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Lynchy99 Samba Member
Joined: June 21, 2020 Posts: 47 Location: Warwickshire
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Posted: Tue May 11, 2021 5:27 am Post subject: Would a 1.5ohm coil perform better than a 3ohm coil? |
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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 |
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Alstrup Samba Member
Joined: July 12, 2007 Posts: 7216 Location: Videbaek Denmark
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Ohio Tom Samba Member
Joined: February 09, 2006 Posts: 1660 Location: Marshallville Ohio
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Posted: Tue May 11, 2021 10:12 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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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. |
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jpaull Samba Member
Joined: February 22, 2005 Posts: 3465 Location: Paradise, Ca
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Posted: Tue May 11, 2021 10:36 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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Ohio Tom wrote: |
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. |
Yes a MSD can drive many coils to that voltage, but the amperage will be less if chooseing a coil not capable of delivering what it receives. The amperage represents the output, not the voltage. |
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Alstrup Samba Member
Joined: July 12, 2007 Posts: 7216 Location: Videbaek Denmark
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Posted: Tue May 11, 2021 1:15 pm Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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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 _________________ https://www.thesamba.com/vw/forum/viewtopic.php?t=435993 |
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Lynchy99 Samba Member
Joined: June 21, 2020 Posts: 47 Location: Warwickshire
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Posted: Wed May 12, 2021 1:00 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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Alstrup wrote: |
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 |
So of my choices to feed the XR700:
1 - Crane coil 1.5ohm with resistor (as OG Spec kit)
2 - 3 Ohm Petronix (built in resistor) (more modern tech?)
Is there a better route?
Does the Petronix just do the same as the old Crane 1.5 ohm set up but in a modern integrated way (i assume so as I have not seen 1.5 ohm set ups on VW being very common)
Obvs number 2 is cleaner set up wiring/maintenance wise.
Thanks for the knowledge/input! |
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modok Samba Member
Joined: October 30, 2009 Posts: 26787 Location: Colorado Springs
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Posted: Wed May 12, 2021 2:31 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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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. |
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Ohio Tom Samba Member
Joined: February 09, 2006 Posts: 1660 Location: Marshallville Ohio
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Posted: Wed May 12, 2021 6:43 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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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..). |
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jpaull Samba Member
Joined: February 22, 2005 Posts: 3465 Location: Paradise, Ca
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Posted: Wed May 12, 2021 7:31 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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Lynchy99 wrote: |
Alstrup wrote: |
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 |
So of my choices to feed the XR700:
1 - Crane coil 1.5ohm with resistor (as OG Spec kit)
2 - 3 Ohm Petronix (built in resistor) (more modern tech?)
Is there a better route?
Does the Petronix just do the same as the old Crane 1.5 ohm set up but in a modern integrated way (i assume so as I have not seen 1.5 ohm set ups on VW being very common)
Obvs number 2 is cleaner set up wiring/maintenance wise.
Thanks for the knowledge/input! |
Lynchy99, you cant remove the ballast resister! Alstrup has it right your box is designed to run on 7 volts. If you remove the ballast you fry the box. _________________ [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. |
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Lynchy99 Samba Member
Joined: June 21, 2020 Posts: 47 Location: Warwickshire
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Posted: Thu May 13, 2021 5:00 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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jpaull wrote: |
Lynchy99 wrote: |
Alstrup wrote: |
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 |
So of my choices to feed the XR700:
1 - Crane coil 1.5ohm with resistor (as OG Spec kit)
2 - 3 Ohm Petronix (built in resistor) (more modern tech?)
Is there a better route?
Does the Petronix just do the same as the old Crane 1.5 ohm set up but in a modern integrated way (i assume so as I have not seen 1.5 ohm set ups on VW being very common)
Obvs number 2 is cleaner set up wiring/maintenance wise.
Thanks for the knowledge/input! |
Lynchy99, you cant remove the ballast resister! Alstrup has it right your box is designed to run on 7 volts. If you remove the ballast you fry the box. |
Hi, OK thats a worry I dont want to fry the box, just to check we are on the same page though the Petronix has a built in resistor so does not need the external one though right?
The manual below if I read correctly says I dont need a ballast resistor if the coil has one fitted, like a blue coil which Petronix has.
So your frying comment refers to the 1.5 ohm coil with no BR? which I was not propising. |
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jpaull Samba Member
Joined: February 22, 2005 Posts: 3465 Location: Paradise, Ca
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Posted: Thu May 13, 2021 9:37 pm Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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If you want to run the 3 ohm, I would do this test:
_________________ [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. |
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Lynchy99 Samba Member
Joined: June 21, 2020 Posts: 47 Location: Warwickshire
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Posted: Fri May 14, 2021 12:54 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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jpaull wrote: |
If you want to run the 3 ohm, I would do this test:
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Thans Jpaul, I dont have a volt meter so I will have to trust that the Petronix 3 ohm is 3 ohm.
My question really was just to clarify that running a 3 ohm coil with internal resistor was ok. Reading the instructions that was my understanding. |
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dgsaz Samba Member
Joined: November 04, 2008 Posts: 579 Location: Phoenix / San Diego
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Posted: Sat May 15, 2021 10:07 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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Here's how I addressed the issue:
Last edited by dgsaz on Sat May 15, 2021 11:36 am; edited 1 time in total |
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dgsaz Samba Member
Joined: November 04, 2008 Posts: 579 Location: Phoenix / San Diego
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Posted: Sat May 15, 2021 10:08 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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Here's how I addressed the issue:
But then:
dgsaz |
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Vanapplebomb Samba Member
Joined: November 03, 2010 Posts: 5412 Location: Holland, MI
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Posted: Sat May 15, 2021 10:41 am Post subject: Re: Would a 1.5ohm coil perform better than a 3ohm coil? |
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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. _________________ 1800 Type 4 Berrien 295
http://www.thesamba.com/vw/forum/viewtopic.php?t=487021 |
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