Here Are the Important Basics of How a Car’s Transmission Works
Transmissions can be complicated. We’re here to ease you into this new and exciting territory.
Your engine makes the power and it’s obviously the first thing you spec-sheet brag to folks. Still, if it wasn’t for your transmission, that power would have nowhere to go. You’d be doing nothing more than charging the battery and beating up on your bearings. No transmission, no burnouts, no fun.
I know engines are important, and a great topic of conversation, but have you ever seen the inside of a transmission? You may or may not already know these are impressive pieces of technology, but that doesn’t mean you know what’s going on inside.
That’s why Car Bibles wants to talk a bit about gearboxes. We’re going to do our best to detail the most common transmission types out there and give you an idea of how things work. That way, you can decide which is the best match for your ultimate dream build, though your options are fast dwindling.
Let’s kick it in to first.
Manual vs Automatic Transmissions
The most apparent difference between manuals and automatics is user input. An automatic is easy to work with as all you need to do is how to slot it into PRNDL and go about your merry way because the transmission mostly shifts itself — you still need to select forward, reverse, neutral, and park. A manual, on the other hand, requires constant input as you must manually change the forward gears. The differences don’t end there, though.
By looking at the anatomy of automatic and manual transmissions, you’ll see that they’re almost alien to one another. Let’s start with the dying manual transmission.
In simple terms, a manual transmission operates with power coming in through the input shaft, which drives the intermediate shaft. The intermediate shaft has fixed gears, actuated by the driver, which drive other gears that ride independently on the output shaft. With the aid of a clutch fork, a collar slides over splines on the output shaft and the selected gear set, mating the two together and transferring power to the remainder of the driveline.
Automatics are a little more complicated. A traditional automatic transmission works with a series of clutch packs paired with planetary gear sets. Unlike a manual transmission with a physical gear pair for each transmission speed, automatics can use the clutch packs in specific sequences to create more gear ratios than there are planetary gear sets. Furthermore, a shift controller and the use of fluid pressure take the place of user input, adding further complexity to the system.
As if that wasn’t enough, the need to interrupt power from the engine to provide smooth operation is handled in very different ways. A manual transmission relies on a clutch mounted to the flywheel. As long as it’s engaged, the transmission’s input shaft is supplied with power from the engine. Depressing the clutch disengages it and interrupts the power, allowing you to shift gears while driving and place the car in neutral without damaging the internals.
Automatics don’t use a clutch to interrupt power from the engine. Instead, they work with a fluid coupling and stator system allowing the engine to spin independently of the system until a high enough engine speed is met to drive the transmission. This coupling is known as a torque converter which is mounted to a flexplate. The most important function, however, is its stall phase or the reason you can leave an automatic transmission in drive when stopped in traffic — unlike a manual which must be returned to neutral or the clutch disengaged.
And then there’s the exotic beast known as the dual-clutch.
A dual-clutch transmission is something of a hybrid between an automatic and manual transmission. It breaks all of the rules by combining everything there is to love about either configuration into a single unit. The idea is that they feature a gearbox similar to that of a traditional manual transmission, being able to shift with paddles or a sequential shifter, but having an automatic’s PRNDL functionality, too, thanks to the inclusion of a dual-clutch module.
This module is attached to shift forks, and as the clutches engage or disengage, these forks move to change the gear of the transmission. Of course, engine speed and throttle input influence these changes, and the driver simply needs to set the transmission in drive for the process to begin. It’s mind-bendingly complex when you get into the oily bits beneath your floorboard, but this technology has allowed manufacturers to produce some of the quickest shifting transmissions ever seen coming off the assembly line.
What Are the Pros and Cons of a Manual Transmission?
Now that we have the basics out of the way, we can talk about what’s important to you. The big pro about a manual transmission is that you immediately build clout as you are, in fact, working with three pedals and not goofing around with two like the rest of the yuppies out there.
Joking! There are a lot of things to like about manual transmissions, though I’ll start with something of a broad stroke: manual transmissions are typically stronger and require less maintenance than automatics. Keeping them up usually only entails changing the fluid every 30,000-60,000 miles or so. And newer manuals can go even longer than that, though to be fair, newer automatics can go fairly long as well.
On top of that, there’s less parasitic loss between the engine’s power and what’s put to the road as it takes less for a manual to transmit such power than an automatic. That, paired with the typically lower weight of a manual, is where the idea that manual cars are quicker. Less parasitic loss means that more power can make it to the ground, and less is required to get the car moving because of the lower weight.
Furthermore, the ability to select the gear that best matches your driving speed gives you more control over the vehicle and, as every enthusiast will tell you at length, makes for a much more engaging driving experience.
Lastly, there just so happens to be a big maintenance advantage, too, as when keeping your RPMs at a specific point is required by a host of different projects, it’s far easier to do in a manual.
Are we saying that all cars need manual transmissions because they are universally superior? Not a chance. First of all, not all manual transmissions are built to such standards with the build quality, materials used, and design of each specific unit determining how much abuse a transmission will take.
Let’s also not forget that there’s an inherent speed drop between gears. The longer it takes you to shift, the greater that speed drop becomes. Not only that, but an automatic can out shift any normal person, effectively eliminating this issue.
What Are the Pros and Cons of an Automatic Transmission?
Ok, so by highlighting the pros of a manual transmission, we took a few shots at automatics. And indeed, many automatics aren’t as strong as manuals and they use up more power, generally provide less control over gear selection, require more maintenance, and weigh a little more. Let’s also not leave out that automatics require cooling, which adds even more complexity to the system. So there, we got it out of the way.
Despite everything that’s “wrong” with an automatic, there’s still too much to love to simply write them off.
All of those “yuppies” we picked on for rocking two pedals instead of three shouldn’t be laughed at. The simple fact is that automatics will out-shift manuals any day of the week. They change gears more quickly, practically eliminating any sign of power loss between gears. So, despite taking a little more power to turn, they waste none of what’s on the table. This is exactly why automatic transmissions are favorable in performance cars you’d find on the drag strip or track where every last millisecond matters.
Furthermore, the lack of user input makes for a far more comfortable driving experience. It might not be as engaging or exciting as working a stick, but there’s no denying that it makes it possible for far more people to get behind the wheel of the car. Ultimately, driving a manual is a lot more demanding of the driver, and that’s not something everyone is willing, or even able, to deal with.
What on Earth Is a CVT?
To make things clear, manual and automatic transmissions offer a set amount of drive gears and a set selection of gear ratios to work with. In the case of a 4-speed transmission, you only have four gears to use to get optimal power out of your engine at given speeds. One way to provide better efficiency and performance is to add more gears by moving up to something like a 6-speed, 8-speed, or so on, as it gives you more gears to better match different driving scenarios.
Or, you can take the idea of a set number of gears and ditch it altogether. That’s exactly what a CVT, or continuously variable transmission, does. It might sound like smoke and mirrors, but the concept is ingeniously simple.
The idea of a gear ratio is to keep an engine in peak efficiency to increase or maintain wheel speed. At some point, the engine will rev out of that peak range, and another gear ratio is needed to bring that engine back to peak efficiency based on wheel speed. No matter how plentiful, set gear ratios still have gaps between gear sets that must be dealt with.
A CVT works by keeping your engine in that peak range without ever shifting gears despite variable wheel speed. That means the engine never leaves the RPM range that offers peak performance and maximum efficiency. And because there is no shifting, the ride is uncannily smooth.
The way this works is by placing a steel or composite band between two pulleys. Each of these pulleys exists in two pieces with tapered faces that can move closer and further from one another. As the distance between the two increases or decreases on these pulleys, the position of the belt changes, and the gear ratio effectively changes with it. Think of it as a sprocket and chain assembly, as you’d find on a bicycle or motorcycle. Only the sprockets can shrink and grow on command to provide optimal performance regardless of wheel speed.
Wild, we know.
So What Does It Mean When a Transmission Is Sequential?
Transmissions, in general, are captivating pieces of equipment. Each variation is an engineering marvel that we could drool over for days on end. I suspect that none, however, will capture your heart like a sequential gearbox.
To better understand them, let’s pedal back to a traditional manual transmission. A manual transmission works by sliding collars over the gears riding on the output shaft with the aid of a clutch fork. Traditionally, the forks are connected to shift rods that are pressed into place as you run through the H-pattern of the gearbox.
A sequential gearbox operates much the same but with some slight variations. Rather than the motion of the forks being dependent on the H-pattern, they are operated by rotating a rod and drums that only require you to press the shift lever forward or back to change gears — unless the car has paddle shifters, then it’s a little different.
In some cases, you can modify a traditional manual transmission to accept a sequential style-shifting assembly. Though, the shifting mechanism is not the only difference between the two.
Double clutching is something we all squirm over every time we watch the Fast and the Furious, but it is a real thing. This technique is used on non-synchronized transmissions because it allows the gears inside to mesh smoothly. Modern manuals feature synchronizers, which are tapered gears that exist between the sliding collar and gears on the output shaft, making it possible for the speeds to synchronize before meshing, creating smoother shifts without the need to match gear speeds manually.
In a sequential transmission, you don’t have this. Instead, there are dog teeth connectors between gears that can simply slam together. The benefit is that this makes for a much faster shift in which you don’t need to lift off the throttle, ultimately cutting power drops. However, because this action is so violent, it means sequential transmissions need to have the dog connectors replaced frequently.
Can’t I Manually Shift My Automatic Anyway?
It’s no secret that some automatic transmission let you shift gears manually. Most cars will at least have the option to put the transmission in low gear, if not a mode allowing you to toggle gears on command, on the car’s gear selector or steering wheel paddles. Still, it’s not the same as a manual. On top of that, some automatic transmissions may even proceed to shift on their own despite being set to run in one gear only, this is in order to protect and preserve the transmission’s internals.
Now, that doesn’t mean you can’t modify automatic transmission valve bodies and other components to function solely by manually changing gears. In fact, that’s a common practice as being able to keep a transmission in a particular gear until the engine can no longer provide optimal power then shift to the next gear at lightning speeds is ideal. You’ll see this type of modification done to any number of performance applications with automatic transmissions. I personally used to own an ‘80s Cutlass with a setup like this and I have to say, it was beyond awesome — aside from the whole grandpa car thing.
The point is that you may have the option to select gears on your own, you might even have the ability to run through gears as you see fit. However, this function and its effectiveness is dependent on the transmission you’re working with from the get-go or if you can modify it to do so.
How Does Gearing Affect the Driving Experience?
We talked briefly about transmission gears and speeds, but we still need to dive a little deeper before wrapping things up.
Here’s the thing, transmission gearing and gear count are definitive to vehicle performance. As we mentioned in our talk about CVT’s, the idea is to keep an engine running at peak efficiency. That peak efficiency exists within a set RPM range, and the best acceleration will occur within that window. The thing is that as wheel speed increases, the engine needs to spin faster and faster to keep up. Eventually, the speeds will surpass that window, and it’s time to shift gears.
Gearing refers to the gear ratios in the transmission. Your first gear ratio might look something like 2.66:1 or 3.10:1, meaning the engine needs to spin 2.66 or 3.10 times to spin the output shaft once. Lower numbers, numerically, associate higher wheel speeds with lower RPM values, whereas the opposite is true with bigger gear ratios.
Things get funny when you start shifting gears, though. Every time you shift up, you’re moving to a lower gear ratio numerically, meaning a slower engine speed is necessary to achieve the speed you’re driving at. That means your RPMs are going to drop, and when they do, they might drop right out of the peak efficiency range we talked about earlier.
Typically, a transmission with fewer gears, or speeds, has a bigger gap between gear ratio sizes, meaning your RPMS will drop quite a bit with each gear shift. By introducing more gears with closer ratios, that gap becomes less significant, making it easier to keep the engine within that peak range to promote optimal performance.
That also means that the transmission will need to shift more frequently, though. This can be a problem in itself, and therefore more gears isn’t a universally superior option. In fact, a lot of drag race applications still run with 2-speed or even direct-drive transmissions. In short, proper gearing is heavily dependent on many factors, including engine performance, driving styles, and other vehicle specs that need to be factored into the selection.
The Car Bibles Transmission Glossary
Welcome to Bible School!
“Speed” — Gears
Refers to the number of forward drive gears a transmission has to offer. If a transmission is a 6-speed, it has six forward drive gears. Keep in mind that the numerical value attached to this descriptor does not include reverse.
A mechanical component that clamps onto a clutch disc mating the engine flywheel to the transmissions input shaft. It remains engaged at all times until the clutch pedal is depressed, interrupting power from the engine, making it possible to shift gears or return the transmission to neutral.
The defining component of a dual-clutch transmission. This is a module that works to provide automatic shifting capabilities to transmission that closely resembles a traditional manual variant.
A component that places a fluid coupling and stator between an engine and automatic transmission. Like a clutch, it can interrupt power from the engine eliminating the need to place the transmission in neutral at stops but also aids other crucial functions to an automatic transmission.
Flywheel and Flex Plate
Two different mechanical components that play similar roles. A flywheel is used to mate a clutch to the engine whereas a flex plate plays a similar role for the torque converter. While similar, a flex plate is actually designed to flex, which prevents the torque converter from bottoming out during operation whereas a flywheel is a solid piece that is necessary for clutch engagement.
Automatic transmissions rely on fluid pressure to activate clutch packs. The valve body consists of various passages that can direct the fluid to engage specific clutches.
Paddle shifters can be attached to sequential, automatic, and dual clutch transmissions. These means of shifting gears on command and are typically mounted to the steering wheel removing the need to ever relinquish your grip.
The Car Bibles Questionnaire
Car Bibles answers all your burning questions!
Q. Does manually shifting an automatic hurt the transmission?
A. No. Keep in mind that automatic transmissions are heavily reliant on fluid pressure for operation. That means shifting gears on your own does not inspire wear and tear on the moving parts.
Q. Does change transmission fluid help shifting?
A. It can. If the fluid in the transmission is old and dirty, it will absolutely contribute to shifting problems. That is why you must inspect and change transmission fluid or oil in some manuals to ensure it can operate optimally.
Q. Does holding the clutch down damage it?
A. Yes. Holding, or riding, the clutch can cause excessive wear on the assembly. If the pedal isn’t depressed all of the way, it will allow the flywheel and pressure plate to spin independently of the clutch disc causing excessive wear. However, keeping the pedal depressed all of the way instead of partway will prevent this from occurring.
Q. Do you need a clutch for a sequential?
A. Only to start moving. Once the car is rolling, there is no need to use a clutch, which part of how lightning-fast shifts are achievable with this type of transmission. Again, this also comes with the drawback of excessive wear on internal components.
Q. What’s faster, dual-clutch or sequential?
A. While dual-clutch transmissions might have an edge on physical gear changes, weight holds them back to a significant degree. Furthermore, they are also likely to absorb more engine power. Seeing as the sequential transmission is lighter and puts more power to the ground, you can say that they are the faster of the two.
Yeah. Your eyes are bloodshot and you’re tired of reading. That’s why we’ve thrown this video in the mix for you. Not only that, but transmissions are complicated and it’s best to get your information from multiple sources. The video below gives a full lecture about how planetary gearsets work.
Auto tech discussions in short form leave out a lot of the variations and nuances that make each part unique. No matter how hard we try, there’s no way for us to cover every last detail in a single article. That’s why we strongly recommend those who want to learn everything they can about transmissions and other automotive components pick up books such as How to Rebuild & Modify High-Performance Manual Transmissions , Turbo Hydra-Matic 350 Handbook, Design of Mechanical Power Transmissions, Automotive Continuously Variable Transmissions, and Auto Fundamentals.