The IMA engine's single overhead camshaft (SOHC) cylinder head uses a compact chain drive in place of a toothed belt, and features a new compact, low-friction VTEC valvetrain that uses a common shaft for both the intake and exhaust rocker arms. Placing all the rocker arms on one shaft eliminates the need for a second rocker-arm shaft, so the valve mechanism can be more compact.
An additional advantage of this design is its narrower included valve angle that better centralizes the stratified air-fuel charge around the spark plug for quick light-off and more complete combustion.
The VTEC rocker arms are a new, compact, low-friction design, adapted from the S2000 roadster, with the VTEC switching mechanism located coaxially with the roller elements. This new design reduces the reciprocating mass of the valvetrain, for more reliable high-rpm operation, and also allowed Honda engineers to reduce valve-spring load by 30 percent.