Tachometer

[1967RS]

My factory in-dash tachometer reads 200 rpm higher at idle and almost 400rpm higher at freeway speeds. Since you guys have helped solve all my other problems,I thought I would throw this one your way. By the way, I have a GM HEI distributor. a friend told me this may be the problem and I need to install a "electronic signal filter" ??

[JohnZ]

HEI started in 1975 in Corvettes, and they had a "tach filter" between the distributor and the tach.

[BlackoutSteve]

Hmmm, now I'm wondering be if I have an MSD (85551) with magnetic pickup, am I'm going to need the same thing?
Will a 6AL make a difference as well?

I just found this..
http://www.msdignition.com/page.aspx?id=3300&terms=tach+filter
..and this..
http://www.msdignition.com/info.aspx?taxid=256&taxid2=267

[melav8r]

Heres a link for a home made filter. I had to make one when I switched to HEI, works great, best $2.00 you'll ever spend.

http://www.camaros.net/techref/electrical/tachfilter/

http://www.camaros.net/forums/showthread.php?t=115010

[dutch]

HEI started in 1975 in Corvettes, and they had a "tach filter" between the distributor and the tach.

John..
  I installed a Pertronix setup in my '68 Zed a couple of years ago and didn't notice if it was a corresponding problem time-wise, but my tach (Sun Super Tach II) started pulsating on a non regular basis about or close to the time of the Pertronix installation. I read to check for wires being in proximity of a plug wire or similar logical stuff that might cause such a problem - but nothing is obvious or has been found to cure the problem.
  Would or can a Pertronix unit (its the newer version - sorry the exact model name escapes me right now) cause the same type of problems as a HEI installation or something similar to this?
  My tach seems to read in the correct RPM range it should when at idle or driving - it just twitches up the scale every secong or two like its getting a signal from somewhere jolting it... or maybe it just needs replacement or a get a rebuild if that is possible somewhere.
  Sorry of this sort of highjacks this thread but it seemed somewhat related...
   Thanks - Randy

[JohnZ]

John..
  I installed a Pertronix setup in my '68 Zed a couple of years ago and didn't notice if it was a corresponding problem time-wise, but my tach (Sun Super Tach II) started pulsating on a non regular basis about or close to the time of the Pertronix installation. I read to check for wires being in proximity of a plug wire or similar logical stuff that might cause such a problem - but nothing is obvious or has been found to cure the problem.
  Would or can a Pertronix unit (its the newer version - sorry the exact model name escapes me right now) cause the same type of problems as a HEI installation or something similar to this?
  My tach seems to read in the correct RPM range it should when at idle or driving - it just twitches up the scale every secong or two like its getting a signal from somewhere jolting it... or maybe it just needs replacement or a get a rebuild if that is possible somewhere.
  Sorry of this sort of highjacks this thread but it seemed somewhat related...
   Thanks - Randy

I don't have any experience with Pertronix units and tachometers - you might want to contact Pertronix and ask them.

[BILL69]

I have had this type of problem with a repop tach and a MSD distributor.   MSD said to use a tach adapter module between the coil and tach,  it did not cure the problem.   MSD then sent me the filter free of charge, it worked.  I don't know if the same type of signal mis-match is occuring with the HEI but it seems to be so.

[rich69rs]

Pertronix conversion and my dash tach worked just fine - no issues.