The compression was also adjusted by the distance the piston is below the deck at TDC.On engines with original standard size pistons that varies from about .010 on some 235's to .035 on 261's.The actual compression ratio and HP is less than Chevy advertised in it's specs by quite a bit in some engines .Chevy also changed the HP ratings to suit marketing stratagies,if Fomoco raised their HP on 6's GM did the same and vice versa.
Big trucks used a net power rating at a lower rpm than the gross HP the cars used.261's for example are rated from 123-150 HP depending on the RPM and model year.The only actual dyno tests of a 261 comes from a 1955 Hot Rod magizine,a stock 261 had about 115 HP at 3600 rpm.