Auto Racing: OHC vs DOHC, better mileage, honda 1

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
That is a fairly broad question. The reason some car manufactures are still producing SOHC vehicles is something I really don’t have much information on. It is probably all relative to cost efficiency. In regards to MPG and performance loss I would say that is not necessarily true. The Honda 1.6L SOHC engines get phenomenal mileage, and can sustain VERY high levels of horse power. Some cars are capable of being converted to DOHC but the cost associated with doing so is general high. There are 3 types of cam variations

SOHC- Single over Head Cam
DOHC- Dual over Head Cam
Internal Cam – Exactly as it sounds, the cam is inside the engine. This is a predominate feature on older V-8’s, I.E. 5.0L Mustang.

If you need further explanation please let me know.