ITB

An ITB (individual throttle body) is a fuel injection and intake architecture where each cylinder receives its own dedicated throttle valve, rather than sharing a single throttle or fuel injector rail. Each throttle body sits directly above its cylinder's intake port, controlling airflow independently. This configuration is uncommon in mass production but standard in high-performance and racing engines, where intake velocity and cylinder-to-cylinder air distribution matter most.

The mechanical advantage is straightforward: a dedicated 45 mm or 50 mm throttle valve for a single cylinder flows more air at lower throttle angles than a shared valve would for four or six cylinders. Less restriction means less pumping loss and faster throttle response. Each throttle opens as its cylinder approaches the intake stroke, allowing tuners to optimize timing and volume per cylinder rather than managing cross-cylinder compromise. Fuel injectors are typically mounted in each body or in the port just downstream.

Complexity and cost trade-offs

The drawback is mechanical and electronic complexity. Six cylinders need six separate throttle cables (or electronic actuators) and six individual fuel supplies or injectors. Idle control becomes more difficult because each cylinder must be balanced; one sticky throttle body creates a misfire. Tuning requires individual trim pots or programmable ECU tables. Manufacturing cost is substantially higher, which explains why ITBs appear mainly on track cars, vintage competition engines, and expensive sports cars rather than daily drivers.

Historically, ITBs dominated competition in the 1960s and 1970s, particularly in Formula 1 and club racing, when they were sometimes called "carburettor stacks" or "velocity stacks." Modern electronic fuel injection made them technically feasible for road cars in the 1990s onwards, but fuel economy regulations and OBD emissions compliance favored simpler, centralised injection. Today ITBs remain a tuning and restoration choice: classic engines retrofitted with individual throttles and modern ECUs, or bespoke build platforms like motorcycle-derived powerplants in kit cars.

ITBs should not be confused with multi-point fuel injection (MPI), where injectors spray fuel but all cylinders share intake manifold runners. An ITB engine has both individual throttles and individual intake paths. Performance gains depend heavily on engine speed range and intake manifold design; the benefit is most pronounced above 5000 rpm in naturally aspirated engines, where scavenging efficiency and dynamic pressure become critical.