Thread: Hei rpm ?

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Hi
I'm collecting stuff to work on my cars.

What is the easiest / cheapest way to get rpm?

I've got HEI in the dodge and Chevy.

Thanks

Not sure I understand the question

Press the little pedal on the right to the floor! 8-)

Ok, I guess I deserved that.

I'm thinking the old rpm/dwell/timing units won't be compatible with the tach output on the HEI.

Can I get any old tach guage and hook up? I'd expect it would have to be from an 8cyl at least


I'd like to do this on the cheap, but if I have to get a quality guage, so be it.

Tachs work the same with point and HEI distributors.

Ok, as I recall, and it's been a decade since I played with points, they sink IE they ground the coil. So, the HEI tach outtput is probably NPN?

Originally Posted by firebird77clone

Ok, as I recall, and it's been a decade since I played with points, they sink IE they ground the coil. So, the HEI tach outtput is probably NPN?

Not sure what you're trying to say.. But, you cannot ground any coil! Motor won't have any ignition system with a coil connected to ground.

What's "NPN"?

Thats what I was trying to figure out, Just what are you asking.
I put a MSD box in my 72 Vette with a cable drive tech.
The problem I had with doing that is you have to remove the resister wire,
that went from the old point style distributor or your engine will break up
when revving it.
Kurt

Coils produce spark when they momentarily ground through the points.

NPN refers to transistor type.

NPN output in negative
PNP output is positive

Originally Posted by firebird77clone

Coils produce spark when they momentarily ground through the points.
NPN refers to transistor type. NPN output in negative PNP output is positive

Actually, the spark occurs when the points open. This collapses the magnetic field in the coils primary windings and induces an voltage output of the coil secondary windings. In HEI, Electronic controls turn on / off the flow of electricity to induce the voltage in the coil. Modern Tachs can handle either type of ignition. Old tachs used a separate control box to convert the coil pulses into a usable / controlable signal.

We are splitting hairs, and I really don't wish to discuss electron theory, just wanted to know how to hook a tach to HEI, and if one of those old dwell/timing/rpm meters would work, or if I needed to get a tach

GM HEI has a tach plug on it

Originally Posted by firebird77clone

We are splitting hairs, and I really don't wish to discuss electron theory, just wanted to know how to hook a tach to HEI, and if one of those old dwell/timing/rpm meters would work, or if I needed to get a tach

Sorry if my basic explanation was interpreted as theory.. What you're calling "old dwell/timing/rpm meter" IS a tach. The RPM Meter is the Tach. Try connecting the lead to the distributor cap connection marked TACH. I think there should be a selector switch for the number of cylinders too. Choose what's appropriate.

Thanks all, sorry if I posted the question poorly

More research yields the answer I'm looking for:
On the HEI the tach output comes directly off the negative side of the coil. An NPN transistor switches the coil to ground.