What is "Spark Plug Indexing"?
Drifting
Joined: Apr 2008
Posts: 1,316
Likes: 2
From: St. Louis Mo
Yes the piston DID run that close to the spark plug thats how horsepower was made back then there were NO aftermarket cylinder heads of any kind there were no computers to work out cam profiles we couldn't "build " an engine on a computer,no cnc,no extrude hone.all the benefits taken for granted afforded by powerful computers the internet where information is shared was not there,take away all the benefits of todays technology ,the close tolerences in machine work computer controlled cnc for exact repeatability...Cylinder heads were ground till they hit the water jackets then epoxyed or welded back up they put as much piston
in the engine as would physically fit 14 sometimes 15 to one big flat tappet cams...Yes my friend I was there i saw it plugs were that close and it was never said or implied that spark plugs had to be "backed out of the head" the ground strap was merely positioned upby whatever means(washers,plug selection ect.)indexing the plugs is putting the ground strap in whatever position deemed necesary for your application however when the term is used it is generally understood to mean ground strap up for clearance of piston crown as that was the term used to describe the practice...I'm not sure what is meant by "index the plug is about position of the spark" just how does one change the position of the spark??? the spark goes from the center electrode to the ground strap. And lastly
"you have other problems",again not sure what you mean I think it's a dig or insult,rest assured I take no offense even if it is.... as I consider this a friendly ,sometimes heated debate just stating what I know as true and factual..period <(small letters)
in the engine as would physically fit 14 sometimes 15 to one big flat tappet cams...Yes my friend I was there i saw it plugs were that close and it was never said or implied that spark plugs had to be "backed out of the head" the ground strap was merely positioned upby whatever means(washers,plug selection ect.)indexing the plugs is putting the ground strap in whatever position deemed necesary for your application however when the term is used it is generally understood to mean ground strap up for clearance of piston crown as that was the term used to describe the practice...I'm not sure what is meant by "index the plug is about position of the spark" just how does one change the position of the spark??? the spark goes from the center electrode to the ground strap. And lastly "you have other problems",again not sure what you mean I think it's a dig or insult,rest assured I take no offense even if it is.... as I consider this a friendly ,sometimes heated debate just stating what I know as true and factual..period <(small letters)
I didn t mean it like that at all. I, actually, looked up spark plug indexing on Wikpedia(Wiki) and it does state that it is a debatable topic. It says that "installing them so that the open area of the gap, not shrouded by the ground electrode, faces the center of the combustion chamber, towards the intake valve, instead of the wall." Then it also says "Others believe that this is useful only to keep the ground electrode out of the way of the piston in Ultra-high-compression engines, if clearance is insufficient." 
Safety Car
Joined: May 2008
Posts: 4,237
Likes: 11
From: Lacombe Louisiana
Back in the day(long long ago) before computer programs and cnc designed and manufactured pistons and heads,builders would put clay on top of the pistons to check valve and plug clearances.It was common to fly cut pistons for clearances.Indexing has always meant directing to open part of the gap towards the incoming fuel/air mixture.It was pretty common years ago before multi discharge ignition systems but prob only really helped in top series cars like pro stock,fuel and alcohol funny cars and dragsters where massive amounts of fuel were flooded into the combustion chamber.They still index them in NASCAR though.
Race Director
Joined: Jul 2007
Posts: 18,681
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From: Reno is so close to Hell you can see Sparks , State Of Confusion
St. Jude Donor '12-'13-'14
NO tuners/ shops bother with this/nothing gained you are100% right question was what is spark plug indexing it's positioning the ground strap up so it doesn't contact piston PERIOD nothing more it doesn't position the spark plug to ignite the mixture better or put a better spark no hp gain, there is no "better postion"to ignite the mixture there was an extremly small percentage of racers that needed to index plugs they were for the most part race engines with absoultely every mdification done in a quest for hp which included the most compression hence indexing plugs was necessary as all out race engines they generally made good hp guys saw this thought there was some new trick so back then many indexed plugs for no other reason other than monkey see monkey do
Advanced
Joined: Sep 2005
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From: Santa Barbara Calif
Melting Slicks
Joined: Mar 2004
Posts: 3,365
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From: Farmers Branch TX
Cruise-In VI Veteran
St. Jude Donor '09
Indexing refers to all pointing in the same direction and when refering to spark plugs and the internal combustion engine that direction is ground electrode is pointing up away from the crown of the piston as with ultra hi compression if not indexed the piston will contact the ground electrode if it happens to by chanceto have ground electrode pointing downthey sell small soft copper washers to bring the electrode into the desired position (up) no hp gains just saves plug from gettin kilt.Not needed on 99.9% of cars.Wires always break like pullin teeth ,get a tube Di-Electric grease don't be cheap on spark plug end,and wipe a little on coil end Don't take plugs out when any alum.engine is hot. For extra insurance wipe a small dab anti seize on spark plug threads have a rag ready it tends to be messy.Delco wires over the years have proven themselves again and again other wires may be as good none better and delco has the longevity to prove it. delco IRRIDUMS LAST LONGER BUT $$ OR NGK REASONABLE PRICE DON'T LAST AS LONG.ALSO MANY RESPECTABLE NAMES ON THE FORUM LUV THE NGKThewre are no "Magic" plugs that increase HP don't believe the hype about "E 3 ,Pulstar,bosche fusion to name a few
Some engines built to fit certain classes or engines built with ultra high compression 14 15 to one compression for whatever reason they were built ,were indeed built with the understanding the plugs would be indexed,or in lieu of indexing they sometimes would cut back the ground electrode at the spot where it would 90o over the center electrode.Yes the ground electrode would hit if not indexed or cut back NO there was not "SERIOUS PROBLEMS" .I don't know what applications would benefit from this or how wide spread this practice is in this day and age as engine design along with the amazing cylinder heads we now have with the hi swirl technology may have made this practice obsolete.At one time this was accepted practice to get that last bit of HP out of a race engine.
did not say cut back ground strap was indexing stated "in lieu of indexing" or instead of indexing the ground strap was cut back.Indexing put the ground strap out of harms way cutting it down removed the part of the plug that was in harms way.In addition my friend you are erroneous in your assumption that spark plugs are indexed to propagate flame travel the ground strap in no way hinders flame travel be it wedge, hemi,":semi" hemi (big block chevy) combustion chambers.Plugs have been angled to help flame travel (old style SB chevy)Hi Perf. over the counter chevy (off road only)heads (Known as Turbo heads)were first to use this concept and was found to be just a very small part of the design as others modified their older style heads to angle plug design to find no performance improvement.
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There is NO best position of spark, what is the best positon to ignite the flame front? to ignite A/F mixture?,there is no open part of a spark plug it is not shrouded by the ground strap it doesn't matter if ground strap faces open part of combustion chamber or the cylinder wall the spark is introduced the mixture ignites.where did the dialogue concerning how much the plug protrudes into the combustion chamber come from I stated indexing positions ground strap in an upward position by means of shims so the plug is screwed in not as far generally 1/2 to 3/4 of a turn will position the plug. Shims are used to position the ground strap not pull the plug out of the combustion chamber. A supply of plugs was the way it was done BEFORE shims became available the method used was to mark the porcelain to where the ground strap was: screw in, note position, remove, (if necessary) reinstall after a couple plugs you knew what thickness was needed by where the mark ended up a bunch of plugs was tedious and time consuming and not necessary as shims needed were usually 1 to 5 thous and the amount the plug was pulled out was inconsequential.
There is NO best position of spark, what is the best positon to ignite the flame front? to ignite A/F mixture?,there is no open part of a spark plug it is not shrouded by the ground strap it doesn't matter if ground strap faces open part of combustion chamber or the cylinder wall the spark is introduced the mixture ignites.where did the dialogue concerning how much the plug protrudes into the combustion chamber come from I stated indexing positions ground strap in an upward position by means of shims so the plug is screwed in not as far generally 1/2 to 3/4 of a turn will position the plug. Shims are used to position the ground strap not pull the plug out of the combustion chamber. A supply of plugs was the way it was done BEFORE shims became available the method used was to mark the porcelain to where the ground strap was: screw in, note position, remove, (if necessary) reinstall after a couple plugs you knew what thickness was needed by where the mark ended up a bunch of plugs was tedious and time consuming and not necessary as shims needed were usually 1 to 5 thous and the amount the plug was pulled out was inconsequential.
NO tuners/ shops bother with this/nothing gained you are100% right question was what is spark plug indexing it's positioning the ground strap up so it doesn't contact piston PERIOD nothing more it doesn't position the spark plug to ignite the mixture better or put a better spark no hp gain, there is no "better postion"to ignite the mixture there was an extremly small percentage of racers that needed to index plugs they were for the most part race engines with absoultely every mdification done in a quest for hp which included the most compression hence indexing plugs was necessary as all out race engines they generally made good hp guys saw this thought there was some new trick so back then many indexed plugs for no other reason other than monkey see monkey do
indexing spark plugs on a stock engine will show some improvement providing that particular engine has an inferior head design,like the ones cited in that site,where fuel mixture is restricted to one area or by design the spark plug is somewhat shrouded and to make up for the shortcomings of the head the sparkplug is placed in a position where it ia allowed to work.In this day and age with federally mandated fuel economy (CAFE) Corp.Avg.Fuel Economy if it did ANYTHING increase power,better mileage don't you think automakers both foreign and domestic would have picked up on it??Why is it this little gem of automotive procedure has escaped everybodys scruntity? A guy with a truck, a stopwatch,on a deserted highway timing himself to the hundreth of a second is not something I would submit to the society of automotive engineers as proof positive.A lot of things are said/claimed on the internet a little common sense is needed to sift through the b.s., half truth's the common sense here is NOBODY DOES IT tuners,repair shops,Chevrolet ,Ford Mercedes,Porsche....I'm a retired mechanic,trade school graduate,45 years in the trade ,cars,trucks heavy equipt.foreign,machine shop ,welding,fabricating,crewed on professional drag race car done just about it all but in all those years i never saw in any shop manual the need to or recommend to index spark plugs for all intents and purposes the question asked the correct answer on this forum where American hi performance drag and road racing are the topics we discuss is still position the ground strap away from the piston crown.
Yes the piston DID run that close to the spark plug thats how horsepower was made back then there were NO aftermarket cylinder heads of any kind there were no computers to work out cam profiles we couldn't "build " an engine on a computer,no cnc,no extrude hone.all the benefits taken for granted afforded by powerful computers the internet where information is shared was not there,take away all the benefits of todays technology ,the close tolerences in machine work computer controlled cnc for exact repeatability...Cylinder heads were ground till they hit the water jackets then epoxyed or welded back up they put as much piston in the engine as would physically fit 14 sometimes 15 to one big flat tappet cams...Yes my friend I was there i saw it plugs were that close and it was never said or implied that spark plugs had to be "backed out of the head" the ground strap was merely positioned upby whatever means(washers,plug selection ect.)indexing the plugs is putting the ground strap in whatever position deemed necesary for your application however when the term is used it is generally understood to mean ground strap up for clearance of piston crown as that was the term used to describe the practice...I'm not sure what is meant by "index the plug is about position of the spark" just how does one change the position of the spark??? the spark goes from the center electrode to the ground strap. And lastly
"you have other problems",again not sure what you mean I think it's a dig or insult,rest assured I take no offense even if it is.... as I consider this a friendly ,sometimes heated debate just stating what I know as true and factual..period <(small letters)
in the engine as would physically fit 14 sometimes 15 to one big flat tappet cams...Yes my friend I was there i saw it plugs were that close and it was never said or implied that spark plugs had to be "backed out of the head" the ground strap was merely positioned upby whatever means(washers,plug selection ect.)indexing the plugs is putting the ground strap in whatever position deemed necesary for your application however when the term is used it is generally understood to mean ground strap up for clearance of piston crown as that was the term used to describe the practice...I'm not sure what is meant by "index the plug is about position of the spark" just how does one change the position of the spark??? the spark goes from the center electrode to the ground strap. And lastly "you have other problems",again not sure what you mean I think it's a dig or insult,rest assured I take no offense even if it is.... as I consider this a friendly ,sometimes heated debate just stating what I know as true and factual..period <(small letters)
Burning Brakes
Joined: May 2008
Posts: 1,147
Likes: 1
From: Mohegan Lake New York
you my friend must be a parochial school teacher... A pseudo intellect with low self esteem ......you're right didn't have the benefit of a higher education...but i'm not stupid...and i use what i have to be a productive member of society....maybe you could contribute to this post instead of pointing out my shortcomings...rest assured everybody else saw them...i come on this forum with one goal to help nobody knows me i reap no benefit get no kudos..don't agree fine make your argument...this debate ,indexing , i doubt anybody in our corvette family will get anything out of this they can use...so i'm guilty ... I did not help anybody
Melting Slicks
Joined: Oct 2007
Posts: 2,798
Likes: 8
From: It's true money can't buy happiness, but it is more comfortable crying in a Corvette than on a bicyc
St. Jude Donor '13
When I index the plugs on my big and small block Chevy engines I mark the electrode on the open side and point that when seated toward the intake valve which is toward the intake runner of that cylinder. If you do not index and by chance the electrode faces away from the valve when it is seated it is called shouded. Indexing keeps the incoming fuel at the open spark, if turned away the electrode blocks the incoming fuel. When I raced my new 1965 GTO back in the day an older racer who was fast told me how he did it. I had some chevys in the past with bad valve quides and fouled plugs, when I indexed them they lasted much longer before fouling.
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Drifting
Joined: Sep 2004
Posts: 1,727
Likes: 19
From: Carson City, NV
I can believe that spark plug indexing was played with 90 years ago with flathead engines and low voltage coils in order to get the best combustion, but these days with electronic ignitions, high output coils, and fine tuned heads I doubt it makes any difference whatsoever.
As far as piston to sparkplug contact, I can't believe it. If that were true, they would flycut or notch the pistons. Clearance has to be more controlled and predictable than positioning the plug in a certain orientation.
As far as piston to sparkplug contact, I can't believe it. If that were true, they would flycut or notch the pistons. Clearance has to be more controlled and predictable than positioning the plug in a certain orientation.
Advanced
Joined: Sep 2005
Posts: 83
Likes: 0
From: Santa Barbara Calif
In attempting to squeeze out that last bit of potential output from the Ford Y-Block (and other engines), there are those items that can be applied that may not be immediately measurable but they will be done because they will not be detrimental to the power output. One of these items is spark plug indexing which tends to vary in its benefit depending upon the application it is being applied. Indexing the spark plugs simply insures that the spark plug ground electrode is in a specific orientation within the cylinder to promote a more beneficial flame front when the fuel is ignited. In those cases where detonation is a risk, spark plug indexing is of a proven benefit in that the flame front will not be split by a worst case scenario of the spark plug electrode being in a position to divide the flame path. Indexing also allows the ground electrode or strap to be intentionally positioned away from the higher heat areas it would be subjected to if randomly placed closer to the piston and/or exhaust valve which can increase the propensity for pre-ignition. In a normal situation where a set of spark plugs are randomly installed, the spark plug straps end up running at a variety of temperatures simply from the random placements of the straps. In this case, indexing would provide a more accurate visual means towards optimizing jetting and ignition timing by eliminating potential variances in the spark plug readings themselves. And if running a domed piston that’s marginally close to the spark plug, then indexing also helps in preventing the ground electrode from contacting the piston and potentially closing the gap.
Advanced
Joined: Sep 2005
Posts: 83
Likes: 0
From: Santa Barbara Calif
Published by tedeaton on 11 Jun 2009 at 09:00 pm
Spark Plug Side Gapping
Included on that list of old racers tuning tricks is spark plug side gapping. What’s being discussed here is not a true side gapped spark plug but a gapping procedure or electrode modification that allows the spark to actually fire off of the very edge or ‘side’ of the spark plug’s ground electrode. In lieu of a better description, this gapping process will simply continue to be called ‘side gapping’. Side gapping is performed even though the end result may not be measurable but a logical thought process will maintain that doing this will not be detrimental to efficiency or performance and in fact, should help it.
Like spark plug indexing, spark plug side gapping will vary in its benefit depending upon the application to which it is being applied. While this procedure tends to be very effective on two stroke engines, the benefit on four stroke engines is less predictable depending in part to the combustion chamber design and air flow characteristics of the engine in question. On both the two and four stroke engines, the immediate benefit is typically observed in engine starting where it’s quicker or with much less starter effort. The general premise behind side gapping lies in getting the spark to fire directly on the end of the grounding electrode instead of having the spark shrouded by the electrode strap. Doing this is typically done in conjunction with spark plug indexing in order to get as much exposure of the spark to the compressed fuel mixture as possible.
A typical out of the box spark plug will have the ground electrode (strap) completely covering the center post consequently shrouding or covering it to some degree. Side gapping will simply modify this strap so the end of it is exactly in line with the centerline of the spark plug thereby putting the spark in a position to fire directly on the end of the ground electrode which in turn permits the spark to be better exposed for igniting the fuel mixture. A variety of methods can be employed to shorten or remove the excess amount of ground electrode strap so that the new end is centered directly over the center electrode. A grinder or file are equally effective but in either case, care must be taken to insure that the center electrode (tip) or other portions of the spark plug are not unduly damaged or altered in the process of working on the ground strap. After the strap is appropriately modified, it will need to be deburred and then the gap reset to the desired specification. It’s important to note that the bottom edge of the electrode where it has been shortened does need to keep a sharp edge and not be rounded in order to promote a more decisive and consistent length spark.
Spark plug side gapping is just one more tuning aid that when used with the myriad of other seemingly unimportant tuning tools accumulatively adds to a measurable increase in power output that by itself would not be significant. More simply said… “every little bit helps”.
Spark Plug Side Gapping
Included on that list of old racers tuning tricks is spark plug side gapping. What’s being discussed here is not a true side gapped spark plug but a gapping procedure or electrode modification that allows the spark to actually fire off of the very edge or ‘side’ of the spark plug’s ground electrode. In lieu of a better description, this gapping process will simply continue to be called ‘side gapping’. Side gapping is performed even though the end result may not be measurable but a logical thought process will maintain that doing this will not be detrimental to efficiency or performance and in fact, should help it.
Like spark plug indexing, spark plug side gapping will vary in its benefit depending upon the application to which it is being applied. While this procedure tends to be very effective on two stroke engines, the benefit on four stroke engines is less predictable depending in part to the combustion chamber design and air flow characteristics of the engine in question. On both the two and four stroke engines, the immediate benefit is typically observed in engine starting where it’s quicker or with much less starter effort. The general premise behind side gapping lies in getting the spark to fire directly on the end of the grounding electrode instead of having the spark shrouded by the electrode strap. Doing this is typically done in conjunction with spark plug indexing in order to get as much exposure of the spark to the compressed fuel mixture as possible.
A typical out of the box spark plug will have the ground electrode (strap) completely covering the center post consequently shrouding or covering it to some degree. Side gapping will simply modify this strap so the end of it is exactly in line with the centerline of the spark plug thereby putting the spark in a position to fire directly on the end of the ground electrode which in turn permits the spark to be better exposed for igniting the fuel mixture. A variety of methods can be employed to shorten or remove the excess amount of ground electrode strap so that the new end is centered directly over the center electrode. A grinder or file are equally effective but in either case, care must be taken to insure that the center electrode (tip) or other portions of the spark plug are not unduly damaged or altered in the process of working on the ground strap. After the strap is appropriately modified, it will need to be deburred and then the gap reset to the desired specification. It’s important to note that the bottom edge of the electrode where it has been shortened does need to keep a sharp edge and not be rounded in order to promote a more decisive and consistent length spark.
Spark plug side gapping is just one more tuning aid that when used with the myriad of other seemingly unimportant tuning tools accumulatively adds to a measurable increase in power output that by itself would not be significant. More simply said… “every little bit helps”.
