Lone Ranger

A larger plug gap can cause a slight timing advance.

True or false?

Rapidfire plugs and some iridium plugs for LT1 for instance, came gapped @ 0.060 but factory spec for the LT1 is 0.050. AC even recommends 0.060 gap for the LT1 application on Rapidfire #14 plugs, but specifies 0.050 gap for the OEM 41-906 plug

dizwiz24

False!

I believe its possible that a larger plug gap would act as a slight timing retard.


Why?

Shorter plug gap means it takes less time for the spark to arc that distance.

We are talking about miniscule differences (although at 6000 RPM times 8 cylinders- maybe it does make a difference), but if I had to pick an answer thats what I would pick.

Someone who knows more might be able to shed more light on the subject.

65Z01

A wider plug gap should slightly retard the ignition, for a given coil setup.

The reason is that the breakdown potential of the charge is relatively constant, so if you increase the gap, the potential gradient (volts/inch) is decreased. This means that the coil must charge up to a higher voltage before the charge can be ionized to create the spark which initiates the charge burn.

At low rpm the charge rate of the coil likely so quick that this effect is negligable. At high rpm the effect may indeed significantly retard the ignition. If you know the charge rate of your ignition you can do the math to see if the effect is significant within your power band.

CFI-EFI

The charge up rate for a higher voltage, necessary to jump a wider gap, would be an immeasurably small amount of time longer, retarding the spark. The timing is the one thing that changing the plug gap will not EFFECTIVELY change. However the charge up rate to a certain voltage for a certain coil, is constant. The time it takes is unchanged, but at higher rpms, the crankshaft turns more degrees per a given unit of time. This is true regardless of the plug gap.

RACE ON!!!

65Z01

Hmmm, I'm not so sure it's negligible at high RPM.

Consider the engine turning at 6k RPM or 100 revs/sec, which gives 10mSec per revolution of the crank.

With 360deg of the crank rotation per revolution I come up with 27 uSec for each degree or crank rotation!

I wonder what is the buildup rate for our HEI coil?? And what effect does the resistance of the wires have on EMF buildup at the spark plug.

This is an interesting thread.

OK, the duration of the spark is around 1/4 of a uSec, so indeed a change here is only a small fraction of one degree of crank rotation.

Ref: http://www.centuryperformance.com/spark.asp

rocco16

False.
The spark will START at the same time, and will take the identical amount of time to cover the first .050". That extra .010" gap just takes another .000000000000000000000000000000000000000 01 second to complete (how long does it take for electricity to cover .010" at a speed of 186,000 miles/second?), but by then the mixture should already have been ignited.
Net effect: no timing change.

Larry
code5coupe

CFI-EFI

With all that, did you start out questioning my statement and end up pretty much agreeing?

I DID say:Meaning that any change in timing will be greater at higher rpms.

RACE ON!!!

Lone Ranger

Let's examine another concept: extended electrode. Some after market plugs, including the AC Rapidfire's I'm using, have a slightly longer electrode. This is intended to get the spark a little closer to the mixture, or at least a little farther into the chamber.

Seems to me this would have to have an advancing effect on timing, but would it be negligible or significant?

pwrful c4

The wider the gap the hotter the spark you need.

Nathan Plemons

And the larger gap exposes the spark to the fuel mixture better. As such it has been said that to make the most horsepower you want to run the largest gap possible that you can reliably fire.

Fuzzy Dice

Very interesting thread to me...I run "el cheapo" Autolites in my 93 and they are gapped at .030...my motor is stock and this gap makes it idle a tiny bit rougher but at WOT it screams...I replaced my plugs after 20 months and the old ones looked absolutely brand new...