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HEI's And Spark Plug Gaps

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Mar 9, 2009
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Yemen
Several years ago I went to Los Angeles and on the way back my 454" engine started missing (???). As I was dead tired and night was rapidly approaching I continued on my way and made it home about 2 hours after dark. The next morning I used my IR gun to see which cylinders weren't firing and found the front 4 were dead. I pulled the plugs (AC Delco R45XLS) and found they were gapped to .055"...............010" more than I had gapped them about 15,000 miles before. I discovered the AC Delco plugs erode VERY quickly and found an HEI just isn't powerful enough to fire .055" plug gaps. The Standard Motor Products ignition catalog recommends the usual .035" plug gaps even on the more powerful HEI's and I suspect it's because they know the HEI's just aren't capable of firing across big gaps. Okay for .045" gaps IF those gaps are maintained but mine had exceeded that amount.

Since then I have switched to expensive Denso #5305 plugs with their .015" fine-wire iridium center electrodes and gap them to .035" to ensure they fire. The wider plug gaps were intended to light off very lean mixtures but as I'm running a #3310 Holley I just don't need huge plug gaps.

Oh yeah, and since I made the switch to the expensive Denso iridium plugs I have found their gaps have only increased about .002" in 30,000 miles so I will continue buying them. Their long lifespan alone makes their additional cost worth it. If I recall correctly I bought them for something like $5.50 each so I got a terrific deal as they usually cost $10 or more.
 

LLC5

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Wa.
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98 black 6spd convert.
Several years ago I went to Los Angeles and on the way back my 454" engine started missing (???). As I was dead tired and night was rapidly approaching I continued on my way and made it home about 2 hours after dark. The next morning I used my IR gun to see which cylinders weren't firing and found the front 4 were dead. I pulled the plugs (AC Delco R45XLS) and found they were gapped to .055"...............010" more than I had gapped them about 15,000 miles before. I discovered the AC Delco plugs erode VERY quickly and found an HEI just isn't powerful enough to fire .055" plug gaps. The Standard Motor Products ignition catalog recommends the usual .035" plug gaps even on the more powerful HEI's and I suspect it's because they know the HEI's just aren't capable of firing across big gaps. Okay for .045" gaps IF those gaps are maintained but mine had exceeded that amount.

Since then I have switched to expensive Denso #5305 plugs with their .015" fine-wire iridium center electrodes and gap them to .035" to ensure they fire. The wider plug gaps were intended to light off very lean mixtures but as I'm running a #3310 Holley I just don't need huge plug gaps.

Oh yeah, and since I made the switch to the expensive Denso iridium plugs I have found their gaps have only increased about .002" in 30,000 miles so I will continue buying them. Their long lifespan alone makes their additional cost worth it. If I recall correctly I bought them for something like $5.50 each so I got a terrific deal as they usually cost $10 or more.



You and your damn nonsense. Exactly what school/training center did you learn this information about HEI at? GM, AC Delco, Napa, what?


A good condition HEI distributer set up can put out 80K plus volts and over a 1/4 amp, enough to kill a human being under the correct conditions, which is more than enough for your wimpy .055" gap. HEI was originally designed to fire .060-.080 gaps, and they did that nicely. Your 010" gap differential at 15K is normal, the non iridium/platinum plugs were designed to be replaced at 10-12K miles.

You do of course know that you are never supposed to check the gap on iridium or platinum plugs right? Thought so.

Oh, BTW what V-8 engine has 4 front cylinders, and what exactly are their cylinder numbering?
 
Last edited:

Tom Bryant

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There is nothing wrong with being passionate about your point but let's please refrain from name calling. I'm enjoying these lively discussions but I won't let them turn nasty.

Tom
 
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There is nothing wrong with being passionate about your point but let's please refrain from name calling. I'm enjoying these lively discussions but I won't let them turn nasty.

Tom


Name-calling and character bashing is the American way. If anyone doesn't want to read what I post they can put me onto their IGNORE list. The problem is they DO want to read what I post so they can attack me. Sad, huh? In all the years I have been a member of a forum I have NEVER bashed anyone or called them names because I have way too much class to lower myself to that level. I will definitely disagree with someone if I feel I am right and they are wrong but I will never belittle someone for their views even if their views seem absurd to me. What sets me apart from most folks is I will freely admit when I make a mistake and I will willingly change my position if I'm proven wrong.
 

LLC5

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Messages
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Wa.
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98 black 6spd convert.
Name-calling and character bashing is the American way. If anyone doesn't want to read what I post they can put me onto their IGNORE list. The problem is they DO want to read what I post so they can attack me. Sad, huh? In all the years I have been a member of a forum I have NEVER bashed anyone or called them names because I have way too much class to lower myself to that level. I will definitely disagree with someone if I feel I am right and they are wrong but I will never belittle someone for their views even if their views seem absurd to me. What sets me apart from most folks is I will freely admit when I make a mistake and I will willingly change my position if I'm proven wrong.


Well guess what, you were wrong. Now admit it.

Sorry about the name calling, I got heated when I read another one of your your totally incorrect posts.

But sincerely, I am sorry about the name calling and I will edit it now......
 

kpic

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Mar 29, 2014
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Columbus, NC/NE Georgia
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1997 boosted silver coupe
A good condition HEI distributer set up can put out 80K plus volts and over a 1/4 amp, enough to kill a human being under the correct conditions, which is more than enough for your wimpy .055" gap. HEI was originally designed to fire .060-.080 gaps, and they did that nicely. Your 010" gap differential at 15K is normal, the non iridium/platinum plugs were designed to be replaced at 10-12K miles.

Toobroke..
As LLC5 stated a properly set up GM HEI will fire .060-080" gaps.

The short form, the larger the gap, the bigger the fire ball which ignites the charge air quicker and increases performance. MPG improves also.
The MPG improvement is subject to proper inflation pressures. :chuckle

The advantage is:
  • 1. narrow-gap risk: spark might be too weak/small to ignite fuel;
  • 2. narrow-gap benefit: plug always fires on each cycle;
  • 3. wide-gap risk: plug might not fire, or miss at high speeds;
  • 4. wide-gap benefit: spark is strong for a clean burn.
#3 is a warning what happens when a wide gap is applied with a poorly set up or poorly maintained HEI.
 

dirtypants

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May 14, 2015
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69
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Spokane
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81 4spd Corvette Blue dark met
spark plug gap with improved HEI

Toobroke..
As LLC5 stated a properly set up GM HEI will fire .060-080" gap; the larger the gap, the bigger the fire ball which ignites the charge air quicker and increases performance.

agree..I installed a Performance Distributors DUI..HEI..that has tuned mechanical and vac advance..and they recommend spark plug gap of .055. I fudged a little and went with .053, using Autolite AP25.. a little colder than the stock AC Delco R45TS..starts immediately, runs like a raped ape.
Nick
 

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