I have the formula’s based on article from Auto Speed http://autospeed.com/cms/title_DIY-Deto ... ticle.html that has the following:
Doing just conversions to get frequency in the KHz and converting radius to bore (or diameter) in mm, I got the following formula:Bandpass Frequency
The reverberation resonance generated by detonation generally lies in the range between 2 and 12KHz. The following equation can be used to estimate the detonation resonant frequency for a specific engine.
Detonation resonant frequency = 900/ (3.14 x cylinder radius)
Where the resonant frequency is measured in Hertz and the cylinder radius in meters.
900/(3.14*(Bore/2)/1000)/1000 = KHz
Then converting meters to inches, for those across the pond, I go the following formula:
900/(3.14*(Bore/2)*0.0254)/1000 = KHz
This way the frequency value is more accurate then picking a value from a list.
I also read in Master Thesis by Bridge M. Revier Phenomena that Determine Knock Onset in Spark Engines http://dspace.mit.edu/bitstream/handle/ ... sequence=1 that if you use the second harmonic it reduces the interference from other engine noise. If this can also be put in the design of the inputs this should help.
If someone has better formulas for calculating the correct frequency then by all means use them, the more accurate the better.
Note: Knock should not be used if you are pounding (racing) you engine. If you want to pound the engine use higher-octane fuel and remove the knock. It should be used if you are on the street and using low-octane fuel and do not want to damage your motor!!!