10 fastlane
techtorque
MAKING THE MOST
OF MODERN FLAT
TAPPET
CAMSHAFTS
Story and Photos by Wayne Scraba
IS THE FLAT TAPPET CAMSHAFT A DINOSAUR ?
Roller lifter cams are today ’ s norm . But flat tappet cams still see considerable use . Up until a few short years ago , NASCAR mandated flat tappet cams on their Cup Cars . In NHRA Stock Eliminator , there are still plenty of good oldfashioned flat tappets in use . The same applies to countless hot rods , vintage musclecars , and street machines .
Flat tappet cams that were once considered state of the art are no longer . What we ’ re seeing today are cams with more radical opening and closing rates similar to the “ square nose ” cams developed years ago for NHRA Stock Eliminator racers ( see the accompanying photos ). These cam profiles place more area under the curve , which makes for improved cylinder filling as well as enhanced exhaust pumping .
Modern valve spring and valvetrain parts allow the use of radical flat tappet cams . A good example is the beehive-style spring . The beehive configuration allows the valvetrain to handle more RPM and more aggressive cam profiles . The spring ’ s oval / multi-arc wire shape places the maximum area of the wire at the point of highest stress . This allows the spring to handle valvetrain loads more efficiently and provides better heat dissipation for longer life . Beehive valve springs are also lighter and use a smaller , lighter retainer than traditional springs .
Of course , flat tappet cams do limit the spring pressure you can safely use , particularly in comparison to a solid roller . Spring rates for flat tappet cams vary by application . For example , the seat pressure on a hydraulic grind will be less than 135 pounds with an open pressure of 350 pounds maximum . A solid cam can have a seat pressure less than 160 pounds and an open pressure of 385 pounds .
Pushrods are another part that has undergone change . For decades the standard small block Chevy pushrod was a 5 / 16-inch diameter job . It was deemed sufficient for almost any cam you could stuff in the engine . In today ’ s world , most pro engine builders will advise you to use the biggest , stiffest pushrod you can get away