This glossary of automotive terms was created to serve the Tinker community of DIYers. Whether you're a seasoned mechanic or just starting to explore the world of auto DIY, your most valuable tool is knowledge. Before you pick up that wrench, you need a resource to understand all the technical jargon you’ll encounter in automotive repair manuals, forums and discussions.
With this Tinker glossary you will learn to speak the cryptic language of car fluidly. It provides easy-to-understand explanation of automotive terms and concepts, ranging from core engine components to advanced performance features.
All-Season Tires
Most new cars and trucks sold are equipped with all-season tires. All-season tires are the most common type of tire because they feature tread patterns and materials that perform well in a variety of road conditions, including dry, wet and light snow. They blend the characteristics of summer and winter tires, making them a practical choice for drivers in moderate climates who prefer not to switch tires seasonally. However, they do not offer the same level of ice and snow traction as winter tires, nor do they provide the optimal grip of summer tires in warm and dry conditions.
Alternator
A crucial component of a vehicle's charging system, an alternator converts mechanical energy from the engine into electrical energy when the engine is running. Usually driven by a belt connected to the engine’s crankshaft, the alternator spins to generate alternating current (AC), which it then converts into direct current (DC) electricity used to charge the car battery and power the vehicle’s lights, radio and other electrical systems.
Bell Housing
A bell housing is the metal casing that connects the vehicle’s transmission to the engine block. It also surrounds and protects the flywheel and clutch if your car is equipped with a manual transmission or the flexplate and torque converter if you have an automatic transmission. Its name comes from its bell-like shape.
Cam Duration
Measured in degrees of crankshaft rotation, cam duration is the amount of time an intake or exhaust valve is kept open by the camshaft. This duration is determined by the shape and size of the camshaft lobes.
A longer duration means the valves are open for a greater portion of the engine cycle. That allows more air and fuel into the combustion chamber and more exhaust gases out, which can increase engine power, especially at higher rpm. On the downside, longer cam duration can reduce performance at lower speeds, as it may cause less efficient filling of the combustion chamber, impacting idle stability and overall smoothness.
Cam Lift
Cam lift refers to the height a camshaft's lobe lifts the valve off its seat. This directly influences the engine's airflow and, consequently, its performance. The greater the cam lift the more air and fuel enter the combustion chamber and the more exhaust gases exit, increasing the engine's output. However, this typically comes at the cost of engine efficiency and smoothness.
Camber
Measured in degrees, Camber is a wheel alignment term (See also Caster and Toe.) that refers to the tilt of the wheels when viewed from the front or rear of the vehicle. If the top of the wheel tilts inward, it's negative camber. This improves grip by maximizing the contact patch of the tire during cornering. On the other hand, positive camber, when the tops of the wheel tilts out, improves vehicle stability on uneven surfaces, making it a popular choice for off-roaders. However, too much camber in either direction can lead to uneven tire wear and adversely affect straight-line stability.
Camshaft
A camshaft is a rod with many oblong lobes or 'cams' positioned along its length. As the camshaft rotates, these lobes open and close the engine's valves, letting air and fuel into the engine and exhaust gasses out. In most engines, the camshaft is driven by the crankshaft, either through a timing belt or chain.
Most modern engines have a single overhead cam (SOHC) or a dual overhead cam (DOHC) design. DOHC allows for more valves per cylinder and potentially higher performance. Another notable variant is the pushrod engine, in which the camshaft is located inside the engine block and operates the valves using a series of pushrods and rocker arms.
Carburetor
Patented in 1888 by Karl Benz, one of the founders of Mercedes-Benz, the carburetor is a mechanical device that controls the air-fuel mixture entering the engine based on engine load, speed and operating conditions. By atomizing the fuel (breaks it up into tiny droplets), carburetors mix it with air to create the correct air/fuel ratio for combustion. This mixture is then delivered to the engine’s cylinders where it’s ignited by the spark plug. Ultimately replaced by fuel injection systems, the carburetor was used by all car manufacturers up until the late 1980s.
Caster
Measured in degrees, Caster is a wheel alignment term (See also Camber and Toe.) that refers to the angle of the steering axis (relative to the vertical axis of the wheels) when viewed from the side of the vehicle. Most cars have positive caster, which means the steering axis is tilted toward the rear of the vehicle. This improves stability by encouraging the steering wheel to return to center after turning. It also increases tire lean in corners, enhancing grip. However, too much caster can make the steering feel heavy and wear out steering components prematurely due to increased load.
Catalytic Converter
A catalytic converter is a key component of a vehicle’s emissions control system located in the exhaust system between the engine and the muffler. Inside the “cat” is a honeycomb structure coated with a catalyst, typically made of precious metals including platinum, palladium and rhodium. When toxic waste gases pass through it, a chemical reaction occurs that changes them into less harmful substances before they exit the tailpipe.
A catalytic converter tackles three primary pollutants: carbon monoxide (a poisonous gas), hydrocarbons (unburned fuel that contributes to smog) and nitrogen oxides (contribute to smog and acid rain). Through a process known as catalytic conversion, carbon monoxide becomes carbon dioxide, hydrocarbons turn into water and carbon dioxide, and nitrogen oxides become nitrogen and oxygen.
Combustion Chamber
The combustion chamber is the space in the engine between the top of the piston and the cylinder head where the air-fuel mixture is burned to produce power. The chamber’s size and shape are critical aspects of engine design, directly affecting how well the fuel-air mixture burns and therefore also the engine's power, efficiency and emissions. Common types and designs include hemispherical (or 'hemi'), pentroof, wedge and bathtub, which describe the physical shape of each type.
Compression Ratio
The compression ratio of an engine is a measure of how much a piston squeezes the air-fuel mixture in the cylinder before it ignites. It's calculated by dividing the total volume of the cylinder when the piston is at the bottom of its stroke, by the volume of the combustion chamber when the piston is at the top. For example, if the total volume is 10 times the volume at the top of the stroke, the engine has a 10:1 compression ratio.
Contact Patch
A tire’s contact patch is the few inches of rubber actually touching the ground at any given time. Sometimes called a tire’s footprint, it ultimately controls your car’s acceleration, braking and turning.
Larger tires with the biggest contact patch will add traction, but they also add noise and increase rolling resistance, which can negatively affect fuel economy. Conversely, smaller tires have lower rolling resistance and offer better fuel economy, but offer less performance.
A tire’s contact patch also can vary depending on tire pressure. Under-inflation may increase the contact patch but decreases fuel mileage as well as handling and braking response. This is one of the reasons off-roaders “air-down” their tires before tackling a rocky trail. An over-inflated tire will bulge out at its top and reduce the footprint at the tire’s center.
Continuously Variable Transmission (CVT)
A continuously variable transmission, or CVT, is a common type of automatic transmission found in many different vehicles from a long list of brands including the popular Honda CR-V and Acura Integra. Unlike a conventional automatic with a fixed set of gears, a CVT uses a pair of cone-shaped pulleys connected by a belt or chain. The setup creates an infinitely adjustable set of ratios, which vary as needed to keep the engine in its most efficient range of operation, improving fuel economy. Some find their driving characteristics unusual because in some application they lack the distinct shifts of a conventional gearbox.
Detonation (Engine Knock)
Detonation, commonly referred to as engine knock, is a harmful condition that happens when the air-fuel mixture in the cylinders ignites prematurely, causing a sharp knocking or pinging noise. This sends high-pressure shockwaves through the cylinder, which can damage vital engine components over time.
Common causes for detonation include excessive heat (e.g. from an overloaded engine), stale or low-octane fuel, improper ignition timing or carbon deposits in the combustion chamber.
Many modern engines are equipped with knock sensors that detect the onset of detonation and adjust the ignition timing to prevent it.
Final Drive Ratio
The final drive ratio is the gear ratio within the differential, which connects the vehicle’s driveshaft to the axles and wheels. A higher numerical final drive ratio boosts acceleration by allowing more engine revolutions per wheel rotation, while a lower ratio delivers better efficiency at speed by keeping the engine rpm lower. Specifically, it's the number of teeth on the differential's ring gear divided by the number of teeth on its pinion gear, expressed as a ratio. For example, a differential with 41 ring gear teeth and 11 pinion gear teeth would produce a final drive ratio of 3.72:1, which means that it takes 3.72 revolutions of the driveshaft to spin the wheels one full turn.
Gross Vehicle Weight Rating (GVWR)
Every car and truck has a gross vehicle weight rating (GVWR). The GVWR is the maximum weight at which a vehicle is designed by its manufacturer to safely operate. This includes the weight of the vehicle itself plus the additional weight of fuel, other fluids, passengers and cargo. Overloading a vehicle with too much weight strains all its mechanical elements and can lead to dangerous instability. GVWR is different than the vehicle’s curb weight, which is the weight of the actual vehicle. As one example, a 2023 Honda Civic Type R has a curb weight of 3,188 lbs. and a GVWR of 3,968 lbs.
Gross Combined Vehicle Weight Rating (GCVWR)
Overloading a vehicle with too much weight can cause dangerous instability on the road, so every car and truck has a gross combined vehicle weight rating (GCVWR or GCWR). The GCVWR is the weight of a vehicle plus the maximum weight of a trailer it is safely capable of towing. For example, a 2023 Toyota Tacoma TRD Pro has a curb weight of 4,550 lbs. and a GCVWR of 11,360 lbs.
Horsepower
Horsepower is a unit of measure that quantifies power output. Originating in the 18th century to compare the power of steam engines to that of horses, horsepower is defined as the ability to move 550 pounds one foot in one second. In automotive terms, it translates to how efficiently and effectively an engine can convert fuel into motion. The higher the horsepower, the greater the vehicle's potential for speed and acceleration.
Horsepower is closely related to torque, but they are distinct concepts. While horsepower measures the total power output of an engine over time, torque represents the immediate force available. In other words, torque determines how quickly a vehicle can start moving or pull a load, while horsepower reflects how fast and efficiently the vehicle can perform once in motion.
Intercooler
An intercooler is a component used by forced-induction engines (engines with a turbocharger or supercharger) to cool the air compressed by the turbocharger or supercharger before it enters the combustion chamber. This increases the air's density and oxygen content, enhancing combustion and increasing the engine's output and efficiency.
There are two main types of intercoolers: air-to-air and air-to-water. Air-to-air intercoolers use the ambient air passing by the vehicle to cool the compressed air from the turbo or supercharger. They are generally less effective than air-to-water intercoolers, which use a liquid coolant flowing through a radiator.
Limited Slip Differential (LSD)
A differential allows the driven wheels of a vehicle to turn at different speeds. This is necessary because as a car turns, the wheel on the inside of the curve will travel a shorter distance than the outside wheel. A limited slip differential limits the difference in wheel speed so some of the engine’s power is going to the wheel or wheels that have traction. General Motors' trademarked name for limited slip differentials is “Positraction”. Remember Mona Lisa Vito in “My Cousin Vinnie”? Most high-performance vehicles use an LSD to maximize traction when accelerating off the line and out of corners.
Locking Differential
Unlike a standard differential, which always allows the drive wheels to rotate at different speeds, a locking differential can force the wheels to rotate at the same speed. This maximizes grip in challenging or slippery conditions, such as off-road driving, by preventing the wheel with less traction from spinning freely.
Some locking differentials engage automatically when they detect a significant speed difference between the two drive wheels; others require the driver to activate them, typically via a switch on the dashboard or a lever near the transmission shifter.
NOTE: locking differentials should only be engaged in low-grip conditions. In normal driving, each drive wheel needs to rotate independently to navigate corners without binding.
Octane Rating
Octane rating refers to a fuel's ability to resist premature ignition during compression, which can result in potentially harmful detonation, often called 'knocking' or 'pinging.' It's determined by comparing the fuel's performance to a mixture of iso-octane (which resists knocking well) and heptane (which knocks easily). A fuel with an octane rating of 87 performs like a mixture containing 87% iso-octane and 13% heptane. The higher the rating, the more stable the fuel is during combustion.
Roadside Assistance
Roadside assistance is a service that helps motorists get back on the road quickly when they encounter breakdowns or other problems while driving. It's typically offered through auto clubs, insurance companies, or as part of a vehicle manufacturer's warranty program. Coverage usually includes access to a 24/7 call center that dispatches help for a range of needs, such as towing, jump starts, flat tire changes, lock-out assistance and fuel delivery.
Supercharger
A supercharger (or 'blower') is a type of forced induction system that forces more air into the combustion chamber to allow more fuel to be burned, radically increasing the efficiency and power output of an internal combustion engine. A supercharger’s output is called boost and is measured in pounds per square inch (PSI). Boost usually ranges from as little as 5 psi to 25 psi and more in high-performance applications. The more boost the more power the engine makes.
Unlike turbochargers, which are usually powered by exhaust gases, superchargers are mechanically driven, typically by a belt connected to the crankshaft. This direct connection provides immediate throttle response, eliminating the lag associated with turbochargers. Common types of superchargers include roots, twin-screw and centrifugal.
Strut
A strut is a type of shock absorber that is integrated with a coil spring to form a structural component in a vehicle's suspension. Struts are typically used on the front wheels but can also be found on the rear in some designs. The most common type by far is the MacPherson strut, which was designed in the late 1940s by engineer Earle Steele MacPherson. MacPherson struts are used extensively in front-wheel-drive cars because their compact design allows ample space for the drive shafts to connect to the wheels.
The Chapman strut is a similar design implemented at the rear and is typically found in classic sports cars and race cars. It is named for Lotus founder Colin Chapman, who first used the design on his Formula One race cars in the late 1950s.
Toe
Toe is a wheel alignment term (See also Camber and Caster.) that refers to the direction the front of the wheel is pointing when viewed from above. Toe is most often measured in degrees but is sometimes reported in inches or millimeters. Toe-in is when the front of the tire points inward. If it points outward, it's toe-out. Toe-in improves straight-line stability and reduces oversteer. Toe-out is generally less stable but offers sharper turn-in, which can be beneficial in performance driving. Too much toe in either direction causes the tire to scrub against the road surface, leading to faster wear.
Top Dead Center
When a piston in an engine reaches the top of its stroke, it is at top dead center. Similarly, bottom dead center is when a piston reaches the bottom of its stroke. Regardless of how fast an engine is revving, each one of its pistons slows as they approach top or bottom dead center. Once they reach either end, they have to reverse direction. Those two points at the extreme ends of a piston’s travel, where its velocity reaches exactly zero, are defined as top and bottom dead center. Top dead center is found by manually rotating the engine by hand and referring to timing marks on the crankshaft’s pulley.
Torque
Torque is a measure of rotational force, typically measured in pound-feet (lb-ft), is closely related to horsepower. Engines with higher torque generate greater pulling power, which translates into quicker acceleration and a higher capacity for tasks like towing or climbing hills. While horsepower is a measure of total engine output, torque reflects immediate pulling power at a given moment.
Torque Converter
A torque converter is a device used in automatic transmissions to transfer rotating power from the engine to the transmission. Unlike a clutch, which connects the engine and transmission directly, a torque converter uses a fluid coupling. This allows the engine to spin independently of the gearbox at idle, which is crucial for allowing the vehicle to come to a complete stop without stalling the engine.
The torque converter has three main components. The pump (or impeller) is connected to the engine's crankshaft and spins the transmission fluid. This creates centrifugal force, rotating the turbine, which is connected to the transmission. Located between them is the stator, which redirects fluid from the turbine back to the pump.
Most modern torque converters also include a lock-up clutch. It engages at speed, creating a direct connection between the engine and transmission to improve efficiency.
Transaxle
A transaxle combines the functions of the transmission, differential and axle into a single unit. Integrating these components saves space and reduces weight, contributing to improved vehicle packaging and efficiency. Most front-wheel-drive cars use a transaxle to save space under the hood. Transaxles are also advantageous in rear-wheel drive performance cars, where engineers use them to achieve better weight balance across the chassis, improving handling.
Integrating multiple drivetrain components within one assembly does have some downsides, however, including higher manufacturing and repair costs. Many sports cars are equipped with transaxles, including the Porsche 928, Ferrari F12 Berlinetta and the last four generations of the Chevy Corvette.
Transfer Case
A transfer case is a key component in four-wheel drive (4WD) and all-wheel drive (AWD) vehicles. It distributes power from the transmission to the front and rear axles, enabling all four wheels to be driven.
In 4WD systems, the transfer case allows the driver to select between two-wheel drive (usually the rear wheels) for regular driving conditions and four-wheel drive for off-road or slippery conditions. Some 4WD systems also offer high- and low-range gearing, with the low range providing increased torque at lower speeds for better off-road performance.
In AWD systems, the transfer case varies the power split between the front and rear axles according to demand. This is typically done automatically and in conjunction with the vehicle's stability control system to enhance traction and handling, especially in adverse driving conditions.
Turbocharger
A turbocharger is a type of forced induction system that forces more air into the combustion chamber to allow more fuel to be burned, radically increasing the efficiency and power output of an internal combustion engine. A turbocharger’s output is called boost and is measured in pounds per square inch (PSI). Boost usually ranges from as little as 5 psi to 25 psi and more in high-performance applications. The more boost the more power the engine makes.
Unlike superchargers, which are mechanically driven, typically by a belt connected to the crankshaft, a turbo is driven by the flow of exhaust gases at a very high rate, as fast as 250,000 rpm in some cases.
Turbo Lag
Turbo lag is the delay that can occur between pressing the accelerator and feeling an increase in power from the effects of the turbocharger. This occurs because turbochargers are driven by the engine’s exhaust gases, and it takes a moment for them to spool up to a speed that provides significant boost.
Modern cars employ various technologies to reduce turbo lag, such as using smaller, lighter turbochargers that spool up more quickly or employing twin-turbo setups where a small turbo provides boost at low rpm and a larger one takes over at higher engine speeds.
Valve Float
Valve float is a detrimental condition that occurs when the engine's valves fail to close properly at high rpm. This primarily happens when the valve springs cannot keep pace with the rapid movements of the camshaft, causing the valves to 'float' or stay open longer than intended. The consequences of valve float can include loss of performance and even engine damage.
Valvetrain
The valvetrain controls the timing and operation of the engine's valves, which allow air and fuel into the combustion chamber and then let exhaust gases out after combustion. Its purpose is to ensure that the valves open and close at the correct times for optimal performance.
Central to this system is the camshaft, equipped with lobes (or 'cams') that dictate the valves' opening and closing times. A timing mechanism, usually a belt or chain, synchronizes the camshaft with the crankshaft, ensuring the valves open and close in time with the movement of the pistons.
In overhead cam (OHC) designs, the camshaft is located directly above the valves in the cylinder head, allowing precise control, especially in dual overhead cam (DOHC) setups where separate camshafts manage the intake and exhaust valves. In contrast, pushrod engines feature a camshaft in the engine block, and its cam lobes push lifters, which in turn move pushrods to actuate rocker arms that open the valves.
This glossary was created to serve the Tinker community and we will add to it from time to time, so if there is an additional term you’d like explained, please send us a suggestion: support@tinkerdiy.com.
Wheel Horsepower
Different than crank horsepower, a vehicle’s wheel horsepower is how much of its engine’s output is actually reaching the road. Measured on a chassis dyno, “wheel horsepower” and “torque at the wheels” are the engine’s power minus the drivetrain loss – the cumulative power consumed by the transmission, driveshaft, differential, axles, wheel bearings, CV joints and tires. A general rule of thumb is about 10-15% of the factory horsepower and torque ratings is consumed by drivetrain loss. Older cars, especially older cars with automatic transmissions, will exhibit closer to 20%.
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