69z with petronix ignition

[canadair]

the tach in my car reads accurately but as you accelerate it fluctuates and flat lines the tach is new ,changed  to a new seperate harness did the same, is it the petronix ignition.

[rat pack]

The new repro tachs I don't think can handle the breakerless ignitions and neither can originals. Eventually you will burn the tach up. Check with John at Redline Gauges to see if he can rebuild your existing tach to handle the Pertronix ignition. I had my original Stewart Warner tach rebuilt by him with the new electronics and it works fine with the Pertronix kit in my stock distributor..........RatPack..........

[tmodel66]

You need a Tach Filter on it. Simple to build and install. Here are directions.

http://firstgens.com/tech-articles/46-tach-filter.html

[jeff68]

What Pertronix are you using?  If it is the standard Ignitor unit in the stock distributor, then I'm not sure why you are being told that the factory tach won't work or that you need a tach filter.  The tach should work fine since it is still getting its signal from the negative side of the coil.
Am I missing something?

[tmodel66]

I said he needs a tach filter because he said it jumps all over the place on acceleration. The tach filter will stop that.

[canadair]

called petronix they recommended a 10k half watt resister tried it today it worked.

[tmodel66]

That's basically what the filter is with a 270 ohm 1/2 watt.

[jeff68]

So why would an original tach need a tach filter after installing a Pertronix (assuming it is just an Ignitor unit and nothing else was changed at the same time)?   Isn't the Ignitor just an electronic switch that takes the place of the mechanical switch (points)?  How/why would it introduce 'noise' into the system that needs to be filtered out?  I'm just trying to understand what effect the Ignitor is having on the tach.

[tmodel66]

Read this link.

http://firstgens.com/tech-articles/46-tach-filter.html

[jeff68]

^^How about a cut-and-paste.  That link requires forum registration.  I really don't feel like registering for another forum.

[68Zproject]

If you're tach is troubled by a bouncy needle, or the needle "floats when you get on the gas, this little filter will solve the problem.

Why the tach bounces: A tachometer "counts pulses" If the pulses from your ignition are not smooth, the tach may see more or less than there really are, and this can cause erratic needle movement. With most aftermarket high performance tach's, this may not be a problem, but if you have a factory tach, and add an HEI or other high powered electronic ignition, you may get a bouncy needle. This device will smooth out the tach needle on any engine.

What the filter does: This is a simple "low pass" filter. It contains a resister and a capacitor. Low frequencies are passed through to the tachometer, but high frequencies are sent to ground. In this filter, the cutoff is about 10,000 rpm with a V8 engine.

Will it affect my ignition: No, it can't. It is a completely passive device. The resister isolates the ignition system from any effect.

Can it hurt my tach: Again, no it cannot. In fact, it's ability to free your tach from unwanted hi frequency, high voltage spikes may help extend the life of your tach!

What if I want to build my own: If you can solder, and you have a Radio Shack nearby, no problem. You'll need a 270 ohm, 1/2 watt resister, a 1 ufd electrolytic capacitor, and some wire, terminals, and shrink tubing.

[jeff68]

^Thanks for the explanation.  I hate to keep asking questions, but where would the unwanted high-frequency high-voltage spikes come from when using something basic like the Pertronix Ignitor installed in an otherwise stock ignition system?  The Ignitor isn't an HEI or high-powered ignition system, it's just an electrical switch installed in place of the points.  I'm asking because I have an Ignitor installed in my stock system.  Also, I'm a ME and not an EE.  I'm wondering if a filter should be installed when using a Pertronix Ignitor even if there are no signs of tach issues.

[JohnZ]

When Corvettes went from point ignition to the HEI distributor in 1975 (and went from a mechanical tach to an electronic tach at the same time), they added a "tach filter" as well. Don't know the specifics as to why, but they wouldn't have added the cost of the filter if it wasn't necessary.

[68Zproject]

Beats me, I just copied their explanation from that site.  I didn't want to join another forum, but I knew the guy from TC and I needed some wiring diagrams so I joined up.