Practical Transmission Tuning: Flex Plate, Torque Converter & Lockup Clutch

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Flex Plate, Torque Converter & Lockup Clutch

14.34

00:00	While you don't have to know absolutely everything there is to know about the internals of a transmission in order to make calibration changes, it absolutely helps.
00:07	So, in this section of the course, we'll be diving deep into the inner workings of an automatic transmission to ensure we have a really good foundation of knowledge to build our calibration skills upon.
00:18	While some car enthusiasts have an idea of what a torque converter, clutch, and gear are, when we start talking about stators, compound gear sets, and brake bands, things may get a little fuzzier.
00:27	And ultimately, even if you have a general idea of what all these parts are, a fuller understanding of how they operate and interact with each other will help allow you to make the best use of them.
00:37	In the following modules, we're going to break down automatic transmissions, explain how each component functions, and then build things back up to explain how all these parts work together to transfer power from the engine, through the torque converter, then the transmission, and out to the rest of the drivetrain.
00:52	Once we've accomplished that, we'll discuss gear ratios and the shift process.
00:56	Then I'll review the integration and interaction between the Transmission Control Module, or TCM for short, and the Engine Control Module, or ECM for short.
01:06	Of note, the ECM is sometimes referred to as a PCM, which stands for Power Train Control Module, or ECU, which stands for Engine Control Unit.
01:15	All these terms refer to engine control and can be used interchangeably.
01:18	The key item these devices share is information about torque provided from the engine to the transmission, and torque requests from the Transmission Control Module back to the Engine Control Module.
01:30	Let's dive right in, starting with the components of a transmission.
01:33	We'll start on the engine side with the flex plate.
01:35	While this isn't technically part of the transmission, without it we won't be getting very far.
01:40	The flex plate bolts to the engine's crankshaft on one side, and the transmission's torque converter on the other.
01:46	Flex plates look a lot like a flywheel you would see on a vehicle set up for a manual transmission.
01:51	They still usually have a ring gear for starter engagement, , but they're less thick and heavy duty looking, since a clutch disc won't be clamped to it by a pressure plate.
01:59	And why is it called a flex plate? Because it's actually designed to flex.
02:02	The torque converter deforms a bit as rotational speed, fluid pressure, and torque applied to it vary.
02:07	In order to allow for that deformation, but prevent the torque converter from bottoming out on and damaging the transmission, the flex plate provides a careful measure of compliance.
02:18	As power levels increase, thin OEM flex plates can become a weak point, and this is something we'll start to look into once we start discussing upgrades.
02:26	Next, in line in the torque flow path from engine to drivetrain is the torque converter.
02:31	Among other things, the torque converter serves a similar purpose to a clutch on a manual transmission, in that it can decouple the engine from the drivetrain when required.
02:39	For example, when we start the engine, we don't want the vehicle to immediately take off, so we need to allow the engine to rotate while the drivetrain, wheels, and tires don't move.
02:49	This is also important when you're driving and slow the vehicle to a stop.
02:53	Again, we want the engine to keep rotating while the drivetrain comes to a stop, and the torque converter is what decouples or disconnects the transfer of torque between the engine and transmission to allow for that.
03:05	The torque converter can also allow for a measure of slip, much like slipping a clutch, as we transition the vehicle from being stationary to driving.
03:13	In addition to that basic coupling and decoupling function, the torque converter provides a performance benefit by allowing partial and progressive coupling.
03:21	At the start of an acceleration event, the torque converter can allow the engine to rev up quicker than the drivetrain.
03:27	This gets the engine into an operating range where it can generate more torque and horsepower more quickly.
03:33	Because the torque converter doesn't have a direct physical link between its halves, it also acts as a buffer between the engine and drivetrain, reducing shock when engine output suddenly increases or grip level suddenly changes due to a bump in the road or a surface change that causes a massive shift in drivetrain speed.
03:50	This is more kind to driveline parts like axles than a clutch-dumping manual transmission setup, and it also provides a smoother ride for vehicle occupants.
03:59	Now, let's take a look inside a torque converter to get a better understanding of how it operates.
04:04	Depending on the model, we might find a stamped or billet housing, single or multiple stators, and the unit may have a torque converter clutch, which looks like a typical clutch disc you find in a manual transmission vehicle.
04:15	The number of discs or stators found inside a converter varies, but the general idea of how they operate is the same.
04:21	For a more detailed look, let's now break down the converter to its individual parts, starting with the engine side and working our way back from right to left.
04:29	The first piece all the way at the right is the front cover, which simply seals fluid into the torque converter assembly and attaches the converter to the flex plate.
04:38	As we've discussed, the flex plate is attached to the engine via the crankshaft, so this is our source of torque input.
04:45	Next, is the clutch assembly, which when activated, essentially locks the input and output sides of the converter together.
04:51	The input side is called the turbine side, and the output side is called the impeller side.
04:56	Looking at an exploded view of the parts, it may seem a bit confusing to understand how a clutch in that location would lock the impeller and turbine together.
05:05	What we have to remember is while we're looking at the torque converter in pieces here, a functioning converter has the impeller and front cover welded, bolted, or otherwise mechanically linked as a unit at all times.
05:17	So, when the torque converter clutch assembly locks the turbine side to the front cover, it's effectively locking it to the impeller as well, because the front cover and impeller are mechanically linked at all times, despite appearing separated in the exploded view.
05:32	The next item to the left is the stator, which has a one-way clutch sometimes called a sprag clutch, the hub of which is mated to the transmission case via a shaft.
05:42	This causes the stator to remain stationary while the turbine and impeller rotate during periods of large speed differentials between the impeller and the turbine.
05:51	Then once the impeller and turbine speeds are more closely matched, the one-way clutch can freewheel.
05:57	We'll get into more detail on this soon and how it's helpful, , but first let's walk through the last big piece of the converter first, which is the impeller, also called the pump.
06:05	As the name implies, this section pumps transmission fluid from the center outward using centrifugal force caused by the rotation of the impeller.
06:14	Do your best not to confuse this with the transmission pump though.
06:17	The transmission pump is a traditional pump driven by the engine that transfers fluid into the torque converter and provides lubrication and pressure for automatic transmission functions.
06:28	Now, that you know the pieces of this torque converter puzzle, I'm going to cover how it all comes together, how torque gets from the engine to transmission and how fluid flows through the torque converter as well.
06:38	We ran through the parts in order from what I'll refer to as the front or engine side of the converter to the back or transmission side of the converter, but that's not actually the path torque or fluid takes through the system.
06:50	The transfer of torque all starts with engine crankshaft rotation.
06:54	Torque is passed from the crankshaft through the flex plate into the front cover of the torque converter.
07:00	Because the flex plate bolts to both the crankshaft and the torque converter, it's a critical component in terms of fitment compatibility.
07:07	Then because the front cover is welded, bolted or otherwise attached to the impeller pump, anytime the engine is rotating, the impeller pump portion of the torque converter is spinning at the same number of revolutions per minute as the engine.
07:21	If we look at this assembled torque converter for a power glide, turbo 350 or turbo 400 transmission, we can see the impeller and front cover are effectively one piece in this design since they're welded together.
07:34	And here's a billet example for the turbo 350 and 400 showing the two pieces bolted together with the same result.
07:41	The front cover and impeller move as a unit and contain the fluid.
07:45	The fluid pumped out of the outer channels in the impeller pump move into the inlets of the turbine side, driving it to rotate as well.
07:53	We can think of this like setting two box fans in a row, like they're spooning each other, and a small gap of an inch or so between them.
08:01	If we turn the fan in the back on, the second fan will spin up too.
08:05	It won't match the speed of the powered fan, but just by blowing on the second fan without any direct physical connection, it will create motion.
08:13	The torque converter operates in a similar fashion, but uses fluid motion instead of air.
08:18	That's why it's called a fluid coupling.
08:20	The two halves aren't physically connected, but one side can create motion in the other via fluid transfer.
08:27	Moving rearward, the turbine side of the torque converter is connected to the transmission via a shaft.
08:32	So, turbine speed will always match transmission input speed, and the torque from the motion of the turbine passes right into the transmission.
08:41	At this point, you might be thinking, this seems like a really inefficient and overcomplicated way to transfer torque, and you'd be right.
08:48	Up until lockup clutches and stators were added to torque converters, they were far from ideal.
08:53	Now, let's talk about why this extra complication makes sense in a modern automatic.
08:58	First, we have the torque converter lockup clutch, which comes in varying designs, but the net effect is the same.
09:04	When the clutch is engaged, rather than the turbine side of the converter spinning slower than the impeller pump side and energy being lost, clutch lockup causes a mechanical link between the two sides, transferring all torque from the engine through the converter to the transmission.
09:20	This maximizes efficiency under conditions where slip is not desired, such as steady state low load cruising.
09:27	Let's take a quick look at how the actual connection works.
09:30	Here's a holding style clutch bolted to the turbine.
09:34	When engaged, it applies pressure between the friction disc and the front cover.
09:38	Instead, some converters use a braking clutch attached to the front cover, which when engaged, brakes on a drummed hub on the turbine to link them together.
09:47	In either instance, or in other slightly different install cases, a link from the turbine via clutch to the front cover is created.
09:55	And because the front cover is welded, bolted, or otherwise attached to the impeller, the converter input and output sides are now linked.
10:03	Moving on, we save the best for last, the stator.
10:06	It doesn't look like much, but the simple device causes torque multiplication, giving automatic transmissions a performance advantage and making the torque converter more than just a fluid coupling.
10:17	The stator provides a massive benefit during large slip events, where the impeller pump spins much faster than the turbine side of the torque converter.
10:25	This commonly occurs while launching the vehicle or flooring it from a roll.
10:29	While the two sides of a torque converter work well together when rotating at similar speeds, fluid exiting a much slower turbine doesn't make its way back into the impeller side at an appropriate angle.
10:40	This slows fluid motion and reduces acceleration.
10:43	During this condition, the stator helps by redirecting the fluid exiting the turbine side to more efficiently re-enter the much faster spinning impeller.
10:53	Fluid then circulates out of the impeller back into the turbine more efficiently, which brings the turbine up to speed more quickly.
11:00	Once the turbine side accelerates up to near impeller speed, the stator's redirection of fluid is no longer required or a benefit.
11:07	As turbine and impeller speeds come close to matching, the angle at which fluid contacts the stator when exiting the turbine is such that it drives the stator to freewheel on its one-wheel clutch instead of remaining stationary.
11:19	This prevents the stator from significantly impeding flow into the impeller once the turbine is up to speed.
11:26	We've talked a bit about fluid passing from the impeller pump side to the turbine side and back, but let's get a little more in-depth on the impact that has.
11:34	Understanding how this all works really helps inform not only our transmission tuning decisions, but part selection as well.
11:41	As the engine revs up, the transmission fluid pump and impeller pump speeds increase.
11:46	This means the fluid flow in the torque converter increases, which increases the coupling effect between the input and output halves of the torque converter.
11:53	At a certain RPM, the coupling of the torque converter is sufficient, the forward motion is possible, and eventually, if the converter is properly selected for the engine package, engine speed and transmission speed become equal or relatively close to it in conditions where high efficiency is desired.
12:11	You've probably heard stall speed mentioned, and here's what that really means.
12:14	It's the engine speed at which the torque converter will start transmitting enough engine torque to cause forward motion of the vehicle.
12:21	Stall speed varies with many factors, including engine torque output, gear ratio, vehicle weight, and the construction of the torque converter itself.
12:29	We can think of it this way.
12:31	As engine speed increases and more fluid pumps through the converter, more torque is transferred from the input to output side of the converter.
12:38	But how much torque has to make its way through the converter to the transmission before it's enough to start the vehicle moving forward? If it's a really heavy vehicle, moving it will take a lot more torque than a light vehicle.
12:50	For this reason, a converter designed for a lighter vehicle would have a higher stall speed if it was installed in a heavier vehicle, since more torque transfer than the design called for is needed before the heavier vehicle will start to move.
13:04	If you tune an engine and its torque output is increased substantially, but the converter is left stock, engine speed when preparing to launch will generally increase as well.
13:13	This occurs because for a given impeller RPM, the amount of torque the converter will transmit to the turbine side and out to the transmission has remained the same, but the amount of torque the converter receives from the engine has increased.
13:26	We'll be discussing considerations for torque converter selection and various upgrade options in the component upgrade section of the course.
13:34	But first, this has been quite a big module, so let's review what we've learned before moving on.
13:39	The flex plate connects the engine's crankshaft to the torque converter, but allows for a little compliance to account for slight torque converter deformation and avoid transmission case contact.
13:48	The torque converter itself is made up of a front cover, clutch assembly, turbine, stator, and impeller.
13:55	Much like a clutch in a manual transmission, the converter transfers torque from the engine to the transmission, while also providing the ability to decouple the engine from the drivetrain when necessary, like during startup and idle.
14:08	The torque converter allows for controlled slip, which helps smooth torque application to the tires, especially when launching the vehicle, flooring it from a roll, going over bumps, and other transient conditions.
14:19	The converter incorporates a stator that redirects fluid flow for optimal efficiency during changing operating conditions, and provides the torque multiplication effect that gives automatic transmissions a key performance advantage.