as stated earlier SOHC are suppose to be more efficient than OHV engines. DOHC engines are more efficient than SOHC engines. the DOHC engines (4 valve per cylinder) have a better design to allow for better spread of the fuel mixture and burn of the flame in the cylinder. and to 984.6gt. have you ever heard of someone putting the cams in wrong(intake cam on exhaust side and visa versa)?? i've had to correct those types errors many times because people don't pay attention to what they are doing. I'm not saying that is what happened but something that may have happened.