Deepening a spark plug

[Alaskaracer]

Why are you concerned about indexing the spark plugs? There is no gain to be had and the only valid reason for it is to gain clearance from the piston.....unless you have an issue with the plug hitting the piston, it's a waste of time imho

[ELS]

Why are you concerned about indexing the spark plugs? There is no gain to be had and the only valid reason for it is to gain clearance from the piston.....unless you have an issue with the plug hitting the piston, it's a waste of time imho

I'm concerned about the depth of the plug in the combustion chamber in this thread.
but I'll give my reasons on it anyway:
more CR, although insanely small, deepening the plug will raise CR, if I get .2cc from this, I could raise my CR from (for example) 11.81 to 11.88
which is pretty close to a 0.1CR increase, which isn't nothing.
anti-fouling, with the plug shrouded in the spark plug hole it is easier to foul it than if it is exposed, also that decreases stability, stray fuel droplets may quench the spark on certain combustion cycles and retard ignition or even stop it entirely.
anti-detonation, with the plug located closer to the center of the bore it decreases the chance of detonation, if I move it 3mm, in an 79mm bore, that's 1.2% closer to the center, again that isnt nothing.

and about indexing:
if the ground electrode is for example towards the piston, the fuel in the compression cycle would move upwards, and could de-atomize when it tries to turn around the spark plug electrode, which then from inertia could land on the spark plug electrode and cause problems, but with it located at the top, that would only possibly happen on the intake cycle, which would give more time for the fuel to vaporize, and even then - somewhat less likely.
if the electrode is indexed towards the exhaust side, it could possibly melt over time, especially in lean conditions, the electrode indexed away from the exhaust valve, it would make the plug less likely to foul because of the heat, that said, indexing away from the exhaust valve is preferable because the exhaust valve side of the combustion chamber is already hotter than the rest, so igniting the mixture more towards that side doesn't make much sense.
and then there's fouling from intake vortexes around the plug, depends on the head and where those vortexes are, but you could index the plug to reduce fouling from those.

all of these are minuscule effects, but if you don't collect change, wont end up with a dollar

[David Redszus]

Assuming a SCR of 11.81, a reduction in chamber volume of 0.2cc will result in a SCR of 11.83.
The DCR will go from 9.33 to 9.35, with a compression temperature at TDC going from 837.65F to 838.86F.
A delta of only 1.21F deg. or 0.144%. Fuel evaporation variations will produce much larger changes from cycle to cycle.

A far as plug index, examine the probable squish jet flow pattern. Both before TDC and after TDC, the plug
is in a quiescent location with little velocity directed toward the plug.

Perhaps a conversation with a piston designer and fuel chemist might produce more significant returns.

[Alaskaracer]

Why are you concerned about indexing the spark plugs? There is no gain to be had and the only valid reason for it is to gain clearance from the piston.....unless you have an issue with the plug hitting the piston, it's a waste of time imho

I'm concerned about the depth of the plug in the combustion chamber in this thread.
but I'll give my reasons on it anyway:
more CR, although insanely small, deepening the plug will raise CR, if I get .2cc from this, I could raise my CR from (for example) 11.81 to 11.88
which is pretty close to a 0.1CR increase, which isn't nothing.
anti-fouling, with the plug shrouded in the spark plug hole it is easier to foul it than if it is exposed, also that decreases stability, stray fuel droplets may quench the spark on certain combustion cycles and retard ignition or even stop it entirely.
anti-detonation, with the plug located closer to the center of the bore it decreases the chance of detonation, if I move it 3mm, in an 79mm bore, that's 1.2% closer to the center, again that isnt nothing.

and about indexing:
if the ground electrode is for example towards the piston, the fuel in the compression cycle would move upwards, and could de-atomize when it tries to turn around the spark plug electrode, which then from inertia could land on the spark plug electrode and cause problems, but with it located at the top, that would only possibly happen on the intake cycle, which would give more time for the fuel to vaporize, and even then - somewhat less likely.
if the electrode is indexed towards the exhaust side, it could possibly melt over time, especially in lean conditions, the electrode indexed away from the exhaust valve, it would make the plug less likely to foul because of the heat, that said, indexing away from the exhaust valve is preferable because the exhaust valve side of the combustion chamber is already hotter than the rest, so igniting the mixture more towards that side doesn't make much sense.
and then there's fouling from intake vortexes around the plug, depends on the head and where those vortexes are, but you could index the plug to reduce fouling from those.

all of these are minuscule effects, but if you don't collect change, wont end up with a dollar

NHRA Pro Stock and Comp Eliminator are some of the most refined engines you can get. A local engine builder here, MadCap Racing Engines, used to build V. Gaines engines when he ran that class.....virtually none of the drivers/builders I know bother with indexing the plugs. There is virtually no gain from it at all. Your story about indexing isn't exactly accurate either. As I stated, the only reason to index a spark plug is to gain clearance from the piston. It's better to notch the piston in this regard, during the build. It's been proven time and time again that the gains made from doing it are not even worth the effort. Dyno results have also shown no gains.

However, if you feel the need to do so, it's not going to cost you any performance either. I do agree with getting the plug electrode and ground strap unshrouded, that should help some....

[ELS]

NHRA Pro Stock and Comp Eliminator are some of the most refined engines you can get. A local engine builder here, MadCap Racing Engines, used to build V. Gaines engines when he ran that class.....virtually none of the drivers/builders I know bother with indexing the plugs. There is virtually no gain from it at all. Your story about indexing isn't exactly accurate either. As I stated, the only reason to index a spark plug is to gain clearance from the piston. It's better to notch the piston in this regard, during the build. It's been proven time and time again that the gains made from doing it are not even worth the effort. Dyno results have also shown no gains.

However, if you feel the need to do so, it's not going to cost you any performance either. I do agree with getting the plug electrode and ground strap unshrouded, that should help some....

dyno curves don't show throttle response, misfires, and other short anomalies

[Alaskaracer]

dyno curves don't show throttle response, misfires, and other short anomalies

You missed my point. Those engines are very highly refined. If there were gains to be made, they would do it. Pro Stock teams will spend thousands of dollars for just a few hp. If indexing the plugs gained them anything at all they would do it. It does not.

[David Redszus]

There are some chamber designs that might benefit from spark plug indexing.
When the squish jet is aimed at the plug, it could blow the flame kernal off the plug and into the
ground strap or wall where it quenches.

A very long spark duration could help eliminate possible misfires.A larger spark plug gap may
reduce misfires as well. Or, use a plug that does not have a ground strap.
Or a smaller center electrode.

Heat range is a much more significant variable. Since all cylinders do not run the same,
perhaps a different heat range is needed for each cylinder. Or even a different style plug.

Perhaps the real problem to be addressed is not well served by plug indexing.

[hoodeng]

ELS, was the photo of the head you are working on a test head or the one you are actually developing?

[Bill Chase]

dyno curves don't show throttle response, misfires, and other short anomalies

You missed my point. Those engines are very highly refined. If there were gains to be made, they would do it. Pro Stock teams will spend thousands of dollars for just a few hp. If indexing the plugs gained them anything at all they would do it. It does not.

Makes a guy wonder if the reason they don't do it is ease of repeatability. Thrashing in the pits between rounds probably wouldn't be the time to play with indexing spark plugs. More a case of not worth the time and effort, easier ways to pickup 1-3 hp. Time spent between rounds tweaking the shocks, tire pressure, reading the plugs and making jetting or ignition timing changes would be more productive? Maybe that's the reason they don't do it?

[Walter R. Malik]

I got a par of 13 degree head castings from "All-Pro" and the spark plug type recommended left 4 full threads, (and a couple half threads), showing on the combustion chamber side.

I viewed this as a good thing and lowered the plug seating area on the heads some .200" so, the spark plug tip would reside closer to the center of that combustion chamber when the heads were finished.

I guess I could have simply used plugs with 1 inch or 7/8" reach instead of the 3/4" reach ... now that those are plentiful.

[Alaskaracer]

dyno curves don't show throttle response, misfires, and other short anomalies

You missed my point. Those engines are very highly refined. If there were gains to be made, they would do it. Pro Stock teams will spend thousands of dollars for just a few hp. If indexing the plugs gained them anything at all they would do it. It does not.

Makes a guy wonder if the reason they don't do it is ease of repeatability. Thrashing in the pits between rounds probably wouldn't be the time to play with indexing spark plugs. More a case of not worth the time and effort, easier ways to pickup 1-3 hp. Time spent between rounds tweaking the shocks, tire pressure, reading the plugs and making jetting or ignition timing changes would be more productive? Maybe that's the reason they don't do it?

I know a couple of engine builders that do pro stock stuff. I've been told from them there is zero gain....only reason to do it is if there is an issue with piston clearance. As far as time goes, they have a ton of stuff prepped and ready to go, so time isn't an issue if there is a problem. They can swap engines between rounds with time to spare if needed.

[hoodeng]

I'm thinking there is a whole world of improvement for that head in that photo, without the gain of .2cc or indexing. Although, it might have been the angle of the photo shot.

[ELS]

I'm thinking there is a whole world of improvement for that head in that photo, without the gain of .2cc or indexing. Although, it might have been the angle of the photo shot.

of course, the combustion chamber is abysmal, but the spark plug was a problem anyway.

[hoodeng]

I was thinking more is to be gained from around the seat area.