Mark123
In large radial engine aircraft engine power (Torque) was directly measured and indicated on seperate dedicated (BMEP) guages. However there is no guage in a light aircraft which directly measures engine power. This is because for a non turbo/supercharged simple flat 4 or 6 cylinder light aircraft engine running normally, there is a very strong correlation between the induction manifold pressure and the power output. POH cruise charts show the percentage of power developed at a variety of RPM and MP combinations. Most will have a column for % of power and corresponding MP's at maximum RPM....or the takeoff condition. If at sea level your engine is only showing say 25 in at full RPM than it is only making about 75% of it s maximum horsepower....which IMO would be a good reason to abandon the takeoff.
Bottom line there is no way a normal smooth running engine with correctly calibrated engine guages could display full MP (close to field baro) and full RPM and not be making full power for those conditions.