Spark Plugs / HEI conversion for 454 with 781 heads
01-16-2008, 02:15 AM
#1
Racer
Thread Starter
Spark Plugs / HEI conversion for 454 with 781 heads
I have a 72 454 that I just replaced the engine. It has the 336781 oval port heads. I also added an HEI distributor. I know the basics about the plugs. They are 1/2 deep 5/8 tapered plug, and should be around a .045 gap. Please advise on heat range and plug brand that would work best.
Thanks
John
Thanks
John
01-16-2008, 11:53 AM
#4
Racer
Thread Starter
Thanks, however HEI was not available in 72 as you are probably aware. Are you suggesting same plug, wider gap?
01-16-2008, 12:06 PM
#6
Team Owner
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The ignition system not withstanding, is the engine a direct replacement for what was in the car stock or is it an after market engine? The reason I ask, is because all you have to do is go into a Napa or Oreilly and get a set of NGK plugs for a 1972 Corvette with a 454, and ask the person to look up the recommended gap for that application. Always check the get, even if they claim to be pre-gapped.
I just like NGK plugs, and have always had good luck with them in any car or truck I have installed them in.
If the engine is after market, or the heads are after market, contact both manufacturers and ask them the recommended plug style and gap.
01-16-2008, 12:35 PM
#7
Burning Brakes
While I dont the answer to the poster question, I disagree with your answer. If he asks the counter guy for plugs for a 72 454 he will get plugs for a points ingition system. And plugs for a points ignition system have a gap smaller than a HEI system. You dont want to take a plug designed to be gapped at .035 and increase it to .045. I have no idea about heat range.
Last edited by 71406; 01-16-2008 at 12:40 PM.
01-16-2008, 12:41 PM
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While I dont the answer to the poster question, I disagree with your answer. If he asks the counter guy for plugs for a 72 454 he will get plugs for a points ingition system. And plugs for a points ignition system have a gap smaller than a HEI system. You dont want to change the gap on a plug from .045 on a plug that was designed to be gaped at .035. I have no idea about heat range.
Okay, I see what's being said. I'm not familiar with points plugs enough to have known the gaps are a lot smaller. I didn't know the difference would be that wide.
01-16-2008, 01:53 PM
#9
Racer
Thread Starter
While I dont the answer to the poster question, I disagree with your answer. If he asks the counter guy for plugs for a 72 454 he will get plugs for a points ingition system. And plugs for a points ignition system have a gap smaller than a HEI system. You dont want to take a plug designed to be gapped at .035 and increase it to .045. I have no idea about heat range.
I think the biggest hang up at this point may be the heat range. I think I can go with a 75 Chevelle, they had this particular head and also had hei
John
01-16-2008, 04:06 PM
#10
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Don't sweat the gap issue. The HEI system has more energy capability than a points system, and your cylinder doesn't know or care what you have for an ignition system, anyway. The cylinder merely sets up the conditions that the ignition system and components need to respond to (primarily gap breakdown voltage, but also the heat range requirement of the plug). Run any gap you want (.035 to .045), and the HEI will respond with the necessary breakdown voltage. Don't lose any sleep over this. 
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01-16-2008, 04:29 PM
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69427 has the correct comments above. A few other points regarding HEI and plug gap:
The HEI was developed and released as part of the emissions control package in 1975. The engines at that time were low compression engines, and they were set up to run very lean. A lean mixture is harder to "light off" than a "correct" mixture, so the HEI system was used so that a much wider plug gap could be used. This would ensure that the lean mixture would ignite and burn.
When a wider plug gap is used, there is more resistance across the gap, and more energy is required to jump the gap. Increased compression will also increase the resistance across the gap. The wide plug gap could be used on these engines because the compression was low. If you use a wide gap with high compression, you can get the resistance across the plug gap so high that the plug simply won't fire: the HEI spark will jump at the point of lowest resistance. This can become the plug wire boots, the inside of the distributor cap, or right through the plug wires if plug gap resistance (from the combined effects of gap and compression) is high enough.
Keeping this in mind, it is important to note that you cannot run an "HEI Plug Gap" (some gaps in the mid-70's were in the .060" range, if I remember right) on a high compression engine, even though you have the HEI system. You will, in fact, get plug misfires from the high energy spark jumping any point of lower resistance rather than across the plug. For this reason, most high performance applications with HEI (or other high energy systems such as MSD) use plug gaps in the .035 - .040" range. This assures that the spark stays at the plug and does not move to inside the cap... If you have a modest compression engine, you can run a wider gap.
The HEI was developed and released as part of the emissions control package in 1975. The engines at that time were low compression engines, and they were set up to run very lean. A lean mixture is harder to "light off" than a "correct" mixture, so the HEI system was used so that a much wider plug gap could be used. This would ensure that the lean mixture would ignite and burn.
When a wider plug gap is used, there is more resistance across the gap, and more energy is required to jump the gap. Increased compression will also increase the resistance across the gap. The wide plug gap could be used on these engines because the compression was low. If you use a wide gap with high compression, you can get the resistance across the plug gap so high that the plug simply won't fire: the HEI spark will jump at the point of lowest resistance. This can become the plug wire boots, the inside of the distributor cap, or right through the plug wires if plug gap resistance (from the combined effects of gap and compression) is high enough.
Keeping this in mind, it is important to note that you cannot run an "HEI Plug Gap" (some gaps in the mid-70's were in the .060" range, if I remember right) on a high compression engine, even though you have the HEI system. You will, in fact, get plug misfires from the high energy spark jumping any point of lower resistance rather than across the plug. For this reason, most high performance applications with HEI (or other high energy systems such as MSD) use plug gaps in the .035 - .040" range. This assures that the spark stays at the plug and does not move to inside the cap... If you have a modest compression engine, you can run a wider gap.
01-16-2008, 05:35 PM
#12
Racer
Thread Starter
Excellent Information! Thank You
69427 has the correct comments above. A few other points regarding HEI and plug gap:
The HEI was developed and released as part of the emissions control package in 1975. The engines at that time were low compression engines, and they were set up to run very lean. A lean mixture is harder to "light off" than a "correct" mixture, so the HEI system was used so that a much wider plug gap could be used. This would ensure that the lean mixture would ignite and burn.
When a wider plug gap is used, there is more resistance across the gap, and more energy is required to jump the gap. Increased compression will also increase the resistance across the gap. The wide plug gap could be used on these engines because the compression was low. If you use a wide gap with high compression, you can get the resistance across the plug gap so high that the plug simply won't fire: the HEI spark will jump at the point of lowest resistance. This can become the plug wire boots, the inside of the distributor cap, or right through the plug wires if plug gap resistance (from the combined effects of gap and compression) is high enough.
Keeping this in mind, it is important to note that you cannot run an "HEI Plug Gap" (some gaps in the mid-70's were in the .060" range, if I remember right) on a high compression engine, even though you have the HEI system. You will, in fact, get plug misfires from the high energy spark jumping any point of lower resistance rather than across the plug. For this reason, most high performance applications with HEI (or other high energy systems such as MSD) use plug gaps in the .035 - .040" range. This assures that the spark stays at the plug and does not move to inside the cap... If you have a modest compression engine, you can run a wider gap.
The HEI was developed and released as part of the emissions control package in 1975. The engines at that time were low compression engines, and they were set up to run very lean. A lean mixture is harder to "light off" than a "correct" mixture, so the HEI system was used so that a much wider plug gap could be used. This would ensure that the lean mixture would ignite and burn.
When a wider plug gap is used, there is more resistance across the gap, and more energy is required to jump the gap. Increased compression will also increase the resistance across the gap. The wide plug gap could be used on these engines because the compression was low. If you use a wide gap with high compression, you can get the resistance across the plug gap so high that the plug simply won't fire: the HEI spark will jump at the point of lowest resistance. This can become the plug wire boots, the inside of the distributor cap, or right through the plug wires if plug gap resistance (from the combined effects of gap and compression) is high enough.
Keeping this in mind, it is important to note that you cannot run an "HEI Plug Gap" (some gaps in the mid-70's were in the .060" range, if I remember right) on a high compression engine, even though you have the HEI system. You will, in fact, get plug misfires from the high energy spark jumping any point of lower resistance rather than across the plug. For this reason, most high performance applications with HEI (or other high energy systems such as MSD) use plug gaps in the .035 - .040" range. This assures that the spark stays at the plug and does not move to inside the cap... If you have a modest compression engine, you can run a wider gap.
01-16-2008, 05:36 PM
#13
Racer
Thread Starter
Thanks for the info!
Don't sweat the gap issue. The HEI system has more energy capability than a points system, and your cylinder doesn't know or care what you have for an ignition system, anyway. The cylinder merely sets up the conditions that the ignition system and components need to respond to (primarily gap breakdown voltage, but also the heat range requirement of the plug). Run any gap you want (.035 to .045), and the HEI will respond with the necessary breakdown voltage. Don't lose any sleep over this.
