Subaru EJ20 ITB Kit: What Actually Works on a Boxer Engine
I get asked about a Subaru EJ20 ITB kit more than almost any other boxer enquiry, and the conversation usually needs a hard reset before we even talk parts. Individual throttle bodies are one of the most effective induction upgrades you can make to the right engine — and one of the most expensive mistakes you can make on the wrong one. The EJ20 sits awkwardly across both camps, so this article is about telling you straight which side of the line your engine is on, and what a properly engineered kit actually involves.
I’m Graham Martin. I design and build induction systems and bespoke race engines here in Northampton, and I calibrate them too — which means I see the dyno consequences of badly specified throttle bodies long after the glamorous bit is over. Let’s do this properly.
First question: is your EJ20 turbo or naturally aspirated?
This is the single most important decision, and it comes before brand, bore size or budget. The vast majority of EJ20s on the road — the EJ205 and EJ207 — are turbocharged. Conventional ITBs are a naturally aspirated induction concept. They open the intake tract straight to atmosphere at each cylinder, which is exactly what you don’t want when a turbo is trying to build manifold pressure.
On a turbo EJ20 you hit two problems immediately. First, you’ve nowhere sensible to put the boost — open trumpets and forced induction don’t coexist without a sealed plenum, at which point you’ve largely defeated the point of separate throttles. Second, you introduce suction and spool behaviour that makes throttle response and idle control a nightmare. Unless you’re running something exotic and factory-engineered for it (the SR20 in the Pulsar GTI-R is the usual example), running open ITBs on a turbo car is impractical.
So if you’ve got a turbo EJ20, the honest answer is: don’t buy an ITB kit. Spend the money on a larger single throttle body and a properly designed plenum manifold instead — and only after your supporting mods (turbo, fuelling, exhaust) are sorted. That’s where the gains actually live. If you’re building a turbo car and want the induction side done right, talk to me about a bespoke intake manifold spec’d around your combination rather than chasing the ITB look.
If you’re running a naturally aspirated EJ20 — a high-RPM track build, a hillclimb car, a stripped road car chasing response and noise — then ITBs are genuinely on the table. The rest of this article is for you.
What’s actually on the market for an EJ20 ITB kit
The budget end is dominated by far-East billet kits. The OBX Racing Sports EJ20/EJ25 kit and the BLOX Racing equivalent are the two you’ll see most. The BLOX unit runs a 66.5mm outer body with a 70mm flange OD and a 48mm multi-throttle setup, billet 6061-T6 aluminium throttle bodies, lightweight aluminium funnels, an included fuel rail, and it retains the stock fuel pressure regulator and throttle position sensor with equal-length runners. On paper that reads well.
Be careful with the numbers, though. Manufacturer wording across these kits is inconsistent — “70mm” is usually the housing or flange diameter, and the actual butterfly bore is the figure that matters for airflow. Verify the real throttle bore before you assume it suits your engine.
At the premium end, Tomei produce an EJ ITB system that costs several times what the budget kits do. You’ll find forum chatter claiming the cheap kits are “the same thing for a quarter of the price” — treat that as speculation, not fact. They are not the same thing once you’ve measured concentricity, butterfly sealing and spindle quality.
For a UK and motorsport audience, Jenvey Dynamics is the name that matters. Jenvey design and manufacture EFI throttle bodies and induction systems in-house — housings, spindles, butterflies, levers and fuel rails — for everything from kit cars and track-day machines to S2000-spec rally cars and World Touring Cars. Their complete kits arrive built, balanced and ready to fit with the manifold, throttle bodies, interconnecting links, fuel rails and air horns. A dedicated off-the-shelf “EJ20” Jenvey listing isn’t always catalogued, so an EJ application is typically built as a bespoke or supplier-assembled system. That’s exactly the territory we work in — read more on how to buy an ITB kit that actually fits and performs.
Bore sizing: the bit most people get wrong
Bigger is not better. The most common ITB mistake I see is oversizing, and it shows up clearly on the dyno: if you only gain at the top end — or gain nothing at all — the throttles are probably too big and you never reached optimum runner velocity. A correctly sized ITB system sits above the standard manifold curve across the whole rev range, with the largest gains at the top, not a flat line that suddenly perks up at 7,000 rpm.
As an indicative rule of thumb (and I stress indicative — every engine is different):
- 40mm ≈ 265cfm, suits ~350–500cc per cylinder
- 42mm ≈ 304cfm, suits ~450–600cc per cylinder
- 45mm ≈ 362cfm, suits ~550–700cc per cylinder
- 48mm ≈ 408cfm, suits ~650–800cc per cylinder
A 2.0-litre four works out to roughly 500cc per cylinder, which points at 42–45mm bores for a road and fast-road N/A EJ20 — not the 48mm that a lot of budget kits ship with. Big bores belong on big-capacity, high-RPM race builds.
But bore alone is a blunt tool. What really matters is the port’s minimum cross-sectional area at the choke point, the taper of the runner, and how far the throttle blade sits from that MCSA. Get the taper and velocity right and a 42mm system will out-drive a poorly matched 48mm one everywhere below the redline. This is why I design around your actual ports and target rev range rather than picking off a chart — the same philosophy behind every custom component we specify.
The boxer-specific problems: throttle actuation and packaging
Two EJ-specific headaches catch people out. The first is throttle actuation. Early EJ20s are drive-by-cable; later WRX and STI cars are drive-by-wire (DBW). Most ITB kits are built around cable actuation, so if you’re working with a DBW STI you either convert to a throttle cable and rewire the TPS, or you engineer the electronic throttle into the system. Neither is trivial, and it needs planning before you order anything.
The second is packaging. The boxer layout puts intake ports facing outward on both banks with very little vertical room and a lot of ancillaries in the way. Equal-length runners and trumpet length both matter for the pressure-wave tuning that gives ITBs their midrange, and on an EJ you’re fighting the bay for every millimetre. A kit that ignores this gives you mismatched runner lengths and a torque curve full of holes. Related: if you want the lightest possible runners on a serious build, see our notes on a carbon intake manifold for a race engine.
Don’t forget the airbox and calibration
Open trumpets sound fantastic and ingest hot underbonnet air, grit and rain in equal measure. Every 10°C of intake temperature rise costs you roughly 3% power, so feeding your ITBs cold, clean, filtered air through a properly volumed carbon composite airbox isn’t optional on a serious build — it’s where some of the easiest gains hide.
And ITBs absolutely require standalone or fully remapped engine management. Eight separate throttle plates change load sensing, idle control and transient fuelling completely. Bolting them on and hoping the factory map copes is the fastest route to a car that runs worse than standard. We handle bespoke calibration for OEM and aftermarket ECUs as part of the build, because an ITB kit that isn’t mapped properly is just expensive jewellery.
FAQ
Can I fit an ITB kit to a turbo EJ20?
In practice, no. Conventional ITBs are a naturally aspirated concept and don’t play well with forced induction — you get spool and suction issues and nowhere to contain boost. On a turbo EJ20, a larger single throttle body and a well-designed plenum manifold give better results.
What bore size should I run on a 2.0-litre N/A EJ20?
For road and fast-road use, roughly 42–45mm bores suit ~500cc per cylinder. Larger 48mm bores only make sense on high-capacity, high-RPM race builds — oversizing kills runner velocity and loses you midrange.
Are the cheap OBX/BLOX kits as good as Tomei or Jenvey?
No. They share rough dimensions but not the machining quality, butterfly sealing or spindle precision. For a serious build, a Jenvey-based or bespoke system gives repeatable, properly balanced results.
Do I need to change my ECU for ITBs?
Yes. ITBs change load sensing and transient fuelling fundamentally, so you’ll need standalone management or a full remap and proper calibration. Don’t skip it.
If you’re building a naturally aspirated EJ20 and want induction that actually feeds the engine across the whole range, get in touch. I’d rather tell you honestly whether ITBs suit your combination than sell you a kit that disappoints on the dyno.
