Spark Plug Gap
Small gap = easy to jump, so a weak ignition system will make a spark. Bad thing about a small gap is a small spark, obviously. This makes it more difficult to light off the entire mixture cleanly, so you can get a miss when the engine is operating in a lean mix setting, such as idle.
Some boosted cars have to close the gap because at high boost settings, you can get "spark blowout". Its not really blowout in the literal sense, so much as a bunch of air crammed into the chamber, which insulates the spark plug so much that it cant jump the gap any more.
Wider gap means a nice big spark. UNLESS, the gap gets so wide that the coil does not have the energy to jump the gap. Then, again, you get a miss.
Most of the time, you run the maximum gap that pulls hard to redline. Hopefully, it is enough gap to idle well too, but on high boost applications it may not be.
On a stock NA car, you run the plug's recommended gap. Its pretty much that simple.
Some boosted cars have to close the gap because at high boost settings, you can get "spark blowout". Its not really blowout in the literal sense, so much as a bunch of air crammed into the chamber, which insulates the spark plug so much that it cant jump the gap any more.
Wider gap means a nice big spark. UNLESS, the gap gets so wide that the coil does not have the energy to jump the gap. Then, again, you get a miss.
Most of the time, you run the maximum gap that pulls hard to redline. Hopefully, it is enough gap to idle well too, but on high boost applications it may not be.
On a stock NA car, you run the plug's recommended gap. Its pretty much that simple.
Pole Position
Joined: May 2008
Posts: 346
Likes: 0
From: australia
First off, you must not attempt to re-gap an Iridium plug. Trying to do so will damage the plug's electrode.
For copper plugs...too big a gap will create problems on boost, the spark is literally blown out by the air pressure before it has a chance to ignite the mixture, resulting in a misfire. A decent sized gap must always be maintained though as too small a gap will create problems at idle
and reduce the efficiency of the combustion process. Only adjust a gap by 0.1mm at a time and retest...if you go below 0.8mm and still have a miss-fire on boost I would suggest there is another problem not related to the sparkplugs (unless you are not using a plug in the correct heat range).
Normally Aspirated
Copper - 1.1
Platinum - 1.1
Iridium - 1.1
Stock TT
Copper - 1.1
Platinum - 1.1
Iridium - 1.1
TT @ upto 1bar
Copper - 0.9 7 heat range
Platinum - 0.9
Iridium - 0.8 6 heat range
TT/hybrids @ upto 1.3bar
Copper - 0.8
Platinum - 0.8
Iridium - 0.8 7 heat range
Single Turbo/Big Twins @ upto 1.3bar
Copper - 0.8
Platinum - 0.8
Iridium - 0.7 7 heat range with HKS USA Twin Power DLI
For copper plugs...too big a gap will create problems on boost, the spark is literally blown out by the air pressure before it has a chance to ignite the mixture, resulting in a misfire. A decent sized gap must always be maintained though as too small a gap will create problems at idle
and reduce the efficiency of the combustion process. Only adjust a gap by 0.1mm at a time and retest...if you go below 0.8mm and still have a miss-fire on boost I would suggest there is another problem not related to the sparkplugs (unless you are not using a plug in the correct heat range).
Normally Aspirated
Copper - 1.1
Platinum - 1.1
Iridium - 1.1
Stock TT
Copper - 1.1
Platinum - 1.1
Iridium - 1.1
TT @ upto 1bar
Copper - 0.9 7 heat range
Platinum - 0.9
Iridium - 0.8 6 heat range
TT/hybrids @ upto 1.3bar
Copper - 0.8
Platinum - 0.8
Iridium - 0.8 7 heat range
Single Turbo/Big Twins @ upto 1.3bar
Copper - 0.8
Platinum - 0.8
Iridium - 0.7 7 heat range with HKS USA Twin Power DLI
Last edited by v8soarer91; Jan 15, 2011 at 12:47 AM. Reason: spelling
I thought at first that you couldn't gap iridiums, but you can. As long as you are careful not to crunch the electrode. Use a pair of pliers to bend the ground strap without prying against the electrode itself and use a flat feeler gauge to measure. To close the gap, just press the strap against a piece of wood. You just have to be careful.
I thought at first that you couldn't gap iridiums, but you can. As long as you are careful not to crunch the electrode. Use a pair of pliers to bend the ground strap without prying against the electrode itself and use a flat feeler gauge to measure. To close the gap, just press the strap against a piece of wood. You just have to be careful.
More often then not, people are trying to pry against the center electrode to open the gap (HUGE no-no!), or they force too large of a feeler gauge in when measuring the gap and cause damage. Done properly, there is absolutely no reason iridiums can't be re-gapped.
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yup, i thought you couldnt gap them either but you can, when the store doesn't carry the regular bkr7eix but only the bk7eix -11 version the gap is 11mm and I like to gap it down to 8mm. if you get the version without the -11 on it, it already comes at 8mm or .032", these for me are a drop in (your setup may vary).
just tap it down firmly if you need to gap them, the tip is resistant to moving but once you apply enough pressure its easy to overshoot. never make contact with the iridium center part.
just tap it down firmly if you need to gap them, the tip is resistant to moving but once you apply enough pressure its easy to overshoot. never make contact with the iridium center part.
Last edited by Ali SC3; Jan 15, 2011 at 07:34 PM.
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