Hi5,
Detonation does not occur at a single fequency, but is affected by the pressure in the combustion chamber at time of detonation.
The following FFT analysis of a detonating engine shows the knife shaped light blue marks at around 6kHz spread themselves over a range of frequencies.
The forged piston may have a dampening affect on the signal, but will not affect the frequency of knock.
The marks between 2 and 4kHz are just engine noise.