[1beautifuldaughter]

Anybody know if the addition of petronix ignition would affect factory 71 gto hood tach?? The tach worked fine when I got the car ,, then before petronix was added it started so that the needle would stop at and not pass about 2000 rpm, Car is not used much so its hard for me to remember if it did anything else funny before petronix was added, But now it seems to not be reading proper,, seems low compaired to engine speed. thinking I should try to hook it up to my 68 buick and see if its the same, anyone out there know this stuff, also anyone who services a factory tach? if the tach turns out to be the issue. I have heard that a aftermarket replcement can be spotted from 100 ft away, is that true? thanks

[orejudge]

I have had several pertronix with factory tachs and work fine. They read off the coil so should not be a problem.

[Peter Serio]

GM-HEI including all Modern transistorized and electronic conversion kits manipulate the dwell angle in real time to optimize spark performance.

That said all of the original factory Hood tachometers use the old GM circuit which is not capable of doing anything more than de-coding the opening and closing of a set of points. On all GM V8 engines that came stock with points ignition the dwell angle is fixed at 30 degrees.
Therefore the tachometer circuit is based on that figure.

I have seen some people run their (old) original in-dash or hood tach(s) with pertronix and then claim that it works "just fine" but in reality the error after the conversion is typically greater than 500/800 RPM as it's reading a changing dwell; if it even works at all (many do not). I would not trust it. The points tachometer was never designed for that.

There is a completly different circuit for the tachometer that is needed to de-code the RPM signature signal off of anything that "plays" with the dwell angle.

[tekheavy]

I installed the Pertronix II ignition a couple of months ago. My in dash tach works fine. I'd assume a hood tach should work just as good.

[hakansan]

After installing a Pertronix II ignition my hood tach stopped working...

Håkan

[1beautifuldaughter]

Originally Posted by Peter Serio

GM-HEI including all Modern transistorized and electronic conversion kits manipulate the dwell angle in real time to optimize spark performance.

That said all of the original factory Hood tachometers use the old GM circuit which is not capable of doing anything more than de-coding the opening and closing of a set of points. On all GM V8 engines that came stock with points ignition the dwell angle is fixed at 30 degrees.
Therefore the tachometer circuit is based on that figure.

I have seen some people run their (old) original in-dash or hood tach(s) with pertronix and then claim that it works "just fine" but in reality the error after the conversion is typically greater than 500/800 RPM as it's reading a changing dwell; if it even works at all (many do not). I would not trust it. The points tachometer was never designed for that.

There is a completly different circuit for the tachometer that is needed to de-code the RPM signature signal off of anything that "plays" with the dwell angle.

Wow ,that sounds like, If your input is more than your output your upkeep will be your shortfall. lol So is there an inline addition of a converter or something or is it just a problem to live with, Will it harm a factory tach?

[Old Goat 67]

Listen to Peter. He knows of what he speaks!

Charles

[1beautifuldaughter]

Originally Posted by Old Goat 67

Listen to Peter. He knows of what he speaks!

Charles

Thanks , Yes I can tell just by his verbiage,

[OVERULD]

I added Pertronix on my car shortly after I bought it and my in-dash tach works just fine. If there were a 500 to 800 RPM error it would certainly show up at idle and I have not experienced that at all. It still idles at the same RPM as it did before the conversion.

BJ

[Old Goat 67]

Sir,

There is more to electronics than just the idle RPM.

Why would you suppose there was two different tachs for points and transistorized ignitions back in the day?

Just for glee? I think not.

Charles

[OVERULD]

Charles:
I'm sure I don't know as much as many have forgotten when it comes to automotive electronics and I humbly defer to the greater knowledge of those with more experience than I. A better way to have worded my post might have been that to my untrained eye there was no difference in the indicated RPM at the tach. So until this thread was posted, I was gleefully unaware of the potential for any difference.

BJ

[1beautifuldaughter]

Originally Posted by OVERULD

Charles:
I'm sure I don't know as much as many have forgotten when it comes to automotive electronics and I humbly defer to the greater knowledge of those with more experience than I. A better way to have worded my post might have been that to my untrained eye there was no difference in the indicated RPM at the tach. So until this thread was posted, I was gleefully unaware of the potential for any difference.

BJ

Very well said, I wasn't going to say anything but I think someone needs to switch to decaf.

[Old Goat 67]

BJ,

I will accept that explanation.

I just hope Peter will, I value his expertise here.

We have been loosing too many knowledgeable people here because of comments that appear to want to pick an argument or question their hard earned knowledge. I've been on the phone with three this last week that have had it. We are less informed because of that sort of thing.

Charles

[1beautifuldaughter]

well I'm just a new guy who knows alot about cars but , for sure not everything ,And I have not yet been able to help anyone else here ,But this site has helped me an aweful lot. Info I probably wouldnt be able to learn elsewhere,, Its true written words have a way of being seen two ways. Sometimes I question myself after the fact. and sometimes I feel like the " Monkey in the middle ". Very easy to misunderstand things written or take them out of context, I just figure we all know that, ( I use the term "we" loosely), Anyway, I didnt know there was two different tachs, But now I do I guess, lol , so the question that remains is if I figure my tach isnt working right because of the petronix, How do I resolve the problem, is there something that can be added in the wiring ? Can it be converted ? Could it have damaged my tach? I would hate to go back to points because its running awsome. but would like to have the tach working, ,OH, and stop looking at me in that tone of voice !!

[Old Goat 67]

I do not know your name but,

Peter is capable of modifying your tach (No, I am not on his payroll), but I don't know the cost.

I modified mine myself, although I doubt it is as good a job as he can do. I know it is FAIRLY accurate, but not truly dead on. Then again, I do not do racing, so it is a sort of mute point. As long as its closer than it was, I'm happy. Probably within 100-200 RPM according to my tests with a waveform generator.

Mine would work seemingly at low speeds, but at higher RPM's was way off. Reading a spike compared to a square wave requires different circuitry. Check Peter's website and contact him.

There are others also. Google is your friend.

I do trust Peter, he's been here for a long time. Honest guy.

Charles

[gtohurstjudge]

My car has had the old fashioned original type Pertronix unit that they
first came out with for about twenty years and in that time I have had two different
hood tachs and they read correctly throughout the entire RPM range. It may be that
the newer style pertronix units work differently and won't work, or it may be the brand of
aftermarket hood tach that is being used. My hood tachs came from Performance Years.

[1beautifuldaughter]

Original pontiac hood tach

[PB]

In my old GTO, I had pertronics and reading through this thread, I now realize what was going on. Like BJ's, my tach appeared to work fine at lower RPM, but I knew it just wasn't quite right. At speed, and at highway speeds, sometimes the needle would jump around, bounce all over the place. I recently installed my original dist (unitized) in my lemans and hooked up the in dash tach. It runs so smoothly in comparison to my old GTO w/pertronics. Learned something new today! Thanks fellas.

[Old Goat 67]

PB,

There you go. That's what this place is all about, people helping people.

Sure beats the days before the WWW.

Charles

[jstites]

I have three pontiacs with pertronix. The oldest one is aproximatly 11 years old ( first gen pertronix ) and works perfectly with the factory hood tach. I have two others that are jst a couple years old and the tachs are never accurate, especially at high rpm's. I'm no expert, I just know what my results have been. This discussion is an eye opener.