What spark plug gap do you run?
Thread Starter
Race Director
Joined: May 2001
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From: Guelph, Ontario
What spark plug gap do you run?
There seems to be a big discrepancy as to the actual spark plug gap that the plugs for the LS1 should run, so I'm curious as to what everyone here is running?
I've been running NGK TR55ix plugs at the preset gap (.059) for the past couple of years with no problems, the engine runs very smooth with no misfires. Before that I ran NGK copper plugs with a .050 gap and that also worked well for me too. But I understand GM now has a TSB stating the LS1 should be running a .040 gap. (FWIW I tried a .040 gap for a while on my 98 LS1 Firebird Formula and it also seemed to work well for me, scan tool testing showed no misfires)
But I've always figured that for normally aspirated (and relatively stock) engines that you should run the highest gap that your ignition system will properly fire as that will allow for more horsepower (although I doubt there would be more than a 1 or 2 hp difference if even that)
I've been running NGK TR55ix plugs at the preset gap (.059) for the past couple of years with no problems, the engine runs very smooth with no misfires. Before that I ran NGK copper plugs with a .050 gap and that also worked well for me too. But I understand GM now has a TSB stating the LS1 should be running a .040 gap. (FWIW I tried a .040 gap for a while on my 98 LS1 Firebird Formula and it also seemed to work well for me, scan tool testing showed no misfires)
But I've always figured that for normally aspirated (and relatively stock) engines that you should run the highest gap that your ignition system will properly fire as that will allow for more horsepower (although I doubt there would be more than a 1 or 2 hp difference if even that)
Safety Car
Joined: Jan 2001
Posts: 4,386
Likes: 689
From: Boise ID
Kind of depends on the plug. The TSB you mention is from 2003
A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.
A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.
Melting Slicks
Joined: Apr 2007
Posts: 2,167
Likes: 55
From: Kempner TX
St. Jude Donor '08-'09
I run BR7EFs at .35 all the time but that's also for nitrous. You won't get a misfire the closer you gap them. Your more than likely to get spark blowout the wider you gap them and I've even had that happen to me on me 2003 KB Cobra running TR6s. Gap your corvette plugs between .45 and .55 and you will be ok... Especially for a simple daily driver.
Thread Starter
Race Director
Joined: May 2001
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From: Guelph, Ontario
I'm disappointed that this topic hasn't gotten more responses 
Is anyone here running the NGK TR5ix plugs at their preset gap of .039? I might try those out next when it comes time to replace my TR55ix's (which have been in the car for almost 3 years and 32,000 miles now)
Is anyone here running the NGK TR5ix plugs at their preset gap of .039? I might try those out next when it comes time to replace my TR55ix's (which have been in the car for almost 3 years and 32,000 miles now)
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Team Owner
Joined: Mar 2008
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From: Canon City Co
St. Jude Donor '11,'13
"I voluntarily took 60 days off from OT"
Kind of depends on the plug. The TSB you mention is from 2003
A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.
A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.
..Only AC Delco Iridiums....get that foreign NGK crap OUT of your GM vehicle 
........................................ .......
Tech Contributor
Joined: Dec 2003
Posts: 19,384
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From: Horncastle Lincolnshire, England
2023 C5 of the Year Finalist - Unmodified
I fitted NGK TR55s iridiums and the advice was to stick with the pre-gapped value which is less than the stock plugs.
There's loads of discussion. Some say stick with the traditional 0.060 but apparently that was prone to poor idle performance which is why the aftermarkets are gapped smaller. I thought the preset gap for TR55s was 0.040 but I could be wrong.
There's loads of discussion. Some say stick with the traditional 0.060 but apparently that was prone to poor idle performance which is why the aftermarkets are gapped smaller. I thought the preset gap for TR55s was 0.040 but I could be wrong.
Last edited by DeeGee; Feb 12, 2010 at 02:24 AM.
Thread Starter
Race Director
Joined: May 2001
Posts: 15,323
Likes: 2,077
From: Guelph, Ontario
I fitted NGK TR55s iridiums and the advice was to stick with the pre-gapped value which is less than the stock plugs.
There's loads of discussion. Some say stick with the traditional 0.060 but apparently that was prone to poor idle performance which is why the aftermarkets are gapped smaller. I thought the preset gap for TR55s was 0.040 but I could be wrong.
There's loads of discussion. Some say stick with the traditional 0.060 but apparently that was prone to poor idle performance which is why the aftermarkets are gapped smaller. I thought the preset gap for TR55s was 0.040 but I could be wrong.
My idle is very smooth with the TR55ix plugs so I don't know why GM decided to drop the gap for the LS1 to the lower setting.
Race Director
Joined: Aug 2003
Posts: 13,421
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From: Lake Mary Florida
A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed. The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01mm (0.040 inches) when the spark plug is made. The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.
Many new plugs are pre-gapped and can be damaged by changing it.
