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Power Starts Here - Throttle Guide

July 22, 2026

Home - Guides - Power Starts Here - Throttle Guide

What individual throttle bodies are, why they respond the way they do, and how to spec a set for your engine - throttle size, inlet length and linkage. Here's how each choice shapes power and torque, so you can tune your setup to your engine and how you drive.

What is an ITB system?

A standard engine breathes through one throttle. Air passes a single butterfly, collects in a shared plenum, then splits down the manifold to the cylinders - every cylinder drawing from the same box of air, along a different length of pipe.

Individual throttle bodies - ITBs, also called independent throttle bodies - give each cylinder its own butterfly and its own intake path, all of equal length. Nothing is shared, so nothing has to be averaged out: every cylinder sees the same air, and the engine is no longer mapped around whichever one breathes worst.

How ITBs work

Three things change when you take the manifold away, and together they account for how an ITB engine feels.

The throttle moves to the headOn a direct-to-head setup the butterfly sits a few inches from the inlet valve rather than the far side of a manifold. There is far less volume to fill, so the engine answers the moment your foot moves - the response ITBs are known for.
Atmospheric, not vacuumA plenum sits under manifold vacuum, and every valve event sends a pulse back through it that disturbs the next cylinder's fill. Give each cylinder its own throttle and its runner sits at atmospheric pressure with nothing to disturb it. Fit an airbox over ITBs and it stays atmospheric too, because the throttles are downstream of it.
Equal paths, individual controlSame bore, same length, same airflow to every cylinder - so fuelling and timing can be trimmed cylinder by cylinder instead of being compromised to protect the weakest one.
An idle you can live withLong-duration cams and a shared plenum work against each other. Most of the pressure pulsing that upsets idle disappears when each cylinder breathes on its own, so an ITB engine will usually idle more cleanly on aggressive cams than the same engine on a manifold.

What you gain

Power and torque across the rangeNot only at the top end - inlet length lets you place the gain where you want it.
Throttle response that tracks your footThe single most commented-on change after fitting ITBs.
Induction noiseThe engines people remember for the way they sound - the E46 M3, Ferrari F355, McLaren F1, Skyline GT-R R32 - were all ITB engines.
A cleaner idle on modified enginesParticularly worthwhile once you are running serious cam duration.
Room to tunePer-cylinder trim, and inlet length you can change - see the section below.
An engine bay worth opening the bonnet forAnodised to your choice of finish, in your own combination of colours.

ITBs were once the preserve of homologation specials and race engines. Modern manufacturing puts the same layout within reach of a road or track build, without giving up the engineering that made them worth having.

Choosing your throttle size

Throttle bore is matched to your engine's power. As a starting point (based on 100mm extensions and 40mm ram pipes):

45mm
Throttle bore
Up to 250 hp
50mm
Throttle bore
250 - 350 hp
55mm
Throttle bore
350 hp +

Guideline figures only - engine specification, rev range and application all play a part. Send us your engine details and we'll recommend the right size.

Extension & ram pipe tuning

Inlet length - your extension (runner) plus ram pipe (trumpet) - moves where the engine makes its power. Shorter inlets favour top-end peak power; longer inlets build low-end torque.

Short inlet - 40mm ram pipe on 70mm extension

Short inlet

70mm EXTENSIONS · 40mm RAM PIPES

Higher peak power, with less bottom-end torque.

Best for
  • Long-track
  • Circuit racing
Recommended baseline
Medium inlet - 40mm ram pipe on 100mm extension

Unsure on length? Start here - it gives you the baseline to tune from.

Medium inlet

100mm EXTENSIONS · 40mm RAM PIPES

An even spread of more power and more torque across the range.

Best for
  • Track days
  • Rallycross
  • Street racing
Long inlet - 60mm ram pipe on 130mm extension

Long inlet

130mm EXTENSIONS · 60mm RAM PIPES

Higher torque in the lower rev range, with less peak power.

Best for
  • Rally
  • Hill climb
  • Drifting
  • Time attack
AT Power Tuning RingNot sure which length?

Our Tuning Ring lets you test an extended inlet length for power and torque before you commit to a set.

You can order additional sets of extensions and ram pipes to make one setup versatile across different events - and finish them in any of our anodised colours.

Know your throttle

The main parts, the options for each, and the different names they go by - including the terms our US customers use.

Labelled AT Power throttle body assembly showing the fuel rail, adaptor manifold, TPS fitting, linkage, ram pipes and extensionsCross-section naming the ram pipe, extension, throttle housing and manifold adaptor, with the sizes and types available for each
The assembly, part by part
Ram pipe / trumpet / bellmouth40mm open ram pipe60mm open ram pipe40mm mesh ram pipe60mm mesh ram pipeThe flared intake on top. 40mm or 60mm, open or mesh.
Extension / runner / velocity stack70mm extension100mm extension130mm extensionSets the inlet length - 70, 100 or 130mm. Aluminium for round & naturally aspirated; 3D-printed nylon for oval or boosted throttles.
Throttle housing / throttle bodyRound injected throttle housingRound non-injected throttle housingOval injected throttle housingOval non-injected throttle housingOval or round, injected or non-injected - with AT Power's patented Shaftless technology for unobstructed airflow.
Manifold adaptor / intake manifoldIntake manifold adaptor plate mounting throttle bodies to the cylinder headMounts the throttles to your engine - Universal, Twin, DCOE or IDA.
Fuel railFuel rail assembly with injectors, mounting brackets and end fittingsSits at the top - AN6 fittings as standard, with injector brackets to suit your injectors.
LinkageCentre pull, left pull and right pull throttle linkage arrangementsCable-pull throttle linkage, suited to our motorsport throttle cable - centre, left or right pull for twin setups.

Ram pipes, trumpets & velocity stacks

The same two parts go by different names either side of the Atlantic, and the names get used interchangeably. If you searched for one of these, here is what it maps to in our range.

Velocity stackThe US name for our extension - the tube between the throttle housing and the ram pipe that sets your inlet length. 70, 100 and 130mm, aluminium for round and naturally aspirated engines, 3D-printed nylon for oval or boosted.
Ram pipeThe flared mouth that sits on top. 40mm or 60mm, open or mesh. Mesh keeps debris out of the engine without measurably costing airflow.
Trumpet, intake trumpet, bellmouth, air hornAll the same part as the ram pipe. "Trumpet" and "bellmouth" are the common UK terms; "intake trumpet" and "air horn" turn up more in the US.
Runner, inlet tract, stackAll the same part as the extension. It is the combined length of extension plus ram pipe that moves your power band - see the inlet length section above.

Ram pipes and extensions are available as spare and replacement sets for AT Power throttles, in every anodised finish, and in mixed lengths if you are experimenting with inlet length. Talk to our team for sizes and pricing.

Talk to our team Shop throttle bodies

Guidance only - the right combination depends on your engine, targets and how you use the car. Our team can spec a setup with you.

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