ShopTalk Forums | Aircooled VW Tech & Restoration Help

I have nothing but praise for the Bosh blue coil. My father had on his 67 and 74 Beetle. The 67 was only five years old when it was stolen, but my father kept his 74 Beetle for twenty five years and had over 250,000 miles on it, with the original Bosh blue coil. The only thing that kill that car was the New York City road salt. It ate the car like a cancer. But, the engine was a reliable till the end. My 1970 Beetle has a Bosh blue coil and I have no problems with it. I only replaced the coil because it was all scratched up and I was looking to dress up the engine.
I don't agree with replacing your original Bosh blue coil if you have flooding issues. If your engine floods a lot then fix it. If the float needle is dirty then clean it. If it is damaged then replace it. When you start compensating for a problem all you do is invite more problems. If fuel is leaking into a engine from a carburetor issue and that problem is not fixed, then you are going to have more engine problems later on. Using "special" coils is never the answer.

How much spark do you need and why do you need it? There are three different things to look at when looking at secondary voltage.
1. Secondary Available Voltage = max out put
2. Required Firing Voltage = actual KV
3. Reserve Voltage. = what is left over
Secondary Available Voltage is what the secondary side of the coil is capable of producing.
Required Firing Voltage is what it actually takes to jump the plug gap.
Reserve Voltage is the difference between the Available and Required voltage.
Why is all this Reserve Voltage needed?

nybug wrote:I don't agree with replacing your original Bosh blue coil if you have flooding issues. If your engine floods a lot then fix it. If the float needle is dirty then clean it. If it is damaged then replace it. When you start compensating for a problem all you do is invite more problems. If fuel is leaking into a engine from a carburetor issue and that problem is not fixed, then you are going to have more engine problems later on.

Replacing the weak Bosch ignition coil with one that is actually suited for the job of fuel ignition in real world ignition conditions is NOT compensation for a problem. It's fixing the weak ignition system which manifests itself with many symptoms, including the inability to fire spark plugs in a flooded engine. I use this as a dramatic example as it's a concept that most people can easily grasp. Flooding can happen even when there aren't any problems. After a hot engine has been shut off, the heat from the engine can heat up the gasoline in the carb float bowel and force it into the intake manifold and flood the engine. This is a real world engine condition that the stock Bosch ignition coil CANNOT HANDLE!

People have a little harder time grasping the other problems caused by a weak ignition system, such as increased ignition lag time and difficulty igniting lean and excessively rich fuel mixtures. There is simply NO advantage to using a weak ignition coil.

nybug wrote:Using "special" coils is never the answer.

In this case a variable permeability ignition coil is the CORRECT answer to the problem.

The conventional cylindrical autotransfomer used for ignition coils is only suited for a CONSTANT electrical load that does not vary. However, the spark plug electrical load is constantly varying. It is is the WRONG type of transformer to use for an automobile ignition system!!!!!

A variable permeability air gapped transformer is well suited for VARIABLE non-constant electrical loads. The spark plug electrical load on the ignition coil is constantly varying. A variable permeability ignition coil is PERFECT for the application of an ignition coil!

So why have these crappy cylindrical non-variable permeability autotransformers been used for ignition systems for so many years? They are CHEAP to produce!!! That is their ONLY good quality. Performance wise they SUCK! Note that most suto-manufacturers do not use them any more in favor of better performing E-core and C-core designs.

There is nothing particularly special about a variable permeability air gapped core inductor which is used for ignition coils. It is highly likely that you are using an air gapped core transformer in your computer's switching power supply. At some point between my computer and yours, several variable permeability air gapped core transformers will be used in the electronic equipment that make this internet possible. Air gapped core inductors have been used in power supplies with variable electrical loads since the early days of radio. Kettering's original drawing for his famous ignition system, that is used in air cooled VWs, appears to show an air gapped core variable permeability ignition coil.

The air gap is what makes the variable permeability ignition coil work
Jacobs CMD Ultra Coil Core showing 0.030" air gap.

It's just a 0.030" gap in the center leg of the ignition coil. This is what you are referring to as a "Special" ignition coil.

Here is a Mallory #30440 variable permeability ignition coil. But you can't tell by looking whether or not it is a variable permeability air gapped ignition coil.


I really don't understand why people are so resistant to use a superior ignition coil that uses an external ballast resistor, is shaped a little differently, and is actually suited for the job of fuel ignition in an engine. I also don't know where anyone every got the idea that a Bosch ignition coil was actually a good ignition coil design for the application.

Most people have never used a variable permeability ignition coil in their air cooled engines and have NO basis for comparison. Nor do most of them have even basic electrical knowledge about magnetic theory and transformers. Even when confronted with data that shows how poorly the standard ignition coil performs, they are often still adamant that a variable permeability ignition coil is not needed and won't make any improvement.

Scott Novak

david58 wrote:How much spark do you need and why do you need it? There are three different things to look at when looking at secondary voltage.

1. Secondary Available Voltage = max output:

The commonly advertised open circuit ignition coil output voltages are IRRELEVANT!!!

You need to know the maximum output voltage UNDER ACTUAL SPARK PLUG LOADING.

This means using a load resistance no greater than 1,000,000 ohms connected across the spark gap to simulate a new park plug, 500,000 ohms to simulate a spark plug in severe use or one that is beginning to foul, and 250,000 ohms to simulate a fouled spark plug. Some alternative fuels cause even greater electrical loading.

2. Required Firing Voltage = actual KV:

This varies with engine conditions such as air temperature, cylinder and cylinder head temperature, barometric pressure/altitude, humidity, fuel mixture, RPM, throttle position, compression ratio, volumetric efficiency, dynamic compression ratio, spark timing, spark polarity, spark plug condition including electrode wear and contaminant loading on the insulator, spark plug heat range, electrode material, electrode shape, electrode diameter, and spark plug gap width to name a few variables. Obviously this varies from engine to engine, and it can vary dramatically on the same engine when changes are made.

3. Reserve Voltage. = what is left over:

Open circuit voltage must NOT be considered when considering the necessary reserve voltage. You must determine the worst case spark plug loading that you can reasonably expect in normal operation. You can reasonably expect that your engine may become flooded on a hot day when you park your car after a long drive. Someone off roading or racing can expect fuel sloshing that can dump gasoline down the carb barrels and cause flooding conditions. The ignition coil must be able to put out a sufficiently high voltage under this SEVERE loading to be able to fire the spark plugs under these conditions.

In addition, engine use and engine maintenance must be considered. People that change their spark plugs often do not need much reserve voltage. People that don't change, or at least remove, file the electrodes flat and regap their spark plugs often, must have a much higher reserve voltage. Likewise, endurance racing conditions where you can't stop to change spark plugs also requires a larger reserve voltage.

You also need to factor in the reduction in spark voltage while starting the engine. This varies depending on the size, quality, and condition of your battery, when using a Kettering inductive switching ignition system.

Using an ignition coil with built in ballast resistor, that cannot be bypassed during starting, results in the lowest possible spark voltage and requires the highest reserve voltage. Internal ballast resistors also heat up the ignition coil and lower the output voltage. Unfortunately this also requires an ignition coil with a higher turns ratio that reduces its spark current. Needless to say an ignition coil with an internal ballast resistance is a VERY BAD THING and should only be used for a doorstop.

The output voltage of a well designed capacitive discharge ignition systems is not as dependent upon battery voltage. Some of these CD ignition systems will put out full spark voltage with as little as 7.5 volts.

You also need to consider that the Kettering ignition system used in stock Air Cooled VWs does NOT vary the dwell time and as a result, the ignition coil output voltage drops as the RPM increases. You must have enough reserve voltage at the highest RPM that the engine will operate at. At low RPM you could use a much larger spark plug gap than you could at high RPM. But since you can't change the spark plug gap while driving, you must use a much small spark plug gap to be able to fire the spark plugs at the highest RPM that will be used.

Using an adaptive spark ignition system that varies the dwell with RPM along with a variable permeability ignition coil, means that the output voltage will remain nearly constant with RPM, until you reach the RPM limit of the ignition system. This dramatically reduces the reserve voltage necessary, and it allows for more efficient use of the ignition coil output, as there can be a better impedance match and more power transferred into the spark, and less power wasted inside the ignition coil.

In an air cooled VW where the ignition switch is does not have a provision to bypass the ballast resistor during startup, using an adaptive spark ignition system can remedy this. Ballast resistors are normally not used on the ignition coil because the ignition system limits the ignition coil primary current electronically. These adaptive spark ignition systems can produce very high output voltage while starting, even with a reduced battery voltage.

You can see that there is a lot to consider and there are not easy answers to these questions.

Scott Novak

So what coil would be better than the Bosch Blue coil?

You also need to factor in the reduction in spark voltage while starting the engine. This varies depending on the size, quality, and condition of your battery, when using a Kettering inductive switching ignition system.

Using an ignition coil with built in ballast resistor, that cannot be bypassed during starting, results in the lowest possible spark voltage and requires the highest reserve voltage. Internal ballast resistors also heat up the ignition coil and lower the output voltage. Unfortunately this also requires an ignition coil with a higher turns ratio that reduces its spark current. Needless to say an ignition coil with an internal ballast resistance is a VERY BAD THING and should only be used for a doorstop.

The condition of the starter is also important, one that is dragging ( drawing to much amperage ) takes away from the primary voltage and makes the engine hard to start. The condition of the electrical system has a lot to do with if a car will crank under different conditions. Voltage tests during cranking should be done to find these problems.
I agree that the Bosch coil had it's place, and still does if that is what you want to use. There are better coils today than there were back in the day.

david58 wrote:I agree that the Bosch coil had it's place, and still does if that is what you want to use. There are better coils today than there were back in the day.

Unless your goal is to only use period correct parts, spending money on a Bosch ignition coil is throwing away good money after bad. Why reward a company for producing a bad design?

An acceptable application for a Bosch ignition coil would be as a gas heater ignitor where the load on the ignition coil is constant.

Not to mention that using a variable permeability ignition coil may be the difference between being stranded and getting to your destination on time. In the long run, a Bosch ignition coil will cause more pollutants from your engine and it will also cost you gas mileage. Using a Bosch ignition coil will cost you more money in the long run.

Blue 1/2 Moon wrote:So what coil would be better than the Bosch Blue coil?

Personally I recommend some Jacobs ignition coils. Read the following for my recommendations on which ones to buy and which to avoid.
High Energy Ignition Systems Jacobs MSD Mallory
http://www.shoptalkforums.com/viewtopic.php?t=122428

Many of the other brands of variable permeability ignition coils have excessively high turns ratios that degrade performance. The Mallory #30440 ignition coil, previously pictured here as an example, has a 1:101 turns ratio that is excessively high. It does not perform as well as lower turns ratio Jacobs ignition coils and it is very noisy, so you wouldn't want to mount it inside a vehicle. However, it still much better than most other ignition coils.

I don't recommend ignition coils with turns ratios much over 1:90. A 1:60 turns ratio works very well if you are using a small cap Bosch distributor.

Scott Novak

Thanx...

Scott Novak wrote: People have a little harder time grasping the other problems caused by a weak ignition system, such as increased ignition lag time and difficulty igniting lean and excessively rich fuel mixtures. There is simply NO advantage to using a weak ignition coil.

Scott Novak

The only thing that I agree on is that I don't agree with your logic. I am a Packaging Mechanic in a large Pharmaceutical. I have had to in the past un-repair repairs that other "mechanics" have done to the companies machines. There illogical modifications to machines (in an bizarre attempt to solve a running problem) only caused more problems.
If particular machine has a history of always being reliable and then there is some problem, they are the first to replace a motor with a bigger one or go into the machines programing and change all the factories presets, just because the line is not running as fast as it normally does, or whatever the problem is.
They don't bother to look for some kind of mechanical binding or jamming. They go right to these unbelievable modifications. Then I have to come in a un-repair what they damaged then look for the real problem.
When you work with machines (whether a Pharmaceutical machine or even a old air cooled engine, you learn to listen, see or even feel the way the machine should run. You know when there is something wrong.
My Bosh blue coil work as great a my fathers did. It is better to have a coil like that so as to not mask other problems. If my engine is flooding I want to know why. I don't want to just change a coil and then go obliviously on my way not knowing that there is a problem.

Well Nybug. If youre good at listening, you can put your own words in a corner in notime flat, if you try out some of the recommended ignition set ups.
Your argument that "it runs fine" is worth nothing if you have not compared it to anything. I have done comparisons both on and off the dyno. And the verdict is clear.
But I do agree on the fact that there are some people that mixes parts the wrong way.
If I had to choose a Bosch coil today, it would be the red version with external resistor, because I saw that some of those are still made in Germany, and they do have more reserve voltage than the blue has.
T

farmer wrote:
If I had to choose a Bosch coil today, it would be the red version with external resistor, because I saw that some of those are still made in Germany, and they do have more reserve voltage than the blue has.
T

Whatever makes you happy.

We need to make sure we're all keeping our ststements clear here. There is no doubt that the Bosch blue coil and even the older (and I assume inferior, even 6V, black Bosch coils) have served many a vw owner well since the 1930's. Coils are, by and large, a trouble free item compared to many other parts. I have 50 year old Bosch black 6V coils running today with little complaint.

I think what has been proposed here is a couple of things:

1. Newer technology coils can offer substantial advantages in performance and life with little downside.

2. Bosch coils, particularly those manufactured outside of Germany, may not be all we have come to think of relative to the Bosch name. That would really be ashame, I think.

3. Given the current pricing of Bosch coils (way up from the $12 I used to pay), more expensive setups may be a viable and valuable upgrade.

I don't think anyine is saying that the traditional Bosch coils don't work. But the actual testing by members here suggests there may be a better alternative.

On my T4 hotrod car, I'm inclined to try this newer technology. On my stock '57, I'm sticking with the 50 year old Bosch black 6v.

doc

doc wrote:1. Newer technology coils can offer substantial advantages in performance and life with little downside.

In this case a variable permeability ignition coil is VERY OLD technology. I learned about variable permeability air gapped inductors in electronic school in 1972. They have been around since the early days of radio. Kettering's original ignition system sketch from 1908 shows an air gapped core ignition coil.

The only thing that has changed over the years is better insulating materials, and a larger variety of steel lamination formulations to obtain the desired paramagnetic characteristics desired. Transformers ARE old technology.

There is NO performance downside whatsoever to using a variable permeability ignition coil. They run cooler than the Bosch Blue coil. Because of the lower operating temperature, a variable permeability ignition coil can be more reliable than a Bosch Blue coil and it also has less voltage losses than a Bosch Blue coil. As such, a lower turns ratio can be used which produces more spark current and improves it's resistance to fouling.

The cost to make a variable permeability E-core ignition coil is only slightly more expensive. Nearly all automakers now use either E-core or C-core designs. In large volumes a variable permeability ignition coil may now be cheaper to produce than a Bosch Blue coil which is not produced in such huge quantities any more.

General Motors even has a patent on a variation of a variable permeability ignition coil. It is a little more unusual because they use a different formulation for the primary core than they do for the secondary core.

People somtimes seem to forget that VW wasn't making the best engine in the world. They were making a very inexpensive one and they made MANY tradeoffs that degraded performance and sometimes reliability too.

Scott Novak

Above: Plain old BOSCH racing sticker on the rear.

Hi Scott,

Of course tradition has a lot to do with our choice of cars or parts. I'll never forget the first time I walked up to a 917 and saw those Blue coils, and it impressed me. Just like on my street Ghia -- and I mean just like. Not something special. The factory could have anything they wanted on that 1100hp car. They used twin coils (and a capacitive spark booster). It got the job done at 8 grand.

Over the years I've installed and debugged all types of ignition systems on race and street cars. I think the new ignitions that will fire old dishwater are great, and that's what I want my wife to drive.

But in combat at the track, I have to keep it tried and true. I've even used plain old black coils in a pinch and I know that's not politically correct for tuners. Don't tell anybody.

A Subaru engine is better than my old aircooled, and any suspension on late model cars is better, or why would a guy want a 79mm stroke when he could have an 84? The spiral never ends.

We Blue Coil guys aren't just contrary sorts. I believe you and I think you know your stuff.

But I might have to make up a new sticker for the rear end of our Ghia. It'll have a Bosch spark plug and a Blue Coil on it and the words:

You just got beat by Bosch!

FJC

I ran the Jacobs Bug Pack, I think the word Onmi was thrown in there also, on an otherwise stock 2.0 Westy with FI. I was able to gain two mpg across the board, almost three, with carefully documented charting.

Thanks Scott, for the info. Let the purists have there way. All you can do is present the material. Good reading.