Would a 1.5ohm coil perform better than a 3ohm coil?

[Lynchy99]

Hi, So my car (1835cc bug, cam heads etc) came with a Crane Cams XR700 ignition which uses a 1.5 Ohm Crane came coil with a Ballast resistor..

The ballast resistor is breaking which lead me to look at it.

Now I have a Petronix 3 Ohm coil with a built in resistor which I have put in the car (not knowing about the Ballast Resistor) and have been using since, starts great always been a bit stuttering at mid range...

So I can get rid the Ballast resistor and just use the 3 Ohm Petronix but this made me think... would a 1.5ohm coil (with a new BR) actually perform better than a 3 ohm coil as it should give more power out at start up?

I can run either just thinking is there is a performance advantage to be had with the 1.5ohm set up? Any one know?

Thanks

[Alstrup]

Short answer is YES! you need a 1,6 ohm coil with resistor.
This link will give you the answers
https://mgaguru.com/mgtech/ignition/ig207.htm
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[Ohio Tom]

Early (low voltage) systems that used a ballast resistor would bypass it on startup only. You are cranking so the battery voltage is already low so spark energy is low.

Modern ignitions (MSD and others) don't care about coil resistance and ballast resistors. They give full energy no matter what. They drive the coil to saturation so resistance doesn't really affect much. (search Google about driving coils/transformers to saturation).

Folks ask me what coil to use with their MSD. I say use any coil you want. It will drive it to saturation regardless. Even a stock coil can be pushed to 20,000 volts with no problem.

[jpaull]

[Alstrup]

Yes. But the XR700 only requires about 7 volt. I think feeding it with a constant 12-14 volts will kill it. I may be wrong.
I fully agree on the MSD, Crane, Pertronix CDI´s
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[Lynchy99]

[modok]

a 3 ohm coil VS
1.5 ohm coil and a 1.5 ohm resistor, vs
0.7 ohm coil 2.3 ohm resistor
should perform the same, if the specs of the coil are the same otherwise.

The external resistor could be a better system because the coil runs cooler,
but it's just not that big a deal.
I haven't really seen one way outlast the other.

[Ohio Tom]

OK. Electronics lesson.

V=IR.
Voltage = Current x resistance.

W= V x I
Wattage (total power) = Volts x Amps.

So, the comparson of a 3ohm coil compared to a 1.5 coil with a 1.5ohm resistor does not give the same energy to the coil.
the coil + resistor only has 1/2 of the voltage of the system applied to the coil.
Current is the same in either system due to the total 3ohm load giving 4.0amps.

So the 1.5ohm coil has 6V x 4A = 24 Watts.

The 3ohm coil has 12V across it, so: 12V x 4A = 48Watts.


However, what kills coils is heat and heat is measured by the Wattage.

Traditional points fire ignition applies 12V to the coil and for the "dwell" duration to allow the field to build up. Then the current is broken, and the field falls causing a spark.
The Dwell is amount of "ON" time and can cause overheating if excessive. Leaving your ignition in the on position without the motor running can overheat a coil.

An MSD type ignition hits the coil with 300V.
No real Dwell needed as the coil field gets driven to max real fast. Then the MSD cuts the current and the field falls... Many times per spark event.
Since the duration is so short for these pulses, the average wattage (heat) is low.

Coils need to be able to handle the increased voltage both on the input side (300V) and the output side (wires, rotor, plugs, etc..).

[jpaull]

[Lynchy99]

[jpaull]

If you want to run the 3 ohm, I would do this test:

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[email protected] MPH 1/4 Mile & 8.1 @ 83.7MPH in 1/8 Mile with Mild Type 1 VW Mag Case 2234cc commuter engine in stock weight bug w/only .491 total lift(CB2292 Cam), 42x37 heads, 48idf's, Street tires, Belt on, Mufflers, Pump gas, video of the run here: https://youtu.be/M3SPqMOKAOg

Transmission by MCMScott:
Rhino case, Klinkenberg 4.12, Superdiff, 002 mainshaft with 091 first idler. Weddle 1.48 Third & 1.14 Fourth.

[Lynchy99]

[dgsaz]

Here's how I addressed the issue:

[dgsaz]

Here's how I addressed the issue:

But then:

dgsaz

[Vanapplebomb]

In most cases it is 6 or 1/2 dozen the other.

The 3+ ohm coils have the resistor built into the coil housing. When running points or most points replacement modules, lower resistance coils use an external resistor to limit current so the coil doesn’t overheat or fry your module. So either way, the primary windings sees roughly the same current and voltage.

The biggest factor in output is the turns ratio of the coils.

Now... if you have a coil built to handle higher current and your are running a trigger system that also doesn’t require current limiting, then it is a different story.
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