indexing plug ground electrodes

[grumpyvette]

now Ive posted much of this info before, but I recently helped tune a guys 13.7:1 compression big block chevy where the guy could not get the car to run correctly at first because he had failed to check a few clearances and on several cylinders the piston domes were barely contacting the spark plug ground electrodes and closing the gaps down to less than .010-.020 thousands.
this should help,your quench distance should be between .038-.044,
you might want to INDEX the plugs so the ground strap faces up minimizing the chance the piston can contact the plug ground electrode at TDC
when he failed to index the plugs (something he had never encountered before and was totally un-aware of as a potential problem)
just by chance most of the plugs indexed so the ground electrodes didn,t contact the piston domes but a few did, and it seemed to be a random deal to him because sometimes the plugs worked sometimes they failed, he never figured out the cause. well the only reason I spotted it almost instantly was EXPERIENCE, having been that route about 38 years ago with my first real high compression 489 BBC where the pop-up piston domes made indexing the plugs mandatory

spark plug info

spark plug info..SO!...read thru the links theres a ton of good info, but its spread over all the links, if you can get that skill mastered your well ahead of most guys who fail to take the time and effort to learn that skill guys frequently ask me what plug gap to use, Provided your ignition...

garage.grumpysperformance.com

read the links below, if you want to take the time to index the spark plugs in your heads you'll need to carefully mark where the electrode ground is on the plugs insulator where you can easily see it with a permanent marker and use spacer washers to insure that the open area of the plug faces the area between the valves, keep in mind,in most cases plug indexing is done for clearance issues relating to domed pistons, but it can help ignition speed in a few cases especially in some combustion chamber designs,indexing the open gap slightly favoring the exhaust valve location, WITH THE GROUND ELECTRODE INDEXED UPWARD TOWARD THE COMBUSTION CHAMBER ROOF , can help in some cases, remember the piston compressed nothing until both valves are seated and the ignition and burn will occur while the valves are fully seated if you've got everything correctly timed
remember in some high compression applications, its fairly common for the pop-up piston dome to contact the ground electrode and bend it, closing the gap if the plugs ground electrode is placed in the lower 270 degrees of its possible indexed rotation on the plug when its installed , especially ,if you failed to correctly check clearances, between the piston and combustion chamber.
now IM sure you can find info indicating the ground should be indexed differently, but think about where ignition occurs, the pistons rapidly swinging from btdc to after tdc and theres very little volume or space around the plugs ignition point and having the open part of the plug facing the piston as it approaches and moves away makes sense as the flame front has ignite far more in that direction than the couple ccs of combustion chamber volume between the plugs ground electrode and the chamber roof, remember the valves have been fully seated for quite some time on that compression stroke before ignition takes place, and the quench and squish between the piston and head are forcing everything (the compressed f/a mix) towards the open part of the plug under those conditions
in this example directly below the plugs GROUND electrode is ALMOST EXACTLY OPPOSITE, OF THE IDEAL INDEXED location , as its located wher the piston has the best chance of contacting the ground electrode,

example , now ideally youll want the ground electrode facing the roof of the combustion chamber for max clearance and flame propagation
similar too...THESE TWO PICTURES POSTED BELOW

In theory having the ground electrode between the green liines is the idea. having it located at or closer to the red location is ideal , in theory,the idea is to have the ground electrode face toward the chamber roof and the open arc of the ignition face slightly to the exhaust valve but as long as the ground electrode is located toward the chamber roof it works fine
if your using high compression pistons like these youll soon see the reason for indexing plug ground electrodes, to gain max clearance to the piston dome, this is also one reason why the multi ground electrodes are a waste of money, as the more ground electrodes the more the spark is shielded from igniting the compressed F/A charge, packed in around it.

http://performanceunlimited.com/documents/plugsidegapping.html

side gaping or cutting back on the ground electrode to increase the flame exposure helps on race cars with very high compression ratios, BUT ITS ALSO A USEFUL TIP TO GAIN ADDED COMBUSTION CHAMBER CLEARANCE, BETWEEN DOME PISTONS AND PLUGS
http://performanceunlimited.com/documen ... pping.html

OBVIOUSLY A PLUG SOCKET DESIGNED TO HELP INDEX PLUGS IS HELPFUL AT TIMES
http://www.jegs.com/i/JEGS-Performance- ... 0/10002/-1
,this can in some cases smooth the idle slightly and add a few hp, in other applications its basically wasted effort, but in some cases the engine clearances make it mandatory, a good deal depends on the compression, intake runner and combustion chamber design, and it should be obvious that if your not running up close to the limits on your ignition advance curve and octane limits on your fuel you probably won,t notice the slight improvements in ignition,only careful testing in your engine will tell you if its worth the effort in your combination.....now I have guys that tell me its always wasted effort, and if you use the 2-4 electrode ground strap plugs is not going to change, well thats true, if you have several ground straps on the plugs you shroud the spark enough that youve effectively reduced its efficiency too ignite a clean flame front, so yeah if you use those its wasted effort[/color]

related info
http://www.bullittarchive.com/5005.htm

http://dodgeram.info/Engine-Gas/SparkPl ... dexing.htm

http://www.eatonbalancing.com/blog/2009 ... -indexing/
there are several types of spacer washers for both taper seat and standard plug designs that allow you to index plugs
http://www.shop.speedunlimited.com/p-58 ... -seat.aspx
Moroso #71900 Spark Plug Index Washers, Copper, Tapered Seat, for 14mm Plugs, Sizes .010, .021, .032 in., 10 of Each, Set of 30.

M28-71900

http://www.jegs.com/i/Moroso/710/71910/10002/-1

http://www.summitracing.com/parts/MOR-71900/?rtype=10

 

[Indycars]

I chose not to consider the whole picture, but just a few factors that would determine if spark plug indexing would have
an effect. Comments are based on a SBC/350 cu in and the spark duration of two MSD coils. Also assuming total spark
advance is set to 35 BTDC and the squish band equal 0.040 inches. The squish band is equal to the
"Deck Clearance" + "Compressed Head Gasket Thickness", see picture below.

Since the intake and exhaust valves have been closed for a considerable time there would be little turbulence in the cylinder
when the spark occurs at 35 BTDC. Therefore the only turbulence that could be generated in the cylinder during the spark
discharge would be from the squish area when the piston is within 0.060". I wanted to examine just how close the piston
would be when the spark was last available. As you would can see it depends on the RPM and the duration of the spark.
The two coils I used for this example have a duration of 200 uS and 450 uS (uS=Microseconds or x10-6).

As you can see from the table, the distance the piston travels during the spark is 1.2 degrees to 18.9 degrees. That means
the piston will still be 0.360" to 0.088" from TDC. If we accept the general statement that quench is only effective at 0.060" or
less and the squish band is zero, because of the stresses on the components. Even then, looking at the table, the closest the
piston comes to the head during the spark is 0.088" @ 7000 RPM.

As I see it from the condition stated above, there seem to be little performance gains from indexing the spark plugs.

You still might have to index the spark plugs for clearance reasons.

Comments or questions about how I arrived at the numbers in the table ???

 

[grumpyvette]

I chose not to consider the whole picture, but just a few factors that would determine if spark plug indexing would have
an effect. Comments are based on a SBC/350 cu in and the spark duration of two MSD coils. Also assuming total spark
advance is set to 35 BTDC and the squish band equal 0.040 inches. The squish band is equal to the
"Deck Clearance" + "Compressed Head Gasket Thickness", see picture below.

Since the intake and exhaust valves have been closed for a considerable time there would be little turbulence in the cylinder
when the spark occurs at 35 BTDC. Therefore the only turbulence that could be generated in the cylinder during the spark
discharge would be from the squish area when the piston is within 0.060". I wanted to examine just how close the piston
would be when the spark was last available. As you would can see it depends on the RPM and the duration of the spark.
The two coils I used for this example have a duration of 200 uS and 450 uS (uS=Microseconds or x10-6).

As you can see from the table, the distance the piston travels during the spark is 1.2 degrees to 18.9 degrees. That means
the piston will still be 0.360" to 0.088" from TDC. If we accept the general statement that quench is only effective at 0.060" or
less and the squish band is zero, because of the stresses on the components. Even then, looking at the table, the closest the
piston comes to the head during the spark is 0.088" @ 7000 RPM.

As I see it from the condition stated above, there seem to be little performance gains from indexing the spark plugs.

You still might have to index the spark plugs for clearance reasons.

Comments or questions about how I arrived at the numbers in the table ???

lets take it a point at a time, on that 350 sbc

"Since the intake and exhaust valves have been closed for a considerable time there would be little turbulence in the cylinder
when the spark occurs at 35 BTDC. Therefore the only turbulence that could be generated in the cylinder during the spark
discharge would be from the squish area when the piston is within 0.060".

while I see what your saying I can,t ignore the fact that the pistons moving at 1000rpm at almost 600 feet per minute, and at well over 4000 feet per minute at 7000rpm , thats going to create a good deal of motion in the compressed air / fuel mix with or without quench or squish, in the combustion chamber, but having both quench and squish designed into the wedge chamber design , and a good deal of heat and inertia in those trapped and rapidly compressing gases as the time theres a great deal of gas motion


"As you can see from the table, the distance the piston travels during the spark is 1.2 degrees to 18.9 degrees. That means
the piston will still be 0.360" to 0.088" from TDC. If we accept the general statement that quench is only effective at 0.060" or
less and the squish band is zero, because of the stresses on the components. Even then, looking at the table, the closest the
piston comes to the head during the spark is 0.088" @ 7000 RPM."


while I agree with the basic premiss, on spark duration Id point out that gasses ignite from the spark but generally burn and increase in pressure from the ignition to generally well past 24-30 degrees AFTER TDC

 

[Indycars]

Rick
"Since the intake and exhaust valves have been closed for a considerable time there would be little turbulence in the cylinder
when the spark occurs at 35 BTDC. Therefore the only turbulence that could be generated in the cylinder during the spark
discharge would be from the squish area when the piston is within 0.060".
Grumpy
while I see what your saying I can,t ignore the fact that the pistons moving at 1000rpm at almost 600 feet per minute, and at well over 4000 feet per minute at 7000rpm , thats going to create a good deal of motion in the compressed air / fuel mix with or without quench or squish, in the combustion chamber, but having both quench and squish designed into the wedge chamber design , and a good deal of heat and inertia in those trapped and rapidly compressing gases as the time theres a great deal of gas motion

Rick
Ok, there will be turbulence, but without any directionality. Like when the quench area becomes effective, shooting gases
across the chamber towards the plug. One factor for indexing the plug is so that the Air & Fuel (AF) will flow across the
plug gap like in the picture, without some general and repeatable gas flow direction, there is little to be gained from keeping
the ground on the plug in the lower 270 degrees. I'm referring to the statement about putting the ground in the lower
270 degrees, but maybe this was for clearance reasons. See 2nd picture below.


Rick
"As you can see from the table, the distance the piston travels during the spark is 1.2 degrees to 18.9 degrees. That means
the piston will still be 0.360" to 0.088" from TDC. If we accept the general statement that quench is only effective at 0.060" or
less and the squish band is zero, because of the stresses on the components. Even then, looking at the table, the closest the
piston comes to the head during the spark is 0.088" @ 7000 RPM."

Grumpy
while I agree with the basic premiss, on spark duration Id point out that gasses ignite from the spark but generally burn
and increase in pressure from the ignition to generally well past 24-30 degrees AFTER TDC

Rick
I'm not sure what the point is here ??? If MORE AF was ignited initially with the longer spark duration, then maybe the burn
would be complete sooner.

The longer the spark duration the more AF that is ignited, so that it can ignite even more AF. Kinda like a chain letter, but
it starts with 100 letters instead of just 1 letter.

I hope you see that I'm playing the devil's advocate on this discussion. I have certainly over simplified the situation, so
that I could narrow the topic down to something that would not require something the size of a novel.

 

[grumpyvette]

in most cases plug indexing is done for clearance issues relating to domed pistons,
the burn time varies with the rpms and speed the gases are compressed and the speed the piston moves those gases around in the combustion chamber. at low speeds the burn takes about .0040-.0045-( forty thousands to forty five thousands of a second), the idea is to have about 80%-85% of the burn, result in PRESSURE building AFTER TDC, but the time limitations mean that the burn normally is effectively completed by about 24-30 degrees past TDC both because the fuel air mix is exhausted AND the piston rapidly moving away from TDC greatly increases the cylinder volume its burning in, thus reducing the pressure as the piston drops away from TDC.

at 1000rpm, the piston in a 350 sbc is traveling at an average of 580 feet per second ,at 7000rpm is traveling at an average of 4060 feet per minute but the true piston speed varies with the angle of the connecting rod

lets say just to keep things simple, the piston speed stays fairly consistent and the piston moves a set distance per degree of rotation, (it does not but follow along here)
if that were true the piston moving at 1000rpm would move about .01933 inches per degree of rotation and that would take about .000322 seconds per degree of rotation, if we look at the typical ignition curve with its 6-8 degrees advance at 1000rpm we find that about .00225 seconds of the cylinder burn time occurs before TDC, or about 1/2 the burn occurs before TDC. but because the piston is not at a consistent speed the effective pressure builds at and after TDC.
speed the engine up to 3000rpm and your advance might be at 34 degrees but your working with both a faster more energetic burn and 1/3rd the time so the same basic pressure curve before and after TDC remains, after about 3000rpm the squish, quench and piston speed compressing the fuel/air mix becomes consistently MORE energetic , the burn speeds continue to speed up, and the squish throws the flame front faster and faster so theres no need to advance the ignition advance
REMEMBER as the engine rpms increase the cylinders have less and less time to fill and scavenge and the piston falls away from TDC on the power stroke faster and faster making the production of and use of cylinder pressure more difficult , usually on a standard low compression V8 the peak torque occurs before about 5000 rpm,

upping the compression changing the cam timing and increasing exhaust scavenging efficiency helps increase that rpm limitation,increasing the squish area and centering the plug location helps increase the burn speed lowering the need to advance the ignition timing like on this vortec combustion chamber design(below)

one reason hemis work so well with NITRO METHANE FUEL is the centrally located spark plug, cross flow breathing and the lack of squish which better matches the nitro slower and longer burn characteristic,s

BTW some of the older guys will remember that the SBC used to have 13/16" spark plugs and they were changed to the smaller 5/8" style in the early 1970s, and if your wondering why they changed it was because they found that smaller diam. plug threads allowed slightly larger coolant passages and allowed moving the plug location slightly closer to the exhaust valve and also allowed a slightly changed angle all of which allowed lower emissions, better control on combustion and less chance of stripped or cross threading plugs

related info
viewtopic.php?f=52&t=4081&p=13147&hilit=efficiency#p13147