spark plug gap formula?

[Alxsmt]

just wondering if any1 knew if there was a formula. i gaped mine at .060 but have been having issues with my plug wires arking and glowing blue (hard to see at night but its there) i was told for compression/modified engines the cap needs to be smaller and my big gap may be causing too much resistance..

lowered the gap to .045 and its better but still there.. how low should i go?

ive also tried new plugs, different coils, different wires (even some 8.5mm ford racing lol)

there is also a slight miss at part throttle i think.. when you hold a
"steady" rpm say 3k the rpms change give or take 100rpms and when they go down is when you can hear a miss.. but it could be just the raspy sound of the engine deaccel?

[sprucegagt]

Where are your wires glowing blue? Did it stop after changing the wires?

[SappySE107]

Boosted engines need less gap to keep the spark from being blown out. .045 should be stock for the 3400. I would consider the heat range more than the gap for your setup but I am not a spark plug expert.

[Joseph Upson]

Your plug gap needs to be smaller than recommended for the same stock engine whenever you increase the compression ratio or boost the engine unless at the same time you increase the ignition system performance by changing to higher voltage coils. I looked this subject up yesterday on the net in regards to my own engine.

The newer engines have gaps considerably larger than previous, plugs from one of my 3500 engines (E85 compatible model I believe) had a gap of about .060 and the 3900 ~.042 which I changed to .035 since the OE coils for the 3500-3900 are probably a little more powerful than the old stuff.

The denser air charge from boosting or increasing compression works just like a resistor and therefore increases the resistance between the ground strap and electrode of the spark plug making it more difficult for the spark to jump the gap. Since eletricity travels the path of least resistance, if there are any areas along the wire path that pose less resistance than the spark gap that's where the spark is likely to occur.

You can find the formula for calculating resistance vs. air gap in a physics book, that's the last place I recall seeing it.

[sprucegagt]

When did anybody mention boost? Stock plug gap on a 3400 is .060". I also don't believe that the stock system won't handle an increase in compression while keeping the plug gap untouched. Now the heat range may need to be adjusted, but nothing else. DIS is the strongest ignition system I know of for the street. In fact, I have witnessed stock GM DIS Coils being used on a blown, high compression 590 big block Ford. The engine was used strickly for pulling and it never misfired.

I think the OP has bigger problems than spark plug gap.

[SappySE107]

I mentioned boost because he doesn't have it and it takes a lot of compression/turbulence to create an issue with the gap. You are right, .060 is newer spec and .045 was gen 2 3.1. I doubt very much he has a gap issue. I have gapped mine to .075 before using DIS-4 on my 3.1 and I did find the point where it was too much gap. It will not skip or anything, but flat out suck for power.

[sprucegagt]

Agreed. You can have only so much gap before the spark gets weak or fails to make the jump.

But back to the OP. If your getting spark outside of the wire then they are definitely bad and if they are arching to the coil then they could be bad as well.

[Joseph Upson]

Here is the file I was looking for. It implies that wide gaps were better suited for carbs and that on FI engines no matter what, if boost or high compression is applied the gap should be closed some. It has some good info in it.

[1meanz]

was reading through this, and for shits and giggles i checked my plus since its been missing, gettin bad gas milage, and ect. well mines gapped to over .85....... i think i found my problem! i gap my plus to .60 on all my 660s and i havnt changed them in this one yet, gets i need to.

[Alxsmt]

its just where ever the wire is in contact with something. before i changed the gap to .045 i was having the whole wire light up in blue (more than one). it wouldnt shock me.. there is no way 4 sets of wires could be bad... 2 gm, one napa, and one ford racing (one set of the GMs were installed new with the engine, maybe 6k)

right now just right at the spark plug there is a slight glow on 2 wires up front and a slight glow in the middle of one wire too... not sure about the back but i can see a blue glow. i also tried to get video of it but it wouldnt pick it up, it has to be DARK to see it

maybe i will grab a video of the miss i have, maybe some1 can verify it. it should pull a low 14 qrt mile as the car sits now, maybe even faster. its not bad at all if it is a miss

[Superdave]

I run mine at .060, no problems using stock coils and Accel wires.

Sorry, i completely forgot to answer your PM.. lol

[Alxsmt]

ok, i imagine i should be just fine at .060 or close to it.. 10.5 isnt that much more than 9.8 i wouldnt think.. maybe i will go out and buy new coils for the hell of it, i have to get thing done... what size wires are you running? what is too big?

has any1 ever revved there engine at night from under the hood with no light? as in more than one good rev in a row

[34blazer]

when i had my 99 GA the wires did the same thing. seems to be pretty normal considering the amount of voltage going thru them

[Alxsmt]

just ordered universal 8.5mm msd wires... with a LOT of wire seperators. i guess i will see where those get me..

[Superdave]

i think mine are 8mm, i use the Accel V8 cut-to-fit set with the straight boots. They hold up very well.