory with an LSD rear differential.
Now, back to the center differential. Our center differential uses a Viscous Coupling as a form of an LSD to help ensure a 50/50 split in torque being distributed front vs. rear. Without the viscous coupling, the center diff would be an open differential. This means that if the front tires have less traction than the rear, the front would get most or all of the torque sent to those wheels.
Imagine you are stuck in the snow. With an open differential, if the front tires had the least resistance, ONLY one of the front tires would be spinning on this AWD vehicle as the rear wheels would not spin. So, you wouldn’t be able to get un-stuck with an open front AND center differential.
Now imagine the car in this example all of a sudden had a viscous coupling like what is in our AWD DSM’s. Now, the front wheel with the least resistance would spin as well as the rear wheel with least resistance. Now one front wheel and one rear wheel are spinning.
Maybe we are still stuck in the snow. So, let’s go ahead and lock the center differential by either welding the center diff, using a spool, or by installing our VCE. Now, ALL FOUR wheels would spin in 4×4 mode and we would be able to crawl up out of the snow.
The interesting thing that most people don’t think about when talking ‘Locking Center Diff’ is that even though the center differential is locked, each wheel on each axle (front and/or rear) still rotates at a different speed when turning.
For example, the left rear wheel would still spin slower than the right rear wheel when turning left. But, the left rear wheel would be turning at the same speed as the left front. Because of our viscous coupling and center differential design, the left/right wheel speeds would be different even though the front/back wheel speeds would be the same.
Differentials are needed when the car is turning because, assuming no tire slip, the inside wheels turn slower than the outside wheels and the rear wheels turn slower than the front wheels. If you did not have a differential, your car would not want to turn and would suffer significant tire-wear when it was forced to turn.
Sometimes, people make the whole “locked center diff” thing sound worse than it really is because they forget about this. At first glance, it may seem like normal driving on the highway or rural back road would be hell on the drivetrain. But, it’s not as bad as you may think.
All 4 wheels aren’t going to be completely locked and rotate at the same speeds and at the same time. Instead, only the front wheel on one side will be locked with the rear wheel on the same side, even when turning. The LF wheel would spin slower then the RF when turning left, which would spin the LR wheel at the same speed, also while still slower than the RR wheel. I will elaborate more on this later.
I mainly did all of my research to ensure that occasional weekend driving wouldn’t kill the transmission with using my VCE. I found a lot of reputable people in the DSM world, say that they hadn’t experienced any transmission failures, “even after 5 or more years of daily driving, road racing, auto x-ing, and drag racing with a locked center diff.
But, of course I also found that a lot of other people thought it would greatly reduce the longevity of the transmission and other driveline parts. The only other thing that a locked center diff would blatantly affect in a negative manner would be the annoyance of the tire chatter. The chatter of the tires skipping across dry pavement would increase tire wear. Tire wear wouldn’t necessarily greatly improve thought, as either wheel on either side of the front and rear axle, would still be able to turn at a different speed to achieve the difference in distance each wheel would need to travel.
But, the right wheel on the rear axle would need to turn slower than the wheel on the right of the front axle, but couldn’t because the center differential is locked. So, this would mean the rear tires would wear quicker than they would without a locked center differential, whether the difference in tire wear is greater or barely affected.
“Problems using a welded center instead of an LSD/VC unit include wearing out tires, drive shaft, CV shafts, and engine mounts more quickly, and slipping or chirping of the tires when turning in tight areas.” – Jacks Transmission
Note that Jacks Transmissions does not say anything about transmission wear. The difference between a VCE and a welded center differential is that the VCE still relies on the fragile 2 gear spider design.
In the end, my conclusion was this. There is no possible way to test the impact that a locking center differential would have on a specific area of a specific driveline part, with a specific type of locked differential- not even close. So, like plenty of others have done in the past, I decided it was up to me to find out exactly what will happen via my own personal experience. As I said earlier, I am choosing to go with the VCE to lock my center differential. The why is practically common sense, but you may find my reasoning is interesting.
I’ve always had a thing for drifting, but I never pursued the interest because I am so devoted to DSM’s. DSM’s aren’t known for drifting. That’s an area that 240sx’s, Miata’s, 350z’s, and other RWD vehicles strive in. So naturally, there has been a 1G drift missile project on my bucket list of things I want to do before I die. I bought a VCE a couple of years ago and thought about putting it in #VirginMary. But, I was afraid of the possible reliability issues it may cause. With Mary as my daily driver, I never put it in.
I sold it at some point when I needed some quick cash. Just recently, I came across one on our Eat Sleep DSM Facebook group for $20 shipped. I sent the funds to the seller over PayPal and it arrived some short time later. I never had a plan with it, I just wanted it because it was $20 and it’d be nice to have around for something down the road as these are very hard to find these days.
Today, I put my transmission on #PoisonIvy and had to make the decision on whether or not to install it into the transmission before huffing it up onto the engine. After all of my days-worth of research, I decided to install it. I don’t plan on drifting Ivy or anything like that, but I strongly have the urge to do just one RWD burnout video or record a brief drift session in an open parking lot. Just because.
But that’s definitely not the only reason I wanted to try the VCE. There are actually several other potential advantages to run a VCE. The most obvious reason would be a gain in performance at the drag strip. With a locked center differential, an AWD DSM would have a more consistent launch and would give you more traction while launching. With my LSD center AND rear differentials, locking the center diff should allow for perfect straight line acceleration because each front/rear wheel on either side would rotate at the same speed.
The other thing that I would benefit from is when breaking an axle. Let’s say I broke a RF axle while on my way home from the grocery store. Without a locked center differential, the differentials would be trying to send most of the torque to the RF wheel as it would have the least amount of resistance with a broken axle. With a locked center diff, the RF axle would be getting the same amount of torque as the RR axle. With no torque actually being applied to the ground on the RF due to the broken axle, 100% of the torque would be applied to the RR wheel.
But, this can also be very dangerous in the right situation. If I broke a RR axle under hard acceleration at the track, the rear end of the car could come around and nose me into the wall if I were a shitty enough of a driver. Even the best driver would have a difficult time keeping the car out of the wall. The other three working axles would be applying the power to the ground, so the left side of the car would try passing the right side of the car and turn hard right.
Another benefit of using a locked center diff is to be able to drive the car in FWD if needed. If I were to blow my rear diff, break a rear axle, break my driveshaft, or blow my transfer case, I can remove the rear axles and run the car in FWD until I can either afford to buy new parts, or get the car home without calling a flat bed.
The reasoning why I chose to use a VCE and not a welded center diff was simply because I had a VCE and had no real intention on locking my center differential prior. It is a very quick way to lock your center differential with a VCE.
To install a welded center differential, you have to remove the transmission, pull the bell housing side off of the transmission, remove the center differential, have it professionally welded, then reinstall the center differential if you can figure out how all the little tiny pieces of the trans goes back together. And, it’s permanent.
With a VCE, you simply remove the 5th gear cover off of the end of the transmission, remove a clip off the Viscous Coupling, pull the Viscous Coupling off, install the VCE, reinstall the retaining clip, and bolt the 5th gear cover back on. If I decide for any reason to not use a locked center differential, it’s literally 30 minutes of work or less and doesn’t require you to remove the transmission from the engine in the car.
If you weld your center differential, you still need to keep your Viscous Coupling in place. You actually can’t properly weld the center diff without the Viscous Coupling in place. You can also use a VCE with a locked differential. Most people will tell you that it is not necessary because your center diff is already locked with the welded diff. But, the VCE could weigh much less than the Viscous Coupling unit depending on which design VCE you have, so you would be removing a good chunk of weight by using a VCE with your welded center diff.
There are two designs of VCE’s. One looks like a regular Viscous Coupling, but is welded inside. The other type of VCE is a stock Viscous Coupling milled down on a lathe so only the teeth and gears are left, then the two gears welded together. This allows you to remove a lot of weight from the unit. I use this type in my transmission currently.
How To Remove VCU & Center Diff, And Weld Center Diff
First, remove the end cover (11 bolts, 9 short and 2 long).
This is what you will see.
Remove this pin on the shift fork and pull it off (keep it all together will help on reassembly).
Pull the snap ring off the VCU.
After you pull the VCU off, there is a small ball bearing in the shaft (remove it and keep it safe for reassembly).
Remove the two nuts of the gears.
Pull the three bolts out on the side (be sure to keep the spring and ball bearing all together).
You should not be able to pull the second layer cover off.
Remove the bolts holding the center diff in.
Remove your center diff.
The parts in the picture below will not be reused. Pull them out and sit them aside.
Now you can weld the center diff. See diagram below.
Remove everything with and “X”. Weld gear 6 to case 4, then weld gear 10 to case 12.
Then simply reverse all the steps to reassemble and your good to go.
Limited Slip Differentials and Spider Gears
Kazz, Quaife or Cusco: These are all types of limited slip center differentials. They usually offer a better limited slip feature, but they are not any stronger than a 4 spider gear center diff.
4 Spider Gears Center Differential: A factory center differential comes with only two spider gears. It is by far the weakest link on DSM trannies. It is the reason why DSM drivetrains have reputation for being weak. Upgrading the center diff is a MUST on anything running faster than 13 sec (and you can break it even before that point). The most common upgrade is to convert it to 4 spider gears, but cost is usually between $400 and $500. But this does not in any way effect your limited slip feature. You still rely on a crappy viscous coupling.
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