Will my rotor and cap handle my new coil - Mallory Notes

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MRRAGPICKER
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Will my rotor and cap handle my new coil - Mallory Notes

Post by MRRAGPICKER » Thu Apr 24, 2008 8:58 am

I have a pertronix 60,000 volt coil. Will a mid 1970's oem rotor and cap handle it?

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MNAirHead
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Post by MNAirHead » Thu Apr 24, 2008 1:55 pm

I've been successful on my stock 70s bug

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MNAirHead
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Post by MNAirHead » Thu Apr 24, 2008 1:56 pm

Iv'e been successful after thousdands of miles.

Scott Novak
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Post by Scott Novak » Thu Apr 24, 2008 5:39 pm

The voltage that the distributor cap and rotor see is dependent on your spark plug gap and fuel mixture density inside the spark plug gap, and the resistance of your spark plug wires and rotor.

ANYTHING that increases the fuel mixture density, such as raising the compression ratio, improving air flow so you have a greater percentage of cylinder fill, supercharging, etc. will also raise the ionization voltage. Decreasing altitude also increases the fuel mixture density. What works in Denver may not work in LA.

The stock carbon core ignition wires and distributor rotor have a very large amount of resistance. There is a significant amount of voltage dropped across them. Let's say that you had 10,000 volts across your spark plug gap. You would have maybe 2,000 volts dropped across your rotor and spark plug wires. So the inside if the distributor cap might see 12,000V. (These are just gross approximates for the example)

If you change to a high energy ignition system, the current through the spark plug wires and rotor will increase and also the voltage dropped across the spark plug wires and rotor would increase.

If you tripled your spark current, you would approximately triple the voltage dropped across the spark plug wires and rotor. So now you'd have 6,000 volts dropped across the spark plug wires and rotor, and the inside of your distributor cap would now see 16,000 volts. And that's all without increasing the spark plug gap!

So before you do anything else, toss those stock carbon core spark plug wires in the trash, and replace them with low loss magnetic suppression wires. Also replace the stock rotor, which has an internal resistor built into it, with a NON-resistor rotor, or modify the rotor by grinding a slot between the brass rotor contacts and soldering a jumper wire between them, clean off the flux with 99% pure isopropyl alcohol, then cover the jumper wire with high temperature rated epoxy to help keep it in place.

With low loss ignition wire and a resistorless rotor your voltage losses will be very small, and increasing your spark current won't raise the voltage inside your distributor cap very much.

The stock Bosch distributor cap can only handle about 28,000 volts before crossfire.

The stock Mallory small cap can handle about 32,500 volts, which is about 16% higher.

A Mallory Comp 9000 distributor cap can handle about 59,000 volts which is 111% more than the Bosch distributor cap.

With a low compression engine, say 7.1:1, low loss ignition wire and a resistorless rotor with a Bosch distributor you can probably open your spark plug gaps to 0.040". I don't think I'd want to push it much further.

If you don't open your spark plug gaps, there isn't any point to using a higher voltage ignition coil. You would be much better off with a lower turns ratio ignition coil that would provide you with a higher current spark. You don't need to develop any more than 28,000 volts with a Bosch distributor cap, because the distributor cap can't handle any more than that.

Scott Novak

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Post by MRRAGPICKER » Thu Apr 24, 2008 10:34 pm

Thanks for the in depth answer. I knew that it was questionable on how much the stock distributor could handle. I already have gutted my rotor. I soldered in a 16 gauge jumper. Is that fat enough?

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Post by Scott Novak » Fri Apr 25, 2008 12:12 am

You could get by with a 20 AWG wire for the rotor resistor bypass. With a high energy ignition, you might see currents of about 1 amp at most. The duty cycle is so low that the current is probably flowing less than 25% of the time, so it just won't heat up a low resistance wire. The quality of your solder joints is more important than the gauge of the wire.

Scott Novak

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MNAirHead
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Post by MNAirHead » Fri Apr 25, 2008 8:29 am

Scott...

ARe solder joints a resistance point?

Do you suggest clamped terminals (using the correct quality tools)

Thank you.
Tim

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raygreenwood
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Post by raygreenwood » Fri Apr 25, 2008 8:34 am

The stock wires...were not carbon by theway, they were wire core...all theway through type 4. These had less real resistance than the suppression wires (carbon core).

What Scott is outlining is quite dead on. The current loss in the stock system was enormous. In general the stock aircooled coil before digijet (digfant 1) was about 18kv in output. From bits and peices I have read and heard...that loses down to about 5-7kv at the plugs.

All of teh ACVWwere seriously underignited. The high compression type 4 and the high compression twin carb 1500 s in the type 3....were the worst because of using the same system and high compression.

But...bear in mind that stock electronic ignition systems that came later on vanagon and WC cars used basically the same cap, material and structure. Those coils put out 40-50KV.

But..they were like the ACVW's...heavily resistored. That may be why they never cross fired much.

One of the best bets...is that unless your coil requires a certain level of resistance to reach stauration...get rid of the chain of resistors in wire and rotor and use resistor plugs...or keep ONE resistor (either rotor or wires) and use non-resistor plugs. Ray

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Post by Scott Novak » Fri Apr 25, 2008 9:03 am

Tim,

When solder joints are properly made they are fairly low resistance and should last for many years. Current through a good solder joint is not likely to develop much heat.

A cold solder joint can be highly resistive and can fail quickly. Also, as a cold solder joint can have a very high resistance, current through the cold solder joint can cause it to become hot enough to burn phenolic and epoxy.

Worse yet, a cold solder joint can become intermittent.

The Crimped terminal versus Soldered terminal dabate again. If done properly, a crimped terminal can form a gas tight pressure weld which is also lower in resistance than a solder joint. If you are really picky, you will crimp bare terminals and then add heat shrink over the terminals.

As badly as most people solder, crimping will often yield better results.

Scott Novak

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Post by Scott Novak » Fri Apr 25, 2008 9:46 am

raygreenwood wrote:The stock wires...were not carbon by theway, they were wire core...all theway through type 4. These had less real resistance than the suppression wires (carbon core).
Ray is correct. The stock wires were metal core with resistor spark plug ends. I forgot about that.

I have have been using Magnetic Suppression ignition wire since 1975. I have never used stock ignition wire on any of my VWs.
raygreenwood wrote:But...bear in mind that stock electronic ignition systems that came later on vanagon and WC cars used basically the same cap, material and structure. Those coils put out 40-50KV. But..they were like the ACVW's...heavily resistored. That may be why they never cross fired much.
The ignition coils may have been capable of putting out more than 50,000 Volts. But they didn't do that with the stock small Bosch distributor cap. The spark plug gap was small enough to limit the voltage to under 28,000 volts, which would prevent the crossfiring inside the distributor cap.
raygreenwood wrote:One of the best bets...is that unless your coil requires a certain level of resistance to reach stauration...get rid of the chain of resistors in wire and rotor and use resistor plugs...or keep ONE resistor (either rotor or wires) and use non-resistor plugs. Ray
Good quality low loss magnetic suppression ignition wire won't require any extra resistance for proper RFI/EMI noise suppression. I'd agree, use Non-resistor spark plugs if you can find them. If I had an ignition coil that required any external resistance in the spark plug wires, I'd chuck the ignition coil in the trash.

If you are using a breakerpoint or simple point eliminator module, you should only need a ballast resistance connected between +12 volts and the ignition coil positive, unless the ignition coil already has a built in ballast resistor.

If you are using a Jacobs, MSD, or Mallory ignition system, you probably won't need any ballast resistance, as the ignition system usually electronically limits the ignition coil primary current.

This is what a Jacobs Omni-Torquer ignition did to a resistor rotor that I installed by mistake.
Image

Scott Novak

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Post by fortyeye » Fri Apr 25, 2008 10:19 am

If you are using a breakerpoint or simple point eliminator module, you should only need a ballast resistance connected between +12 volts and the ignition coil positive, unless the ignition coil already has a built in ballast resistor.
Most everyone has read this before but Scott brings up an important reminder here. Make sure (from the manufacturer) whether or not your points replacement module will function with your coil choice based on primary resistance of the coil. All Bosch Blue coils are not the same. Some have internal primary resistors and some require an external ballast resistor. The two things that will render your Pertronics replacement module (probably Comp-u-fire too) junk are improper primary resistance of the coil, and (like those nimrods on TV a few years ago) hooking the module wires up backwards. Once they're smoked (probably without smoke) ... you'll end up buying another one.
AKA clearsurf

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Post by sagaboy » Fri May 02, 2008 11:21 am

I notice that the bruned-out tip of the rotor is about 1.5mm longer than the normal standard one--is that a possible cause for it to burn-out?

Last week when I was at the spare-part store--the sale guy showed me 3 rotor, a) standard tip b) wider tip c) longer tip--1.5mm more--any one had any experience using the longer tip ones?
Scott Novak wrote:This is what a Jacobs Omni-Torquer ignition did to a resistor rotor that I installed by mistake.
Image

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Post by Scott Novak » Fri May 02, 2008 11:51 am

The rotor tip width doesn't have anything to do with a rotor burning up. The higher spark current, of the Jacobs ignition system that I was using, caused the resistor inside to burn and eventually burned the rotor itself until it fell apart. That's why you need to bypass the internal resistor inside the rotor, or upgrade to a Mallory distributor which doesn't have a resistor inside the rotor. The only reason the resistor is in there is for electromagnetic noise suppression. If you are using magnetic suppression ignition wire there is absolutely no need for a resistor rotor.

A shorter rotor tip is only useful for weak ignition systems. It allows the voltage to build up higher before the spark jumps the gaps between the rotor tip and the distributor terminal and the spark plug gap.

A high energy ignition system doesn't benefit from a wider gap between the rotor tip and the distributor terminal.

If the longer rotor tip fits inside your distributor cap, without rubbing against the distributor terminals, you could use it.

However, if you are using a multi-sparking ignition system, a wider rotor is important. The multi-sparking can continue after the rotor tip has past the terminal and the spark is forced to jump a huge gap, which can cause buildup on the inside of your distributor cap.

Mallory rotors have the brass rotor tip screwed on to the rotor. That makes it easy to cut your own rotor tip from 1/1/6" thick brass sheet, and make it with a wider tip.

The following rotors are for Mallory Comp 9000 distributors, but you can cut your own rotor tip for the small cap Mallory distributors.

Modified Rotor Tip with wider edge to handle multi-sparking ignition systems and electronic retard on Right.
Left:Centrifugal Advance Rotor Tip . . . Middle: Vacuum Advance Rotor Tip . . . Right: My Modified Rotor Tip
Image

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Last edited by Scott Novak on Thu Feb 19, 2009 7:53 am, edited 1 time in total.

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Post by sagaboy » Sat May 03, 2008 2:49 am

Scott Novak -- great info :lol:

This morning I took out a bunch of original rotors, connecting plug wires and VW spark plug connector.

I measured the rotors and got resistance reading from 6.5 Maga ohm, 5K ohm, 1K ohm and no resistance -- all differend boch parts number all with various tips length and width--all which I didn't notice earlier until this post :(

I measured the orignial VW spark plug connector, they consisit of: 500 ohm, 1 K ohm and 5 K ohm and none -- they still have the rating at the tip of the connector--and I never notice the differences all these years.

I happened to have a bunch of new 2006 mercerdes plugs wires and they measured 1 K ohm.

This morning, I replace the 6.5 Maga ohm rotor from my bug and replace it with the 500 ohm one--went for a spin and I can feel the surge in engine power of more than 10% -- I am going to the spare parts shop and get non-resistance ones and replace the current 5k ohm spark plug connector and new non-resistance wires as well, plus increase the plugs gap to what "Scott Novak" suggested.

I think I am going to get another surge of engine power.

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Post by Scott Novak » Sat May 03, 2008 5:33 am

Let me be clear that my spark plug gap suggestion of 0.040" is about the maximum gap that you want to use with a low compression engine and still maintain a safety margin. This may be a conservative suggestion. It would be best if you actually measure the voltage at the output of your ignition coil, at full load and full throttle. Then you can know for sure how close you are to the 28,000 volt limits of your small Bosch distributor cap. Of course this means that you must measure this while your engine is on a dynamometer.

If you increase the compression ratio of your engine, the voltage that it takes to jump the spark plug gap will also increase. As your distributor cap will also see this voltage inside, you have to make sure that you don't exceed the voltage capability of the distributor cap. You may have to use a smaller spark plug gap, especially if you are using a turbo. However, if you are using a turbo, you should be using a distributor with a larger distributor cap to handle the higher voltages. The spacing between the terminals is the critical factor. The wider the spacings the better.

Left: Stock VW Cap, . . . . . . . . . . Middle: Mallory Comp 9000 Cap, . . . . . . Right: Mallory #271 Small Cap
Image

Also take note, that by using a NON-resistor rotor, NON-resistor spark plugs, and Low Loss Magnetic Suppression wire, you can increase your spark plug gaps by about 0.005". So for a stock 7.1:1 compression ratio engine, using a stock Bosch or Bosch Blue ignition coil, you can increase your spark plug gaps from 0.024" to about 0.029". If you check your plugs frequently you might push that to .035". But without a better ignition coil, and, or high energy ignition system, don't try more than a 0.035" spark plug gap, or you might start missing at higher RPM.

I wasn't aware that Bosch made a resistorless rotor to fit their small cap VW distributors. If anyone finds one I would like to know the part number.

I do have a resistorless aftermarket rotor for a Bosch VW distributor. But I don't know what company manufactured it.

Scott Novak

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