Tuning glossary

58 terms, defined in plain English — and each one linked to the calculator or guide that puts it to work. If a word on this site is doing work you cannot see, it is explained here.

A

AFR air-fuel ratio#
The mass of air per mass of fuel entering the engine. It is fuel-specific: 14.7:1 is chemically correct for petrol but about 9.8:1 for E85, so an AFR number only means something once you say which fuel it refers to. Tuners generally prefer lambda for that reason. AFR ↔ Lambda →
AKI anti-knock index#
The octane number on US pumps, the average of the RON and MON ratings — which is why US "91" and European "95" describe similar fuel. Always check which scale a figure uses before comparing. E85 explained →
Aspect ratio tyre profile#
The middle number in a tyre size: sidewall height as a percentage of section width. On a 225/45 R17 the sidewall is 45% of 225 mm, about 101 mm. It is a ratio, not a fixed height, so the same profile number gives a taller sidewall on a wider tyre. How to read a tire size →

B

Backspacing#
The distance from a wheel's mounting face to its inner rim edge, measured in inches. It describes the same geometry as offset but from the other side, so it changes when width changes even if offset does not — which is why offset is the more portable figure. Wheel Offset →
Blow-off valve BOV, dump valve#
A valve that vents intake charge to atmosphere when the throttle closes, stopping pressure backing up into the compressor. It protects against compressor surge; a recirculating version returns the air upstream instead, which mass-airflow systems usually need. Turbo Size →
Boost#
Intake pressure above ambient, normally quoted in psi or bar. Because it is measured relative to the air around you, the same gauge reading means a higher pressure ratio — and more compressor work — at altitude than at sea level. Boost & Compression →
Brake bias brake balance#
How braking effort is split front to rear. The ideal split follows the load on each axle, and because braking transfers load forward, the ideal is not a fixed percentage: it moves with deceleration, centre-of-gravity height and wheelbase. Brake Bias →
BSFC brake-specific fuel consumption#
Fuel burned per unit of power per hour, the constant that turns a power target into a fuel-flow requirement. Roughly 0.45–0.50 for efficient naturally aspirated petrol, 0.55–0.65 boosted, and higher again on ethanol because you burn more of it for the same energy. Injector Size →

C

Camber#
The tilt of a wheel seen from the front. Negative camber leans the top inward, which keeps the tread flatter on the road as the body rolls in a corner. Too much wears the inner edge and costs straight-line braking grip. Alignment Converter →
Caster#
The fore-aft lean of the steering axis. Positive caster gives straight-line stability and adds camber to the outside wheel as you steer, at the cost of heavier steering effort. It is usually set by the chassis rather than adjusted. Camber, toe & caster explained →
Centre bore hub bore#
The hole in the middle of a wheel that locates it on the hub. A wheel whose bore matches the hub is hub-centric and carries load through that fit; a larger bore needs a centring ring so the studs are not doing the locating. Bolt Pattern (PCD) →
Charge cooling#
Cooling the intake charge as fuel evaporates in it. Ethanol's latent heat of vaporisation is far higher than petrol's, so E85 chills the charge noticeably — denser air and more knock margin, before any octane benefit is counted. E85 explained →
Closed loop#
Running with the ECU correcting fuelling from live oxygen-sensor feedback, normally at idle and cruise. Under full load most systems switch to open loop and follow the tuned table instead, which is why a wideband is the only way to check power-fuelling. AFR ↔ Lambda →
Compression ratio static CR#
Total cylinder volume at BDC divided by what remains at TDC. Getting it right needs five numbers people routinely forget to include: chamber volume, gasket bore and thickness, deck clearance and piston dish or dome. Compression Ratio →
Compressor map#
A plot of pressure ratio against airflow for a given compressor, with efficiency islands and the surge and choke limits marked. Sizing a turbo means putting your airflow and pressure-ratio requirement on that plot and checking it lands inside an efficient island. How to size a turbo →
Correction factor SAE J1349#
A multiplier that restates measured power as it would have been in a standard atmosphere, so runs on different days can be compared. Above about ±5–7% the correction starts hiding real problems like heat soak, so quote the standard with the figure. Dyno Correction →
Cross-weight wedge#
One diagonal pair of corner weights as a percentage of the total. At 50% the car behaves the same turning either way; away from 50% it has a built-in directional preference, which is a tuning tool on an oval and a mystery handling complaint everywhere else. Corner Balance →

D

Dead time injector latency#
The delay between an injector being commanded open and fuel actually flowing, typically around a millisecond and voltage-dependent. It matters most at small pulse widths, which is one reason very large injectors idle poorly. Injector Size →
Deck clearance#
How far the piston crown sits below (or above) the block deck at TDC. It is a small volume with a large effect: ignoring it can put a compression-ratio calculation a full point out, the difference between pump fuel and detonation. Compression Ratio →
Density altitude DA#
The altitude in the standard atmosphere whose air density matches today's, folding temperature, pressure and humidity into one number. A naturally aspirated engine loses roughly 1% of its power per 100 m of density altitude — and a hot humid day at 600 m can breathe like 1,800 m. Density Altitude →
Detonation knock#
Uncontrolled auto-ignition of the end gas after the spark, producing a pressure spike that hammers rings, pistons and bearings. Distinct from pre-ignition, which starts before the spark; both destroy engines but from different causes. Boost & Compression →
Displacement#
The total volume the pistons sweep in one full cycle, from bore, stroke and cylinder count. It sets the baseline for how much air the engine can move, which is why airflow, carburettor and injector sizing all start here. Displacement →
Duration#
How many crank degrees a valve stays open. Advertised duration is measured at whatever small lift the grinder chose so it flatters the cam; duration at .050″ lift is the industry yardstick and the only one worth comparing across brands. Camshaft Specs →
Duty cycle#
The share of available time an injector spends open. Size injectors so peak demand sits at or below 80% and you keep headroom for cold starts, poor fuel and next year's power; size them for 95% and you have built in a lean condition exactly when it hurts. Injector Size →

E

E85#
Nominally 85% ethanol, 15% petrol. Its knock resistance and charge cooling let a boosted engine run more boost and timing, but its stoichiometric ratio is about 9.8:1, so it needs roughly 30–35% more fuel volume — injectors and often the pump have to grow. E85 Mix →
Effective compression ratio#
Static compression ratio combined with the pressure ratio of whatever is feeding the engine. It, not static CR, is what decides whether you detonate: a 9:1 engine at 14.7 psi behaves thermodynamically closer to 18:1. Boost & Compression →
ET (elapsed time)#
How long a drag run took, measured from the moment the car breaks the stage beam. It rewards traction, gearing and driver as much as power, which is why trap speed is the better power estimate of the two numbers on a slip. Trap Speed HP →
ET (offset) Einpresstiefe#
The offset figure stamped on a wheel: the distance from its mounting face to its centreline, in millimetres. Confusingly the same two letters mean elapsed time at a drag strip — context decides which one someone means. Wheel Offset →

F

Flex fuel sensor#
A sensor that measures the actual ethanol content of the fuel in the line so the ECU can scale fuelling and timing to it. It exists because pump "E85" is anything from about E51 to E83 depending on season and region. E85 explained →

I

Ignition timing#
How far before top dead centre the spark fires, in crank degrees. More advance extracts more work from the burn until it starts detonating; how much you can run depends on compression, charge temperature and fuel octane. Boost & Compression →
Intercooler charge cooler#
A heat exchanger between compressor and engine. Compressing air heats it, and hot air is both less dense and closer to knock, so cooling the charge recovers power twice over — once in density and once in the timing you can then run. Turbo Size →

K

KPI kingpin inclination, SAI#
The inward lean of the steering axis viewed from the front. With caster it sets how the wheel gains camber and lifts the car as you steer, producing the self-centring you feel. It is a design dimension, not an adjustment. Camber, toe & caster explained →

L

Lambda λ#
Air-fuel ratio expressed as a fraction of stoichiometric for whatever fuel is burning, so λ 1.00 is chemically correct on petrol and on E85 alike. Because the target does not move when the fuel changes, it is the safer unit to tune in. AFR ↔ Lambda →
Load index#
The number after a tyre size giving its maximum rated load, on a lookup scale rather than directly in kilograms. Dropping below the figure your vehicle was homologated with is both a safety and, in most countries, a legal problem. How to read a tire size →
LSA lobe separation angle#
The angle between the intake and exhaust lobe peaks, ground into the camshaft and unchangeable. With the two durations it sets overlap: a tighter LSA at the same duration means more overlap, more lope and a higher powerband. Camshaft Specs →

M

Mean piston speed#
Average piston velocity, from stroke and rpm — bore does not appear. Production engines mostly live below 20 m/s, well-built performance engines 20–25, and beyond 25 m/s is race-engine territory. It is the best one-number sanity check on a rev limit. Mean Piston Speed →
MON / RON#
Two octane test methods: RON under mild conditions, MON under load and heat. MON always reads lower, and the US AKI figure is their average — so a European 95 RON and a US 91 AKI are roughly the same fuel. E85 explained →
Motion ratio#
How far the spring moves for a given wheel movement. Its effect on rate is squared, so a 0.7 motion ratio leaves only about half the spring rate at the wheel — a much bigger reduction than most people expect. Spring Rate →

O

Overlap#
The window around TDC where both valves are open, set by the two durations and the LSA. A little scavenges the cylinder at high rpm; a lot lets exhaust back into the intake at low rpm, which is exactly why a big cam lopes at idle and pulls hard up top. Camshaft Specs →

P

PCD bolt pattern, pitch circle diameter#
The stud count and the diameter of the circle through their centres, as in 5×114.3. Patterns cluster close together — 5×112 and 5×114.3 are 2.3 mm apart — so measure rather than eyeball, because a near-fit loads studs unevenly. Bolt Pattern (PCD) →
Poke#
How far a wheel and tyre sit outboard of the arch. It is the visible consequence of width and offset together: a wider wheel at the same offset grows in both directions from the mounting face, so poke and inner clearance move at once. Wheel Offset →
Power-to-weight ratio#
Power divided by mass, usually hp per tonne or kg per hp. It predicts straight-line performance far better than power alone, which is why a light 300 hp car and a heavy 400 hp car can feel near-identical. Power-to-Weight →
Pre-ignition#
The charge igniting before the spark, from a hot spot such as a glowing deposit or an overheated plug. Unlike detonation it is not a fuel-octane problem and it destroys pistons far faster; the two are often confused because both sound like knock. Boost & Compression →
Pressure ratio#
Absolute outlet pressure divided by absolute inlet pressure — so 14.7 psi of boost at sea level is a ratio of 2.0. Compressor maps are drawn against it rather than gauge boost, which is why the same boost is harder work at altitude. Turbo Size →

Q

Quench squish#
The tight gap between piston crown and flat head area at TDC, which squirts charge into the chamber and cools the end gas. A well-chosen quench height suppresses knock, letting an engine run more compression than the number alone suggests. Compression Ratio →

R

Ride frequency#
The natural frequency of a corner, from wheel rate and the mass it carries — the number that actually predicts how a car will feel. It is why spring rate alone tells you little until motion ratio and corner weight are in the sum. Spring Rate →
Rocker ratio#
The leverage between camshaft lobe and valve. Lobe lift times rocker ratio gives net valve lift, so a 1.6 rocker on 8 mm of lobe lift opens the valve 12.8 mm — one reason two cams with the same duration can flow very differently. Camshaft Specs →

S

Scrub radius#
The distance at the road between the steering axis and the tyre's contact centre. Lower offset wheels push it outward, which amplifies bump-steer and kickback through the wheel and adds load to bearings — the hidden cost of aggressive fitment. Camber, toe & caster explained →
Speed rating#
The letter after the load index, giving the maximum speed the tyre is rated for. Like load index it is a homologation figure as much as a performance one, and fitting below the original rating can invalidate insurance. How to read a tire size →
Spool#
A turbo accelerating to the speed where it makes useful boost. A compressor sized for a high power target has more inertia and needs more exhaust energy, so chasing peak power almost always costs response low down. Turbo Size →
Stoichiometric stoich#
The air-fuel ratio that burns all the fuel with all the oxygen and nothing left over: about 14.7:1 for petrol, 9.8:1 for E85, 14.5:1 for diesel. Lambda 1.00 is stoichiometric by definition, whatever the fuel. AFR ↔ Lambda →
Surge#
Flow breaking down at the left edge of a compressor map, where the wheel is asked for too much pressure at too little airflow. It sounds like fluttering and it is destructive — a sign the compressor is too large, not too small. How to size a turbo →

T

Toe#
Whether wheels point inward or outward viewed from above, quoted in millimetres, inches or degrees. Converting between them needs the tyre diameter, which is why a spec in degrees and one in millimetres only agree for one tyre size. Alignment Converter →
Trap speed#
Terminal speed at the end of a drag run. It depends mostly on power and weight and very little on how well you launched, which makes it the most trustworthy dyno most people own — and a better power estimate than elapsed time. Trap Speed HP →

V

Volumetric efficiency VE#
How completely a cylinder fills compared with its swept volume. A stock engine is nearer 80% than the 100% people like to assume; well-developed naturally aspirated heads reach the high 90s, and forced induction exceeds 100% by definition. Engine Airflow →

W

Wastegate#
A valve that bypasses exhaust gas around the turbine to cap boost. Internal gates are packaged in the housing and can struggle to hold low boost on a large turbine; external gates flow better, which is why high-power builds tend to use them. Turbo Size →
Wheel rate#
The effective spring rate measured at the wheel rather than at the spring — spring rate times motion ratio squared. It, not the number printed on the spring, is what the tyre and the chassis actually respond to. Spring Rate →
Wideband wideband lambda sensor#
An oxygen sensor that reads accurately across a broad range instead of only switching around stoichiometric like a factory narrowband. It is the minimum instrumentation for tuning anything under load, because that is where narrowbands stop telling you anything. AFR ↔ Lambda →

Why a glossary and not fifty pages

Every term here could have been its own page. It would have made fifty-eight pages of three sentences each — thin, near-duplicate, and worse to use than one list you can search in place. So this is one page, with a permanent link (#lambda, #lsa) for every term if you want to point someone at a single definition.

Definitions are deliberately short. Where a term needs real explanation, the link at the end of each entry goes to the guide or calculator that does the teaching properly.