spark plug gapping - Pontiac GTO Forum
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post #1 of 15 (permalink) Old 04-04-2012, 01:45 PM Thread Starter
 
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spark plug gapping

Hey guys! Spark plug gapping is something so small but also so crucial. What is the correct gapping for a 67 Pontiac 400 with a crane cam ,HEI distrubtor and sterling pistons. Thank you.
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post #2 of 15 (permalink) Old 04-04-2012, 01:58 PM
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It's less about "correct" than it is "optimal"... You want a gap that's wide enough to get a good fat flame started, but not so wide that being able to get a spark generated across the gap becomes iffy. Compression ratio and cam (dynamic cylinder pressure) along with ignition system (available "oomph" to create a spark), fuel type, combustion chamber characteristics, plug type, even head material (aluminum vs. cast iron) all come into play.

For an HEI system on a 400, I'd start with .045 on the gap and see how it acts.

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post #3 of 15 (permalink) Old 04-04-2012, 02:03 PM
 
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Yep 045 to 060 I been told.



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post #4 of 15 (permalink) Old 04-04-2012, 08:06 PM
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.045" is what I would run. But that's just me. Run what you wanna.
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post #5 of 15 (permalink) Old 04-05-2012, 05:10 AM
 
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Hi. I'm one of the new guys on the block just reading along here. For my 2 cents my drag racing guy runs NOS and is into it pretty big checks the gap after almost every run and plays with it based on the apperance of the burn on the plugs. As Bear stated it is what is "optimal". Back in the day for tune-up specs the books gave a .030" gap for the 335 hp 400 running AC RS44's and the 360 hp was .035" with AC RS45's. Course if you try to find that info now the books just recomend .035". Running HEI ignition would change that and you would need to experiment with what works best. Takes time. Hey thanks for putting up with my dribble.

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post #6 of 15 (permalink) Old 04-05-2012, 01:09 PM Thread Starter
 
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thanks for all the feedback guys! great info once again!
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post #7 of 15 (permalink) Old 04-05-2012, 02:23 PM
 
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Not to hijack but my motor is similar. What part number spark plug do you recommend and what brand?

1969 GTO clone, stock 400 motor and 400TH transmission
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post #8 of 15 (permalink) Old 04-06-2012, 04:04 AM
 
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pontiacsniper - being a GM guy I have to say AC/DELCO for the brand. When I was running my car and maintaining it I found that AC's lasted about twice as long as the other brands of the 60/70 era. I'm going to be getting into the maintainance again on the car as I get it restored so I will probably find out the technology has got better on other brands. So if someone wants to let us in on the techo stuff it would be nice.

Bill

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post #9 of 15 (permalink) Old 04-06-2012, 01:18 PM Thread Starter
 
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Original Goat is correct i used AC/Delco i got all 8 for like 16 bucks. As soon as im by th car again ill see what number they are.
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post #10 of 15 (permalink) Old 04-24-2012, 02:04 AM
 
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spark gap is based on what plug you use and what distributor you have.
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