Upgraded ignition problems? Check your spark gap!

[Chuck.H]

(reposted as new thread like I meant to the first time)

This has been covered before (I did a search and came up with a few posts) but I
think it's worth re-mentioning:

Anybody who's upgraded their ignition system to MSD, etc. be very conscious that many of the higher energy systems recommend a big spark plug gap like .045 or more...

While a bigger gap can be a performance boost, it can also cause misfires if the voltage finds an easier path to ground than thru the spark plug gap (as mine did).

I lowered my plug gap from .045 to .035 and the misfiring stopped immediately.

I'm still debugging where the voltage is misfiring -- I have a new cap, rotor and wires and if one of them were 'bad' I suppose it would still misfire even with the .035 gap. I guess I should check my rotor phasing next.

This is on my Jeep I was upgrading to a Ford TFI coil, but I had similar MSD problems in the past that were never resolved and I wish I knew this then.

HTH,
Chuck.H
'89 TurboLookTarga, 292k miles

[stlrj]

A bigger gap is also a good way of finding out if your ignition system needs attention. My 86 3.2 has had its plugs gapped at 0.060" for years but when I initially tried the larger gap, it too shorted and missed until I discovered I needed to upgrade to HEI carbon core ignition cables and a larger diameter HEI Jeep Cherokee cap and rotor.

What I can't understand is why the factory would install an ignition coil that is fully capable of firing plugs set at 0.060" but with stock Beru wires, cap/rotor that are only capable of firing a spark plug gap < 0.030".

Joe
74 911 w/86 3.2

[rnln]

Hi Joe,
What is the advatage of bigger gap?

[stlrj]

No missfires, better mileage, more torque.

[stlrj]

Originally Posted by stlrj

No missfires, better mileage, more torque.

"Why use larger spark plug gaps? The question keeps popping up so I thought try to better explain the reasoning behind using larger spark plug gaps.

An analogy would be would be trying to light damp kindling for a fire with a small match in windy conditions. It might take some time to get it the kindling ignited. The amount of time needed to ignite the kindling would not be very predictable.

If instead of a match, you used a propane torch to light the kindling, it wouldn’t matter if it was windy or if the kindling was a little damp. The kindling would be lit almost immediately and the kindling would soon set the logs on fire. You bypass this period of unpredictability by using a larger starting flame.

Use a small spark plug gap with little current and try to ignite a fuel mixture in turbulent conditions, with the varying fuel mixture that you will see inside an engine. The small flame kernel, that starts inside the small spark plug gap, would take some time before it reached the critical energy mass to really get the fuel mixture burning. The amount of time to ignite the fuel mixture would not be very predictable, especially with varying fuel mixtures and throttle positions.

If instead of a small spark plug gap, you used a huge spark plug gap with lots of current, the spark would light a larger flame kernel inside the larger spark plug gap and the larger flame kernel would spread to the rest of the fuel mixture more quickly. The variability of the flame propagation would be reduced. As such, your ignition timing would be closer to optimum under all throttle positions and fuel mixtures, and you would achieve greater gas mileage, horsepower, acceleration, and reduced hydrocarbon emissions.

A larger spark plug gap moves the flame kernel, inside the spark plug gap, closer to the center of the combustion chamber, which reduces the distance the flame must travel. Projected core spark plugs also move the flame kernel, inside the spark plug gap, closer to the center of the combustion chamber.

With the flame kernel being closer to the center, it is probably in a more turbulent region, and the flame kernel, inside the spark plug gap, might spread to the rest of the fuel mixture sooner.

A larger spark plug gap is also supposed to be beneficial in igniting leaner fuel mixtures.

Imagine how much faster the flame would propagate through the combustion chamber if the spark went all the way across the combustion chamber. There would be a much shorter distance for the flame to travel. It could really improve high RPM operation. The practical implementation of this is using two widely gapped spark plugs inside the combustion chamber.

The longer spark, in a larger spark plug gap, also has a higher voltage across it. There will be more power in the spark to ignite the fuel. There is a better power transfer from your ignition system into the spark.

Ultimately, to determine diminishing returns, you need to know when you consume more power, to generate the electricity to power the longer spark distance, than any power increase the larger gap provides. This also includes measuring engine output at part throttle conditions.

Likewise, you must determine when the engine consumes more gasoline, to provide the power for the longer spark distance, than any gas mileage increase that the larger gap yields.

I suspect that the ignition system will be the limiting factor of the maximum spark plug gap, in most engines. This also includes the electrical and physical limitations of the spark plugs. The same projected core spark plug that works great in a low compression normally aspirated engine, would probably be destroyed inside a fire breathing turbocharged or NOS monster engine.

Larger spark plug gaps require a higher voltage before a spark will occur. As such, you need to use better and thicker insulation on the spark plug wires.

You may also need a larger distributor cap with greater internal spacings to prevent crossfiring or arcing to ground. You have to pay more attention to the inside of your distributor cap and rotor. You should clean it with solvent to remove any traces of contamination, including fingerprints which are conductive. Likewise, you need to keep contaminates off of the spark plug insulator. This means washing your hands before installing the spark plugs and trying to screw them in using the tip of the spark plug and not touch the insulator.

This higher voltage also places greater stresses upon the insulation inside the ignition coil. A coil with marginal insulation may fail.

You have to keep the spark plug gap small enough so the voltage is low enough that no crossfiring or arcing to ground occurs inside your distributor cap.

It’s unlikely that you’ll exceed the spark plug’s maximum voltage before you exceed the maximum voltage that your distributor cap can withstand.

The general consensus seems to be that .040” to .045” is about the largest spark plug gap that you can use with the Bosch small distributor cap, with a low compression engine. Higher density, caused by higher compression ratios, requires a higher voltage to jump the same size spark plug gap. So higher compression engines must keep the spark plug gap smaller with the same size distributor cap.

If you are using the maximum possible spark plug gap, you must also service the spark plugs more frequently. As the spark plug electrodes wear, the voltage to jump the spark plug gap will increase. If you don’t replace or file the center electrode flat and re-gap the spark plug soon enough, you risk crossfiring and arcing to ground inside the distributor cap.

In a normally aspirated engine, the two worst case conditions for your distributor cap will probably be when:

1. The barometric pressure is highest, the air temperature is the coldest, the engine is dead cold after sitting out in the cold overnight, and the spark plug electrode is dead cold.

2. The barometric pressure is highest, the air temperature is the coldest, your engine has been running at high RPM and the ozone has built up to it’s maximum level inside the distributor cap. A vented cap will reduce the ozone. Both Bosch and Mallory make vented caps.

The only way to know how well larger spark plug gaps will work with your engine and ignition system, is to experiment with larger and larger spark plug gaps until there is a reduction of horsepower, acceleration, and gas mileage.

Scott Novak"

http://www.openroad.ca/volkswebbin/viewtopic.php?pid=215313

[stlrj]

Originally Posted by stlrj

You may also need a larger distributor cap with greater internal spacings to prevent crossfiring or arcing to ground.

http://www.openroad.ca/volkswebbin/viewtopic.php?pid=215313

That's why I use a Jeep Cherokee cap and rotor. It has a greater internal spacing (larger diameter)to prevent crossfiring or arcing.

Joe

[rnln]

alright, sounds so good. Now, can you help me more specific? Since plugs for our car is cheap, I am ok to place them more often.
1- What gap size is the safest to gap for stock 3.2 engine (88)?
2- What kind of wire you use?
3- what year/model of Cherokee to use the cap/rotor?

[fred cook]

If you suspect a crossfire or grounding situation with your plug wires, simply park the car in a dark place (outside at night, etc.), start the engine and look around the plug wires and distributor for small lightning flashes! If the charge is going to ground prior to reaching the plugs, it will create the discharge flashes. Easy to do and costs nothing! Often times you will find one wire is the culprit.

[stlrj]

Originally Posted by rnln

alright, sounds so good. Now, can you help me more specific? Since plugs for our car is cheap, I am ok to place them more often.
1- What gap size is the safest to gap for stock 3.2 engine (88)?
2- What kind of wire you use?
3- what year/model of Cherokee to use the cap/rotor?

Chrysler LH ignition wires on my 86 3.2











Here's my 86 3.2 with a Jeep Cherokee distributor cap/rotor : 84-2000 Jeep Cherokee 4.0
http://www.jcwhitney.com/autoparts/ItemBrowse/c-10101/s-10101/p-100000354213/mediaCode-ZX/appId-10556625/Pr-p_CATENTRY_ID:10000035421310556625


This conversion allows the use of conventional HEI ignition wires without the need of modifying the cap connectors or boots to fit a Beru cap.

Joe









Jeep Cherokee cap and Chrysler HEI ignition cables-extreme wet performance


With the stock cap-not waterproof like above example.


Hard to beat OEM Chrysler ignition wires for extreme wet performance.



Cheers,

Joe

[rnln]

oh man.. you must be a handy man... Let's see if I understand. We have several options:
1- Leave everything stock and try to gap the plugs between 0.04" and 0.045". (lowest gain)
2- Use Chrysler LH ignition wires (carbon core) with stock Beru crimps and stock cap/rotor. (mid gain)
3- Use Chrysler LH ignition wires w/ crimps, Chrysler cap/rotor and do all the modifications you did above. (best gain)

Did I get it? If I did, will it be noticeable gain if I choose option (2)? I don't think I am good enough to do the option (3).
Thanks.

[stlrj]

Originally Posted by rnln

oh man.. you must be a handy man... Let's see if I understand. We have several options:
1- Leave everything stock and try to gap the plugs between 0.04" and 0.045". (lowest gain)
2- Use Chrysler LH ignition wires (carbon core) with stock Beru crimps and stock cap/rotor. (mid gain)
3- Use Chrysler LH ignition wires w/ crimps, Chrysler cap/rotor and do all the modifications you did above. (best gain)

Did I get it? If I did, will it be noticeable gain if I choose option (2)? I don't think I am good enough to do the option (3).
Thanks.

#1 Won't work very well even if you gap at 0.040"

#2 I had some luck with that combination gapped at 0.060"

#3 Bingo! 0.060" gap and you'll never have to worry about your ignition again.

[fred cook]

I have the MSD 6A unit on my '80 SC. I am using the stock rotor and cap along with a set of Magnecor wires. The plugs are gapped at .045". It starts easily, idles well and runs great! Thus far I have not had any issues with crossfire or grounding with the Magnecor plug wires. In fact, other than the once a year tune up/valve adjustment session, the only time I have to open the engine lid is to add a little oil from time to time! The mods that STRLJ have done are interesting and show a lot of ingenuity, but not really necessary.

[stlrj]

It's only necessary if you want to take full advantage of a 0.060" gap which a HEI system is more than capable of firing. If you're content with a 0.045" gap, nothing wrong with that.


Cheers,

Joe

[rnln]

Thanks stlrj

[stlrj]

Originally Posted by rnln

oh man.. I don't think I am good enough to do the option (3).
Thanks.

The hardest part is cutting the Cherokee rotor and JB Welding it to your stock rotor, then using a hack saw to cut off the top of your distributor cap.

If you can wait, I plan on making a complete kit- cap, rotor, ignition cables with modified ends to fit the 911 engine for $200.


Cheers,

Joe

[rnln]

Thanks Joe. I definitely will look for you if I decide to do it this way.