Classic cars forum & vehicle restoration.
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Ashley
Joined: 02 Jan 2008
Posts: 1426
Location: Near Stroud, Glos
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 Posted: Tue May 27, 2014 3:44 pm Post subject: |
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Modern ignition coils are a nightmare, half of them don't work or pack up if the under bonnet temp rises too high, so I'd use what Simmbbc recommends because they're a good company.
I use Accuspark 12 Volt General purpose coils in preference to the Lucas because I've had so much trouble with them, but probably the best you can buy is a Bosch or Beru for a 12 Volt VW Beatle. The oil is vacuumed in, they can be run upside down without it all running out and they withstand higher temperatures without problems.
Old cars are better off with general purpose coils. When new, their primary resistance was 4 to 4.5 Ohms and the secondary gave less than 15K. They draw very little current, which is good for dynamos that don't produce much.
Sports coils aren't necessary and offer no benefit below 15,000 rpm and because the rise time is much quicker and volts far higher (40,000 instead of 15,000) they're more like to cause arcing, especially of rotor arms. Primary resistance is typically 1.5 Ohms and they draw three times the current as well.
Old combustion chambers like a long lasting slow rising spark from a general purpose coil such as Simon sells for £11. I have one on my mildly tuned (120 bhp TR) and it revs happily to 5,500 without issue. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4104
Location: South Cheshire
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| Posted: Tue May 27, 2014 4:28 pm Post subject: |
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| Ashley wrote: | Modern ignition coils are a nightmare, half of them don't work or pack up if the under bonnet temp rises too high, so I'd use what Simmbbc recommends because they're a good company.
I use Accuspark 12 Volt General purpose coils in preference to the Lucas because I've had so much trouble with them, but probably the best you can buy is a Bosch or Beru for a 12 Volt VW Beatle. The oil is vacuumed in, they can be run upside down without it all running out and they withstand higher temperatures without problems.
Old cars are better off with general purpose coils. When new, their primary resistance was 4 to 4.5 Ohms and the secondary gave less than 15K. They draw very little current, which is good for dynamos that don't produce much.
Sports coils aren't necessary and offer no benefit below 15,000 rpm and because the rise time is much quicker and volts far higher (40,000 instead of 15,000) they're more like to cause arcing, especially of rotor arms. Primary resistance is typically 1.5 Ohms and they draw three times the current as well.
Old combustion chambers like a long lasting slow rising spark from a general purpose coil such as Simon sells for £11. I have one on my mildly tuned (120 bhp TR) and it revs happily to 5,500 without issue. |
Hi Ashley
I agree with you regarding the challenges of sourcing decent ignition components these days, but a couple of your other points are technically inaccurate.
Firstly the resistance of a coil has no bearing on the HT voltage; the voltage in the secondary rises until it's large enough to fire a plug, it will be the same voltage regardless of the coil.
What the manufacturers refer to to as sports coils is a misleading term, basically you mean low resistance coils? The heart of any conventional ignition system is a bit of a compromise ; you ideally require maximum current, long dwell and a fast collapse of the coils magnetic field.....current is limited by the engine voltage, condition of the points and the coil resistance, dwell by the number of cylinders and engine revs, magnetic collapse the clean break of the contact breaker.
So whilst a classic engine may not have high revs, it certainly has occasions where the coil voltage is low (cold starting) . The Speed of coil collapse will be significantly faster with a transistor controlling the coils primary current rather than a contact breaker.
You will only get more wear on the rotor arm if the plug gaps are opened up, as this forces the ignition secondary to work at a higher voltage, which as I am sure you will appreciate has many benefits. The additional wear probably equates to changing a rotor arm every 20,000 miles rather than 25,000.
I'm not quite sure what you mean by "older engines like a long lasting slow rising spark" possibly a typo? ....as the secondary voltage rises it's gonna spark as soon as it can jump that plug gap ! It stops when the cylinder pressure exceeds the coils ability to continue sparking, which is also down to the drop in voltage of the oscillating condenser / primary tuned circuit , once this drops below 100v not much happens in the secondary, as the piston will be further up the cylinder ( more compression, requiring a higher voltage to spark)
Cheers
Dave |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4104
Location: South Cheshire
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| Posted: Tue May 27, 2014 4:32 pm Post subject: |
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| ukdave2002 wrote: | | Ashley wrote: | Modern ignition coils are a nightmare, half of them don't work or pack up if the under bonnet temp rises too high, so I'd use what Simmbbc recommends because they're a good company.
I use Accuspark 12 Volt General purpose coils in preference to the Lucas because I've had so much trouble with them, but probably the best you can buy is a Bosch or Beru for a 12 Volt VW Beatle. The oil is vacuumed in, they can be run upside down without it all running out and they withstand higher temperatures without problems.
Old cars are better off with general purpose coils. When new, their primary resistance was 4 to 4.5 Ohms and the secondary gave less than 15K. They draw very little current, which is good for dynamos that don't produce much.
Sports coils aren't necessary and offer no benefit below 15,000 rpm and because the rise time is much quicker and volts far higher (40,000 instead of 15,000) they're more like to cause arcing, especially of rotor arms. Primary resistance is typically 1.5 Ohms and they draw three times the current as well.
Old combustion chambers like a long lasting slow rising spark from a general purpose coil such as Simon sells for £11. I have one on my mildly tuned (120 bhp TR) and it revs happily to 5,500 without issue. |
Hi Ashley
I agree with you regarding the challenges of sourcing decent ignition components these days, but a couple of your other points are technically inaccurate.
Firstly the resistance of a coil has no bearing on the HT voltage; the voltage in the secondary rises until it's large enough to fire a plug, it will be the same voltage regardless of the coil.
What the manufacturers refer to to as sports coils is a misleading term, basically you mean low resistance coils? ........The heart of any conventional ignition system is a bit of a compromise ; you ideally require maximum current, long dwell and a fast collapse of the coils magnetic field.....current is limited by the engine voltage, condition of the points and the coil resistance, dwell by the number of cylinders , points gap and distributor cam profile, and engine revs, magnetic collapse; the clean break of the power to the primary (contact breaker opens)
So whilst a classic engine may not have high revs, it certainly has occasions where the coil voltage is low (cold starting) . The Speed of coil collapse will be significantly faster with a transistor controlling the coils primary current rather than a contact breaker, a crude analogy is that contact breakers are a slow puncture, whilst electronic ignition is a blow out!, both deflate the tyre.....
You will only get more wear on the rotor arm if the plug gaps are opened up, as this forces the ignition secondary to work at a higher voltage, which as I am sure you will appreciate has many benefits. The additional wear probably equates to changing a rotor arm every 20,000 miles rather than 25,000.
I'm not quite sure what you mean by "older engines like a long lasting slow rising spark" possibly a typo? ....as the secondary voltage rises it's gonna spark as soon as it can jump that plug gap ! It stops when the cylinder pressure exceeds the coils ability to continue sparking, which is also down to the drop in voltage of the oscillating condenser / primary tuned circuit , once this drops below 100v not much happens in the secondary, as the piston will be further up the cylinder ( more compression, requiring a higher voltage to spark)
Cheers
Dave |
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Ashley
Joined: 02 Jan 2008
Posts: 1426
Location: Near Stroud, Glos
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| Posted: Tue May 27, 2014 4:51 pm Post subject: |
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Dave
A coil with a 4.5 Ohm primary draws less than 2 Amps from the battery and a 1.5 Ohm coil about 8, which is too much for a dynamo when the lights and heater and wipers are on IMO, so this is my reason for recommending this coil.
http://www.ebay.co.uk/itm/MGB-AccuSpark-12v-Standard-Ignition-Coil-NEW-/150500606162?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item230a8900d2
It has a 3 Ohm primary and 30K Volts and is the nearest to original equipment. It's also more reliable than the modern Lucas copy equivalent. I've had about seven go in the last few years, some leaked all their oil out, some misfire and pack up when hot, so are incorrectly labelled and draw 7-8 amps suggesting they ought to be used with a ballast resistor and so on.
This never used to happen years ago, so I'm guessing it all relates to rubbish pattern parts. Accuspark's stuff is very good value and reliable.
I'm just a customer BTW. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4104
Location: South Cheshire
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| Posted: Tue May 27, 2014 5:18 pm Post subject: |
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Not quite true as the coil only draws current during the dwell periods, and even then it only draws maximum current for part of the time..... you can more or less half the total current draw from a basic ohms law calculation.
I agree with you regarding dynamo's , as you may be aware I do a bit with Morris 8's....now here is a challenge 6v electrics with the standard 3 ohm coil !!! Cars that now have to have an additional tail lamp and nobody uses the original 24watt main beam bulbs they all use 36watt, 70 year old Lucas switches....the result is that when all the electrics are running with these additional loads anything less than a perfect dynamo really can't cope! My SE would misfire with main beam and wipers on  in these conditions you could forget trying to get the trafficator's to lift....a move to any of the transistor assist ignition systems offers significant improvements.
Cheers
Dave |
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kevin2306
Joined: 01 Jul 2013
Posts: 1359
Location: nr Llangollen, north wales
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Ashley
Joined: 02 Jan 2008
Posts: 1426
Location: Near Stroud, Glos
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| Posted: Tue May 27, 2014 5:31 pm Post subject: |
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I believe those Bosch VW coils are the best made ATM, but the Accuspark I linked to give no trouble either.
Although it's all got rather technical, the fact is that coils rarely gave trouble in the old days. The ones on my Bentley lasted over fifty years, but what you buy now can be useless and also make you lose faith in your own judgment. I had so many coils go I really began to think I must be to blame. Two on my Healey with a question mark over a third.
My TR3A and the Healey 3000 have Accuspark and the Bentley a Bosch VW one.
I love those Morris 8s and learnt to drive on a Series 2 Dave. |
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kevin2306
Joined: 01 Jul 2013
Posts: 1359
Location: nr Llangollen, north wales
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| Posted: Tue May 27, 2014 5:40 pm Post subject: |
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nothing wrong about getting technical, its how we learn!
not doubting the accuspark but I think I will order the bosch.
Once ive given the car a decent shake down period I am hoping to get some good adventures in her so worth striving for decent reliability IMO.
thanks for the advice
Kev |
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