[Dacious]

396 was a standard Big Block size, but most big-block Vette's had 427s. 66 'vettes did come with them. 396s were usually used in trucks and standard-type cars although the Chevelle SS used them.

Camshafts are a shaft with 'cams' which are elliptical lobes or bumps to open the valve and allowing it to close gradually. They are turned by a gear, belt or chain off the crankshaft, and critically timed to operate the intake and exhaust valves so they don't hit the piston in the cylinder to let fresh air-fuel mix in and burnt gasses out.

Older motors (setting aside flatheads which haven't been made since the fifties) used overhead valves, where the camshaft opens both intake and exhaust valves, is in the engine block, and operates the valves via lifters and pushrods. Examples of this include the current cast iron GM V6, and aluminium V8 and Harleys which on the Sportster 4 cams - one per valve!. Advantages include lighter and simpler cylinder head castings and simplified drive, lower tolerances required in manufacture. Disadvantages include heavier valve trains with lower rev limits and heavy wear on components in high-performance applications. The intake and exhaust port shape and cylinder head/piston top shape is severely compromised meaning higher-octane fuels are needed for the same performance and economy as more advanced designs. Fitting multi-valves per cylinder serverely constrained by the restrictions on the valve train. 'Valve float' where the springs lose control and can't shut the valve is a problem, and a precursor to piston/valve contact which is usually catastrophic. 1920s technology

The next is a single overhead camshaft (per bank of cylinders - so one on each head on a v-engine like the older Mitsubishi V-6), where the cam sits in the head and again operates all the valves. You can easily get multi-valve setups, but the valves still need rockers to operate them, the cylinder head shape is compromised by the need to design the valvetrain components correctly to reduce wear on the cams and rockers which have high loadings on the bearing parts. In high-performance applications the rockers bend and break, they still weigh a lot at high revs needing stiffer springs which cause more wear problems to contol them etc. 1930's technology. Generally used in economy vehicles where performance is no issue.

Double overhead cams means on one cylinder bank there are two camshafts. So quad cams on a V6 or V8. An exhaust cam operates all the exhaust valves. An intake cam operates all the intake valves. They are sited directly above the valve, and push on it through a lifter.

These are a natural for multi-valve setups. 4 smaller valves have more valve lift area than 2 big ones and can intrude less into the cylinder for any given cam-lift while still flowing more air-fuel mix meaning more power, and reduced piston-valve clearances can be used while maintaining control of the valves so they don't touch the piston at high revs.

Advantages include better combustion chamber/piston shape for better burning meaning lower octane fuels can be used without sacrificing performance. That's how come my wife's 1800cc shopping trolley can get 42mpg at a constant 65mph and do 115 mph, and happily run on on 93 octane standard unleaded fuel in all conditions with no pinging, fluffing or farting.

The lack of anything except a clearance adjusting pad between the valve and the cam means lower reciprocating weight, which means lower string tension to control and return the valves to their seat, and less wear, friction and stress. The intake and exhaust ports can be straighter and larger, which increases efficiency of the motor for more power.

The disadvantages are a more complex, taller and heavier head casting, and a more complex drive arrangement and bigger parts count. Friction is also higher - a Cosworth V8 Formula 1 motor loses nearly 60hp just to drive it's four cams and 32 valves.

End of the day, every serious performance vehicle today or in design, from Ferraris, Lamborghinis, street and racing bikes, Formula 1 cars etc all use 4 or 5 valves per cylinder operated by double overhead cams. Twin cams with 2 valves per cylinder were in use by the Italians in the late 30's. Twin cams with 4 valves were in use by the 50's on MV Augustas and the like. GM US is about to introduce a new V6 (made here in Australia) for passenger cars which is quad-cam, 4-valve per cylinder and around 20-30% more powerful than the old one, which has been jiggered with about as much as it can to keep it modern.

Yamaha used 3 intake and 2 exhausts valves in the 80's.