Designed by some of the engineers behind Honda's winning F1 and CART engines, the S2000's two litre DOHC straight four (a configuration chosen for compactness) boasts such race-bred features as roller rockers, forged shallow skirt pistons, ladder frame main bearing stiffener and fibre reinforced aluminium for the cylinders.
It's also smaller and lighter than Honda's regular two litre engine, and at 237bhp sets a new record for highest specific output for a normally aspirated production car engine.
Sources : http://www.pistonheads.com/doc.asp?c=120&i=5155