TR7 ignition coil overheating?

[TR7 Fan]

Hello,

I installed a Lumenition optronic ignition system.

Situation is:

Lumenition optronic

High tension lead from distributor to ignition coil: “High temp. double silicone”, measured resistance: 11,5 Kohm.

High tension leads from distributor to plugs: “Lucas silicone speed lead”, measured resistance:: 6 to 8 Kohm.

Ignition coil: Bosch  0221 119 030

Ignition coil connected via “eureka wire” from the car.

Eureka wire resistance:1,9 ohm / Bosch primary resistance: 1,4 ohm. Total resistance:3,3 ohm.

Spark plugs: NGK BP5ES

Engine to be fitted with K&N air filters;  Carb jets: BAL;  Inlet and exhaust ports: slightly polished;  Tubular exhaust system.

Voltage at primary side of ignition coil when ignition switch is “ON”:  about 5,7 volt (engine not running), battery voltage 12,2 volt.

When trying to tune the carbs, the coil is getting hot , I measured about 50 degrees (Celsius) after some 20 minutes running.

I can barely hold my hand against it.

The engine is running well stationary.

What temperature should a coil have ?

Regards,

Luc.

[glang]

You can also measure coil secondary resistance from HT connection to the case and it should be between 5 and 20k ohms. Also check what the supply voltage to the coil is with the engine running as I don't expect it to still be 5.7 volts but it certainly shouldn't be battery voltage.

In the end though I don't think that temperature is excessive as there's quite a bit of current passing through the coil and it generates heat which is normally dissipated by airflow when driving....

[RobPearce]

As glang says, I think you're probably OK. Twenty minutes with no airflow is enough to get the coil up to its ultimate running temperature. If the EI box were a clever one with dwell control then the coil would run cooler at idle, but I'm pretty sure Lumenition is a crude type, so the coil gets even hotter at idle than in normal driving. And 50 degrees is not hot - the coil is expected to tolerate a lot more than that. The six cylinder cars had it bolted to the engine block!

[TR7 Fan]

Thanks for the replies.

So I don't have to worry, maybe just when I am in a traffic jam with hot weather.

Forgot to mention: Secundary coil resistance is about 12,7 Kohm, supply voltage to coil when running: about 9 volt.

Regards,

Luc.

[Howard]

The ballast resistor inside the wiring loom was designed to reduce the coil voltage to 6v according to the information I have read. Since energy is proportional to voltage squared this would result in the energy dissipated increasing from 6x6x1.4=50 watts to 9x9x1.4=113 watts so it will get a lot hotter.

 

[glang]

Think its difficult to measure the coil supply voltage running as the coil is charging and discharging rapidly to produce sparks but all these figures sound reasonable🙂

[RobPearce]

The whole "reduce to 6V" thing is an unhelpful over-simplification. Luc has measured the engine stopped coil voltage at 5.7V, which is as near as dammit 6V. So it's correct. Any other time you measure it you'll get more than 6V because, while the points are open, the coil is not grounded and there's no potential divider to give you 6V, so you have 12V there.

In fact, Howard's result is backwards. When the engine is stationary, and the points closed, Luc is getting 6V, so 50W. With the engine idling, meter reading 9V, it's actually only 25W in the coil. Why? Because (to a first approximation) the 9V reading means the dwell is roughly 50%, so the coil is seeing 6V for half the time and nothing for the other half.