Spark plug gapping?
Thread Starter
Instructor
Joined: May 2006
Posts: 153
Likes: 0
From: Gautier MS
Spark plug gapping?
86 corvette. I understand the gap should be set at .035.
Will it hurt anything to set it at .040? Any change in the way my car will run or air/fuel? .035 just doesn't look right. Thanks
Matt
Will it hurt anything to set it at .040? Any change in the way my car will run or air/fuel? .035 just doesn't look right. Thanks
Matt
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
GM doesn't set their gap dimensions on looks!!!!!!!!!!!!!!!!!!!!!
I have been setting my plug gaps at .035 for as long as I have been driving cars (45 years) and they all drove fine! .040 will still work.
I have been setting my plug gaps at .035 for as long as I have been driving cars (45 years) and they all drove fine! .040 will still work.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
That is very true. I've had no trouble with the .035 gapping at all, nor .040.
Le Mans Master
Joined: Jun 2006
Posts: 5,642
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From: One day you're a Comet...the next day you're dust... Arkansas
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Team Owner
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From: SE NY
Cruise-In II Veteran
Our stock HEI is know to drop off (voltage) above 4k RPM, which usually isn't a problem with stock TPI intake.
I have modified the stock TPI intake to breathe well into the 5k RPM range, so I installed a HyperTech coil/cap/rotor and now gap about .047". This setup puts out 55kV well into the 5k RPM range and so will support wider plug gap.
I have modified the stock TPI intake to breathe well into the 5k RPM range, so I installed a HyperTech coil/cap/rotor and now gap about .047". This setup puts out 55kV well into the 5k RPM range and so will support wider plug gap.
Safety Car
Joined: Apr 2001
Posts: 3,525
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From: Houston TX
What is the detriment of going with a smaller gap? I have asked this question before, but no one seems to have an answer.
I currently run 0.031" and would like to go down to 0.028" or 0.029". Thoughts?
Aaron
I currently run 0.031" and would like to go down to 0.028" or 0.029". Thoughts?
Aaron
Melting Slicks
Joined: Dec 2004
Posts: 2,954
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From: Obama=death for America
First, why do you want to run a smaller gap? Is the car not running properly? Or do you just feel like shortening the gap? Unless your car is modded that requires the gap to be altered from its factory setting, leave it at the factory's recommendation. Those gap numbers are there for a reason.
Race Director
Joined: Oct 2001
Posts: 19,653
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From: Melbourne Florida
Cruise-In IV Veteran
I drag race my car constantly and at WOT the tighter gap is a dream...I never have to worry about a miss and it runs smooth as glass at WOT...I replaced my last set of plugs after 2 years of hard racing...they came out looking perfect and new...I almost put them back in but since they were already out I went ahead and put a new set in...wth, I use Autolite's for like $10.00 a set...lol
Safety Car
Joined: Apr 2001
Posts: 3,525
Likes: 2
From: Houston TX
First, why do you want to run a smaller gap? Is the car not running properly? Or do you just feel like shortening the gap? Unless your car is modded that requires the gap to be altered from its factory setting, leave it at the factory's recommendation. Those gap numbers are there for a reason.
The only detriment I know of is with the tighter gap you will get a tiny bit of rough idle...my 93 calls for .050 but I have gapped my plugs at .030 for 4 years now...
I drag race my car constantly and at WOT the tighter gap is a dream...I never have to worry about a miss and it runs smooth as glass at WOT...I replaced my last set of plugs after 2 years of hard racing...they came out looking perfect and new...I almost put them back in but since they were already out I went ahead and put a new set in...wth, I use Autolite's for like $10.00 a set...lol
I drag race my car constantly and at WOT the tighter gap is a dream...I never have to worry about a miss and it runs smooth as glass at WOT...I replaced my last set of plugs after 2 years of hard racing...they came out looking perfect and new...I almost put them back in but since they were already out I went ahead and put a new set in...wth, I use Autolite's for like $10.00 a set...lol
Thanks for the feedback,
Aaron
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
The reason for wider gap, cut-back electrodes, indexing the plugs and such is to expose more of the cylinder charge to the flame front, resulting in a more complete burn.
So long as your HEI is not dropping off in spark intensity with high RPM, there is no advantage to gapping tighter.
Are you sure it's "snuffing" the spark at 5,800rpm or is the spark intensity level dropping off?? Put her on a spark analyzer and run up the RPM should show the story.
So long as your HEI is not dropping off in spark intensity with high RPM, there is no advantage to gapping tighter.
Are you sure it's "snuffing" the spark at 5,800rpm or is the spark intensity level dropping off?? Put her on a spark analyzer and run up the RPM should show the story.
Last edited by 65Z01; Feb 12, 2007 at 08:05 PM.
Safety Car
Joined: Apr 2001
Posts: 3,525
Likes: 2
From: Houston TX
Thanks for the input.
Aaron
Racer
Joined: May 2003
Posts: 304
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From: beaverton oregon
Throwing my oar in the water...
'95 LT1...
Confusion...
In the engine compartment, the emissions sticker says .05...
Here the recommendation is .035-.040
Are we talkin' .035-.040 for an L98 only?
Any idea for the optimum on LT1?
Thanks in advance...
Jeff
'95 LT1...
Confusion...
In the engine compartment, the emissions sticker says .05...
Here the recommendation is .035-.040
Are we talkin' .035-.040 for an L98 only?
Any idea for the optimum on LT1?
Thanks in advance...
Jeff
Safety Car
Joined: May 1999
Posts: 3,651
Likes: 5
From: Atlantis
Cruise-In I Veteran
I gap mine at 42~45.
.032~.035 was used with the old point style ignition systems.
Remember those? THe ones you could hang onto a bare plug wire with the engine running?
I want a nice, big, fat spark, not a little, teeny weenie spark.
The HEI will throw a spark about TWO inches long at atmospheric pressure.
It can jump 40-45 thousandths easily at 150~180 psi.
I'd suppose GM has a multi-million buck 'Spark Adjuticating Machine' at their disposal to test their ignition systems.
I'd also bet there is some nice math that can predict the heat generated by spark gaps of various dimensions at different pressures and voltages.
Sometimes, I like to rely on information generated by the engineers particularly when the engineers have gigantic bucks at their disposal to test the theory they come up with. Not to mention a boatload of PhD's that can resolve just about everything to a boxfull of numbers.
.032~.035 was used with the old point style ignition systems.
Remember those? THe ones you could hang onto a bare plug wire with the engine running?
I want a nice, big, fat spark, not a little, teeny weenie spark.
The HEI will throw a spark about TWO inches long at atmospheric pressure.
It can jump 40-45 thousandths easily at 150~180 psi.
I'd suppose GM has a multi-million buck 'Spark Adjuticating Machine' at their disposal to test their ignition systems.
I'd also bet there is some nice math that can predict the heat generated by spark gaps of various dimensions at different pressures and voltages.
Sometimes, I like to rely on information generated by the engineers particularly when the engineers have gigantic bucks at their disposal to test the theory they come up with. Not to mention a boatload of PhD's that can resolve just about everything to a boxfull of numbers.