Replacement Coil and Spark Plug Wire Questions

[Mattglas]

I wanted to add a bit of pep to my engine so I ordered a new Bosch Super 18,000v coil from John Hervey. I received the package today and to my surprise, found that he had sent me a 40,000v Pertronix flame thrower coil instead. Anyways on his website he insists that 8mm spark plug wire is used or there will be issues with EMI. Although I just updated my spark plug wires to Bosch ultra premium 7mm from DPI and Bosch claims that they have some advanced protection against EMI. So does someone with experience know if my current wires will work? I'm not really keen on shelling out $90 on new wires when I just replaced them. Also on the topic of spark plug wires does it matter if they are touching each other or other wires on the harness?

[kings1527]

Here's what I know about the DeLorean and its ignition system: it's most reliable when you're using OEM equipment, from a regular Bosch cap and rotor, to Bougicord wires, to the stock coil, right down to the silver tip HR6DS plugs. There have been lots of instances where people get creative and ambitious with trying to eek out more from the PRV only to find weird things subsequently happen. Reliability and consistency goes a long way with this engine. When you're getting strange signs and symptoms happening and you're running mix/match ignition components outside of OEM, any vendor would probably swap it out for OEM stuff and troubleshoot it from there. It seems rare that the headache is worth trying to squeeze out a few more horses.

[DMCMW Dave]

.... any vendor would probably swap it out for OEM stuff and troubleshoot it from there. It seems rare that the headache is worth trying to squeeze out a few more horses.

Exactly.

And the "few extra" horses are not there. The spark only ignites the fuel, and pretty much any spark will do it. A hotter spark won't make the fuel burn any better, the fuel is doing the work once it's lit. The only reason for going to a "hotter" spark is to overcome conditions where compression is higher, for example turbo or supercharging, and it might be harder for the spark to jump the plug gap. But there is usually enough margin in the stock system for those conditions.

The factory coil is fine for the stock system and stock plug gap. A so-called high voltage coil has the capability of higher voltage, but it takes a certain voltage to jump a certain plug gap and that's all there is. A hot coil by itself won't do anything at all it only takes about 15-18kV to jump the stock plug gap, and that's what will come out of the coil. So - to really get the higher voltage out of the coil, you would have to open up the spark plug gap. (If you were do this with the stock coil, you would indeed see spark failure).

Problem is, there is also a gap in the distributor between the rotor and the cap. The rotor has a resistor in it for RFI suppression. When you raise the plug gap, yes the voltage goes up, but then Ohms law kicks in. E=IR - higher voltage across the same resistance = higher current. Current that the rotor isn't designed to handle, and higher voltage that the cap isn't really designed for either. Take a look back at GM and Ford when they first went to HEI ignitions (1970s) and you'll notice that the cap diameter got huge for a while to address this issue. Then they went to coil packs and coil on plug to handle this problem. We don't easily have that option.

It's a system, and if you start screwing with one part of the system, other parts are affected, and if you don't address them Bad Things Happen. You'll find that the cap/rotor life is lowered, and when they go bad the car will run poorly.

Since I don't see anyone making a better cap and rotor, you will end up with shorter life in the OE components and other potential performance issues. (If you don't open up the plug gaps, you probably won't see any of these problems, but there really isn't any reason to spend the money either.)

[DMC-81]

I can't comment on compatibility, but if he sent you the wrong coil, just call/ email him and he should exchange it with the correct one that you ordered, or return it and get the OEM one.

[content22207_2]

As one of the earliest proponents of HEI (High Energy Ignition -- a General Motors trademark since the 1970's) in the online DeLorean community, and a HEI user since 2002, a few comments are in order:

First & foremost, spark voltage is limited by the spark plug gap. As soon as the spark jumps the gap, that is the most voltage it will ever carry. Your coil can produce a bazillion volts, but if it only takes 20,000 volts to jump the gap then the remaining .98 bazillion volts will never be discharged.

That said, don't go widening spark plug gaps without making other changes to the ignition system to allow HEI in the first place. For example, widening spark plug gaps without increasing coil discharge voltage will lead to misfires -- spark simply won't jump the gap. The coil is a backwards transformer. The more primary voltage into the coil, the more secondary voltage out of the coil. 10 volts plus is standard for cars with HEI. Stock DeLorean primary voltage is 6-8 volts -- not at all adequate for HEI, no matter the coil.

The limiting factor for DeLorean ignition is the Bosch coil -- it can not handle more than 8 volts primary without overheating. Performance coils such as Pertronix or MSD can handle full charging voltage, but not OEM Bosch.

I'm not the only Talk member running HEI, jumping spark plug gaps in the .04" plus range. Some members definitely disagree with HEI, but to make blanket statements that it doesn't work is simply incorrect. HEI was already widespread in 1981. It's become the industry standard since. 6-8 volts into a low winding coil jumping .025" plug gaps was standard for breaker point ignitions of the 1960s. By the end of the century General Motors was running full charging voltage into high winding coils jumping .06" plug gaps (1999 Chevrolet Silverado for example).

As far as spark plug insulation thickness is concerned: silicone does absolutely nothing to stop EMI. It's only purpose is to keep secondary voltage from arc'ing to the engine block. The thicker the insulation, the better it holds up for that purpose.

This SAE paper from the early 1970's does a good job explaining the benefits of HEI. That all car companies ultimately adopted HEI bears it out. GM was of course the first, but by the late 80's/early 90's everyone was on board:

40140 40141 40142
40143 40144 40145
40146 40147 40148
40149 40150 40151
40152 40153 40154
40155 40156 40157
40158

The stock ignition setup does work, but it's really 1960's breaker point caliber. There simply was no precedent for it by the end of the century.

When in doubt, just look at park plug gap specs. The wider the gap, the higher ignition voltage. For example, Eagle Premiers with 3.0 PRV's have .035"-.040" plug gaps.

Bill Robertson
#5939

[content22207_2]

Take a look back at GM and Ford when they first went to HEI ignitions (1970s) and you'll notice that the cap diameter got huge for a while to address this issue.

Can't speak for GM, but in Ford's case that is incorrect. Duraspark caps increased in diameter to accommodate larger rotor button blades, to increase discharge time at each terminal (notice how much wider a 6 cylinder Duraspark rotor blade is compared to DeLorean):

40159

Note also that Duraspark ECU dwell times *DECREASE* as RPM's increase, to allow the coil to continue discharging longer.

Large diameter Duraspark caps, wide Duraspark rotor button blades, decreasing Duraspark dwell times, and .044" plug gaps are all designed to maximize fuel burn.

Duraspark cap compared to DeLorean (for maximum discharge time from the wider rotor button blade):

40160

Bill Robertson
#5939

[content22207_2]

FWIW: Bosch ignition ECU dwell is 44 degrees at idle, increasing to 46 degrees at 3,000 RPM. Ford Duraspark ignition ECU dwell is 50 degrees at idle, *DECREASING* to 44 degrees at 3,000 RPM.

Bill Robertson
#5939

[Mattglas]

I only wanted to keep it because shipping it back would be expensive and it's a good deal but it sounds like more hassle than its worth. I'll just see if I can get a refund and get one of Hervey's stock ones. Also, will it cause problems if the spark plug wires are near the wiring harness?

[content22207_2]

A Pertronix coil will work just fine in a stock ignition setup. Won't be HEI, but it will work every bit as well as a Bosch coil.

Note that Bosch coils typically do not have terminals clearly labeled for positive ("+") and negative ("-") as the Pertronix coil does. "15" is positive and "1" is negative. You also can use wire colors: White/Yellow is positive and White/Gray is negative. Do not wire the new coil up backwards -- it won't work well at all. And don't use the label on the side to position the coil -- proper wiring polarity may well position the label towards the firewall.

Bill Robertson
#5939

[PJ Grady Inc.]

A Pertronix coil will work just fine in a stock ignition setup. Won't be HEI, but it will work every bit as well as a Bosch coil.

Note that Bosch coils typically do not have terminals clearly labeled for positive ("+") and negative ("-") as the Pertronix coil does. "15" is positive and "1" is negative. You also can use wire colors: White/Yellow is positive and White/Gray is negative. Do not wire the new coil up backwards -- it won't work well at all. And don't use the label on the side to position the coil -- proper wiring polarity may well position the label towards the firewall.

Bill Robertson
#5939

I just checked nine NOS coils I have on the shelf and they all look like this. Notice the - and + signs adjacent the 1 and 15. What are your thoughts Bill?
Rob40172

[content22207_2]

My thoughts are Bosch coil terminals have confused people for decades:

40173 40174 40175 40176

Bosch may have added + and - signs later, but an old Volkswagen mechanic told me that they used to only be labeled 1 and 15.

As long as the end user wires his coil up correctly, what does it matter? I've helped owners who wired up coils backwards, which doesn't work well at all. However it's marked, 15 or +, just make sure that's where White/Yellow is attached.

Bill Robertson
#5939

[PJ Grady Inc.]

My thoughts are Bosch coil terminals have confused people for decades:

40173 40174 40175 40176

Bosch may have added + and - signs later, but an old Volkswagen mechanic told me that they used to only be labeled 1 and 15.

As long as the end user wires his coil up correctly, what does it matter? I've helped owners who wired up coils backwards, which doesn't work well at all. However it's marked, 15 or +, just make sure that's where White/Yellow is attached.

Bill Robertson
#5939
Well that was before the Delorean's time. I got all those coils from various NOS dealer inventories over the years and have never encountered that on a Delorean. This forum needs info as pertaining Delorean's not old Volkswagan's ;)!
Rob

[content22207_2]

I'm a little confused why you've picked coil terminals as a red line in the sand. As stated, as long as an owner wires his coil up correctly it's all good.

FWIW: Pertronix coils only have "+" and "-", no numbers. Absent wiring changes, an owner must attach White/Yellow to + and White/Gray to -.

Bill Robertson
#5939

[PJ Grady Inc.]

I'm a little confused why you've picked coil terminals as a red line in the sand. As stated, as long as an owner wires his coil up correctly it's all good.

FWIW: Pertronix coils only have "+" and "-", no numbers. Absent wiring changes, an owner must attach White/Yellow to + and White/Gray to -.

Bill Robertson
#5939
No red line Bill...it's just a fact that the Delorean coils have + and - markings. Does being wrong on occasion threaten you? If so I'll overlook future any future errors as it's not that important to me to always be right. No one is ever always right anyway regardless of how much they think they know. It was a minor correction...not an indictment. At the end of the day we're all still humans correct?
Rob

[Farrar]

Delorean coils have + and - markings

My early '81 didn't.

[Farrar]

it's just a fact that the Delorean coils have + and - markings.

In Bill's defense, he did use the word "typically."

Now I'll step back and say to the OP that the coil you have should work fine even though it's not Bosch NOS.

[content22207_2]

I suspect the controversy isn't coils -- it's HEI. That is a modification that does not have universal agreement one way or the other. It has prompted spirited debate in the past.

For my part, I fully concede that stock ignition is fully functional. A DeLorean should be able to live its entire life running stock ignition.

But HEI's detractors need to likewise concede that it is fully functional as well. Properly installed (that's the kicker -- there are plenty of HEI installations that are not done right), a DeLorean can live its entire life running HEI as well.

Each owner needs to decide for himself whether HEI is a modification worth making. I am very happy with mine, but I can't make that decision for anyone else.

I can provide information so owners are able to make informed choices. John Hervey unfortunately does not provide complete information on his website (for example, simply replacing the Bosch coil with a Pertronix or MSD unit, absent other changes, will not increase secondary voltage, assuming the Bosch coil was healthy to begin with). Don't know if Hervey provides that information in direct communication.

Bill Robertson
#5939

[bluscreen]

Exactly.

The factory coil is fine for the stock system and stock plug gap. A so-called high voltage coil has the capability of higher voltage, but it takes a certain voltage to jump a certain plug gap and that's all there is. A hot coil by itself won't do anything at all it only takes about 15-18kV to jump the stock plug gap, and that's what will come out of the coil. So - to really get the higher voltage out of the coil, you would have to open up the spark plug gap. (If you were do this with the stock coil, you would indeed see spark failure).



Sorry to resurrect here, but this thread seemed most appropriate.

That step-up Bosch coil that Hervey sells is listed as 18k volt, which falls in your 15-18k and seems like a safe and reasonable "upgrade." Since the bump in voltage is so moderate, are there any negative consequences here? He notes it has 1.5 ohm coil resistance compared to the stock .7. Not sure what that affects...

Thanks for your input!

[Bitsyncmaster]

Sorry to resurrect here, but this thread seemed most appropriate.

That step-up Bosch coil that Hervey sells is listed as 18k volt, which falls in your 15-18k and seems like a safe and reasonable "upgrade." Since the bump in voltage is so moderate, are there any negative consequences here? He notes it has 1.5 ohm coil resistance compared to the stock .7. Not sure what that affects...

Thanks for your input!

My stock coil measured 0.387 ohms but maybe it was faulty because others have stated the stock coil is about one ohm.

If your using a 1.5 ohm coil I would think you should bypass at least one of your ignition resistors.

[David T]

My stock coil measured 0.387 ohms but maybe it was faulty because others have stated the stock coil is about one ohm.

If your using a 1.5 ohm coil I would think you should bypass at least one of your ignition resistors.

There have been more than a few instances on this forum where an owner used a non-Bosch coil and later reported early failure. It would seem if you stay with the Bosch coil it is more reliable. There doesn't seem to be any good reason to use anything else. Getting a "hotter", higher voltage coil doesn't show any measurable improvement but it does seem to decrease the reliability. For any owner looking for cheap, "bolt on" HP, there is no magic bullet. You certainly won't find it in an ignition coil. BTW, increasing the spark gap to get a "hotter" spark will increase the stress on the rest of the secondary ignition system. The cap, rotor and wires were not made for that and you can expect them to break down quickly. As for the need for a "hotter" spark, all it is for is to ignite the A/F mixture. Anything more is unnecessary. While it IS true you need a hotter spark at higher combustion pressures (like when the motor is under load during acceleration or going up a hill) the stock parts are more than enough as long as they are in good condition.

[bluscreen]

There have been more than a few instances on this forum where an owner used a non-Bosch coil and later reported early failure. It would seem if you stay with the Bosch coil it is more reliable. There doesn't seem to be any good reason to use anything else. Getting a "hotter", higher voltage coil doesn't show any measurable improvement but it does seem to decrease the reliability. For any owner looking for cheap, "bolt on" HP, there is no magic bullet. You certainly won't find it in an ignition coil. BTW, increasing the spark gap to get a "hotter" spark will increase the stress on the rest of the secondary ignition system. The cap, rotor and wires were not made for that and you can expect them to break down quickly. As for the need for a "hotter" spark, all it is for is to ignite the A/F mixture. Anything more is unnecessary. While it IS true you need a hotter spark at higher combustion pressures (like when the motor is under load during acceleration or going up a hill) the stock parts are more than enough as long as they are in good condition.

If you're referring to Bitsyncmaster here, please note that he has modified more of his ignition components to better utilize a higher voltage coil. Otherwise, in my case, I'm talking about going from the stock 12k volt Bosch coil to an 18k volt Bosch coil in an attempt to gain a bit of longevity from the coil and overcome any possible resistances in the ignition components that may have come with age / new/different spark plugs. I might be mistaken here though.

[dn010]

I am almost positive I posted this a few times before, but I'll be another one to say [back when I was running a coil] I tried going from OEM to some higher voltage aftermarket coil. This was over a decade ago... My engine ran horribly and was even worse in the rain. After many frustrations not knowing what the problem was and realizing the last thing I did prior to all my troubles was changing the coil, wires, etc.; I changed the coil back to the OEM and the engine ran fine from the moment I turned the key to start with the original coil reinstalled. I kept the 8MM wires and fancy +4 spark plugs installed but got rid of the coil.

I should have learned my lesson years earlier to this event when I installed a 40,000V coil in my Mustang and ended up melting distributor caps and rotors. There is no reason for this upgrade.


There have been more than a few instances on this forum where an owner used a non-Bosch coil and later reported early failure. It would seem if you stay with the Bosch coil it is more reliable. There doesn't seem to be any good reason to use anything else. Getting a "hotter", higher voltage coil doesn't show any measurable improvement but it does seem to decrease the reliability. For any owner looking for cheap, "bolt on" HP, there is no magic bullet. You certainly won't find it in an ignition coil. BTW, increasing the spark gap to get a "hotter" spark will increase the stress on the rest of the secondary ignition system. The cap, rotor and wires were not made for that and you can expect them to break down quickly. As for the need for a "hotter" spark, all it is for is to ignite the A/F mixture. Anything more is unnecessary. While it IS true you need a hotter spark at higher combustion pressures (like when the motor is under load during acceleration or going up a hill) the stock parts are more than enough as long as they are in good condition.

[bluscreen]

Do you know the part number or where to get an OEM replacement Bosch coil? I don't see it in any crossover list, and all of the auto parts stores nearby sell off-brand replacements. I know delorean.com has NOS ones, but I understand this one is still made by Bosch so why not get a new one?

[Bitsyncmaster]

Do you know the part number or where to get an OEM replacement Bosch coil? I don't see it in any crossover list, and all of the auto parts stores nearby sell off-brand replacements. I know delorean.com has NOS ones, but I understand this one is still made by Bosch so why not get a new one?

My blue Bosch coil (I assume this was OEM) has a number "engraved" on the bottom of the coil.

0 221 122 001

[bluscreen]

My blue Bosch coil (I assume this was OEM) has a number "engraved" on the bottom of the coil.

0 221 122 001

Thank you! I'll check it out.

[MikeWard]

Do you know the part number or where to get an OEM replacement Bosch coil? I don't see it in any crossover list, and all of the auto parts stores nearby sell off-brand replacements. I know delorean.com has NOS ones, but I understand this one is still made by Bosch so why not get a new one?

DCUK stock new OEM Bosch coils http://www.deloreanclub.uk/bosch-ignition-coil.html

I'm sure there would be stockist within the US though if you didn't want to pay for shipping from the UK, that said the exchange rate is in your favour at the moment.

[bluscreen]

DCUK stock new OEM Bosch coils http://www.deloreanclub.uk/bosch-ignition-coil.html

I'm sure there would be stockist within the US though if you didn't want to pay for shipping from the UK, that said the exchange rate is in your favour at the moment.


Thanks for this info! I already purchased one from AutohausAZ, but I'll check out DCUK!