TheSamba.com :: View topic - Are German Bosch blue ignition coils worth the extra expense ?

Tue Apr 11, 2023 7:07 pm

Here is an article I wrote on the topic of ignition and coils - that was published in the Karnataka Vintage and Classic Car Club newsletter - The Starting Handle.

A primer to contact breaker (inductive discharge) ignition coil systems
Byas Nambisan

I often get asked – “why is my coil so hot?” or “will a performance coil improve my system?” and other questions around coil, plug wires, spark plugs, and the overall ignitionq system. Recently, while trouble shooting the wasted-spark set up of 2CV, I was pointed to an interesting article by Graeme Dennes on the 2CV system that could be generalized for most inductive discharge ignition systems in our vintage cars (and bikes which use them). So, I thought I’d do a write up on the ignition system drawing on the work of Graeme Dennes, Richard Atwell, and other sources and texts.

The inductive discharge ignition system that we use today is almost completely based on the system invented by Charles F. Kettering for the 1910 Cadillac and first manufactured by Delco (Dayton Engineering Laboratories Company) – the company founded to manufacture the first order of 5000 kits for Cadillac. Hence, you will hear many an oldtimer refer to the distributor as a “delco”. The system consists of a power source (battery) that is switched thru the ignition switch, an ignition coil (with or without a ballast resistor), a distributor with a cam that periodically opens a contact breaker (points) switch (or electronic switch in later models), a capacitor (usually called condenser in this application), coil and plug wires, and spark plugs.

So how does this system work?

Let’s start with the ignition coil. It is an electrical iron cored transformer that is usually housed in a round metal canister with a Bakelite top that has 3 terminals on it. Its purpose is to produce high-voltage pulses from a low-voltage DC supply for firing the plugs. The inside of the coil consists of a low resistance primary winding of about 200 turns and a secondary winding that has far more turns – 15,000 or more. This ratio of 15000/200 = 75 (as an example) is called the step-up ratio and is indicative of the extend to which the voltage in the primary system can be stepped-up in the secondary.

The two windings are insulated in an oil-impregnated paper insulation and the ignition coil container with the windings in it are filled with an insulating fluid. This fluid provides insulation and conducts the heat away from the windings. Coils may be oil filled (Lucas coils are typically oil filled) – with transformer oil (which is toxic so wear gloves) or newer safer variants, or with an asphalt or asphalt epoxy mix (as on many Bosch coils). The oil filled coils should preferably be mounted vertical to reduce likelihood of leakage, while the asphalt/epoxy filled coils can be mounted in any position.

The ignition coil has 3 terminals on top. The positive (+ or marked 15 in Bosch coils), negative ( - or marked 1 in Bosch coils) and the central output (marked 4 in Bosch coils). The primary coil is connected between the +ive and -ive terminals. If the coil has a built in resistor, the resistor is wire in series with the primary coil. If you measure the resistance between the + and – in a coil, a good non-ballast coil will have a reading of 0.6 to 1.5 ohms while a typical ballasted Bosch or Lucas coil will have a 2.9 to 4.5 ohm resistance. The secondary coil will have a higher resistance (measured between + and the center terminal) of typically 7000-11000 ohms. Check up on the specs for the coil for your car and measure your coil to make sure it is still within specs.

When you turn the ignition on and the points are closed a 12V DC current flows thru the primary coil. This sets up a powerful magnetic field that flows thru the iron core and surrounds both the primary and secondary coils. When the points open, this DC current is interrupted and the magnetic field starts collapsing rapidly. This collapsing field does two things – it induces about a 200V-300V potential in the primary coil and a much larger potential (200V x the step-up ratio of 75 = 15,000V) in the secondary coil.

To initiate and complete the combustion of the fuel-air mixture in the cylinder a spark that is strong enough and of sufficient duration (typically 1.5 - 2 milliseconds) to sustain the combustion is needed. The voltage needed to first initiate the spark is dependent on the conditions inside the cylinder and is typically around 10,000V. Once the arc is established, the voltage needed to sustain it is lower. As the magnetic field collapses, the induced voltage in the secondary starts rising. Once this voltage exceeds 10KV a spark is initiated. The remaining energy from the secondary will continue to keep the spark going for the required duration.

It is important to note here that the voltage required to generate a spark is not dependent on the coil but only on the conditions in the cylinder (engine load, cylinder pressure, spark plug gap, plug material and shape of electrode, fuel-air mixture, etc.). So, even a “high performance coil” will initiate a spark at the same voltage as the stock coil. The duration of the spark has an important relation to the selection of the plug wires which we will come to further down.
So, the collapsing magnetic field – triggered by the points opening creates energy in both the primary and the secondary coils. We looked at how the energy in the secondary is used to power the spark. What happens to the energy and potential induced in the primary? If nothing was done, this energy would release itself by jumping as spark across the open points – eroding the contact points. The role of the condenser is to absorb this initial 200V spike – slowing down the voltage build up and preventing arcing and damage to the points. This stored energy is released back to the points when they close the next time – in a controlled manner which prevents arcing and damage. So, if the condenser value (typically 0.22+/-.03 uF) is too small, it will not adequately prevent arcing when the points open. Conversely, if the value is too high, it will create arcing and damage when the points close. This is an important point to keep in mind as I have seen many installations where multiple condensers are fitted – perhaps in the belief that if one is good, two must be better.

As mentioned earlier, for proper combustion a spark duration of 1.5 to 2 milliseconds is needed. The energy from the coil is transmitted to distributor by the central coil wire or lead, then thru the rotor to one of the contact points on the distributor cover from where it travels thru the spark plug wire to the plug itself. One would think that having no resistance in any of these components through which the energy travels would be a good thing and result in the best spark. But that is not the case. If the energy is delivered too quickly, the spark duration will be too short and will not be enough for complete combustion. If the resistance is too high, then not enough energy will be transmitted quickly enough and will result in a weak spark (or no spark at all). So proper matching of resistance is a must. But this has been done by the factory and it will be wise to stay within that specs.

For example: On aircooled VWs, the plug wires are relatively low resistance but the rotor within the distributor has about a 5.3Kohm resistance and the removable plug connectors at the end of the spark plug wire have a 1Kohm resistance. The spark plugs are of the non-resistor variety. So, the system has a total of about 6Kohm. Replacing the non-resistor spark plug with a resistor type will increase the resistance unless a corresponding decrease is made elsewhere. If the resistance is so high that no spark can happen, the coil energy will dissipate internally within the coil by breaking down the insulation and arcing (called “flash over”) across the primary and secondary coils. This can lead to coil overheating and failure.

Key takeaways – a plug wire is not just a plug wire. It is common to see mechanics fit just about any plug wire to a car. The consequences of poor choices are not immediately apparent and can show up in other areas (like overheating coil). Plug wire damage is not something that is readily apparent (other than rodent bites or external abrasions). It is easy to assume that a wire is good if it “looks ok”. I have fallen for that bad assumption. The wires are affected internally by several factors including time, vibration, movement, handling, etc. They may develop faults within that may be hard to see, but can be more easily measured by checking the resistance for change vs. factory spec. Graeme Dennes in his article recommends that plug wires be changed every 2 years. I’d say check your plug wire resistance every year and replace if there is a fault. Handle your plug wires gently. When pulling them off, do not tug on the wire but on the end connector.

After reading the Dennes article, I decided to check the plug wires on my VW convertible. The four plug wires were within spec – 0.95Kohms on three of them and 1.02Kohms on the fourth. Since those were ok, I walked away satisfied and did not check the short coil wire – until today as I was writing this article. Just checked it now to take a picture – it was at 10Kohms (!). I will need to order a new set shortly.

Flash overs are one cause of coil failures. These can also be caused if the engine is run with the coil powered and contact breaker connected, but with the coil lead disconnected. The energy triggered by the contact points must find some way to dissipate and if the coil lead and therefore the spark plugs are disconnected, there will be internal arcing (flash over) and damage. Coils can also be damaged if the ignition is left on without starting the car – causing the coil to turn very hot. Age, heat, and vibrations can also cause coil damage. Good coils in a proper set up should not overheat under proper usage. Not in India and not even in the hot California weather.

It is good to periodically check the coil resistances for proper values. Unfortunately, correct resistance does not mean there is no damage to the insulation. Only real way to be sure your coil is not defective is to compare operation with a known good coil. I have standardized on using Bosch blue coils of 3ohm resistance on all my cars and keep a new one as spare and as reference for comparison.

Wed Apr 12, 2023 7:08 am

Thanks for that writeup bnam-
The last paragraph of your article:

bnam wrote:
It is good to periodically check the coil resistances for proper values. Unfortunately, correct resistance does not mean there is no damage to the insulation. Only real way to be sure your coil is not defective is to compare operation with a known good coil. I have standardized on using Bosch blue coils of 3ohm resistance on all my cars and keep a new one as spare and as reference for comparison.

seems to justify my theory that the resistance test on a coil won't necessarily tell you the whole story.
Like you said, the only real way to be certain is to make the best educated guess you can based on these symptoms, replace the part and go for a drive. :D
Unfortunately in my case these may result in me being stranded on the side of the interstate again.

Wed Apr 12, 2023 7:34 am

bnam wrote:
So, the system has a total of about 6Kohm.

Did you see a formula for this somewhere showing the resistance values? If so, I'd like to see this. Or, are you inferring this from other aspects to balance out other values?

Wed Apr 12, 2023 9:02 am

heimlich wrote: bnam wrote:
So, the system has a total of about 6Kohm.

Did you see a formula for this somewhere showing the resistance values? If so, I'd like to see this. Or, are you inferring this from other aspects to balance out other values?

No. No formula.

I measured 3 sets of good quality wires. 2 were Bosch and the 3rd a set sold by VW heritage. They all measured single digit ohms for the coil wire and one kilo ohm for the plug wire. I measured 5.3kohm for the rotor. I did not measure the carbon contact to rotor resistance. That added about 6-7K.

Fri Apr 14, 2023 7:56 am

Got my new Bosch 00012 coils in today! Last 4 in the country (for a reasonable price anyway :D )

On a side note...
If, like me, you are using an el cheapo Harbor Freight multimeter to test coils, and you get an abnormally high reading like this, which under normal circumstances would indicate a bad coil....:

MAKE SURE to take a measurement of the (ridiculously high) internal resistance of the meter itself :shock: :

and subtract from the coil resistance reading!

Fri Apr 14, 2023 8:57 am

The honest truth is that Bosch components were never better than anything else going around in the day. In fact they had rather a bad reputation honestly, which is where the joke came from: " Why do Germans drink warm beer?"..... "because they have Bosch refrigerators.

In this day in age who knows....even China can make a decent ignition coil...you just don't know which China one is the good or bad one.

Fri Apr 14, 2023 10:58 am

passionre wrote: The honest truth is that Bosch components were never better than anything else going around in the day. In fact they had rather a bad reputation honestly, which is where the joke came from: " Why do Germans drink warm beer?"..... "because they have Bosch refrigerators.

In this day in age who knows....even China can make a decent ignition coil...you just don't know which China one is the good or bad one.

Not their automotive division. The Bosch blue coils were ALWAYS better for aircooled VW than ANY other replacement coil. All of their components were the best to use on any aircooled VW as they worked directly with VW in the engineering and manufacturing process and produced what the auto manufacturer specified, and still do. Bosch is VW's #1 supplier.
I certainly won't stop you from cheaping out on a Duralast coil though if you want.
The joke about Bosch refrigerators refers to their appliance division. Don't know anything about that division and don't care.

Fri Apr 14, 2023 1:32 pm

I just had to ditch a Bosch blue coil that was toast. Lasted maybe 6 years? Bad coils are common now, as are bad Bosch branded points and condensers. The latter I only read about here since I use compu-fire modules instead of points.

Fri Apr 14, 2023 1:58 pm

vamram wrote: I just had to ditch a Bosch blue coil that was toast. Lasted maybe 6 years?

Are you sure you weren't using a Harbor Freight multimeter to test it? :D

I really have no idea whether it was my coil or something else that failed a couple weeks ago leaving me stranded on the interstate. According to some on here the signs point to coil- which would have been a Bosch blue coil I bought in a tuneup kit about 10-12 years ago for $55-$60 along with rotor, condenser, points, and plug wires.
I'm not sure whether I'd put one of these new Bosch coils back on my car or resell them for a profit. May keep one and sell the rest. I have two Beru coils coming in from heimlich that I will probably use one of those instead.

Fri Apr 14, 2023 2:27 pm

sb001 wrote:

MAKE SURE to take a measurement of the (ridiculously high) internal resistance of the meter itself :shock: :

and subtract from the coil resistance reading!

This little detail right here is easily overlooked by too many people that would see that high reading and think the coil was bad otherwise! Glad to see the attention to detail.

Not many think to test their test equipment to get a sane calibration :D

Fri Apr 14, 2023 3:18 pm

Gorgeous! Going to install one of these Berus.
As for the 4 Bosch coils, maybe I'll just buy 4 more bugs... :P

Fri Apr 14, 2023 3:22 pm

sb001 wrote:
As for the 4 Bosch coils, maybe I'll just buy 4 more bugs... :P

I know of a couple Autosticks.

Fri Apr 14, 2023 3:40 pm

heimlich wrote: sb001 wrote:
As for the 4 Bosch coils, maybe I'll just buy 4 more bugs... :P

I know of a couple Autosticks.

Do you know of a new job for me where I can afford them? :wink: :D

Fri Apr 14, 2023 3:44 pm

sb001 wrote:
Do you know of a new job for me where I can afford them? :wink: :D

I'm sure we can find you something.

Thu Apr 20, 2023 7:40 pm

passionre wrote: The honest truth is that Bosch components were never better than anything else going around in the day. In fact they had rather a bad reputation honestly, which is where the joke came from: " Why do Germans drink warm beer?"..... "because they have Bosch refrigerators.

In this day in age who knows....even China can make a decent ignition coil...you just don't know which China one is the good or bad one.

Can confirm
They could have been worse, or they could have been better.

Why are you using garbage tester to test garbage coils?
because it's fun :wink:
I'm cool with it

Sun Apr 23, 2023 5:07 pm

I suspect a lot of petronix stuff is made cheap in China and simply packaged with a flashy sticker and box to impress gullible folk who love to buy shiny, pretty new things.

A coil that works is all you need, spending big bucks and you won't find any improvement in performance, Manufacturers have become experts at marketing and advertising completely Bogus claims for decades.
That said Bosch are better quality components in general than any other manufacturer for bug parts in my 15 years daily driver experience.

Mon Apr 24, 2023 12:55 pm

Quick question: do coils go "bad"?

I replaced my Bosch blue with a BERU blue (bought from Heimlich) thinking my rough idle issue was the coil... it wasn't.

Is it worth saving the Bosch blue for a future use?

Mon Apr 24, 2023 3:13 pm

bhartwell59 wrote: Quick question: do coils go "bad"?

I replaced my Bosch blue with a BERU blue (bought from Heimlich) thinking my rough idle issue was the coil... it wasn't.

Is it worth saving the Bosch blue for a future use?

Sure. May as well have a backup... I actually kept mine that may have caused my stalling on the interstate last week, even though I have no real proof (I replaced mine with a Beru as well.) Problem is there is a chance a coil could measure good when cold, but cut out when warmed up, as might have been the case with mine- you'll never know unless you drive the car with the suspect coil until hot and see if it stalls out, then take a measurement immediately. Not worth putting myself in that position again!

Tue Apr 25, 2023 10:27 am

bhartwell59 wrote: Is it worth saving the Bosch blue for a future use?
sb001 wrote: Sure. May as well have a backup
Hahaha.... and we have come full circle!
The OP for this thread was asking almost 3yrs ago if running a Bosch Blue coil as a primary and keeping the black OE coil as a spare was a good idea.

Now we are keeping the Bosch Blue as a spare. :lol:

Tue Apr 25, 2023 11:09 am

I dunno if they are still the best, but I am not gonna pay $80 for a Bosch coil, at least not today.
I tested my coil Sunday, and the primary read 3.9 ohms. I could not get any reading at all from the secondary, so I assume it had "gone bad".
I bought one from Jbugs, a Beru, and some other items.
I subsequently received a notice that said "some of your order may NOT be delivered on time"....
So, no coil this weekend?
Argh!!!!