virtual tach no start avital 4103

This would be impossible to do.  The starter is a high current device and the voltage will ALWAYS drop when cranking.  It is impossible to use voltage monitoring to control the crank time.   To compensate for this, you program in a crank time that "should" work.  The remote start engages, fires the starter for as long as you programmed, and then a few seconds later checks the voltage of the system.  If it is above x.xx volts the remote start knows the car started, if it is below x.xx volts then it knows the attempt failed.

Hooking up a tach reference is the ONLY way to provide closed loop monitoring and control crank time.  When a motor cranks it is typically around 200-300RPM.  When it starts it is generally between 8-1200rpm.  Since the tach signal references motor speed under both conditions it is easy to electronically determine when the motor has started. 

The main problem with voltage alone is that generally over time motors take longer to crank before they fire up.  So, it may work now, but in 3 years from now the crank time may need to be extended.  The tach wire, on the other hand, will adjust and continue to work.

The virtual  tach was created as a way to combat the issues with starting over time.  As someone else already mentioned it is the lazy way to install a remote start and a tach wire should always be hooked up.