Hot Rods How Much Horsepower Does The Average Flathead Make

Harley may have had one over on Ford. The Standard H-D 45 ci Flathead was rated at 22 to 25 hp, but there was a loop hole in dirt track racing that allowed engines up to 750 cc (45 ci) with a flathead configuration, Overhead valves were limited to 500 cc. H-D would coax 55 to 60 hp out of their 45 ci KR engines, good for 125 mph on the dirt. If Henry's 239 was that efficient, it would have put out around 300 hp!

Stock WR Flathead.


1959 KR

 

They Fords were making 300+ in the mid 50's. They were not 239's though.