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:

HEI_1.jpg HEI_2.jpg HEI_3.jpg
HEI_4.jpg HEI_5.jpg HEI_6.jpg
HEI_7.jpg HEI_8.jpg HEI_9.jpg
HEI_10.jpg HEI_11.jpg HEI_12.jpg
HEI_13.jpg HEI_14.jpg HEI_15.jpg
HEI_16.jpg HEI_17.jpg HEI_18.jpg
HEI_19.jpg

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):

DurasparkRotor.jpg

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):

DurasparkCap.jpg

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?
Rob20160227_140050.jpg