Throttle Body Sizing

Use the calculator above — enter your displacement, peak power target, RPM, and number of cylinders. The key output is not just the diameter, it's the air velocity through the bore at peak power. You want 70–100 m/s at wide-open throttle. Too low and throttle response suffers; too high and you're restricting peak power.

As a rough rule for a naturally aspirated 4-cylinder: a single TB wants to be approximately 55–65mm for a 1.6–2.0L engine at moderate tune, and 60–70mm for a race-developed unit. ITBs are sized per cylinder — typically 38–45mm per body for 1.3–2.0L engines revving to 8000+ RPM.

A single throttle body feeds a plenum that all cylinders draw from simultaneously. It must flow the total peak demand of the entire engine. The plenum acts as a buffer, smoothing out intake pulses.

Individual throttle bodies (ITBs) give each cylinder its own dedicated throttle plate. Each body is sized for one cylinder's demand only — roughly a quarter of total flow on a 4-cylinder. There is no plenum, so intake pulse energy is preserved and can be tuned via trumpet length to create a resonant power peak at a target RPM. The result is sharper throttle response, better high-RPM breathing, and the soundtrack that makes ITBs worth the fabrication effort.

The trade-off: ITBs are harder to tune at part-throttle and idle, require careful balancing with a vacuum sync tool after fitting, and need a standalone ECU or significant mapping work to drive properly.

The Yamaha R6 is the most widely used donor for 1.3–1.6L 4-cylinder conversions. The 80mm centre-to-centre pitch fits many 4-cylinder head port spacings closely, the 38–40mm bore is correctly sized for engines in this displacement range at high RPM, and used sets are plentiful. They are cast in pairs, which means the linkage must be modified for car use, but this is well-documented.

For 1.8–2.0L engines, the Yamaha R1 (45mm) or Suzuki GSXR 1000 (42–44mm) are the usual steps up. The R1 has a secondary butterfly that must be removed or locked open for race use.

The most important consideration is not bore size — it's the centre-to-centre pitch matching your head's port spacing. If the pitch does not match, you need fabricated adapter plates or, as on the PPP-106 project, a machined and welded port re-pitch section. This is achievable but adds significant cost and complexity.

Almost certainly because the TB is too large for the engine. Use the reverse checker above — enter your bore diameter and engine spec and check the air velocity result. If it's below 65 m/s, the bore is oversized.

When air velocity through the throttle is too low, two things happen. First, the pressure drop across the butterfly at small openings is reduced, making the throttle feel vague and non-linear off idle. Second, fuel atomisation suffers — air moving slowly through the bore carries fuel droplets less effectively, leading to poor mixture formation at part-throttle. The engine runs fine at wide-open throttle but feels dead everywhere else.

The fix is usually to fit a smaller TB, not to remap around it. A correctly sized bore is always preferable to a large bore with the mapping adjusted to compensate.

Yes — always. ITBs fundamentally change the intake characteristics of the engine. The original ECU map was written for a different airflow path, different intake pulse behaviour, and almost certainly a different throttle response curve. Running a remapped engine on a stock map after fitting ITBs will result in poor idle, flat spots throughout the rev range, and potentially a dangerously lean mixture at high RPM.

In most cases ITBs also require a standalone ECU rather than a modified OEM unit, because OEM ECUs are often locked to specific fuelling strategies that cannot accommodate the individual cylinder control and TPS-based load sensing that ITBs demand. Megasquirt, Omex, Life Racing, and MoTeC are all commonly used with ITB setups.

Budget for the remap and ECU when costing an ITB conversion — it is not optional and typically costs as much as the throttle bodies themselves.

With a vacuum sync tool — either a 4-gauge analogue set or a digital unit like the Digi Sync. Both are linked in the tools section above.

With the engine at normal operating temperature and at idle, connect a vacuum gauge to the idle port of each throttle body in turn. Adjust the idle stop screws on each body until all four gauges read within 1–2 cmHg of each other. Work from cylinder 3 as your reference — it is the most stable on a 4-cylinder because it has no adjacent shared passage.

ITBs will need re-syncing periodically — cables stretch, throttle shaft seals wear, and heat cycling causes components to shift. On a competition engine running high revs and temperatures, check sync at every engine service. A set of out-of-balance ITBs will idle roughly, make tuning difficult, and mask genuine mapping problems by introducing cylinder-to-cylinder variation.

Trumpet length determines where the intake resonant peak falls in the RPM range. Longer trumpets push the peak down; shorter trumpets move it up. The relationship is approximately:

Resonant RPM ≈ (83,000 × speed of sound) / (trumpet length mm + effective port length mm)

In practical terms: for a 1.4–1.6L engine targeting a power peak at 7500–8500 RPM, you want short trumpets — typically 60–100mm. The R6 throttle bodies come with integral trumpets designed for a 600cc engine at 15,000+ RPM. On a 1400cc engine at 8500 RPM these are too short and the resonant peak will be higher than ideal, which can be addressed by fitting longer aftermarket trumpets or fabricating extensions.

The honest answer is that trumpet length is best optimised on a rolling road with back-to-back tests at 10mm increments. The formula gives you a starting point — the dyno gives you the answer.

For peak power on a naturally aspirated engine, ITBs will almost always outperform a single TB of equivalent total flow area — because ITBs preserve intake pulse energy that a plenum dissipates. The resonant ramming effect in an ITB setup adds real volumetric efficiency at the tuned RPM point, which translates directly to power.

However, a well-designed single TB setup with a properly tuned plenum can get very close to ITB peak power numbers on a road or fast-road engine — and will generally produce better torque across the mid-range because the plenum smooths out the resonant peak into a broader curve.

For a purpose-built competition engine targeting a narrow RPM window at high revs, ITBs are the correct choice. For a fast road or hillclimb car that needs driveable power from 2000 RPM upwards, a single TB with a well-developed plenum is often the more practical solution.