The Rocketdyne F-1 was to be updated with a new turbopump and gas generator assembly, without other major component redesign. This was to result in a thrust increase from around 1,522,000 pounds force for the F-1 to 1,800,000 pounds for the F-1A. Specific impulse at sea level was to have grown from 265 second to between 269 and 271 seconds, and chamber pressure would be increased from 1015psi to 1,161psi.
However, the combustion chamber dimensions and nozzle area ratio were not altered; so why does the higher-pressure F-1A feature a lower listed vacuum specific impulse of 303 seconds compared to the F-1 at 304s to 305s?
I am aware Astronautix applies a 'six second improvement in specific impulse' for both vacuum and sea level Isp - meaning 270s SL and 310s Vac. However other reports disagree; and during my searching the opposition has grown in number.
Why would a pressure improvement with no changes in chamber dimensions cause lower efficiency?
Could it have been a case of testing at sea level (F-1A was never flown), where vacuum trials were actually conducted at imperfect low air pressure rather than none?