If you check the 8.1 sticky you can note mention of the smaller spark plug gap that some are running (0.045" vs. stock @ 0.060").

If I understand correctly, the smaller gap should actually result in slightly increased spark timing, an affect that may not be desirable with a custom tune which pushes the timing to the max without causing pinging.

The reason I bring this up is following my vehicle-specific tuning done by Justin @ Black Bear Tuning, he informed me that post-tune data showed that I was getting a little bit of knock retard (and I believe if the 8.1 pulls any timing it pulls a whole 5 degrees at a minimum!). I speculated it could be that I was running a smaller-than-stock gap and he agreeded this indeed could be the case. As a result, I am considering going back to the stock 0.060" to see if it it makes any SOTP difference.

Does anyone out there have any experience in this situation or comment to be made? Please share what you know...

GM has changed the spark gap from .060 to .045 in all V8's due to the ethanol in gasoline. A smaller gap is needed with ethanol. Most manufacturers are doing this. I can't speak of the effects with a custom tune.

Gap size does not change timing in the least. This is a myth. The coil fires the plug when it's told, no sooner or later. If you increase the gap you could easily still get the knock retard and loose power.

Sparks take the path of least resistance. Air itself is an excellent insulator, meaning it blocks the flow of electricity. Internal combustion engines ignite fuel in compressed air. So the more air crammed into the spark plug gap there is the less likely it is to generate a sucessful spark jump across the gap. By reducing the gap you increase the ability to deliver sucessful ignition consistantly at maximum loads.

The only reason factory gaps are made as large as they are is to reduce hydrocarbon emmissions at part throttle cruise. In order to cause ignition the spark has to heat an oxygen and fuel molecule to the point of combustion. The air/fuel charge in a cylinder at part throttle is lean. Meaning there are fewer fuel molecules than air, compared to what would produce complete combustion of all the availible oxygen. This ensures that most to all of the HC's have ample oxygen to burn with and none is left over to travel out of the engine and pollute the atmosphere. This lean mixture makes it hard to ignite because the fuel is spread thin. By increasing the gap the spark is physically larger. This increases the ability of the spark to catch fire to the lean mixture by exposing more oxygen and fuel combinations to the spark's heat. Therefore the tune can be made leaner and cleaner. A small gap will reduce the ignition, fire catching, ability of a lean charge because less fuel gets hit with the spark. Much has been done since the onset of emmissions control to increase the ignition systems spark voltage so the larger gaps can be used and still maintain acceptable actual ignition of the charge at all loads.

Large gaps reduce the chances of the spark making it from the firing electrode to the ground electrode of the plugs at high loads and are there to reduce emmssions at light loads only. Small gaps have the opposite effect.

Ignition timing is uneffected by how far the spark has to travel when it is sent. When it is sent is the only way to change the timing.

When and why you are getting some knock retard is due to tuning issues, not your plugs gap. I would have to see some data logs and your tune to know why. But I don't do this for a living and you've already contracted with Black Bear so have them fix it. Hopefully they know that there is an automatic spark knock retard at fast throttle openings tuned into the 8.1's tune and unless you remove this nothing done to timing or fuel will eliminate it. That doesn't mean that if you remove this buffer you won't see real knock that it was preventing.

Either your tuner was just going along with you to expedite the conversation or they don't know what they are talking about. Don't waste any time increasing your plug gaps. If it reduces knock it will be because you have less combustion on your leanest and hottest burning cylinder(s). Less combustion also means less power.

Vernon

Great information, Manic. But I must question your unsupported assertion that changing gap does not (cannot) change timing.

Consider this: you write that air acts as an insulator, which I would think we all agree. This implies there is some intrinsic impedance given the distance between the two electrodes and the temperatures and composition of the "air."

Widening the gap should increase this resistance. Shrinking the gap will have an opposite affect. Given that the ignition system is tuned assuming a given, expected impedance, changing this "constant" should have an affect on the performance of the system as a whole as the spark only jumps the gap once the strength of the electrical potential is large enough to cause the breakdown of the dielectric (the air).

Ergo, a larger gap means a larger potential is required to create the spark in an equal time period. Something's gotta’ give...either you turn up the ignition system output potential OR you accept that it takes longer for the spark to bridge the gap. Longer = loss of timing. Now, how much? I have no idea. We could be talking 0.001 degrees or 1 degree. I don't have enough information to make this determination. But theoretically, it should make a difference.

Now, if you're telling us you've done the math (or you've seen what others have said) and gap makes absolutely no bit of difference, that's one thing. But to assert outright that gap has no affect on timing without providing proof or source is questionable.

Kris

Kris I agree with you completely, but what I'm saying is that's like you exampled 0.001 degree. I haven't done any math because it's not enough to be relevent on what we're worried about. Certainly not enough to make or brake spark knock. The change in spark timing due to plug gap change of 0.020" is finute and immesurable with a timing light.

Vernon

The change in spark timing due to plug gap change of 0.020" is finute and immesurable with a timing light.

Fair enough, thanks.