I installed the technoversions tachmatch voltage booster on my early 260z today so I can have the stock oem tach working off of the msd 6a ingition direct. at first it was about 1500 rpms to high, and when I did start playing with resistors, I found that a 35k resistor worked with accuracy. the only problem I have is when the electric cooling radiator fan comes on the tach drops down and then back up to normal after the fan is running. now my thoughts are to maybe add a diode in line with the re