> Main > Gas Index > DIY Repair > Spark Plug Indexing

Spark Plug Indexing
What is it, does it work?

Visit Geno's Garage
for Truck accessories.
Geno's Garage Truck Accessories

Indexing Spark Plugs - Theory and an experiment

From: (Henry LaViers)

    According to Dr. Christopher Jacobs (of Omni-Pak and Pro-Street ignitions from Jacobs Electronics), indexing sparkplugs 'never hurts and sometimes helps'. Indexing is especially useful if the combustion chamber has a 'squish' area where the piston almost touches the head. This squish area 'squirts' a high speed flow of mixture toward the spark plug. The Dodge Magnum heads do have a squish area (also sometimes called a quench area). The spark plug is at the 'outside' (toward the fender) of the head and the spark plug hole is angled toward the exhaust valve. The intake and exhaust valves are in the middle of the chamber. The squish area is 'inside' toward the intake manifold. The combustion chamber is not round. It is 'kidney' shaped with the tip of the indented 'v' of the kidney pointing toward the spark plug. The difference between 'round' and 'kidney' is the 'squish area' where the piston top almost hits the head at top dead center.
     There are good pictures of the inside of the Magnum heads on page 121 of the Magnum Engines book, page 52 of the 1999 Mopar Performance Catalog (W5 head but it is very similar to Magnum) and there is a bad drawing on page 3-58 of the 1989-1996 Ram Chilton Manual.

    For indexing, Christoper Jacobs recommends that you mark the insulator of the sparkplug with a black magic marker (not a pencil as graphite conducts electricity). Draw a line on the insulator on the side where the gap is open - opposite the side where the ground electrode is. Seems like you could also scratch/grind a mark on the top metal electrical terminal so that you could see & feel it. To align the spark plug's open gap toward the quench area, you would need to have the marked line on the 'top' side - away from the fender on that side of the truck. If you are looking down on the installed sparkplug, this means the mark should lie at either 11 o'clock (if the plug went in angling slightly toward the front of the truck) or at 1 o'clock (if the plug went in angling slightly toward the rear of the truck).
     In practical terms, any o'clock position between 10 o'clock and 2 o'clock would be in the ballpark. This would also mean that if the open gap's mark lay in positions between 4 o'clock and 8 o'clock it might be bad - the ground electrode would stand like a fence post partially blocking the spark gap from the squish area.
    Jacobs sells washers to go onto the sparkplug's seat to cause the plug to line up this way. You can also file off a little bit at a time of the start of the first thread to cause the sparkplug to end up turned a little. If you have a lathe you could turn off just a bit of the seat.

    Does indexing the sparkplugs make a big difference?    Probably not. I have read that indexing can increase hp and mpg by 0 - 5%. On 245 hp, that would only be 12 horsepower. At 15 mpg, that would only be three quarters of a mile per gallon. But it is pretty cheap to do if you donate the labor. The trial and error labor might get annoying.

Bosch Platinum Indexing

    I remember saying in a Dirt post awhile back that the new design Bosch Platinum Super4 sparkplug design might be good because you had four times better chance of getting the most advantageous sparkplug orientation.

After reading Christopher Jacobs advice as to how to index sparkplugs, I'll have to eat those words.  It now seems that four ground electrodes would be the worst for indexing - three of the four electrodes would always be in the way and partially block the flow of mixture from the squish area.  On the other hand, if the engine is pinging because the ignition timing is simply too advanced, the three blocking electrodes might slow down combustion ( 'increase lag' ) and might cause the pinging to stop.

But if the engine is pinging because of 'pre-ignition' where a hot spot in the combustion chamber starts a fire on its own before the sparkplug does, the increased time lag would make the pinging worse.

A Tale of Spark Plug Indexing

    Some weeks ago I ordered a set of sparkplug indexing washers for 14mm flat seat plugs from Jacobs Electronics after reading Christopher Jacobs comments about how indexing "might help and never hurts" in his book "High Performance Ignition Systems." ($16.50+6.00S/H / Part no. 380800)
     Jacobs says to aim the open gap of the sparkplug toward the "Squish" area of the cylinder head that is made by the flattened area of the head's chamber. The idea here is that the mixture gets squished outward by the upward moving piston in a "fan jet" of gas moving at supersonic speed. The fan-jet of gas hits the hot spark arc, ignites, and then slams into the ground electrode "telephone pole" which splits & scatters it with high turbulence. This high turbulence promotes a fast and more complete burn.
     In the worse case, the "telephone pole-like" ground electrode is directly between the squish area and the electrical spark. Now the ground electrode blocks & splits the path of the fan-jet of mixture, shields it from the hot core of the spark, and ends up delaying combustion.

    I hadn't gotten around to doing the indexing, but this weekend I planned to make a 150 mile trip to the Wilmington beaches on Saturday - which presented the opportunity to make a 300 mile round-trip mpg test run similar to two others I have already made in June - so I got up early Saturday morning and went to work.
     This kit gives you ten soft copper 0.043 washers that are supposed to "back off" the sparkplug 105 degrees, ten 0.054 washers for 210 degrees, and ten 0.064 washers for 315 degrees. Moroso sells similar washers.
     The Bosch FR8DPX/stk no 4102 sparkplugs had been in there for 26,000 miles. I marked their tops with a dab from a paint marker at the spot where ideally the ground electrode should point so that I could see how my plugs were oriented originally 'by chance' 2 years ago.
     I used a 3 foot length of windshield washer hose to "puff" the dirt out around the plugs before loosening them as I had no compressed air. There was more dirt than I expected. All plugs were still snug, yet came out easily with no hint of freezing - but five turned out to have loose top tips.

    The Bosch plugs have "3-way" electrical contacts at the top: you can screw them off to use a ring terminal, or screw a bolt into the open top threads, or clip onto the rounded outside. Five had backed off about a half turn. I doubt this looseness was causing any electrical contact problem but I didn't like finding them. I tightened the tips up until they bottomed out and them gave them about 20 more ft-lbs.
     Here's how the plugs had "by chance" stopped in their holes originally: (150 to 180 degrees is bad, 0 degrees would be perfect)

#1 45 deg counterclockwise from ideal (gap now 0.045)
#2 180 deg (0.043 gap)
#3 180 (0.043)
#4 170 clockwise (0.043)
#5 30 counterclockwise (0.043)
#6 20 clockwise (0.043)
#7 45 clockwise (0.045)
#8 160 counterclockwise (0.043)

As chance would have it, half were so-so and half were positioned badly.

    All sparkplug noses had some caramel colored deposits on them, and by pre-marking them I could now see that the deposits were on the side facing the intake valve and the 'Squish' area behind it. There was also small black carbon ring deposits surrounding the tiny platinum wire center electrodes these plugs have.
     #8 sparkplug also had the lightest color shade of tan on its ceramic nose. This makes me more suspicious that #8 is running hottest and may be the one to start pinging first under full throttle acceleration. More on that in some future post.
     In theory, now that you know how the sparkplugs are turned you just choose the right washer and screw the plug back in one time. Unfortunately, it didn't work quite that predictably and I found myself screwing in each plug 2 or 3 times - and finally using a file to 'customize' a few washers. The washers also stay in the hole when the plug is removed, and you have to get your pinky finger in there to get them back out. This defeats the time saving nature of using the washers. The other way to index sparkplugs is to buy 16-24 sparkplugs, find the eight that 'by chance' snug to the right spot, and return the rest to the store for a refund. If I do it again I will probably take this route.

    I ended up putting #5 and #6 back in without a washer, using four 0.043 washers, one 0.051 washer, and one 0.064 washer on the rest. All plugs are now within 40 degrees of ideal. I filed a notch on the top metal tip to see where the plug turned to as the metal heat shields block the rest of the plug from view.
     Before putting the plugs back in, I cleaned up the ground electrodes and filed them so that the edges were clean metal and sharp at the tip. This increased the gaps to 0.048. The original Bosch factory sparkplug gap spec ( X in the Bosch number system) had been 1.1 mm (0.043 inches) and I was impressed to find that they had not worn to a larger gap after 26,000 miles of use.
     I also found dirt deposits on the sparkplug silicone boots and cleaned them with safe-for-plastic electrical contact cleaner. I used dielectric grease on the bottom inside of the boot for sealing and twisted the boot back and forth to spread it once the boot was back on. Like others, I wonder if the metal heat shields are a good idea.
     Finished after about 90 minutes, I started my 2wd '95 5.9V8 1500 Club Cab up. The idle might have been a tad smoother than before, but not much. I let the engine idle for 10 minutes.

    On driving a few miles with a heavy foot, I could immediately tell that the acceleration was both smoother and seemed faster. Most times after changing to a new set of plugs I have felt some improvement, but this seemed like about double the normal benefit.  Where NC Highway 64 crosses Jordan Lake, the 65 mph limit 4-lane road is perfectly straight, nearly flat and traffic is seldom heavy. I have "paced off" 233 of my 'surveyor strides," and called that a quarter mile, on the eastbound lane starting at the junction with the Seaforth Boat Ramp exit road.  This "homemade" quarter mile is semi-legal in that you can start from a stop at the turnoff and accelerate toward the 65 mph speed - and who could blame you if maybe you overshoot the mph at little?  The road is not quite perfectly level - it rises 5 feet over the quarter mile's 1,320 feet, so that makes it an uphill 0.4% grade. I don't wish to risk hurting my trans and so do a 'soft' start: my foot is off the accelerator pedal and there is no pre-loading up of the torque converter by holding the brake like you see at a dragstrip. No tire squealing at all - this and the uphill grade - makes the times you see below longer. My truck weighed 5300 lbs at a certified scale with me, a full gas tank, hard tonneau, and tools.

Here are two older sets of timed quarter runs, plus the new indexed sparkplugs runs:

With old 'by chance' plugs, 3.55 diff gears, 3/8 tank of gas, Mobil One 75-90W gear oil in the differential, I ran five passes in the past: 18.75 seconds/18.25/18.25/18.54/18.75    average = 18.51 seconds

With old 'by chance plugs', new 3.21 diff gears/bearing (after 1000 miles of break-in), Mopar 90W gear oil in the diff, and 1/2 tank gas, I ran two passes: 19.06/19.16     average = 19.11 seconds

With the sparkplugs indexed, 3.21 gears, 3/8 tank gas, fresh Mobil One 75-90W gear oil in the diff, I ran: 18.81/18.75    average = 18.78 seconds

All were on hot humid summer days with the temperature near 90, maybe as hot as 100 degrees four feet above the shade-less blacktop road.

In the back of the "Magnum Engines" book published by Mopar Performance there is this formula to "guess-ti-mate" quarter mile elapsed times:

HP = {Weight lbs/ (Elapsed Time seconds / 5.825)cubed}

The 5.825 is a 'constant' or 'fudge factor' that is a good starting point, but you can improve the accuracy by calculating a custom fudge factor for each unique vehicle, gearset, driver style, and track.

Plugging in the numbers from my 230 HP rated '95 V8, the E.T's I ran with the new 3.21 gears, Mopar 90W, and old 'by-chance' sparkplugs, I solve for a new value of C:

230 HP = {5300 lbs/ (19.11/ C)**3}        C = 6.715

Using this new 'custom fudge factor' and the elapsed times from the runs after indexing the sparkplugs;

New 'indexed sparkplug' HP = {5300/ (18.78/6.715)**3}
........................................= 242 HP or a gain of about 5%

After making those 2 Qmile trials, I carefully filled the gas tank with 87 octane full to the filler neck insert sleeve, rocking the truck bed 2-3 times to let out trapped air underneath the roll-over valve, then filling to the sleeve again. Then I drove to Wilmington over I40 East at a steady 60 mph with the cruise control on 95% of the time.

I previously had checked the speedo & odometer accuracy against the mileposts and stayed at a true 58-62 mph. (The speedo reads 10% low and the odometer reads 15% low.) Rpm was about 1490. 40 psi in Goodyear HT 235/85R16 with 5/16ths tread left that had a new specs of 655 rotations per mile.

This is through very flat country, with light traffic, except when going through 25 miles of 8-lane Road-Rager's Raceway between Cary and Raleigh. There was just enough wind to barely sway leaves at the tops of trees. I parked the truck, did a little beach engineering surveying, then started the truck up late to drive the 150 miles back to Durham.

Total miles: 310.1
Total gallons: 13.30
Indexed sparkplug mpg at 60 mph = 23.2

A week ago I had made the same run to Wilmington with the 'by chance' sparkplugs (but with Mopar 90 w gear oil in diff) and got these results:

Total miles:309.5
Total gallons: 13.85
Prior mpg at 60 mph = 22.3

The "indexed" run is about a 4% improvement, but admittedly you can't put a lot of stock in 1-2 test runs - I could easily be off plus/minus 0.2 gallons on the fill-ups - even though I made them with care at exactly the same station and pump. { I also made this same Wilmington test run early last week with the new 3.21 gears at 70 mph and got 18.6 mpg. I am getting to know that stretch of road}

What caused these improvements?

My gut tells me that indexing was the majority of the benefit, but that the other factors helped. Of the 4-5% gain, I'll guess that 3% was due to the indexing.

I found the indexing experiment to be worth while and recommend trying it. (Even if my back is still aching a little from all the leaning over fenders - or was it from twisting around to follow bikini's sway? :`)

- Hank