Horse Power and Torque

Probably more likely some sort of harmonic vibration, either a fan out of balance, or an odd belt situation causing vibration. "Back in the 70's" when I swapped a 340 into my old 70 440-6 RR, I eventually adapted the "hang on" AC onto the SB. I ended up with "just the right" (wrong) belt situation on the AC that at about 3 RPM points that thing would vibrate like you just cannot believe

I was even more ignorant back then than I am now, and it took me a couple-three months to realize that all I had to do was flip the eccentric belt adjuster and that would change the belt length.