Spark Plugs

Plug selection notes:
With standard nickel alloy electrodes, the spark gap between the center and ground electrodes grows about .0002" to .0006" for every 1,000 miles of driving. After 35,000 miles of driving, the gap can grow as much as 0.015" or more.

A spark plug must maintain an even flow of heat from its firing end to avoid becoming a source of pre-ignition, but must run hot enough to burn off conductive deposits.

GENERALLY, multiple electrode spark plugs provide better ignition "quality" at the beginning of the burning of the a/f mixture. That will produce "peak cylinder pressure" earlier in the stroke. Effectively it's close to the same effect as "advancing" the ignition timing a little bit.
Alleged fact - The spark will only spark across one of the electrodes each time it fires - whichever one has the least amount of resistance.

Spark plug installation is critical. Over-torque can lead to shell seal leakage, while under-torque can lead to loosening. Both can result in over-heating and catastrophic failure. Thread lubricants can be used, but they should be designed for high temperature use and the installation torque must be reduced.

The Platinum electrode heats up earlier than standard electrodes to allow for more efficient and smoother running cold starts.

Most platinum plugs can go up to 100,000 miles before they have to be replaced. The same is true for plugs that use other exotic metals such as iridium for their center electrode. Plugs with platinum on both electrodes ("double" platinum plugs) experience even less wear than plugs with only a single platinum or platinum-tipped electrode. [aa1car.com]

A higher voltage spark can cause more-complete combustion and therefore increase cylinder pressure and produce more power. The electrode tip diameter is much smaller for platinum and iridium plugs, which require less voltage than wider conventional plugs to generate a spark at a given gap. Consequently, the spark gap is widened for precious metal plugs to return the required voltage to conventional-plug levels.

Platinum plugs are pre-gapped. Plug damage can occur with conventional gapping tools. A special Bosch gapping tool is sold for bosch plugs.

Bosch's Platinum+2 plugs have a unique "surface gap" side electrode design with two side electrodes. Supposedly increasing the number of side electrodes gives the spark more paths to ground and reduces the risk of misfire, while extending plug life by spreading wear over more electrode surfaces. These plugs are pre-gapped at the factory to a uniform 1.6 mm (.064 in) setting and should not be re-gapped to the specifications for a standard spark plug.
Because the center electrode is recessed and the grounds are on the side, a standard tool will hit the ceramic insulator and can't be used to measure the gap.

A spark jumps more easily from a sharp edge than a rounded blunt edge. So the more sharp edges it has to jump to, the better the odds of the plug firing under all types of driving conditions.

  • 14-mm plugs with a gasket-style seat should be tightened to 26 to 30 ft.-lbs. in cast iron heads, but only 18 to 22 ft.-lbs. in aluminum heads.
  • 18-mm plugs with gasket-style seats should be tightened to 32 to 38 ft.-lbs. in cast iron heads, but only 28 to 34 ft.-lbs. in aluminum heads.
  • 14-mm taper seat spark plugs should be tightened to 7 to 15 ft.-lbs. in both cast iron and aluminum.
  • 18-mm taper seat spark plugs should be tightened to 15 to 20 ft.-lbs. in both types of heads.

Many people on the jeep forums like Champion 4412 truck plugs.
Other forum comments:
I will say that its been my experience that the exspensive gimmicky ones are just that, exspensive and gimmicky but I havent seen any true gains from them.

I tried some that met the FSM's suggested .040 NGK plugs, and the engine ran rough. I also tried some single platinum plugs, and the engine ran rough with them too.

Installation notes:
Jeep Wrangler 4.0L
Gap: .035
Torque: 22 ft-lbs

Anti-Seize. This is goo that you rub onto the threads on the plug that make them easier to remove later. Use it.
Torque can be reduced because of the lubricating effect.

Di-Electric Grease or silicon spray. This goes on the end of the plug that sticks out to help its connection inside the boot on the coil rail to keep those plugs dry and minimize radio noise. I just squirt it into the boots instead of putting it on the plugs, because a greasy plug is difficult to insert.

I broke off my plastic oil dipstick handle when my hand slipped off the wrench and the dip stick fell down into the tube. You might want to remove it first, so don't have to retrieve it later.
If you do break it. You can put some super-glue (I used the gooey kind) on the end of a pencil or dowel and stick it to the top. Don't push too hard or it will go down further. One guy on the internet said jiffy lube broke his and used a vacuum line to suck it out.

See:
jeep.us41.org/sparkplugs
Spark Plubs at aa1car.com/
jalopnik.com
Spark Plug Swap at jeephorizons.com/
Spark Plug Technical Information
Optimization of Spark Plug Life
SPARKPLUGS 101


last updated 10 Sept 2007