Jump to: Tuning basics · Power figures · Fuel · Turbo and air · Exhaust and the law · Fuel system · Engine and drivetrain
Tuning basics
Stage 1, 2, 3, 4
Tuners' shorthand for how far a car is modified. Stage 1 is software only; Stage 2 adds breathing parts such as a downpipe, intake and intercooler; Stage 3 is a bigger turbo; Stage 4 is a built engine. Each tuner draws the lines a little differently, so always read what's included. Which stage should you start with?
Remap, flash tune
New software written into the engine computer that changes boost, fuel and ignition timing. It's the whole of Stage 1 and part of every stage after it. Tuning software explained
Plug-in box, piggyback
A module that sits between the car's sensors and the engine computer and changes what it reads, instead of rewriting its software. Easy to remove, but usually adds less than a remap.
ECU, engine computer
The computer that runs the engine. Tuners read and write it through the diagnostic (OBD) port, or on the bench with the unit taken out when it is locked.
TCU, gearbox tune
Software for the automatic gearbox's own computer. On most tuned cars it raises the clutch torque limits and shift speed so the gearbox can take the extra torque.
Box code, software version
The part and software numbers of your engine computer. A tuner can only flash files it has for that exact code, so ask before you pay.
Power figures
hp, PS, kW
Three units for the same thing. PS is metric horsepower and is about 1.4% smaller than hp; 1 kW is about 1.36 PS. European makers quote PS or kW, American ones hp. hp, PS and kW converter
Crank power vs wheel power (whp)
Crank power is measured at the engine; wheel power is what reaches the road after the gearbox and driveshafts, so it's always lower. Tuners' figures can be either, and you can't compare one with the other directly. Crank to wheel calculator
Torque (Nm, lb-ft)
The engine's twisting force, which is what you feel as a shove when you accelerate. 1 lb-ft is about 1.36 Nm. More torque is also what stresses clutches and gearboxes.
Dyno
A machine that measures a car's power, either with the wheels on rollers or with hubs bolted to the axles. Different dynos read differently, so compare gains on the same one.
Fuel
Octane: RON and AKI
How well fuel resists knock. Europe and Israel use RON; the US pump number (AKI) is about 4 points lower, so US 91 is roughly 95 RON and US 93 roughly 98 RON. Israeli pumps sell 95 and 98. 95 vs 98 fuel cost calculator
E85, race fuel
High-ethanol and racing fuels that allow more boost and timing. Many headline figures use them, but they aren't sold at Israeli pumps. Tuning a car in Israel
Knock
Fuel igniting at the wrong moment, which can damage an engine. The engine computer pulls ignition timing when it senses knock, which is why a hot day or poor fuel costs power.
Turbo and air
Hybrid turbo, big turbo
A hybrid keeps the factory turbo's housing and mounts but has bigger or better wheels inside, so it fits in place. A big-turbo kit replaces it with a larger unit and needs its own pipes and tuning.
Boost (psi, bar)
The pressure the turbo pushes into the engine above the outside air. 1 bar is about 14.5 psi.
Intercooler, heat soak
A radiator that cools the air after the turbo. When it gets hot on repeated pulls (heat soak), the engine pulls power to protect itself; a bigger intercooler is usually the first Stage 2 part. Do you need a bigger intercooler?
Intake
The air filter and pipe in front of the turbo. On most cars it adds sound and a little power, and helps more once the turbo is bigger.
Diverter valve, blow-off valve
A valve that releases boost pressure when you lift off the throttle. Stock valves on some cars leak at higher boost, so tuners often replace them.
Exhaust and the law
Downpipe
The first section of exhaust after the turbo, where the main catalyst usually sits. A bigger one lets the turbo breathe, which is why many Stage 2 files require it.
Catalyst, sports cat
The part that cleans the exhaust gases. A "sports" or high-flow cat (often 200-cell) flows more than the stock one; removing the cat altogether means the car fails inspection.
GPF, OPF (petrol particulate filter)
A soot filter in the exhaust of most European petrol cars from about 2018. Many Stage 2 figures need it removed, which isn't road-legal.
Cat-back
The exhaust from the catalyst back to the tailpipes. It changes the sound far more than the power.
ECE approval, E-mark, TÜV
European type approval for aftermarket parts, shown by an E-mark or a TÜV certificate. It's the paperwork that makes a part road-legal in many countries. Keeping a tuned car legal and insured
Fuel system
HPFP, high-pressure fuel pump
The engine-driven pump that feeds direct-injection engines. At higher power it can't keep up, so Stage 2+ and turbo upgrades often need a bigger one.
LPFP, low-pressure fuel pump
The pump in the tank that feeds the high-pressure pump. Big-turbo builds usually upgrade it too.
Injectors, port injection (MPI)
Injectors spray fuel into the engine. Bigger ones, or a second set in the intake ports, supply the extra fuel a big turbo needs.
Cam follower
A small cup between the camshaft and the high-pressure fuel pump on some VW and Audi engines. It wears, and a worn one can destroy the pump, so check it before tuning. The Golf GTI Mk5 guide
Engine and drivetrain
Forged internals, built engine
Stronger connecting rods and pistons that replace the factory cast parts, so the engine can take the power of a big turbo.
Timing chain tensioner
The part that keeps the timing chain tight. Early versions on some engines fail, letting the chain jump and damage the valves, so it's a common check before tuning. The Golf GTI Mk6 guide
Carbon build-up, catch can
Direct-injection engines collect oil deposits on their intake valves over time. A catch can traps some of the oil before it gets there.
Clutch, DSG
The clutch is often the first thing a tune overwhelms. A DSG is a dual-clutch automatic; its clutches have torque limits set in software, which is why tuned DSG cars need a gearbox tune.
Engine family
Many cars share one engine and most of its tuning parts. Our engine pages put every car with the same engine side by side. Tuning by engine