In my state, PA, they use an inductive pickup on the spark plug wire for cylinder #1 to determine the RPMs and a probe that's inserted into the tailpipe. The pr

In my state, PA, they use an inductive pickup on the spark plug wire for cylinder #1 to determine the RPMs and a probe that's inserted into the tailpipe. The probe is a small diameter (0.5" or less) tube and it shouldn't raise the back pressure by any appreciable amount. Maybe an EE could tell us if it's possible for the ECM to detect the presence of the inductive pickup.