just installed a new 496 mag in my 89 formula i just plugged in the harness and started but i have no tach my guess is the signal is wrong for the tach vdo i had this same prob w my 357 when i put msd ignition in i needed tach adapters but this system is 8 coils no dist anybody got any ideas

I Would Guess Your Tach Is Analog And Your Motor Is Outputting Digital.

I Put 496's In My '89 Scarab, But The Only Gauges That No Longer Worked Were The Oil Temp.

I went from a '97 454 Mag MPI to a 496 HO, no problems. I guess your old engine was a carb? I think MPI/EFI and carb engines have a different harness.

I would compare a schematic of the old engine harness against the new.
I bet the tach wire is on a different pin.
Or you have a broken wire.
Bad grounding or a bad ignition switch can screw with your tach as well...

What ignition are you running? What ignition was on your old motor? My guess is that you have the Merk Racing tachs? These ONLY work on the old style T-Bolt ignition, it's an outboard set up.