Join Date: Mar 2002
Location: Austin, TX, USA
Re: Fyi oer tach msd ignition
The filter Jim M posted is a simple Pi filter that gives a good 2nd order response meaning it will quite sharply filter the fuzz of the tach signal while leaving plenty of signal to feed the tach. The main advantage is that the filter is adjustable and it's cheap. Set the control to the max, start the engine and turn it down until the tach begins to function. You might give it a little nudge beyond that and you should be good. You should be able to put one of those together for less than $10 so it's not much of a risk to try. Someone with a good junkbox of parts could build it for free.
MSD presents some challenges. The multiple spark discharge below 2000 RPM is a challenege for any tach to use. Mosd of those 6AL boxes have a tach feed signal line that should be much cleaner than using the coil - not sure if the OP was using the Tach signal or traditional coil-connected signal.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI