Day: 12 July 2026

I’ve lost count of the number of K20 builds I’ve seen running individual throttle bodies with four little sock filters poking out into the engine bay, then wondering why the dyno graph doesn’t match the hype. Here’s the blunt truth: on a serious K-series build, the carbon composite airbox isn’t an accessory — it’s part of the induction system, and getting it wrong costs you exactly what you paid the rest of the build to find.

This is a guide to specifying a carbon composite airbox K20 setup that does its job: shields the intake from heat, feeds clean cool air, and works with the trumpets rather than choking them. I’m Graham Martin, and at GMR we design and manufacture these properly, around your actual combination — not a universal-fit lid that “should be close enough”.

Why a carbon composite airbox earns its place on a K20

The primary job of the airbox is simple to state and easy to get wrong: enclose the intake and shield it from engine-bay heat, so the engine draws a cooler, denser charge. Density is power. As a working rule of thumb, roughly every 10°C rise in intake air temperature costs you around 3% power — so a charge that’s been sitting in radiated bay heat is quietly robbing you.

This is where open-cone and sock filters fall down. They often flow better than a restrictive stock box, yes, but in a hot engine bay they suffer badly from heat soak. The filter sits in the hottest air available and feeds it straight to the cylinders. A sealed airbox fed by a decent cold-air intake addresses that directly: it prevents the intake from absorbing excessive engine-bay heat and gives the engine a defined, cool source of air.

On ITB setups in particular, this matters even more. You can run individual sock filters on K20 throttle bodies, but in a dirty, hot engine bay they aren’t really doing much good. The ideal setup is ITBs plus a properly designed airbox — you get precisely metered air and you start exploiting the resonance behaviour between cylinders, because the box becomes a shared plenum that lets the pressure waves interact instead of dumping straight into open air.

Single throttle body or ITBs — and what the airbox has to clear

The K20 platform splits two ways. Plenty of builds run a single large throttle body — aftermarket electronic units are typically sized around 70–72mm, for example the Skunk2 72mm ETB that suits the 06–11 Civic Si. That’s a straightforward route to more airflow with a single sealed airbox over a panel filter.

The other route is individual throttle bodies — four smaller bodies, commonly 48–51mm, in place of one 68–72mm unit. Sizing is application-dependent: the bigger the throttle body, the higher the rpm where it makes power, and that has to be matched to your cam and rpm target. As a guide for a road-race K20, given the port is roughly a 47mm equivalent, you want a sensible minimum to account for flow loss across the plate and shaft — going too small to chase low-end response throttles the top end. If you’re weighing up the platform, our piece on the individual throttle body kit UK buyer’s guide covers the sizing trade-offs in detail.

Whichever way you go, the airbox has a non-negotiable design constraint: it must clear the trumpets and never choke them. Intake tract length is a tuning lever — shorter tracts push power higher up the rev range, longer air horns move it down. The trumpet length you’ve chosen sets the airbox internal volume and the trumpet-to-wall clearance you need. Get that clearance wrong and the box simply strangles the air horns it’s supposed to feed.

What “done correctly” actually looks like

Three things separate a real race airbox from a pretty carbon lid:

• Sufficient internal volume. Within reason, larger is better — a generous plenum settles the air and feeds all four trumpets evenly. Done correctly it should never hurt power and usually helps. But “large” still has to package in the bay and respect bonnet clearance, which is where bespoke design beats off-the-shelf compromise.
• Geometry that respects the trumpets. Even radial clearance around each air horn, no flat wall sitting directly over a trumpet mouth, and an organic, flow-conducive internal shape rather than a square box. The air should be able to turn into each trumpet without separating off a hard edge.
• A real cold-air feed. A sealed box fed from hot bay air is just an insulated heat trap. The inlet needs to draw from a cool, high-pressure zone — ahead of the radiator, a bonnet scoop, or a ducted feed.

For the deeper theory on plenum behaviour and feed design that applies just as much to a K20 box, read our companion guide, Carbon Composite Airbox for Motorsport.

Fitment realities on a K20 ITB build

If you’re building around a Jenvey-style ITB kit — say the EP3 (K20) kits — be aware of the integration details before the airbox even goes on. The standard EP3 kit ships with four 51mm tapered throttle bodies, levers, fuel rail and tapered air horns, but standard Honda injectors won’t fit and the water pump housing needs modification. The curved-manifold variant exists specifically to avoid changes to the pulley, but it won’t clear the standard alternator and belt-tensioner unit. These constraints decide how much room you actually have, and therefore what airbox geometry is even possible.

To run a panel filter against ITBs you’ll typically want a carbon dished backplate (the Jenvey route uses a 50mm dished backplate, part ABF-KIT-MH07; their carbon airbox base plates require machining to suit each application). Throttle actuation can be cable linkage or an electronic actuator — both affect packaging around the box. This is exactly the sort of build where a made-to-fit airbox beats a parts-bin lid. Our take on K-platform induction sits alongside our work on the Subaru EJ20 ITB kit and the Peugeot GTi6 ITB kit — same engineering discipline, different platforms.

What the dyno actually shows — read the numbers carefully

There’s a widely quoted EP3 figure worth understanding properly. A developed Jenvey ITB kit on a K20 produced a peak of 251.1bhp — an additional 13.8bhp over a tune that already had a carbon airbox, exhaust manifold and 70mm catback. Read that correctly: the +13.8bhp is the gain of ITBs over a single-throttle-body car that already had a carbon airbox — it is not the gain from an airbox alone. That ITB spec used four 48mm parallel throttle bodies, a curved manifold to clear the pulley, short 20mm billet air horns and an ITG filter on a backplate.

I flag that because the figures get misquoted constantly. A carbon airbox’s contribution is mostly in protecting charge temperature and stabilising the feed — its real-world value is consistency across a long stint, not a headline peak on a cold first pull. None of these numbers are universal; they’re specific to that engine, that tune and that day.

Why the material and process matter

I specify carbon composite for these boxes for concrete engineering reasons, not for looks. It’s lighter than aluminium or steel, corrosion-resistant, and crucially it has low thermal conductivity — so the box wall itself doesn’t conduct bay heat into the air charge the way a metal lid does. Where the heat load is severe, double-wall air-gap construction adds a further insulating barrier.

How the carbon is laid up is just as important as the fibre. The highest-grade method is prepreg carbon laid up and cured in an autoclave — the aerospace-derived process — where the fibres are pre-impregnated with resin in a controlled ratio and consolidated under heat and pressure. That gives you minimal voids, consistent wall thickness and proper structural integrity, rather than the resin-rich, void-prone results you get from cheap wet-lay parts that look the part and crack in service. If you want to understand how digital manufacturing and composites fit a modern motorsport workflow, our partners cover it well in custom race engine components and 3D printing.

FAQ

Do I really need an airbox on K20 ITBs, or will sock filters do?

You can run sock filters, but on a hot engine bay they don’t achieve much — they sit in radiated heat and offer no resonance benefit. A sealed carbon composite airbox with a cold-air feed gives cooler, denser charge and lets the cylinders share a plenum, which is where the ITB resonance gains actually come from.

How big should the airbox be?

Within packaging limits, larger generally helps — a bigger plenum settles and evens out the feed to all four trumpets. The hard constraint is clearance: it must clear your air horns with even radial spacing and not sit a flat wall over a trumpet mouth. Volume and trumpet length are linked, so they’re designed together, not bolted on after.

Single throttle body or ITBs for a track K20?

A single 70–72mm throttle body with a sealed carbon airbox is simpler and very effective for many builds. ITBs (typically 48–51mm) add throttle response and top-end resonance but bring fitment work — injectors, water pump housing, alternator clearance — so choose based on your rpm target, cam and budget for integration.

Is a carbon airbox worth the money over aluminium?

Yes, for the reasons that matter: it’s lighter, corrosion-resistant and has low thermal conductivity, so it doesn’t pump bay heat into your charge. With autoclaved prepreg construction you also get a structurally sound, repeatable part rather than a flexing wet-lay shell.

Get one built around your engine

A carbon composite airbox for a K20 should be specified around your throttle bodies, your trumpet lengths and your actual bay packaging — not bought off a shelf and trimmed to fit. That’s the whole point of what we do at GMR in Northampton. If you’re speccing a build, talk to us about a made-to-fit box, and read our guide to specifying custom race engine components that actually fit and last. Related: if you build your race engine from the ground up, see our guide to bespoke race engine manufacture.

Related: Bespoke Carbon Parts for Your Engine in the UK: What Actually Survives Under the Bonnet

If you’re searching for peugeot tu individual throttle bodies, you’ve already worked out the obvious truth: the standard injection or carb setup on a TU is the single biggest restriction between the air filter and the valve seats. I’m Graham Martin, and I’ve spent enough time on TU-engined cars — Saxos, 106s, 205/106 hillclimb and slalom builds — to know that the difference between a kit that’s “close enough” and one engineered around your actual combination is worth real, measurable power. This is how I’d approach an ITB project on a TU, and what to specify so you don’t end up disappointed.

First, know which TU you’re actually building

The PSA TU engine ran from October 1986 right through to December 2014 — a long production life across a lot of displacements: 1.0 (954 cc), 1.1 (1,124 cc), 1.3 (1,294 cc), 1.4 (1,360 cc), 1.5 (1,527 cc) and 1.6 (1,587 cc). You’ll find them as 8-valve SOHC and 16-valve DOHC, with everything from Solex/Weber carbs to mono-point and multi-point injection on Magneti Marelli, Bosch or Lucas management. Factory output spans 45–125 PS, so the platform you start with matters hugely.

For ITB projects, the engines worth building are clear. The TU5J4 and TU5JP4 (1.6 16v) are the obvious targets — roughly 100–110 hp standard, a strong big-valve head, and the best low-down torque of the family. The 8-valve TU3 (1.4) is the other popular base, especially for budget or class-restricted builds. Know which one you have before you spend a penny: the TU5J4 turns up in pre-2001 Saxo VTS, early Xsara, 106 GTI and early 206, while the TU5JP4 lives in later Saxo VTS, C2 VTS, early C3, 206 XS/Quiksilver, 307 1.6 16v and late 106 GTI. The split affects VVT, sensors and management — and therefore your ITB and wiring choices.

Why ITBs work on a TU

The point of individual throttle bodies isn’t the noise. Each cylinder gets its own short, unobstructed runner and its own butterfly, so throttle response is immediate and the intake pressure waves aren’t shared and smeared across a common plenum. On a high-revving 16v TU that’s exactly what you want — strong column resonance tuned by runner and trumpet geometry, feeding a head that can actually use the air. I’ve written more on the principle in our guide to buying an ITB kit that actually fits and performs, and it applies directly here.

One caveat: ITBs reward a head and cam package that can breathe. On a TU3 with the smaller combustion chambers and modest valves, ITBs help, but the head is the limiter. Combustion chambers on TU3s and other small-valve heads are much smaller than on the big-valve heads (all TU5, TU2J2, TU3J2). The 1.3 Rallye, 1.4 XSi and 1.6 Rallye/XSi run big-bearing heads, and cams from other heads won’t simply drop in — so plan the whole combination, not just the throttle bodies.

The proven TU5 fast-road/competition combination

A combination I see work repeatedly: a TU5 bottom end (any 90 bhp 1.6) with a 1.3/1.4 XSi or 1.6 Rallye big-valve head, Catcams 646 or 640 grind, and a 1.3 Rallye inlet as the starting point. That always needs a remap or a standalone ECU — there is no “fit and forget” here. For slalom and tight technical work, the 1.6 (TU5) gives far more low-down grunt with a Catcams 646 than a 1.3 or 1.4 ever will, which is why I steer most road and sprint builds towards the bigger displacement.

Three real routes to ITBs on a TU

1. Purpose-built bolt-on ITB kits

Several established names make TU kits. Jenvey‘s TU5 1600cc kit (CKPG01) uses four ST45 taper bodies with 42 mm butterflies, an inlet manifold, fuel rail and four 40 mm airhorns, built for the Saxo and 106 GTI and made to order. AT Power takes a different approach with their Direct-to-Head (DTH) shaftless twin-butterfly 38 mm system for the TU5J4/TU5JP4: the patented shaftless, knife-edged blade design removes the central shaft to cut turbulence, with twin-oval housings CNC-machined from billet aluminium and port-matched to bolt straight onto the head face. AT Power claim removing the shaft adds up to 10% airflow versus a conventional shafted ITB — treat that as their figure, not a universal guarantee, but the principle is sound. Indicative pricing on the 106 GTI DTH set has been around £1,265; verify current before ordering.

danST Engineering offers DCOE-type kits for the 106 GTI, Saxo and C2 VTS (TU5 16v), plus — and this is the useful bit for 8-valve builders — a TU5 8v DCOE kit for the 106 XSi / Saxo VTR that can be adapted to older TU3 8v engines. Their kits come complete bar engine management: linkages, fuel rails, genuine Bosch injectors in a range of capacities, air filters and throttle sensors. Bore and trumpet length are buyer-specified, which is exactly how it should be.

2. Bike throttle bodies adapted to the head

A budget route that can work, but it’s fiddly. You’re matching bike-spacing bodies to TU bore spacing, sorting injector placement, building a manifold and adapting the fuel and throttle linkage. The parts are cheap; the engineering time isn’t. If you go this way, get the geometry measured properly rather than eyeballed.

3. A bespoke kit engineered around your build

This is where I sit. When the head, cam, displacement and target rev range are known, the throttle bore, runner length and trumpet profile can be specified to put the torque exactly where you use it — rather than accepting whatever a generic kit ships with. Trumpet length and radius are not cosmetic; they tune the pressure-wave timing, and the right entry radius keeps the flow attached at the bellmouth. We go into the mechanism in our piece on velocity stacks for ITBs, and the same thinking drives our bespoke intake manifold work. If you’ve built a Peugeot 16v before, our GTi6 ITB guide covers the larger XU sibling and the same engineering principles carry across. A bespoke route also means the surrounding parts are made to suit — see how we approach custom race engine components and full bespoke race engine manufacture. Related: if a carbon trumpet or plenum is on your list, our guide to a carbon intake manifold for a race engine covers what actually works, and for the 3D-printed prototyping side our colleagues cover how 3D printing fits the motorsport workflow (@ Ask The Nozzle).

Sizing the throttle bodies

Bigger isn’t automatically better. On a TU5, 38–42 mm covers the sensible window depending on cam, head flow and rev ceiling. Go too large and you lose port velocity at the rpm you actually drive at, blunting throttle response and mid-range. Go too small and you cap top-end. For a fast-road TU5 on a Catcams 646 with a big-valve head, I’d be in the 40–42 mm region; for a peaky, high-rpm sprint engine you can justify larger. The honest answer is that the bore should follow the airflow data for your head, not a number off a forum.

Don’t skip the calibration

ITBs change the fuelling and ignition demands completely. Throttle position becomes the primary load reference (alpha-N), idle control changes, and transient enrichment needs proper attention or the car feels lumpy and lean off-throttle. Every kit above requires a remap or standalone ECU — none of them is plug-and-play. This is the part most people underestimate; it’s also where the power and driveability actually live. Read how we approach it in ECU calibration for motorsport.

FAQ

Can I fit ITBs to an 8-valve TU3?

Yes — danST and others make 8v DCOE-type kits adaptable to TU3. Just be realistic: the small-valve, small-chamber 8v head is the airflow limit, so ITBs sharpen response and free some top-end rather than transforming the engine. Spend on the head and cam alongside the bodies.

Which TU is the best base for an ITB build?

The TU5 1.6 16v (TU5J4 or TU5JP4). It has the strongest head, the most low-down torque and the broadest support in off-the-shelf kits. Confirm whether yours is VVT, because it affects management and sensors.

How much do TU ITBs cost?

A purpose-built bolt-on kit such as the AT Power 106 GTI DTH set has been listed around £1,265, before fitting, injectors to suit and crucially the calibration. Budget for the remap or standalone ECU as part of the project, not an afterthought.

Do I really need a remap?

Yes. There is no exception. ITBs need an alpha-N fuelling strategy and proper idle and transient setup; a standard map will run badly or do damage. Factor a standalone ECU or full remap into every TU ITB build.

Get the base engine, head, cam and throttle sizing matched as one combination and a TU rewards you with sharp, eager response and genuine usable power. If you want a kit engineered to your exact build rather than a universal-fit compromise, that’s exactly what we do in Northampton — get in touch with the spec of your engine and I’ll tell you straight what’ll work. Related: once it’s built, find somewhere to use it with our friends at Trackday Finder’s guide to car track days in the UK (@ Trackday Finder).