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Tue Apr 29, 2014 1:46 am

Just measured the coil resistance after reading a lot of posts. Am using an external ballast resistor. Coil is approximately 0.5 ohms on the primary without the resistor and approximately 3.0 ohms with it and 17.5 K on the secondary. Seems like the standard range for the secondary is max 15 K and recommended around 10 K. Tried to understand what 17.5 K would do but just can't work it out. Bus is running fine but only just re-built the engine and don't won't to do any long term damage.
I have a stack of old coils here which are Bosch, Made in Germany and trying to work out which is the best for the stock 1600 DP.

1 @ 1.0 ohms/9.5 kohms No. 0 221 122 003 KW 12V
2 @ 2.0 ohms/ 9.5 kohms No. 0 221 122 006 KW 12V
1 @ 3.7 ohms/ 10.5 kohms No. 0 221 102 007 322 K 12V

The 003 is blue and the other ones are black like the original ones recommended. The 007 is about 15% bigger than the others. Looks like I could use the 007 without an external resistor which I would like to do as the ceramic casing on the external resistor I am using is cracked so I can't see the element in it lasting too long.

The book I have (Gregorys VW Type 2 Transporter) says the primary should be between 1.1 and 1.3 ohms. Thought this must have been if the bus had a resistor somewhere up the line towards the key but mine has 12V on the feed to the coil not 9V or there about which I think some coils run at and I couldn't see a resistor anywhere in the wiring diagrams. Is this an error in the book or are these systems designed for an impedance that low? None of the other books I have mention what impedance is required.

So the problem is, I can't find decisive information on what is best. Maybe that's just how life is these days.

My guess is that points don't like having a 12 Volt load and will last longer at 9 Volts or there about. There is some mention of how much current draw there is with different impedance but I don't fully understand how that works. Seem to remember coils running at 9 volts from playing with early Toyota Coronas years ago and the opinions of many people here on the Samba seem to confirm that you need an external resistor if the primary is less than 3.0 ohms and you can go without one if 3.0 ohms or more.

There is some info stating tha the lower the primary resistance is, the faster it will charge the coil for the next spark which is more important on an eight cylinder vehicle where the coil has to produce twice as many sparks for the same revolution of the crank but not as important on a four cylinder engine that doesn't pull extreme rpm.

So I am trying to work out if the one I am currently using is better than the others considering the secondary resistance is higher. It isn't or doesn't appear to be a Bosch coil (maybe it came out of a Dihatsu Charade that I was wrecking) and it doesn't have any numbers on it but it appears to be working well.

The number on the large Bosch is 0 221 102 007. I did a search but couldn't see any info or recommendation for any vehicle. There was a similar number on an Alfa Romeo but not the same.

Any info greatly appreciated. Cheers

Tue Apr 29, 2014 5:05 am

2 ohms mifht work & check points4 any showings of overheating, 3 should fine, this coil resistance is what keeps points life up and keeps down excessice current flow, & the v drop that happens when a resistance is added to any current circuit, remove that resistance and v will increase. External resistors added can be a resistor block, or a wire from coil to dizzy, ends up doing the same function as being internal in coil.In stock, i was never worried about the secondary unless i had a weak spark, aftermkt ones to boost v of spark is 1 of your choice to improve performance(?) Fact electronic ign coils r different and usually of specific resistance, never had problems with any of mine so no need to actually dig into their specs, my-2cts.

Tue Apr 29, 2014 9:40 am

My original black Bosch coil that failed was an 021 905 115B . It's primary was 3.0 ohms, and secondary was 5.7k ohms.

My new Bosch 012 was 3.4/7.79k ohms. Works perfect.

Also see:

http://www.ratwell.com/technical/BlueCoil.html

Wed Apr 30, 2014 3:25 am

Thanks for the info. I switched coils to the

black Bosch 3.0 ohms/10.5 kohms No. 0221 102 007

and it didn't seem like the bus was running as smooth or develop as much power and also it didn't fire as easily while cranking so maybe the 17.5 kohm secondary resistance works better than 10.5 kohm.
I have a carburetor issue that I have to sort out, is a 34 pict 4 that takes ages to pick up speed but is Ok when it gets there. Think it's an accelerator pump issue as when I put my foot down too quick it wants to die but is ok if I nurse it up to speed. This carb has always been a bit like that. Wondering what I can do to make it start working properly.
Read way back that the 34 pict 4 were a partial pollution control carb, something to do with automatic accelerator pump adjustments. Seems like more of a top end carb, kind of feels too big for the engine like the engine can't suck enough to get it working properly. I checked out all the jetting a while back but maybe I got it wrong. I am missing the heat riser pipes on the intake manifold, couldn't work out if this was ok in the tropics or not, maybe that is part of the problem although a lot of people seem to run without them here.
Thinking about switching to a smaller carb to get more low rpm control at least for around town anyway. Next size down that I have is a 30 pict 2 but maybe that's too small for this engine. Interested in hearing what carbs people are using on stock 1600 DP and for what purpose and what type of driving experience they are getting.

Wed Apr 30, 2014 5:33 am

A coil is actually a pulse transformer that steps the voltage up, so a higher secondary impedance (resistance) should give you a higher voltage or hotter spark.

Wed Apr 30, 2014 6:32 am

Ahaarr I get it now, so that could be a good thing as long as I don't get the spark going where it's not supposed to go? Will I risk burning the plugs if too high? What to look for if they are?

Was looking at some of the Electronic ignition info. It looks like they might operate with spark voltage quite a bit higher than what i would get with this coil, so I shouldn't be putting the engine at too much risk, am I likely correct?

Wed Apr 30, 2014 6:56 am

Was looking at some posts from Scott Novak where he recommended using coils with less than 85:1 secondary winding. If the primary on the coil that I am using is 1.0 ohms before resistance and the secondary is 17.5 kohms is it likely that the ratio is a lot higher than the standard 100:1 winding and the recommended 85:1 or will it completely not matter considering this is a stock engine that won't be a high rpm engine.

Am just trying to get the smoothest most efficient engine for long haul highway use. Bus and trailer ... gulp.

Wed Apr 30, 2014 7:07 am

Muzooma wrote:
Read way back that the 34 pict 4 were a partial pollution control carb, something to do with automatic accelerator pump adjustments. Seems like more of a top end carb, kind of feels too big for the engine like the engine can't suck enough to get it working properly. I checked out all the jetting a while back but maybe I got it wrong. I am missing the heat riser pipes on the intake manifold, couldn't work out if this was ok in the tropics or not, maybe that is part of the problem although a lot of people seem to run without them here.
Thinking about switching to a smaller carb to get more low rpm control at least for around town anyway.

If you have a Bentley manual, you can measure your accelerator pump output. Then you can adjust the output.
The heat riser is a critical component for smooth driveability. Even in warm humid conditions, the intake air is cold enough to condense the fuel vapor.
The german Solex 34 Pict 3 is an excellent carburetor for the dualport engines.
Colin

Wed Apr 30, 2014 8:32 am

Thanks Colin, am definitely getting the picture now. Have just in the last week had the engine apart again to fix a noise. Sorted and got it going and went for a test drive in the hottest part of the day with the higher voltage coil and it was the best and smoothest performance I had got out of it to date. In the evening I switched to the Bosch 3.0/10.5k coil and the bus was a lot more sluggish, so maybe what was going on is a duel impact of cooler ambient high humidity air and weaker spark acting together.
I will get a heat riser manifold (shame I actually had a duel carb setup off a type 4 but my dad sold them while I was away without realizing what they were :( ) Will then see what I need to do to the carb.
Sounds like jetting it to the same as the 34 pict 3 might be a good place to start as well as the pump measurements you have suggested thank you. I don't have all the vacuum setup on my bus so am going to have to work out how to make the accelerator pump valve work.
I am sure that there is some sort of problem with the accelerator pump but also maybe a jetting problem as even when accelerating really slowly there is still a reluctance in the engine. Or could this be the fuel condensation in the manifold you are talking about. Just trying to get a feel for what does what. If I give the accelerator a double pump it seems to overcome the reluctance for a short burst then back to usual.
My experience with this engine so far is that most of the time it has been an all or nothing engine with seldom moments of smooth controllable running. I have often wondered weather I ended up with a bigger cam in it than stock, never thought to measure the lift with the rocker covers off until just now. But now illumination of the nil heat riser effect is starting to make sense - awesome. Today was the first time in two years that the car put a smile on my face. I am starting to see what it can become. felt like an airplane winding up today, man this engine has some legs when it decides to go.
Also wondering about the exhaust I have. Set of Genie pipes which are leaking slightly where they join fore and aft. Am going to get flanges welded in so they can bolt up together with a flat standard exhaust gasket. Is that a good idea? And what sort of effect on the engine does small leaks like this have so far away from the heads? The leaks sound like the slightest valve chatter but a bit more crisp and not any louder than slight valve noise. Is that any thing to worry about or will fixing them have a major effect on the problems I am seeing?

Wed Apr 30, 2014 9:07 am

[Note: rambling tech opinions follow:]

You can't really infer anything about the turns ratio of a coil from resistance readings. Here's the story:

Primary resistance is somewhat critical. That, combined with any series resistor, must limit non-running primary current flow to a reasonable value, say, 3 or 4 amps. Any more will fry points (or driver transistors).

Coil resistance is determined by the wire gauge used to wind the coil, and the number of turns. If you use large wire, the resistance will be less than with small wire, for the same number of turns. This applies to both primary and secondary windings. Thus, for the same turns ratio, you can get wildly varying resistance readings, depending on the size of the wire used to build the coil.

Bottom line: resistance readings are only useful for comparing a coil's characteristics against published values, for troubleshooting purposes.

A coil works by storing energy in a magnetic field. The field is ultimately limited by the current flowing in the primary winding, the number of turns, and the type of magnetic metal used for the core of the coil. When the points open, this stored energy is dumped into the spark plugs. The voltage on the primary and secondary coil windings varies directly with the coil turns ratio. For example, if you have a 100:1 coil, and the spark plug voltage is 25,000 volts, the open points primary voltage would be 250 volts. This primary voltage has to be handled by the condenser (if you have points) or by the output transistor (for electronic ignitions). Thus, electronic ignitions tend to use coils of higher turns ratios, resulting in a lower primary voltage for the same plug voltage, and thus less stress on the driver transistor.

Plug voltage is determined primarily by plug gap and cylinder pressure. It will be pretty much the same regardless of the type of ignition system used to fire the plug. [That said, capacitive discharge systems can result in higher voltages because the spark is so fast, but we generally don't use CD systems on buses, so we'll ignore that]. Thus, your plug voltage will be roughly the same, regardless of whether you're running points, or electronic. So much for advertising... [What will vary is the duration of the spark: higher energy systems will fire the plug longer, resulting in a higher probability of mixture ignition.]

2nd bottom line: use the proper coil for your type of ignition. For engines using the stock RPM range, and using points, use the stock coil. For electronic, use the coil recommended by the manufacturer of the system. Don't mix and match.

Thu May 01, 2014 3:43 am

Great info Telford thank you! I am a little uncertain on a couple of points you make.

Are you saying that the plug gap and resistance in the plug leads sets up the voltage in the primary coil?

Is not the primary coil voltage a product of the battery voltage and the resistance in the primary circuit? So that if you have 12V on the primary circuit you would have 1200V available at the plug? Trying to understand.

And the dwell angle becomes a time period that allows the coil to charge?
Faster the engine goes the shorter the time period available to charge the coil in between discharges? So is the problem with a coil with more windings to do with charge time being to long at higher revs when the dwell angle gives such a short charge time?

Am wondering isn't the coil very similar to a capacitor with so many windings?

I understand that if the designers thought the engine needed a high voltage coil, they would have put one on it. Doesn't seem like they are that complicated. so no reason they wouldn't.

On my two test drives yesterday, one in the middle of the day with the high secondary resistance coil and one in the evening after sundown with the coil that has the recommended resistance, there appeared to be considerable difference in performance but maybe that was more atmospheric condition affecting the engine than the coil voltages as my bus is missing heat risers on the carb manifold.

I think I understand now that even though the coil with the recommended resistance of around 10 kohms it could have larger guage wire and a lot more windings, maybe 130:1 whereas the coil with the high secondary resistance might be wound with very thin guage wire and might only have 80:1 windings.

However if that is not the case and the high resistance coil is providing a longer spark as you say, maybe thats why i see the difference yesterday? The longer period spark partially overcoming the un-vaporized fuel that the engine is trying to use? So maybe I don't need longer duration spark if iI just fix the rest of the problems.

I am still wondering what is wrong with a longer duration spark? Is this when plugs start burning etc?

Thu May 01, 2014 12:37 pm

Muzooma wrote: Are you saying that the plug gap and resistance in the plug leads sets up the voltage in the primary coil?
Basically. The plug gap and cylinder pressure determine the voltage that the plug will fire at. Add to that wire, rotor, and the gap inside the distributor cap, and you have the voltage the coil secondary must obtain to fire the plug. During this period, the primary voltage will mirror the secondary voltage.

Quote: Is not the primary coil voltage a product of the battery voltage and the resistance in the primary circuit? So that if you have 12V on the primary circuit you would have 1200V available at the plug? Trying to understand.
It is, while the points are closed. Max coil current is (approximately) the battery voltage divided by the coil resistance. Once the points open, it's a whole 'nother story. Remember, the coil is an energy storage device, storing energy in a magnetic field. When the points open, the primary current stops, and the magnetic field collapses.

It's the nature of an inductance, like the coil winding, to develop a voltage as a function of the rate of chance of the current. Specifically:

V = L x dI/dT

where:

L is the inductance of the coil primary (a constant)

dI/dT is the rate of change of the (primary) current with time, in amps/second.

The faster the current changes, the higher the voltage will be.

Google "Electromagnetic Induction" for more details.

Quote: And the dwell angle becomes a time period that allows the coil to charge?[
Mostly. Dwell is the angle of distributor rotation during which the points are closed. What is more important is the time the points are closed. For example, on a four cylinder engine, four plugs fire for every rotation of the distributor. Since a distributor turns 360 degrees, each plug gets 90 degrees of that rotation. If the dwell is 45 degrees, that means the points are closed 50% of the time - 45 / 90.

Quote: Faster the engine goes the shorter the time period available to charge the coil in between discharges? So is the problem with a coil with more windings to do with charge time being to long at higher revs when the dwell angle gives such a short charge time?
If the engine is turning 6000 rev/min (a nice round number), then it turns 100 rev/second, or 10 msec/rev. Since two plugs fire per revolution, that means a plug fires every 5 msec. If the dwell is 45 degrees, then the coil gets primary current for only 2.5 msec. Thus, to function at this speed, the coil must develop maximum primary current in 2.5 msec.

Remember the equation above? It also applies here.

V = L x dI/dT

or

dI/dT = V / L

This means that, ignoring resistance, for a constant primary voltage, the primary current will increase linearly with time. But there is resistance, so the primary current increases at an inverse exponential rate, eventually reaching a maximum value. This is why high performance coils have so little primary resistance. It also means that they have to use other means to limit primary current.

Quote: Am wondering isn't the coil very similar to a capacitor with so many windings?
In a way. No physical device is 'pure'. All have a combination of characteristics - resistance, capacitance, and inductance. As the operating frequency of a device changes, the nature of these characteristics changes. This is what makes it so tough to design things - you're always running up against limitations.

Quote: I understand that if the designers thought the engine needed a high voltage coil, they would have put one on it. Doesn't seem like they are that complicated. so no reason they wouldn't.
Actually, it is. Higher performance costs more money (generally, a lot more - it's not a linear function...)

Quote: On my two test drives yesterday, one in the middle of the day with the high secondary resistance coil and one in the evening after sundown with the coil that has the recommended resistance, there appeared to be considerable difference in performance but maybe that was more atmospheric condition affecting the engine than the coil voltages as my bus is missing heat risers on the carb manifold.
Yeah, environmental factors make a big difference. You need to A-B test coils, one after another, under the same conditions, to draw a meaningful conclusion.

Quote: I think I understand now that even though the coil with the recommended resistance of around 10 kohms it could have larger gauge wire and a lot more windings, maybe 130:1 whereas the coil with the high secondary resistance might be wound with very thin gauge wire and might only have 80:1 windings.
Could be. Can't tell without knowing what's inside...

Quote: However if that is not the case and the high resistance coil is providing a longer spark as you say, maybe that's why i see the difference yesterday? The longer period spark partially overcoming the un-vaporized fuel that the engine is trying to use? So maybe I don't need longer duration spark if I just fix the rest of the problems.
I always recommend against modifying one part to compensate for correctable deficiencies in another part. Always comes back to bite you in the butt...

Quote: I am still wondering what is wrong with a longer duration spark? Is this when plugs start burning etc?
Yup. Longer duration uses up plugs faster.

Fri May 02, 2014 4:54 am

Great knowledge Telford, thank you so much for taking the time to share.

I now have a much better understanding of how that all works.

I read one of Bob Hoovers blog pages yesterday where he described how amazing these vehicles ran when they first rolled off the line. I would really like to be able to achieve that with my bus. At this point all I have got to go on is the seat of my pants. If I don't feel that something is wrong I won't know what to look for. I can't go and buy everything new and that probably wouldn't solve the problem anyway. Going step by step through the four manuals I have is a lot of ground to cover and there is so much variation in the information anyway. And so much info saying there is better ways of doing things. I do tend to agree with Bob, the original designers were pretty clued up.

I guess there is no way to describe what it is like when one of these engines is operating correctly. I have watched a stack of you tube videos of bugs that have been worked up and are good for the strip but not much about an every day driver working perfectly. If I knew what a good one felt like I could get there a lot quicker. And more importantly is knowing the feel of what each part does. I do know they feel totally different than a water cooled engine.

I've just been for another test drive after installing heat risers which I spent all day building. A reasonable improvement, less reluctance in the engine to accelerate. I have the copper heat riser start at no.2 exhaust, past the manifold then to a coupling and then following the exhaust to the tail pipe but not connected anywhere. I have flattened copper pipe wound around the manifold and the heat riser pipe to spread the heat. I have tried to get it as close to the original function as possible. The outlet sounds awesome but a bit loud (like a single pot banger Lol), it won't be staying like that. Will either put it back into the main silencer or it's own dedicated silencer. The inlet manifold is now warm where it goes into the head pipes but still cool below the carb. Thinking of winding some thinner copper wire up around that section. The improvement is not as much as what I was hoping for so I have to search for the next piece of the puzzle.
A couple of years ago I tried a 30 pict 1 carb and the bus accelerated very easily with no reluctance whatsoever. I can't understand why this carb - 34 pict 4 doesn't have the same response. I have tried nearly everything short of re-jetting it. That's why the interest in the ignition system. Do I have all that set up correctly and do I even have enough understanding to set it up correctly. The points are a no brainer ( I think) The coil? think I am about 80 percent there on that one. Plugs are a no brainer, right temp and right gap. Am using high temp good quality silicone leads, seem to be doing the right thing. I guess without throwing it on a machine there is no way of knowing if i have all the ignition components up to scratch as I just completely do not know what they are like when running perfectly.

So far I have rebuilt the bottom end with new bearings. The lash between the gears was minimal, I got the right size cam gear. I don't know what cam I am using but pretty sure it is a stock cam but don't know what from. Is it possible to have a miss match between cam and carburetor?
I balanced crank, flywheel, rods an pistons. Read carefully how to do each.
new guides and valve seats machined in the heads. Guides were originally a bit sloppy and passing oil into the air fuel so I put valve stem seals from another type of engine on the inlet valves and this worked well so have retained them after the head rebuild.
I haven't checked valve timing as was sure everything is stock. Is this something that needs to be done regardless? I couldn't see how I could alter that anyway. I put the marks on the cam gear and crankshaft in the right place. The rocker gear seems to be functioning correctly. I had a noisy exhaust valve on no.4 cylinder but have dropped the clearance to .005" and that seems to have solved that problem. I have installed heat risers to stop the fuel air condensing as they were missing. I did a complete carb overhaul, installed bushes on the shaft and cleaned it really well. The choke is working perfectly. I rebuilt the distributor and it appears to be working correctly with the correct amount of advance. The bus is idling very well but under operation is still slightly reluctant to accelerate easily. Feels like it is running learn or the timing is too retarded. I'm almost there. All pointing towards the carb. Is there anything I have over looked? Only other thing I can think of is this engine doesn't have thermostatic control of the cooling air. Could that be what's causing the reluctance?

Fri May 02, 2014 5:07 am

Looks like I got off topic a bit there sorry.

Back to the ignition system, are you saying that when the points open and the magnetic field collapses the energy built up in the field is suddenly released not only through the secondary winding BUT ALSO in the primary winding causing that to mirror the secondary winding by ratio?

Is there any other way to check if I have the right coil. I see in the Haynes Manual it has a figure for spark length .480". Is that the maximum possible length of the spark and how would I test that?

Fri May 02, 2014 5:37 am

Oh I get it now, the penny has dropped. When the field collapses the energy output through the secondary also inducts the primary. Or is it that the field collapsing inducts both? Yes that's how it must be?

If the coil is charging for 2.5 msec, how long is the usual discharge, obviously a hell of a lot quicker but wondering if there is a measurement?

So if say at 6000 rpm the coil is charging for 2.5 msec, and the coil has to reach the required voltage within that time, why wouldn't coils be marked with that information ie a 1.5 msec coil that outputs 20,000 volts? or whatever it has to be. Is that something that happens in other industries? Maybe I am starting to split hairs now.

Oh I see, so with that inverse exponential setup the coil can reach much higher voltages but takes a lot longer to get there. So as long as it can reach the required voltage in the required time it is adequate.

Ah I see the trade off now. All those reports of engines being difficult to start on a cold morning when the battery is working the hardest and the voltage to the primary coil is the lowest causing the slowest charge time and there fore a weak spark, BUT if you were to slide in a more powerful coil to overcome cold starts you then run the risk of burning plugs when the car is running flat chat down the highway on a warm day when the coil is running at it's maximum. So the stock coil is designed to fit the extremes of operation without doing damage? I get why some vehicles have an external ballast resistor and a bypass for cold cranking now. Awesome!

So basically the stock coil outputs enough energy to get the job done and gets the best life expectancy out of plugs and points and any increase in spark energy, even though increasing engine power, also decreases service life of the parts.

Wow! wonder if anybody has charts on what to expect from different levels of spark energy?/ Engine power, fuel economy, plug and point endurance etc?

This is not rocket science but it feels damn close to me haha!

Fri May 02, 2014 5:43 am

hahaha The samba is the University for Air Cooled Volkswagen!

Fri May 02, 2014 9:47 am

Muzooma wrote: When the field collapses the energy output through the secondary also inducts the primary. Or is it that the field collapsing inducts both? Yes that's how it must be?
Exactly correct. The primary voltage is useless.

Quote: If the coil is charging for 2.5 msec, how long is the usual discharge, obviously a hell of a lot quicker but wondering if there is a measurement?
It's very fast - in the microseconds.