Auto Racing: DOHC and other types of cams, dual overhead cam, dohc motors

Question
i was wondering whats up with OHC and DOHC.

everything i read says that DOHC is prefered over OHC, or whatever, becuase it can get good performance with better mileage, but if that is the case, then why are cars still made with OHC, or some other type of cam?

furthermore can you convert a car from OHC, or whatever,to DOHC?

also an explanation on overhead cam vs other types of cam would be nice also.

Answer
Richard-

  Well there's really only two widely used engines in cars...OHV (overhead valve) and SOHC or DOHC (signle or dual overhead cam).  OHC motors do provide increased efficiency over OHV motors and DOHC motors only increase upon that.  DOHC motors are generally tuned for optimum efficiency and performance since it's easier to do so with an extra camshaft versus a single overhead cam.  But with that extra tuning comes more parts and increased complexity (basically it increases cost) which is why not every car out there sports a DOHC setup.  Whether or not you can convert a SOHC to a DOHC depends on the motor.  If it came from the factory with both options, there's a chance that it's possible.  For instance the Ford 4.6L motor came in both 2 valve SOHC and 4 valve DOHC style and they can be swapped with some work.  As for other types of cam setups, there's only really one other that I know of and that's the spherical valve setup.  It was developed by a company called Coates (maybe Coates Engineering...it's been awhile since I looked them up) and is mainly used on industrial engines although it can be fitted to automotive applications if you've got about $10K burning a hole in your pocket (that was the price they quoted me a few years ago when a customer inquired).  I know they made a custom setup for a 302 Ford that made ridiculous power, but that was a long time ago.  I'm sure they're still around on the web if you want to know more.  The truth is it's been so long I don't remember much about them (I'm amazed I still remember the name).  Hope this helps.

Steve