Found this article about dyno testing standards that Ford used
Ford-men legends
odstSrnope1f4hM htftrl 04ifm:6101a00h26a553M1uaAgm ig m7hfcl ·
CORRECTED HORSEPOWER (A, B or C curve)
While digging through the ASE Papers for the 1966 Fairlane GT 390-4v engine, we noticed an asterisk that said "Max. bhp (brake horsepower) and max torque corrected to 60°F and 29.92 in. Hg atmospheric pressure." This method is called the STD Correction Factor and it presents an inflated perception of horsepower and torque increases without any actual increase in power.
The SAE (Society of Automotive Engineers) correction factor is now the generally accepted "Standard for Comparison" as used by virtually every professional race team and engine builder/manufacture (SAE J1349 and J1995 are the standards and SAE J2723 is the procedure). This is the most accurate and widely accepted form of measurement when it comes to the world of dynoing.
Uncorrected horsepower is NEVER accepted in the world of dynoing and tuning. The correction standards were developed to discount the observed horsepower readings taken at different locations and weather conditions. It is obvious that an engine builder in Colorado could not produce as much horsepower as a shop at sea level. There is just less oxygen for the engine to burn at the higher altitude. What are less obvious are the other weather condition effects on the engine. So in order to compensate for this all advertised horsepower is “corrected” to several different industry standards, here are the main two:
SAE (Society of Automotive Engineers) power is corrected to reference conditions of 29.23 InHg (645' above Sea Level) of dry air and 77°F. This standard requires a correction for friction torque.
STD is corrected to reference conditions of 29.92 InHg (Sea Level) of dry air and 60°F. Because the reference conditions include higher pressure and cooler air than the SAE standard, these corrected power numbers will always be about 4 % higher than the SAE power numbers.
A, B or C
Ford also choose what state of tune or readiness to list the advertised horsepower numbers, known as the A, B and C curve. Prior to 1968, Ford would advertise "Engineering A-Curve" power. That was the maximum output you could get out of an engine, an older engine with looser clearances than a new one and with dynamometer headers. Then there was the B-Curve, which was the engine with no accessories, and the C-Curve was "as installed" in the vehicle, which included back pressure. Uniquely with the Cobra Jet, Ford started advertising the B-Curve power rather the Engineering A-Curve. The 428CJ was capable of approximately 405 hp (A-Curve), 325-335hp (B-Curve) and 310-320 hp (C-Curve). NHRA and others immediately refactored the Cobra Jet to 360hp for class racing.
In 1966 Bud Moore dyno tested three 390 GT engines with stock exhaust manifolds with results in the 290hp range. This was done with three brand new engine with tight clearances and higher-horsepower-robbing friction. Adversely, Ford's A-curve was for a well-used, high mileage engine with looser clearances, and dyno headers, which is not what Bud Moore had. Furthermore, Bud Moore didn't correct to STD like Ford did. These two factors may explain the difference in what Ford advertised and what Bud Moore tested.
It wouldn't matter to Canadian Barrie Poole of the Border Bandits with his 1966 Cyclone convertible 390GT called "The Collector" he won 34 trophies and one Eliminator title.
