Is the tach filter really needed?
Thread Starter
Le Mans Master
Joined: May 1999
Posts: 6,692
Likes: 29
From: Texas
Is the tach filter really needed?
Quite a few years ago, replaced the defective tach filter with a replacement. Not, when rpm's go over 2500+, tach just jumps around. Does this mean the tach filter is bad - again. Do I really need one. Did not observe any abnormalities when not having filter. Thinking about going back to no filter again.
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Wire broke at my orig tach filter. Tried running without and the needle went haywire. Fab'd one from Radio Shak parts and it worked for a while. Then, I found a tach filter from a late 70's-early 80's (Monte, Regal, Cutlass) G-body for $25 on eBay and no issues since. Direct replacement parts.
Safety Car
Joined: Nov 2005
Posts: 3,532
Likes: 1,185
From: Arizona - If you don’t know CFI, STOP proliferating the myths around it...
Is the tach filter really needed? Well, that depends. If your tach signal is clean, then no. example: Electrical noise generated by a modern HEI ignition system can make tach readings appear erratic. The remedy to this is to add a tach filter to “clean up” the noisy signal, and therefore smooth out the reading on the gauge. Hope this helps.
Last edited by Buccaneer; Feb 20, 2018 at 11:18 AM.
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Le Mans Master
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Like others have mentioned, it depends on the noise of the signal. I have a Mallory electronic distributor in my '71 and no problems without a filter.
Le Mans Master
Joined: Dec 2006
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The filter does 2 things.
It filters the HEI output for more accurate display.
It protects the tach circuit board from spikes, etc.
Le Mans Master
Joined: Feb 1999
Posts: 8,351
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From: Tampa Bay FL
Looking at the engine from the passenger side fender, it is just forward and below the base of the distributor (between distributor and carburetor). It is much more visible with air cleaner and EGR valve removed.
Le Mans Master
Joined: Jan 2001
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I had to remove the filter when using the signal from my MSD-6 box. It would read ok up to about 3500 rpm, but as soon as I went above that it would immediately start cycling wildly. I read up on it, removed the tach filter, and it was immediately noticeable on my next outing that the tach no longer bounced, and that it was nice and steady up to my 5800-6000 rpm limit....with no bouncing.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
I had to remove the filter when using the signal from my MSD-6 box. It would read ok up to about 3500 rpm, but as soon as I went above that it would immediately start cycling wildly. I read up on it, removed the tach filter, and it was immediately noticeable on my next outing that the tach no longer bounced, and that it was nice and steady up to my 5800-6000 rpm limit....with no bouncing.
It IS a requirement with the MSD systems.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
The tach filter has nothing to do with radio interference. It is there to clean out the random high frequency spikes [noise] in the vehicle's DC wiring/ignition system. Do you NEED it? Only if you want to be sure that the tach is reasonably accurate.
Can someone's tach work without "jumping" if it does not have the tach filter installed? ........maybe.......
Can someone's tach work without "jumping" if it does not have the tach filter installed? ........maybe.......
Last edited by 7T1vette; Feb 23, 2018 at 02:45 AM.