4-to-6/8 Cylinder Analog Tach Conversion
For All 1987-88 Cougars and Thunderbirds with Analog Cluster
Page Revised: 10 May 2019
If you've swapped over the full analog instrument cluster to your car (or are thinking about it), one of the main roadblocks in having that "perfect" setup is the tachometer. There were two different tachs: one for the 4-cylinder turbo engine (7,000 rpm), and one for the V8 (6,000 rpm). You'll find that the V8 tach is fairly difficult to find, and you're probably wondering if there's a way to convert a 4-cylinder tach over to read accurately for a V8 or V6. This section will help you do just that. Thanks to Tom for the information.
IMPORTANT: This mod is for 1987-88 4-cylinder tachometers ONLY. It WILL NOT WORK on 1985-86 4-cylinder tachometers. If you want to do this mod and you own a 1985-86 tach, so far no technical solution has been made. Currently the only solution is to switch over to a 1987-88 tach and reuse your stock face, or use a 1988 V8 tachometer.
"These tachometers simply react to the pulses fed to it, thus the reason for tachs calibrated for 4 or 8 cylinders. Mustangs had a convenient 4-6-8 switch that allowed instant calibration; our cars were not that fortunate. The 4-cylinder tach will read double the rpms in a V8. There is only one resistor that is different in the Turbo Coupe vs. the V8 tach. It is located near the minus (blue) wire to the tach movement. It is 100k ohm. Changing it to a 75k for a 6-cylinder or to a 50k for an 8-cylinder should be close. The resistor is a 5% tolerance, as noted by the gold end band. Anything between 95k and 105k is still in spec. Either add another 100k across it in parallel or replace it with a 51k (50k is not a standard value)."