Originally Posted by
steveo
knock frequency is pretty much all about the bore size. although temperature has a small effect as well because physics, and probably a few other things, so you do need to keep the width of your bandpass a bit wider than expected.
i have the following calculation that has served me well i forget where i got it. where n = bore mm: knock khz = 900 / ( 1.570795 * n )
also have this spreadsheet i found that apparently calculates it with higher precision if required (attached)
the knock module at a simple level is definitely like a band pass filter, and when you get a signal of sufficient amplitude in there, it simply goes KNOCK!!. it sounds simple but the amount of noise coming out of an engine is really extreme.
i don't think detecting knock is ever a problem, and the stock knock module's band seems wide enough to handle overbores. the problem is these engines are really noisy and clattery and so are the ancient chassis that they are attached to - so there can be lots of other stuff that crosses in that band and cause tons of false knock, especially once modified with fancy valvetrains and looser clearances in the rotating assembly
i had success once by installing a capacitor that functioned as a high pass filter on the sensor itself. it seemed to alleviate some valvetrain noise that was causing false knock but still allow knock detection. i calculated the high pass to start pretty much right at the knock frequency then induced knock by temporarily cranking up timing advance to test the results.
if you want a really good knock system you could probably figure a good way to replace the module with a small programmable DSP so you could configure your own filters
personally when i have run into issues with knock detection on a naturally aspirated engine with an antique ECM, i usually tune timing advance conservatively and then the thing goes right in the garbage can -- especially on a track car or something where fuel is consistent, and a false knock can cost you horsepowers.