Originally Posted by martinbo

Mirage, that's a good question... and I'll answer it in an unscientific layman's fashion becuase I'm too lazy to go and look up the technical details.

The 5 valve head first appeared in Audi products in the mid to late 90's in the A4 1.8 litre engine. If I remember correctly it was a double overhead cam unit with 3 inlet valves and 2 exhaust valves per cylinder. It was subsequently turbocharged with fabulous results ushering in a new era for mass produced turbo engines.

The idea behind multi-valve engines is to maximise control of the volume of air / fuel mixture by utilising the maximum surface area of the cylinder head in the combustion chamber. In multivalve engines, the inlet and exhaust valves require less valve lift (how far a valve opens into the combustion chamber) than a 2 valve head to ingest the same volume of air and fuel. Less valve lift means the valves can open and close faster allowing an engine to perform optimally at high rpm - something that 2 valve single overhead cam engines can't do without severly impacting on the all-round useability of the engine.

5 valve engines were quite in vogue in the late nineties: VW Audi 1.8T, Ferrari V8, Toyota 1.6 RSI, Yamaha R1.

With the adoption of direct injection in the VW FSI engines, the centre inlet valve has been removed as a necessity to make way for the injector that sprays fuel directly onto the head of the piston.

Interestingly, 5 valve engines appear to be out of favour again... both the Yamaha R1 and Ferrari F430 returning to a four valve head.