Tach Issues with upgraded coil
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Joined: May 2021
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From: Lee County, Florida
Tach Issues with upgraded coil
I just cleaned up and basically gave my Delco Distributor out of an 87 car a good cleaning with a repaste for the GM ICM as well as a new brass cap and rotor, I decided to take the stock coil out and put a Davis Unified Ignition 50,000 volt coil in as a good upgrade as well as blue streak 8.5mm wires, installed this dizzy into my 85, the car runs fantastic, but the tach doesn't read past 2800-3000 rpm, after that it jumps all over the place, never had a tach filter issue with this car, threw the stock coil back in to test it, and it completely solved the problem, how would I go about making this upgraded coil work? I'm sure it's not something I necessarily need for a bolt on L98 but I would love to make it work if possible.
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Joined: Jan 2019
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From: Connecticut, USA
I just cleaned up and basically gave my Delco Distributor out of an 87 car a good cleaning with a repaste for the GM ICM as well as a new brass cap and rotor, I decided to take the stock coil out and put a Davis Unified Ignition 50,000 volt coil in as a good upgrade as well as blue streak 8.5mm wires, installed this dizzy into my 85, the car runs fantastic, but the tach doesn't read past 2800-3000 rpm, after that it jumps all over the place, never had a tach filter issue with this car, threw the stock coil back in to test it, and it completely solved the problem, how would I go about making this upgraded coil work? I'm sure it's not something I necessarily need for a bolt on L98 but I would love to make it work if possible.
I have the DUI setup, put in over 20 years ago, but have no problem with the tach.
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Joined: May 2021
Posts: 97
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From: Lee County, Florida
Definitely let me know, I think the higher voltage the coil uses doesn't work with the factory tach filter well, it sucks too, took it out last night to Mexico for a few pulls and it ran like a stabbed rat.
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I just cleaned up and basically gave my Delco Distributor out of an 87 car a good cleaning with a repaste for the GM ICM as well as a new brass cap and rotor, I decided to take the stock coil out and put a Davis Unified Ignition 50,000 volt coil in as a good upgrade as well as blue streak 8.5mm wires, installed this dizzy into my 85, the car runs fantastic, but the tach doesn't read past 2800-3000 rpm, after that it jumps all over the place, never had a tach filter issue with this car, threw the stock coil back in to test it, and it completely solved the problem, how would I go about making this upgraded coil work? I'm sure it's not something I necessarily need for a bolt on L98 but I would love to make it work if possible.
The stock ignition module has a peak current amount that it will conduct, regardless of ignition coil. This is to insure desired energy into the plug-gap, and also to protect the coil primary winding due to changes in alternator voltage and temperature/RPM. Depending on the coil electrical specs, a non-stock coil may output increased or decreased energy to the spark plug, depending on coil inductance/resistance and engine RPM.
While the stock coil should be more than sufficient for your engine (lotsa blown/turbo C4s out there running the stock ignition), it's not obvious yet why you should be seeing any issues with the aftermarket coil. Is the problem at any throttle position at 3000 RPM, or only under 3000 RPM WOT?
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Joined: May 2021
Posts: 97
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From: Lee County, Florida
A couple items: The 50,000 volt rating is for the coil winding insulation/construction. It has no connection to what the secondary/plug-gap voltage is during operation. That voltage is primarily determined by the plug-gap size and the cylinder pressure at the time of ignition.
The stock ignition module has a peak current amount that it will conduct, regardless of ignition coil. This is to insure desired energy into the plug-gap, and also to protect the coil primary winding due to changes in alternator voltage and temperature/RPM. Depending on the coil electrical specs, a non-stock coil may output increased or decreased energy to the spark plug, depending on coil inductance/resistance and engine RPM.
While the stock coil should be more than sufficient for your engine (lotsa blown/turbo C4s out there running the stock ignition), it's not obvious yet why you should be seeing any issues with the aftermarket coil. Is the problem at any throttle position at 3000 RPM, or only under 3000 RPM WOT?
The stock ignition module has a peak current amount that it will conduct, regardless of ignition coil. This is to insure desired energy into the plug-gap, and also to protect the coil primary winding due to changes in alternator voltage and temperature/RPM. Depending on the coil electrical specs, a non-stock coil may output increased or decreased energy to the spark plug, depending on coil inductance/resistance and engine RPM.
While the stock coil should be more than sufficient for your engine (lotsa blown/turbo C4s out there running the stock ignition), it's not obvious yet why you should be seeing any issues with the aftermarket coil. Is the problem at any throttle position at 3000 RPM, or only under 3000 RPM WOT?
