18 volts is correct, depending on the year the set came out and the type of controllers.
12.5 Volts is the original spec for Tyco pro cars, What happened was Early Curve hugger sets were rebranded Tyco Pro sets and there is confusion as to what power supply should be issued.
the issue is Magnatraction verse, Non-magnatraction.
Tyco pro was Non-magnatraction for the most part. the original specs are 60-90 ohm controllers and 12.5 volts. later they changed to 18 volts at 75-90 ohm controllers.
Tyco Hp2 went through about 4 or 5 revisions. but they are magnatraction. The 18 volts carried over to the Hp2 curvehugger versions from 1975 till 1980. but the cars kept changing, Double Flux Plates, then single Flux plates, they tried to reduce the magnetic attraction because the hand controllers would catch fire from the increased amp draw, around 1978 they changed from the Thumb controllers carried over from the Tyco pro era to the Trigger style controllers. the voltage remained at 18 volts and the controllers callibrated to use that voltage, because they needed to be backwards compatible. in 1980ish they went to higher voltage like 20,22,24 volts. then the HP2 had removed the flux/traction plates from the center of the chassis to the rear of the car in front of the rear axle( more traditional) Then the chassis was replaced around 1981 with 440.
If you are using the original controllers, 18 volts is correct. if you are using later controllers depending on the ohms of the controller which can vary from 35-75 ohms, then you may need to play with the voltage level between 18 and 22 volts.
ride height has a big effect as well due to the sensitive nature of the magnatraction flux plates in the early cars, increasing the rear tire diameter may get you into a sweet spot.
lastly the front contact shoes are a major problem the design is REALLY bad and hangs up the Hp2 chassis .
check out some awesome threads here
http://www.hobbytalk.com/bbs1/showthread.php?t=257624
http://www.hobbytalk.com/bbs1/showthread.php?t=257391
applying these modifications will majorly free up the HP2 chassis and stop that ON/OFF throttle feeling you get no matter what voltage you try out.
check your front tires contact with the track if the front end is floating it will explain your voltage issue thats driving you nuts. the front tires must be supporting weight not floating above the track, this requires modification to the front contact shoe support plate.