We are all thankful that the Big Three automakers have finally started the diesel horsepower war. Little by little they have been increasing the amount of horsepower allowed to the ground in an effort to sway the buyer to their truck instead of one of the other two. After all, this is why we pay the big bucks to own a diesel: so we can pull our really big toy haulers out to the desert with all the luxuries of home and all the toys too. There is no doubt that these trucks can handle the task. But the problem is, they are still using the single disc TC in their automatic transmissions. Put enough horsepower and torque to the front side of the stock transmission and enough weight on the truck and you will slip the TC clutch—it’s only a matter of time. And the more horsepower you have, the less time it takes. So, for those who want information that will help them to make an educated decision on which transmission products to purchase, we are here to raise the bar. After all, these aftermarket upgrades aren’t cheap and you certainly don’t want to have to do it twice.
ATS Diesel Performance designs and manufactures aftermarket TC and high performance products for all three brands of diesel pickups. ATS is dedicated to making the best quality products and stands behind them with a written warranty and a serious commitment to customer service. They believe in educating the customer so they can make the best choice for their needs. The more you know, the better off you are and the better you will understand why you are being encouraged to upgrade your brand new tranny.
Getting Schooled
Now let me start off by stating the obvious: in order to really get technical, it is going to take a lot of pages to cover all this territory. So for the sake of space and time, I am going to try to condense this as much as possible without sacrificing the information that we all need in order to understand the way things work. After all, that’s the whole point.
Let’s start with the torque converter. It is the heart and soul of what makes one transmission different from another. The interesting thing about the TC is that most of us have never seen the inside of one. Most parts can be disassembled with common hand tools but a TC needs a special machine to cut it open. Hence, the mystery. So what’s inside a TC? A TC is made up of the following elements:
• Clutch material discs
• Viskus drive
• Billet cover
• Turbine
• Impeller
• Stator
We’ll cover each of these elements in the following sections.
Material Difference
The stock converters come with one clutch disc that locks up when you get up to cruising speed. I start here because this is where the biggest problem lies. Aftermarket TCs will usually have three or more clutch discs. When the TC disc locks up, it’s kind of like letting the clutch out on a standard transmission. Once it is engaged it needs pressure against it in order for it to keep its grip and not slip when power is applied. Think of sandpaper applied to a fresh piece of wood. When it is new and you apply a lot of pressure to it-it will try to grip the wood; you need to lighten your pressure or you will not be able to get the sandpaper to slide across the wood. Well the pressure applied to the clutch disc in the TC is fluid line pressure from the valve body, and the higher you have to make the pressure the harder it is on the seals in the transmission. With only one clutch disc in the TC you will need to apply lots of pressure to keep it from slipping, especially when you start upgrading the power.
So now imagine that sandpaper stuck to both sides of a paint stir stick. Make that a couple of stir sticks stacked together. Now apply the same amount of pressure and where the sandpaper grips in between the stir sticks you can see that it would not take a lot of pressure to get them to hold tight to each other. You have increased the amount of surface area that has the sandpaper on it. Yet if you leave the line pressure at its stock setting you will get a tremendous amount of gripping power without adding stress to the seals in the transmission. You will also get the same smooth shift without the harshness that comes along with increased line pressure, yet with the added gripping power you will not get any slippage.
Every time you add a clutch disc to the TC you increase surface area and holding power. Now choose your clutch material to be the most durable and your adhesive to be the strongest and you have enough holding power to withstand 2,000 ft. lbs. of torque without slipping.
ATS Diesel builds their TC with five discs stacked together. Now picture discs one, three, and five connected to the engine side of the TC and discs two and four connected to the transmission side. The odd number discs of the ATS TC have square tabs that fit into square slots on the outer diameter of the discs and the even number discs are splined to the input shaft of the transmission.
Square not Round
When pressure is applied, it sandwiches the pack together with the square tabs holding the torque of the engine. On all other TC these tabs are rounded and so the surface that holds all the torque on the engine side is much less than with a square tab. Like putting a round peg in a square hole only some of the surface will touch the sides of the hole. After a short time, the torque of the engine will shear off the round tabs and they will fail. Now you have no way to transfer torque to the input shaft of the transmission. It does not take much of a power upgrade to surpass the limits of the single disc and the rounded tabs.
Viskus Drive
Now imagine all five discs packed into a space that is usually meant for a single disc. Yes five discs, very close together. So close as a matter of fact that the automatic transmission fluid (or ATF) that flows between the discs is used to push the adjacent disc along with it. Sort of like when a train goes by at a railroad crossing: you can feel it trying to pull your car in the same direction it is moving. This is the great volume of air moving along with the train. If the train were very smooth on the sides it would slide through the air and create less drag. The clutch discs have slots in them to help push the fluid along and the disc next to it also. This creates the Viskus Clutch Drive.
The Viskus Clutch Drive helps to move your truck down the road, adding an additional 18 lb. ft. of torque (along with the impeller and turbine which we will talk about next). It creates the same kind of drag between each disc that you feel when the train goes by. It just does it inside the TC. Free energy that is captured and taken advantage of is energy that would have been wasted but now is being put to use, which translates into better fuel economy.
Billet is Better
The big flat end is the stamped steel cover on a stock converter. It often balloons out under heat and high pressure. Most companies attempt to strengthen this cover by welding the lugs that hold it to the engine, and more recently, welding a plate on to it for added strength. Some do a billet design in the lug area because the heat of welding proved to deform the cover too. ATS does a full billet cover to prevent warping and to withstand even the most powerful modifications. Since there is no welding, there is no warping.
Fluid Flow
Alright, now we get into the fluid coupling part of the TC. Amazing to me that all this goes on inside of the TC where you can’t even see it. The clutch pack is really about half the TC and the fluid coupling it roughly the other half. The billet cover bolts onto the flywheel of the engine and turns at engine speed. Inside is the impeller, which is connected to the billet cover and moves ATF to the turbine side of the TC. The turbine is connected to the input shaft of the transmission. Think of two fans facing each other. One is running at engine speed. The other starts to move as soon as you let go of the brake. The air from the first fan starts the second one moving. The same thing happens with the impeller using ATF instead of air. As you start to accelerate from a stop it starts pushing the turbine and gets it spinning until both are finally moving at the same speed. This is the basic design without any torque multiplication and works fine except it takes forever to get to cruising speed. This crude design actually has a high efficiency once you get up to speed. But with no torque multiplication it is almost impossible to get moving.
Want torque multiplication? Add a stator. This goes in between the impeller and the turbine and is only needed when they are going at very different speeds. When the speed of the impeller is greater than the speed of the turbine the stator redirects the fluid that is returning from the turbine back to the same direction of the impeller. It is mounted on a one-way clutch which stops it from turning the opposite direction of the engine. So as fluid comes into it, it switches the flow back to the direction of the impeller/engine to increase engine torque. Or recapturing the energy from the ATF that the turbine isn’t using and redirecting this energy to increase engine torque, creating torque multiplication. When turbine speed almost equals impeller speed, the stator freewheels on the one-way clutch, or gets out of the way so it doesn’t restrict the flow of ATF. This creates efficiency. The less the stator restricts the fluid the more efficient the TC will be. This is why ATS has redesigned the stator to allow more fluid flow with less restriction while still redirecting the ATF when needed. You can see the difference between the stock stator and the ATS billet stator.
ATS has reduced the restriction and opened up an area between the veins allowing more efficient flow.
Stall Speed
ATS has also designed the stator to allow for lower stall speeds. Stall speed is defined as the engine RPM at which the TC will not allow RPM to increase any further when the truck is being held in place with the brakes. Ideally your stall speed should be at, or a little below, the peak torque RPM rating of your engine. In a stock Cummins 24V, peak torque occurs at approximately 1800 RPM. Unfortunately, the stock converters stall speed is considerably higher than 1,800 RPM. So with a stock converter you won’t be getting the power to the ground until after you have passed your peak torque rating. Another problem happens when you start to crank up the horsepower to your turbo diesel. Stall speed changes with engine horsepower. So if you increase the power with performance modifications, the already high stall speed increases to a higher RPM as the engine overpowers the TC. This is the basic principal of the TC. I hope I got the main concepts across here without totally boring those of you who glaze over at the first sign of technical talk.
If you have any questions please feel free to contact us at:
ATS Diesel Performance, Inc.
5293 Ward Rd., Unit 11
Arvada, Co 80002
800.949.6002 - Toll Free
303.431.0135 - Fax
www.atsdiesel.com/ToyHauler.asp
-THM
