Practical Transmission Tuning: Shift Tuning

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Shift Tuning

16.57

00:00	Shift Tuning, just like Gear Selection, incorporates multiple transmission tuning goals, like performance, drivability, and tailoring for specific use cases.
00:10	It also uses TCM inputs to help infer what the driver wants from the vehicle at that time, and adjusts shift behavior accordingly.
00:20	For example, optimal acceleration comes from rapid shifts which may feel more harsh, but should not be so harsh that they shock the drivetrain, and cause tires to break traction, if possible, or damage drivetrain components.
00:37	During gentle driving, slower shifts, with lower clutch pressures, ramping up more slowly, improve ride quality and drivability.
00:46	When we talk about the pressure applied to an oncoming clutch or braking device, and the pressure being removed from the prior gear's holding components, some key factors in that transition and potential overlap include how quickly the pressures ramp up or down, the pressure the oncoming clutch plateaus at, and the shape of the pressure trace.
01:14	If you're not familiar with the term pressure trace, it's a visual representation of pressure increasing or decreasing over time, which provides insight on not just how long it takes for pressure to get from A to B, but also the progress it makes along the way.
01:31	Pressure may ramp up very quickly at first, then slowly increase to a plateau, or perhaps the opposite, by starting to build slowly, then ramping up very quickly.
01:43	Despite both scenarios, transitioning between the two pressure values in the same total amount of time, how it happens also impacts the result.
01:53	Clutch pressure traces are very similar to camshaft profiles in a way.
01:56	If we only talk about peak clutch pressure, it's kind of only talking about max lift on a cam.
02:03	But what about the duration and the profile that describes how the cam gets from no lift to max lift? It's much like the duration and shape of the pressure trace, as a clutch is applied by feeding it pressure from 0 psi, rising to peak pressure.
02:16	The specific timing and speed with which that pressure is increased has a big impact.
02:23	I've created four sample traces, with no units specified on purpose.
02:28	I've done this to avoid distraction from the point of the demonstration, which is variety in pressure curves.
02:35	The blue trace is a linear increase in pressure over time.
02:39	In this example, it happens to be the slowest to reach peak pressure, with the green trace reaching peak pressure just before it, and orange and red getting to peak pressure much sooner.
02:51	The orange and green traces are less linear and more curved, while the red trace shows pressure increasing in a stepped fashion.
03:00	If this particular clutch starts engaging when clutch pressure reaches 60, and stops slipping at the current input torque level when the clutch pressure reaches 160, the blue trace will result in contact prior to the green trace.
03:16	But the green trace will achieve lockup before blue, allowing less slip time.
03:22	Red and blue traces will engage more aggressively, with orange being the most abrupt feeling and having the shortest slip time.
03:31	If specific numbers drive the point home more easily for you, let's say we need 3 bar of clutch pressure to achieve clamping in the next gear, given the current engine torque output.
03:41	And we engage that oncoming clutch by increasing pressure to it from 0 to 3 bar clutch pressure in 0.3 seconds.
03:49	That's going to feel far less abrupt than going from 0 to 15 bar clutch pressure in 0.3 seconds, because the force applied per unit time is so different.
04:00	If you've ever driven a vehicle with a shift kit, or heard the term, it's a generic one which could be anything from drilling some holes bigger, removing metering devices, or shimming relief valves in the valve body, all the way up to a comprehensive package of parts and modifications with proper design and engineering behind them.
04:18	As with most things, some of the options are much better than others.
04:22	But the general idea behind what's referred to as a shift kit is by running higher maximum line pressure and clutch pressure, along with, in some cases, ramping up pressure more quickly, we can firm up shifts, make them complete more quickly, reduce slippage, and potentially improve performance in the process, while making the transmission seem more sporty.
04:46	Nowadays, these changes in behavior can often be executed even better by TCM tuning without touching the physical components.
04:54	But if we're working with an older system, or perhaps we're actually needing more flow than the stock parts are capable of, then a combination of mechanical and calibration changes may be required.
05:07	Since many modern TCMs request engine torque reductions from the ECU during shifts, the quicker the shift completes, the less time is spent operating the engine with reduced engine torque.
05:19	More time spent at full engine output will generally lead to improved acceleration, as long as we have the grip of the tires to support it.
05:28	Anytime the transmission is slipping clutches, wear also might be occurring, so some folks love shift kits on the basis of potentially extending the life of transmission components.
05:38	Before this sounds like we never want clutch slip, it's important to keep in mind that a reasonable amount of slip during the engagement process, especially during gentle driving conditions, is not a bad thing.
05:50	Controlled slip does not cause significant wear and can make the driving experience in non-race applications far more comfortable.
05:59	Remember, these are wet clutches bathing in transmission fluid that helps cool and protect them from mild brief slip events.
06:06	And if a little slip was wrecking our transmissions, the OEM wouldn't be doing it.
06:11	If we have the opportunity to data log a vehicle before it's modified, and while the parts are in known good condition, I always suggest doing so.
06:20	When you look at the TCM settings with the context of a data log, you get a more complete picture.
06:26	With a data log and stock calibration in hand, we get to see what an OEM with a massive budget has determined the transmission can accept in terms of slip to provide smooth drivability, and then we can decide how far we want to stray from their settings and log behavior.
06:42	If, on the other hand, we're not working with a reflash-based system and don't have reasonable values to start from for that specific application, then it's a good idea to start by running a bit more pressure than is likely needed.
06:56	While I'd love to say we can know exactly how much pressure we need for every combo in advance, that's not feasible, and that's okay.
07:04	Starting with shifts that are a bit quicker and a little more harsh than necessary, using a bit more pressure than is required is safer for clutch and holding components than it would be to run too little pressure and potentially slip something severely.
07:19	Again, while controlled slip can be perfectly fine during the shift process, excess slip can quickly cause wear or warping, so all things in moderation.
07:29	If we're in a situation where we're working with a high-torque clutch pack on a vehicle where drivability still matters, I understand the temptation to let the clutches slip some during engagement so the car doesn't slam into gear.
07:43	As we're dialing the system in though, we should remember that if we've upgraded to thinner plates, they may accept even less heat than stock before damage occurs.
07:53	Now, earlier I mentioned torque reductions and how speeding up a shift allows us to spend less time with reduced torque, but I'm not saying torque reduction is always a bad thing.
08:04	Without a torque reduction, full engagement of the oncoming clutch or holding component requires pressure to build up to a higher level to accept the additional torque, and the higher the pressure required, the longer it takes for pressure to build to that level.
08:20	When torque is reduced during the shift, clutch pressure on the oncoming components can achieve lock-up more quickly since clutch pressure doesn't have to build up as high to hold a reduced torque level.
08:33	Then as clutch pressure continues to rise, torque can be reintroduced in a progressive fashion, keeping pace with the clutch pressure as holding capacity increases.
08:44	The torque reduction also helps engine speed reduce more quickly on upshifts as the oncoming components attempt to lock the engine into a new longer gear ratio where engine speed will be lower.
08:56	For example, if we're at 4000 RPM in 6th gear and we're upshifting to 7th and the gear ratio spread will result in engine speed being 3000 RPM when 7th gear fully engages.
09:09	The sooner we get the engine speed to drop down and allow full engagement of the next gear, the sooner we can get back to applying full power and acceleration.
09:19	If the engine continues supplying full torque output during the shift, the oncoming transmission components will have a much tougher time holding since their engagement will require essentially breaking the engine to slow it down to match the new gear ratio's relationship to the current vehicle speed or grabbing so hard that the tires slip on the road as the transmission suddenly picks up from being in a new longer gear.
09:46	Some combination of the two can also occur.
09:50	You've likely seen a vehicle incapable of spinning the tires in the middle of 3rd gear be able to chirp the tires as it shifts into 3rd.
09:57	This is often because of what I described.
10:01	Without a torque reduction during the shift or without the torque reduction being sufficient, the current engine speed plus the new gear ratio suddenly induces output shaft and wheel speeds beyond the speed of vehicle travel.
10:16	The tires chirp or break traction briefly and as they regain grip, they actually slow to actual vehicle speed.
10:23	Since the drivetrain is connected to the engine, as the tires grip and slow the wheels down, it also pulls the engine speed down.
10:32	In any situation where the vehicle is traction limited, that sudden jolt of extra wheel speed can cascade into wheel spin through a significant portion of the next gear and may actually reduce acceleration while also reducing vehicle stability.
10:48	At this point, hopefully you're getting the picture I'm trying to paint regarding torque reductions during the handoff between one gear and the next.
10:56	Those reductions have value for multiple reasons in many situations rather than something OEMs do to stop us from having fun or slow us down.
11:06	When we're in the middle of a wide open throttle pull and lose power for no good reason, that's one thing.
11:11	But much like we generally wouldn't full power upshift a manual transmission, the same goes for an automatic transmission.
11:18	Helping the engine slow down to match up with the next gear ratio is simply part of the upshift process.
11:25	So, we've talked about bringing pressures up to the oncoming clutch or holding components to engage the next gear and we also need to reduce pressure on the prior gear's components.
11:35	But as automatic transmissions have advanced, the process has become briefer and overlap is carefully controlled.
11:43	Much like in a relay race where two runners pass the baton, there's a point where the off-going components haven't completely let go and the oncoming components are really starting to grab where the transition takes place.
11:55	Then as oncoming components pressure acting on it increases, it takes over.
12:01	When we think about the time penalty from a slow shift, it may not be a big deal when we only have two or three speeds.
12:07	But 8-10 speed automatics are common today and they shift so often that efficiency is critical to fuel economy and frankly, to avoid annoying the driver.
12:19	All this to say, if the transmission is going to be shifting all the time, it better do a good job of it.
12:25	If we're towing or plowing, shortening upshift times reduces the heat put into clutch and holding components as they slip during gear transitions.
12:35	So, while shorter shifts are great for racing, they can also be great for slowly crawling up a steep hill with a heavy trailer.
12:43	As long as we don't get so greedy and go so short that we have too much pressure applied and engage multiple gear ratios at once.
12:52	We also don't want to shock the drivetrain too harshly.
12:56	So, far I've mostly focused on upshifting because acceleration is what most people think of when they think of improving transmission performance.
13:03	But let's go a step beyond that and touch on downshifting and coasting a bit more.
13:08	I had mentioned that braking behavior gives us an idea of whether the driver wants to coast at speed or slow down more rapidly.
13:16	Downshifting increases engine speed, which increases the engine braking effect caused by engine compression resisting rotation.
13:24	While I wouldn't call this a replacement for the braking system, it does help slow the vehicle down.
13:30	Engine braking can be very useful in everyday driving, and it's a great tool to reduce heat in the braking system on vehicles towing a load.
13:39	Just like an upshift, a successful downshift involves getting the engine to rev to the new speed that matches the gear ratio we're shifting into.
13:48	On an upshift, that means slowing the engine down to match a longer gear ratio.
13:53	On a downshift, that means speeding the engine up to match a shorter gear ratio.
13:57	So, how do we speed the engine up? Just like how the TCM can request a torque reduction from the ECU, it can also request a torque increase.
14:08	On a drive-by-wire throttle vehicle, a torque increase can be induced by briefly increasing the throttle angle.
14:15	This causes an increase in airflow matched by an increase in fuel flow, resulting in the engine making more torque.
14:22	On a throttle-less diesel engine, the ECU may simply inject more fuel to briefly increase engine torque for a downshift.
14:31	On a gas engine with cable throttle, an idle motor or idle valve may be employed to increase engine airflow, or a throttle kicker may briefly move the throttle plate more open.
14:43	Another factor to consider is whether the torque converter clutch should be locked up during the shift process or not.
14:49	In some applications, the clutch unlocks to shift.
14:53	In others, it remains locked.
14:56	If the torque converter clutch remains locked, the shift can have a more firm or positive feel, while unlocking it and allowing some torque converter slip can somewhat damp and extend the engine speed change associated with the gear ratio change.
15:11	Depending on the application, it could benefit from being locked or unlocked during shifts, so there isn't a one-size -fits-all best setting, but it's easy enough to try both and see what gives us the best result.
15:23	There's more to know about tuning both the torque converter and lock-up clutch, so later in this section of the course, we're going to review some trends in their operation as automatic transmissions have developed over the years, give some common examples, and get more specific about how the torque converter clutch is controlled.
15:40	For now, to quickly summarize what we've covered in this module, shift tuning comes down to controlled increases and decreases of the fluid pressure in the circuits applying force to the clutch and holding components in the transmission.
15:54	There's a careful handoff between the current gear and the next one, and the pressure traces and timing of these events have a big impact on shift feel, performance, and efficiency.
16:07	Both upshifts and downshifts cause a change in the relationship between transmission input speed and vehicle speed.
16:15	Modern TCMs request help from the ECU to get the engine speed matched up with the new transmission input speed as the gear ratio changes.
16:25	Upshifts to longer gears involve a drop in engine speed, often assisted by a brief torque reduction.
16:32	Downshifts to shorter gears involve an increase in engine speed, often aided by a brief torque increase, sometimes called a throttle blip.
16:41	Whether we want the torque converter clutch locked up during shifts or not comes down to the controls available, the capability of our lock-up clutch, our application, and preference based on feel and performance.