Practical Transmission Tuning: Gear Selection
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Gear Selection
29.14
| 00:00 | In the Transmission Fundamentals section of this course, we discussed some of the main goals of transmission tuning. |
| 00:06 | Now, in this section, we're going to dive much deeper into the fundamentals of the tuning process. |
| 00:12 | As a reminder, we're looking to maximize performance, improve drivability, and tailor transmission behavior to specific use cases. |
| 00:21 | Being in the right gear at the right time is a huge part of achieving all those goals. |
| 00:26 | So, this makes optimal gear selection a powerful tool. |
| 00:30 | We also discussed gear ratios in the fundamentals section. |
| 00:33 | But now we get to dig deeper and use that information to help make gear selection decisions. |
| 00:39 | Let's start by looking at the widely used GM 4L-ADE for speed transmission. |
| 00:45 | Here you can see I've plotted vehicle speed versus transmission input RPM. |
| 00:50 | Each colored line represents a different gear with labels shown at the top. |
| 00:56 | You'll see first gear has a blue circle next to it, indicating it will be a blue line, then second gear in red, third gear in orange, and fourth gear in green. |
| 01:05 | To create this data, some constants had to be set. |
| 01:08 | So, I've used a 28 inch tire diameter, sometimes referred to as tire height, and a 411 or 4.11 final drive. |
| 01:17 | If the tire was taller than this, or the final drive ratio was a lower number, such as 3.23, then without changing the speed coming out of the transmission, the speed at the driven wheels would be higher. |
| 01:32 | If the tires were shorter, or the final drive was shorter, maybe a 4.44, then the opposite would be true. |
| 01:39 | Vehicle speeds achieved for a given transmission speed would be lower. |
| 01:44 | When considering a final drive change, making a plot like this lets you visualize the effect it will have to make sure you pick the right part before spending your money. |
| 01:55 | For example, if we know our engine red lines at 6,500 RPM, we can look at the 6.5 line on the x-axis and see max vehicle speed in each gear when there's no torque converter slip. |
| 02:08 | With this transmission, tire size, and final drive, that's approximately 53 miles per hour in first gear. |
| 02:14 | 89 in second, 132 in third, and 176 in fourth. |
| 02:20 | In terms of drag racing, if we're hoping to trap near 140 miles per hour in the quarter mile, we'll either need to shift to fourth, which is a long overdrive gear, or we'll need a little longer final drive ratio. |
| 02:32 | So, we can get closer to 140 miles per hour in third gear and avoid a shift to fourth. |
| 02:38 | The shorter the gear ratio, the greater the mechanical advantage. |
| 02:42 | And the higher the torque output at the wheels. |
| 02:46 | So, using a very long overdrive gear, like this fourth gear during a drag pass, can slow the vehicle's acceleration significantly. |
| 02:53 | To figure out just how much longer a final drive we need to finish a drag pass in third gear, we can test different values in the spreadsheet I'm using here, which you can also find in the course resource section linked below, so you can test it out yourself. |
| 03:08 | We may find the perfect ratio isn't something we can buy easily, hopefully there's something readily available which is reasonably close. |
| 03:16 | In this case, a 3.9 final drive ratio swap will get you to about 139 miles per hour at 6500 RPM in third gear if the torque converter is locked up to get every bit of vehicle speed out of that engine speed and gearing. |
| 03:32 | But what if we don't want to lock the torque converter up, or we don't have that option? So, we want to leave room for a bit of torque converter slip, causing engine speed to exceed transmission input RPM by a few hundred RPM and still get to 140 miles per hour. |
| 03:50 | How about a 3.7 gear? Looking at the next chart, we can see at 6000 RPM in third gear, there's potential for 135 miles per hour without slip. |
| 04:02 | And at 6500 RPM, there's potential for 146 miles per hour without slip. |
| 04:07 | If we're only expecting a few hundred RPM worth of slip, we can achieve 140 miles per hour at 6500 RPM engine redline and transmission input RPM of 6250. |
| 04:21 | If, however, we're using a four-speed transmission where fourth gear is only mildly overdriven, then it can be beneficial to use all gears during acceleration runs. |
| 04:31 | Instead of running a longer final drive to avoid fourth gear, you can go in the opposite direction with a shorter final drive, improving wheel torque in all four gears. |
| 04:42 | If you're shifting at 6500 RPM, you can look at the plot and see where the vehicle speed you achieve in each gear meets up with the next gear. |
| 04:51 | For example, in first gear, 6500 RPM shows 59 miles per hour. |
| 04:57 | So, when shifting to second gear, you look across the chart and see 59 miles per hour lines up with just under 4000 RPM in second gear. |
| 05:07 | That means on the 1-2 shift, you'll end up around 4000 RPM when second gear engages. |
| 05:14 | If you can't safely rev your engine any higher, that's as good as it gets in terms of keeping revs up with this gearing combination. |
| 05:22 | But with the shorter final drive, the vehicle speed spread between gears shrinks. |
| 05:28 | And so the RPM drop on upshifts gets smaller as well. |
| 05:33 | In addition to the variability I've mentioned torque converter slip introduces, another caveat of selecting a final drive ratio to achieve maximum speed with maximum acceleration potential is the variability of tire size. |
| 05:46 | As the tire grows with speed and due to increase in pressure that comes with an increase in temperature as we drive aggressively, the vehicle speed we can achieve at a given transmission speed and gearing actually increases. |
| 06:02 | With a typical street tire that's inflated to the recommended pressure, driven on public roads at reasonable speeds, the change in size usually won't be large. |
| 06:12 | Street tires are constructed in a manner that restricts growth as speed increases and tire pressures don't increase as massively during casual driving because there's only so much heat being introduced. |
| 06:23 | When we make plots like this and then compare with real world behavior and data logs, if things don't line up quite perfectly, tire height variability's impact on gearing may be the cause. |
| 06:36 | Measuring a street tire with it set to operating pressure will give us a good enough reading, so there's no need to worry. |
| 06:43 | Keep in mind, people forget to inflate their tires. |
| 06:46 | Tires wear out over time and nobody is retuning the tire size setting on the TCM day to day, so there is some room for error here. |
| 06:54 | We'll be covering an easy method to accurately measure tires later in this section of the course. |
| 07:00 | So, for now, it's just important that we understand that different types of tires used in street or motorsport disciplines see different levels of tire dimension changes while in use. |
| 07:11 | A road race car, for example, can easily gain 10 psi or more in a lap or few as the tires heat up. |
| 07:18 | And if you've ever seen slow motion footage of a drag car launching off the line, then you'll have a good understanding of just how much dimensions can change with wheel speed on a drag slick. |
| 07:30 | We find another good example in the rock crawling world, where the max speed required is quite low by comparison to most other motorsports. |
| 07:39 | And close ratio gearing plus shorter final drives plus low range transfer cases are often used to shorten the total gearing. |
| 07:48 | For example, a low range transfer case might take a one to one output and turn it into a four to one output, giving us one fourth the max speed, but four times the wheel torque. |
| 07:59 | As we talk about selecting gearing for maximum performance in an off-road environment, this is a bit different from gear selection for a road car. |
| 08:07 | On a road car, users expect reasonable economy as well as performance. |
| 08:12 | So, a balance is generally struck involving gearing towards a maximum speed considerably higher than what's needed, and in order to allow for driving at highway speeds without turning higher engine RPM. |
| 08:25 | Keeping engine RPM down improves fuel economy, reduces engine heat and wear, and is more efficient since frictional losses are reduced by slowing the motion of engine and transmission components. |
| 08:37 | Now, that we've talked a bit about final drive selection and low range transfer cases, let's discuss the ratios of the gears in the transmission. |
| 08:46 | While there are racing transmissions designed for relatively quick gear ratio changes, most automatic transmissions don't allow for changes in ratios beyond what the OE intended. |
| 08:56 | This makes choosing the right automatic transmission for our application critical. |
| 09:02 | Then if we need to make small adjustments, final drive changes can tweak things more easily. |
| 09:08 | We always want to do the best we can with the components we have available to us. |
| 09:13 | So, when do we want to be in each of the transmission's gears? That comes back to our main goals of transmission tuning, and which of them are most important at the moment. |
| 09:24 | This is where TCM logic comes in, because it's set up to infer what the driver is prioritizing at the time. |
| 09:31 | One common input used to do this is the position of the accelerator pedal, or in a cable throttle application, throttle position. |
| 09:39 | Because cable actuated throttle bodies are controlled by the position of the accelerator pedal which is pulling the cable, we can determine pedal position by monitoring throttle plate position. |
| 09:51 | If the driver is flooring the accelerator, generally that means they want maximum performance, which is most often achieved by waiting to shift near the top end of the engine's power band. |
| 10:02 | The goal here is keeping the engine in the region where it makes the most power possible when you enter the next gear, rather than shifting sooner at a lower RPM and causing the engine to drop down to an even lower speed after the upshift where the engine makes less horsepower. |
| 10:20 | If the driver is applying a medium accelerator position, perhaps 50%, they probably want a balance of performance, control, and efficiency, which usually means shifting at a moderate engine speed, reducing engine horsepower from peak, and making it more efficient and easier to control. |
| 10:39 | At light throttle, the driver probably wants more fuel efficiency, more precise control of the vehicle, reduced acceleration, and more stable operation. |
| 10:49 | To achieve that, we want shifts to occur even sooner at lower engine speeds. |
| 10:54 | This keeps the engine down in the operating range where it makes less horsepower, is more fuel efficient, and smoother to drive. |
| 11:01 | If the driver isn't pressing the accelerator at all, they're likely idling while stationary, coasting, or braking. |
| 11:09 | Some additional inputs which help TCMs infer what the driver wants to do are vehicle speed and some measure of whether the brakes are being applied or not. |
| 11:18 | Sometimes it's a simple brake on off switch on the brake pedal. |
| 11:22 | Other times it's more detailed info from a brake pedal position sensor or a brake pressure sensor, which in addition to showing an attention to slow down, indicates how rapidly the driver wants to slow down. |
| 11:36 | How rapidly the vehicle is decelerating can also be determined from vehicle speed or a longitudinal g-sensor if it's equipped, and that data is then provided to the TCM. |
| 11:48 | The additional info is helpful in determining what the driver wants to do because, for example, if the is off throttle while in motion but not touching the brakes, they're not trying to rapidly slow down. |
| 12:02 | But if they're using the brakes, we know they want to slow down more quickly. |
| 12:06 | Those bits of information help inform the TCM to make decisions between downshifting to increase engine RPM, which increases the engine braking effect to slow the vehicle down quicker, versus staying in a higher gear and allowing the vehicle to coast as efficiently as possible. |
| 12:26 | At low speeds and light throttle, we can infer the driver wants precision control of the vehicle, doesn't want it to be oversensitive, and high performance is not the goal. |
| 12:38 | In fact, they likely want the opposite. |
| 12:41 | By upshifting out of first gear into a higher gear, mechanical advantage and axle torque are reduced, making vehicle response to throttle input less aggressive and twitchy. |
| 12:53 | In a stop-and-go traffic situation, while parking, navigating speed bumps, etc., it can be actually beneficial to reduce the vehicle's ability to change speed rapidly in order to assist the driver to be smooth. |
| 13:08 | So, while shifting into second gear at, say, 15 miles per hour may be terrible in a max acceleration situation, it can be very helpful when being able to smoothly and calmly move the vehicle into a parking space. |
| 13:22 | These inferences of what the driver may be trying to do, coming from the accelerator or throttle position, vehicle speed, brake, and other inputs I mentioned, relate to something referred to as driver demand. |
| 13:37 | When you get into the tuning process itself, you may find tables related to driver demand, and it all comes back to trying to determine what the driver is doing and how they want the engine and transmission to behave based on the inputs available to the ECU and TCM. |
| 13:56 | When the vehicle is traveling at a constant speed on the highway, say 70 miles per hour at light throttle, fuel efficiency is likely now most important. |
| 14:06 | Perhaps we'll want to get the vehicle into an overdrive gear, if present, in order to reduce engine speed, improve fuel efficiency, and avoid wasted effort. |
| 14:15 | If maintaining 70 miles per hour is requiring significant throttle, and that's not typical for this vehicle, perhaps this tells us the vehicle is going up a steep hill or pulling a heavy load. |
| 14:28 | Not all vehicles have a separate tow mode the driver can select to indicate they are towing, and drivers won't always decide or remember to change drive modes, even when they do exist. |
| 14:40 | Having data on driver pedal input and engine output, typically required to cruise at a given speed, allows us to determine when something is changed, like a hill or increased cargo or towing weight. |
| 14:54 | Based on the higher than usual throttle angle for that speed, if the condition persists for a period of time, we may want to improve mechanical advantage and increase engine RPM, reducing exhaust gas temperatures on a diesel engine. |
| 15:09 | If the vehicle usually requires 20% throttle to maintain 70 miles per hour on a level road, and the vehicle is rapidly accelerating instead, perhaps it's going down a steep hill or maybe a gentle hill while carrying a significant load. |
| 15:25 | Those are just some examples, and as we ponder all the things we can infer solely from accelerator pedal position, vehicle speed, brake input, and engine speed, those few inputs can tell a lot of stories about what the driver wants and the condition the vehicle is experiencing. |
| 15:43 | It's our job to keep this in mind as we make our shift tuning decisions. |
| 15:48 | For each set of conditions, how can we make the driver happy while also keeping the engine and the drivetrain operating efficiently and reliably? The most common choice we make based on what we think the driver wants at a given moment is whether to upshift, downshift, or stay in the current gear. |
| 16:08 | As we prepare to make those decisions and calibrate the tables and values associated with gear selection, there are some additional items to keep in mind. |
| 16:16 | So, I'd like to clarify why my prior charts state they're referencing vehicle speed based on transmission input RPM rather than engine RPM. |
| 16:26 | Transmission input speed may not be something you're used to thinking about, but we do have to remember it's not necessarily the same as engine speed. |
| 16:35 | We've already covered this, but it's worth reiterating that there will be some degree of torque converter slip that can cause engine speed to exceed transmission input speed, and that means engine RPM and trans input RPM are not going to be the same. |
| 16:50 | Now, while you may be used to thinking about shift points based on engine speed, transmission control modules are often tuned to shift based on vehicle speed. |
| 16:59 | For example, at 20 miles per hour, at 15% throttle, upshift from first to second gear. |
| 17:06 | The vehicle speed we choose to execute that shift will depend somewhat on gearing because we do need to be aware of how we layer up and down shifts to avoid excessive auto shifting. |
| 17:19 | While engine speed may not be how the control system is operated, it remains a key factor in the choices we make. |
| 17:26 | As I've shown in the 4L80 examples for final drive changes, we can calculate and graph the relationship between vehicle speed and engine speed based on final drive, gear ratios, and tire size to see what approximate engine speed we'll be traveling at in steady state in each gear at each vehicle speed. |
| 17:48 | We can also review data logs of a vehicle driving, and without doing the math ourselves, or without even knowing the final drive or gear ratios or tire size, we have an opportunity to view the relationship between vehicle speed and engine speed in each gear. |
| 18:03 | In order to do that though, we'll want to review data logs with trustworthy data. |
| 18:08 | To determine conditions under which the vehicle speed data is trustworthy or not, let's consider where the vehicle speed comes from. |
| 18:16 | In some cases, it comes from transmission output speed run through a calculation including final drive and tire size to infer vehicle speed. |
| 18:25 | Sometimes it comes from one, two, or more wheel speeds generated from an anti -lock brake system, which monitors some or all wheel speeds and then reports wheel speed information to other modules like the transmission control module. |
| 18:40 | Like many things, this has changed over time. |
| 18:43 | Older vehicles are more likely to infer vehicle speed from an internal transmission speed that gets run through a calculation based on final drive and tire size entered in the TCM tune, while newer vehicles are fitted with advanced ABS systems that communicate with other modules over vehicle data networks and have the option of using that data. |
| 19:05 | Especially on advanced modern vehicles, we like to think all the data is correct, but are there situations where monitored vehicle speed doesn't represent reality? Actually, there are several, so let's run through a few examples now. |
| 19:19 | The first is a situation where vehicle speed is inferred from transmission output speed. |
| 19:23 | As I mentioned a moment ago, that speed gets run through a calculation involving final drive ratio and tire size. |
| 19:30 | So, what happens if the vehicle has been altered without properly updating that information in the TCM tune? This is a very common scenario. |
| 19:39 | People often throw different sized wheels and tires on vehicles without retuning, likely without realizing the impact that can have on shift behavior. |
| 19:49 | Swapping out the rear diff ratio on a rear wheel drive vehicle is also pretty common. |
| 19:54 | People are generally expecting the vehicle to drive differently after the swap because they've installed a shorter or longer gear. |
| 20:01 | What they often don't realize is swapping the diff ratio without updating the shift points can stop them from getting some of the performance or efficiency benefits they were looking for and can also cause poor drivability. |
| 20:15 | Another cause of incorrect vehicle speed representation doesn't require swapping any parts and instead occurs when there's a loss of traction. |
| 20:24 | Let's look at a burnout example to illustrate the point using simplified data set from a front-wheel drive car that sees the two front wheels driven and the two rear wheels undriven. |
| 20:35 | When the driver starts the burnout, the right front driven wheel accelerates faster than the left front driven wheel due to a non-lock diff. |
| 20:43 | This is a common behavior to keep in mind. |
| 20:46 | Driven wheels aren't necessarily all going to go the same speed. |
| 20:49 | The undriven rear wheel speeds trail behind the front wheel speeds because the front tires are spinning. |
| 20:55 | After a quick burnout, the driver lifts the throttle and the driven tires start to slow the rotation. |
| 21:02 | The undriven wheel speeds continue increasing because the vehicle continues to accelerate since the driven tires are still outpacing vehicle speed. |
| 21:12 | Eventually, the driven tires regain grip and wheel speeds equalize across all four corners. |
| 21:19 | Usually, the speed of undriven wheels will best represent actual vehicle speed during an acceleration event but it's not perfect in all conditions. |
| 21:31 | This is because a loss of traction doesn't just occur while accelerating. |
| 21:36 | When braking aggressively, we can brake traction at any tire, driven or undriven, even on ABS equipped vehicles. |
| 21:45 | When a vehicle is braking, true vehicle speed often best matches the wheels going the fastest. |
| 21:51 | In contrast, while a vehicle is accelerating in a straight line and no brakes are applied, true vehicle speed often best matches the slowest wheel. |
| 22:01 | Again,, there are caveats here like a front wheel drive drag car using the e-brake to do a burnout. |
| 22:07 | If the e-brake is strong enough, the rear tires never rotate but the vehicle does drive forward. |
| 22:13 | So, in that case, the slowest wheels don't represent actual vehicle speed during acceleration. |
| 22:19 | In situations where transmission speed and wheel speeds all don't correlate with true vehicle speed, tools like GPS can give us a better point of reference. |
| 22:29 | I won't go through every possible scenario where TCM vehicle speed may not actually match true vehicle speed but I hope I've given you some things to consider and put the thought in your head that the vehicle speed monitor shouldn't be treated as matching actual vehicle speed at all times. |
| 22:47 | Now, because wheel speed induced monitored vehicle speed error during acceleration is fairly common, we'll go a bit deeper on that topic. |
| 22:55 | During excessive wheel spin, we run into a situation where vehicle speed dictated by driven wheel speeds or transmission output speed indicates that the vehicle is traveling fast enough to upshift into the next gear while actual vehicle speed is slower. |
| 23:11 | In this situation, while the vehicle speed monitor doesn't represent the actual speed of the vehicle well, the monitored vehicle speed can still properly correlate to engine speed as long as the torque converter isn't significantly slipping. |
| 23:26 | So, if our goal is to shift the transmission just before the engine reaches redline and we've chosen the vehicle speed to shift at accordingly, we can still achieve that goal whether vehicle speed is being skewed by wheel spin or not and that's a good thing. |
| 23:42 | If, however, we have the same scenario but are working with a vehicle that gets vehicle speed from undriven wheel speeds, then the engine may be banging into the rev limiter while the TCM is still waiting for the vehicle to reach the speed where it's set to upshift. |
| 23:58 | Many TCMs do have something in place to assist in this situation in the form of a separate function which will initiate an upshift if the transmission is in an automated mode and engine speed reaches a certain level. |
| 24:12 | For example, perhaps at 30 miles per hour a particular vehicle would upshift from first to second gear at 3500 rpm during normal conditions but if the vehicle is in first gear at a monitored vehicle speed lower than 30 and engine rpm reaches let's say 3800 rpm on perhaps a 4000 rpm rev limit, the TCM will decide to upshift based on the engine achieving 3800 rpm regardless of the vehicle speed being below 30 miles per hour. |
| 24:45 | Like many things, there are pros and cons to this and they can vary depending on our use case. |
| 24:51 | In drag racing, for example, if the tires spin terribly in second gear and the transmission upshifts to third just before redline, the engine can bog if the driven wheels and tires suddenly grip and slow to actual vehicle speed in third gear causing engine speed to drop out of the power band. |
| 25:10 | In that scenario, we may actually prefer the engine banging off the rev limiter for a bit until the actual vehicle speed is sufficient to avoid bogging the engine when the transmission upshifts, mechanical advantage is reduced, and the driven tires regain traction. |
| 25:26 | In road race, rally, or off-road applications, downshifts can be just as important as upshifts since they impact weight transfer and balance which are critical to maximizing braking zones and corner speeds. |
| 25:39 | In our brake lock-up scenario from earlier, we discussed how partially locking a wheel can skew monitored vehicle speed but generally we'll want to tune our downshift points based on the vehicle not having brake lock-ups. |
| 25:52 | This means we want to use data from logs where lock-ups didn't occur, then work with the braking system and the driver to minimize brake lock-up events. |
| 26:03 | So, again, this is a scenario where it's good to be aware of the impact that skewed vehicle speed can have but it's not the end of the world and there's still a good path forward. |
| 26:14 | Another relatively common situation where vehicle speed doesn't correlate with engine speed predictably is during significant torque converter slip. |
| 26:23 | If we have a converter that slips quite a bit at high RPM during high engine load, this can make vehicle speed-based shift tuning more interesting. |
| 26:33 | Ultimately, we do the best we can with the tools in front of us, so if we have an upshift override function at a certain engine RPM, that can come in handy here. |
| 26:42 | If not, we can review multiple data logs or time slips, then tweak vehicle speed -based shift points and stick with the settings that get the vehicle down the track the quickest. |
| 26:52 | Here, we may also want to experiment with different torque converter components to see which combination of converter slip and shift points provides the best results. |
| 27:02 | But how do we define what the best results actually are? This can be determined in a number of ways, including but not limited to reviewing time slips, GPS speed over time, longitudinal g-force monitoring, or other performance indicators such as timing acceleration between two locations or two speeds, while also monitoring the TCM for signs of concern. |
| 27:26 | We'll cover more on practical gear selection and how we enter that data during the tuning process section later in the course. |
| 27:33 | But for now, let's go over the gear selection material we've covered so far before moving on. |
| 27:38 | When we want to shift gears depends on what the driver is trying to accomplish at that time. |
| 27:43 | We can guess what the driver is wanting from the engine and transmission based on certain information available to the transmission control module, such as accelerator or throttle position, brake switch position or pressure, vehicle speed, and potentially additional information on some vehicles. |
| 28:01 | With that information in hand, we can set up primary shift schedule tables that determine when the transmission shifts up or down and other settings in the TCM related to when shifts occur to match various driver demands. |
| 28:16 | Because shift scheduling is often based on vehicle speed or transmission output speed, but we often want to decide to shift based on engine speed, a little math is needed to determine the relationship in each gear. |
| 28:29 | This can also be observed in data logs, but remember the caveats I mentioned that sometimes skew monitored vehicle speed from actual vehicle speed. |
| 28:38 | This can be due to wheel spin, brake lockup, significant torque converter slippage, and other factors. |
| 28:45 | Knowing where the monitored vehicle speed comes from on a particular vehicle can help determine when vehicle speed data might not be trustworthy. |
| 28:55 | GPS speed reference data is another helpful tool to identify when monitored vehicle speed doesn't match the actual speed of travel. |
| 29:04 | Now, that we've laid the groundwork for choosing gear ratios and shift points, let's talk about controlling the shifts themselves. |
