My 79 ignition system is an MSD pro billet distributor with a 6AL box.  I am hooked up from the Tach output on the 6AL to the tach wire under the dash.  The tach filter went bye bye with the L48 engine.  The tach moves but reads 3 or 4 times the accual RPM.  Should I get a stock style tach filter, or the MSD tach adapt? (not the mechanical)  Or is the output on the MSD "clean" enough that this is a symptom of a bad tach board?

I would see if I could contact MSD, and see what they recommended. They may have a specific filter for you application...

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Joel Adams
C3VR Lifetime Member #56    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Yuo might consider this filter:  http://www.summitracing.com/parts/NVU-99004-04/

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Sorted out the Tach / MSD 6al issue.  It was the tach board.  Not a big surprise. MSD output on the ignition box is compatible with the corvette tach with no mods.

I'm running the same set up on my 79, however, I am using the tach wire from the distributor plugged into the 6AL. Reads perfectly - also dumped the tach filter with no problems.