DOHC setups use a seperate cam for intake & exhaust valves. Most DOHC engines use two intake & two exhaust valves per cylinder, though there are some DOHC setups that use a single large intake & two smaller exhaust valves. The OH indicates that the cams are located in the head(s) of the engine rather than being located in the block above the crankshaft. This allows the lobes of the camshafts to bear directly on the valve stem(s) eliminating the need for lifters & pushrods & allowing less lash & slop in the mechanicals of the engine, this leads to better efficiency.
SOHC engines use a single cam in the head(s) to control both the intake & exhaust valves, typicaly only one of each per cylinder.
Any gasoline engine is nothing more than a big air pump, the more air that can be run through the system at a time, the more power that engine can produce. Multiple valve setups allow more total surface area for the air to flow through, so typicaly these engines can make more power, usually in the upper rpm ranges as Pete describes above.
Regardless of SOHC, or DOHC, either is superior to the old pushrod configuration as far as efficiency goes. You eliminate a lot of valve train parts and slop.
Mostly referring to auto engines. Do any MC engines currently in production use a pushrod design? I suppose at one time they must have, but I know very little about 4 cycle MC engines except the Burgman.
The big Yamaha's also use pushrods, as do a few others. Pushrods limit the rpm that can be attained from an engine, but reduce the height of the engine. In big twins that want big torque, high rpms are not required - so they work fine for them.
Regardless of the number of valves they are operating, the biggest advantage to dual overhead cams as opposed to single overhead cams - is that the intake and exhaust timing can be totally independent.
Many single cam engines have stronger bottom ends - but that's probably related to having less valve area (ie: fewer valves). More valve area normally equates to more horsepower, less torque.
This is just personal opinion. but when an engine gets as small as a 400 cc 1-cylinder, in a scooter-type application where performance is not the only factor to consider, I think SOHC is superior. There is less less cost up front and, even more importantly, less to go wrong. If there were a significant MPG difference involved (I suspect there is none, but don't know) then I might feel the additional complexity is worth it. But, depending on the amount of MPG gain, I also might not. I simply do not like needless complexity in things mechanical. It gets you every time, sooner or later.
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