HEI Large /Small Cap - Tach Signal

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typopup

typopup

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Hey Folks

I’m thinking about going to an HEI large cap distributor in my 1973 Duster w/ a 408 small block. I’m running an Edelbrock RPM Air Gap intake & a Holley Street Avenger 770 carb. Has anyone got a similar setup … I’m trying to find out if it will fit before buying.

I have a small cap HEI 2-wire distributor but the only way I can get a tach signal is going w/ an inductive pickup … I’m running a Crane Fireball coil but am not getting a tach signal on the negative side. Figured if I go to a large cap there’ll at least be a tach terminal connection.

Thx in advance.
 
typopup said:
Hey Folks

I’m thinking about going to an HEI large cap distributor in my 1973 Duster w/ a 408 small block. I’m running an Edelbrock RPM Air Gap intake & a Holley Street Avenger 770 carb. Has anyone got a similar setup … I’m trying to find out if it will fit before buying.

I have a small cap HEI 2-wire distributor but the only way I can get a tach signal is going w/ an inductive pickup … I’m running a Crane Fireball coil but am not getting a tach signal on the negative side. Figured if I go to a large cap there’ll at least be a tach terminal connection.

Thx in advance.
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I run 408 with a digital small cap due to not much room between firewall and cap for timing clocking on my 66 DART but your setup might have a little more room. I also run 830 cfm 4150 style carb if that helps.
 
Yea! Trying to get a large cap to fit is almost impossible with that firewall set up.
I’ve run Mopar cap perfectly with HEI for years. GM went further apart with terminals to prevent spark jumping but I’ve yet to see it with a good Mopar tan cap.
As for pick up- large cap should have a terminal for that.
 
Put a big block in it. Then it'll fit.
 
typopup said:
That is definitely a fix!!!

Any idea why the e-coil wouldn’t be a good hookup for the tach … that would fix everything if I could figure that out?
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What do you mean "figure it out"? Tach hooks up the same as any other coil. Tach wire to the negative side of the coil. BUT a lot of times with HEI and some other aftermarket ignitions, you need a tach filter to keep the tach from getting spikes and reading wrong. I had to put one on the big cap HEI in my Ford 400.
 
When I hook the inductive clip on a plug wire I get an RPM reading on the Equus tach … when I hook up the tach lead up to the negative side of the coil I do not. I haven’t tried the tach on another vehicle so it may just be a faulty tach.

I’m not a fan of the inductive clip being seen but it has worked so far … I would just like to clean up the engine bay a bit.
 
typopup said:
When I hook the inductive clip on a plug wire I get an RPM reading on the Equus tach … when I hook up the tach lead up to the negative side of the coil I do not. I haven’t tried the tach on another vehicle so it may just be a faulty tach.

I’m not a fan of the inductive clip being seen but it has worked so far … I would just like to clean up the engine bay a bit.
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Post a picture of this inductive tach.
 
RustyRatRod said:
What do you mean "figure it out"? Tach hooks up the same as any other coil. Tach wire to the negative side of the coil. BUT a lot of times with HEI and some other aftermarket ignitions, you need a tach filter to keep the tach from getting spikes and reading wrong. I had to put one on the big cap HEI in my
RustyRatRod said:
Post a picture of this inductive tach.
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The three prong connector is for 12+ / 12- / light ... the four prong connector is for the inductive pickup lead ... the single pink wire four prong connector is for the negative side of the coil (this is the one that doesn't work).
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Not familiar with those tachs, are they actually supposed to be able to connect that way? trigger to coil neg, I mean? Link to destructions on web?

Photo of coil? The TRICK. Some of these modern E core coils have tricky connections. That is, some of them have dual 2 terminal connectors, which leads you to believe that one is pos, the other neg, but on some, each terminal in each connector is pos and neg. SOMEWHERE there is a photo.............

This is not the diagram I was thinking of but it is similar I believe the "C" pin is for the filter/ radio cap. So C and 12+ are the same electrical point, and the NEG black at upper left and the tach at lower right are the same electrical point. In other words, in reality, this coil only has THREE terminals......the high tension, the +12, and the Neg

HEI8_coil.gif
 
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The HEI cap is 5" across. Not all tachs will work with the GM HEI system. As mentioned above, a filter or adapter may be needed.
 
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