HEI spark plug gap - Chevelle Tech
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post #1 of 33 (permalink) Old Aug 8th, 16, 12:21 PM Thread Starter
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HEI spark plug gap

Just curious on what gap you guys are running with HEI. I'm running .045.
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post #2 of 33 (permalink) Old Aug 8th, 16, 1:29 PM
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Re: HEI spark plug gap

I use to gap .04-.045 with both HEI and ignition boxes like msd 6al.

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post #3 of 33 (permalink) Old Aug 8th, 16, 2:32 PM
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.035. Hot dense spark!
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post #4 of 33 (permalink) Old Aug 8th, 16, 3:43 PM
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Re: HEI spark plug gap

Guys that know more than me recommend reducing the gap above 10:1 compression. Do a search on this forum and you will find more info.
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post #5 of 33 (permalink) Old Aug 8th, 16, 4:03 PM
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Re: HEI spark plug gap

For stockers with stock GM or Wells modules I have ran .085-.045" no difference in MPG or power.

I did a Back to Back test with same timing curves GM HEI with White Module VS GM single point that had a good point cam and CS89 NAPA point set. this was a 245 psi cranking pressure 350 shifting at 6900rpm. conservative tune.
other tunes I would shift at 7400-7800..The HEI was a putz up there.. may have needed to tighten the gap past .035"

The HEI was slower by 2 tenths in the 1/4 mile vs the point.. stock Gm coil vs a Ford coil for the single point.

I had to gap the plugs to .035" for that deal.

I now run .035" on about all of the HEI stuff..But I run factory HEI with Factory modules.
Hard to find the old modules.


Some say the factory ones will retard timing at a certain RPM..I have watched my timing with a light all the way to 6500rpm and never seen it retard.

Another note..95 corolla always had a miss fire feeling.. all plugs showed the carbon tracking lines on the porcelain.
Tried 4 different kinds of plugs. plug gap on that car was .032" .

I finally tried setting them at .025".. missfire is now gone and picked up MPG.. was getting 33.5 now getting 35 MPG.
280,000 miles..so I have tried a lot of tweaks on it.

my 10 cents worth.
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post #6 of 33 (permalink) Old Aug 8th, 16, 4:27 PM
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Re: HEI spark plug gap

Quote:
Originally Posted by jeff swisher View Post

my 10 cents worth.
Jeff's 10 cents is worth a couple hundreds of my advice
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post #7 of 33 (permalink) Old Aug 8th, 16, 7:01 PM
 
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Re: HEI spark plug gap

my 6al instructions read ......

These examples are just staring
points to get you going in the right
direction.Every application is
different and should be tested and
tuned.

Compression Spark Plug Gap

Up to 10.5:1...0.050" - 0.060"
10.5:1 - 13.0:1...0.040" - 0.050"
Above 13.0:1...0.035" - 0.045"
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post #8 of 33 (permalink) Old Aug 8th, 16, 7:22 PM
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Re: HEI spark plug gap

Quote:
Originally Posted by jeff swisher View Post
...GM HEI with White Module ...
.
Jeff, van you elaborate on the different modules available for the GM HEI?

For instance; what module comes with the GM PP (performance parts) distributor?
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post #9 of 33 (permalink) Old Aug 8th, 16, 7:55 PM
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Re: HEI spark plug gap

No Idea that is newer stuff I run old stuff that came on factory vehicles.

All I can say is if you have a module then install it and test it out.

I tested a couple many years ago in the late 80's on a big Allen scope.. watched the spark graph You would have a big voltage spike around 45,000 volts at idle then as that spike was finished you would have 6 or so more small spikes each time a little smaller.. than Nothing during dwell time ..then Bam another 45,000.

Now as R's increased the peak voltage was getting less and less by 4500 rpm it was about 15,000 on some modules.

My single point distributor would have less initial but at 4500 RPM it would have more than the HEI.

I can't tell you part numbers as I never cared to look at them,, figured either it will perform or it will not.

Just like today some praise the MSD.. another guy gets the same thing and nothing but trouble from it.

I figured no 2 were the same..The circle track guys all talked up the white Module ,, which was from Pontiac 400 and some Oldsmobiles when lean burn was the way to go from factory and plug gaps from factory were in the .060" or more.

Took a good module to last in that environment.. so I used those and tighter gaps.

You know out of all those old factory GM modules I only had 1 quit on me.
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post #10 of 33 (permalink) Old Aug 8th, 16, 9:47 PM
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Re: HEI spark plug gap

Higher compression will take more voltage to jump the gap...Either reduce the gap or increase the coil output..It's that simple. It's very easy to see on an oscilloscope. Thats why when checking for a good spark you should really use a spark tester that stresses the coil. It only takes about 3 KV to fire a plug not under compression. A GOOD distributor like a DUI that has a GOOD MODULE and GOOD coil will allow you to open up your gap and restore your idle quality.

Note the different in the spark line height on this waste spark system in the link below... One coil fires 2 cylinders at the same time but one is on the exhaust stroke and the companion cylinder is on the compression stroke.

https://encrypted-tbn1.gstatic.com/i...Rmkzea8aMiYchw

Last edited by Importtech; Aug 8th, 16 at 10:16 PM.
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post #11 of 33 (permalink) Old Aug 9th, 16, 10:47 AM
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Re: HEI spark plug gap

Formerly at 10.75:1 street, I gapped at .043. I still do , although I;m down to 10:1 ( yet cranking PSI or DCR remains the same if not higher)
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post #12 of 33 (permalink) Old Aug 9th, 16, 9:32 PM
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Re: HEI spark plug gap

PLEASE, this has been gone over, and over, and over.............10.00:1 and lower, .045, more compression, more pressure from boost/nitrous, close it down, this is for ALL HEI, MSD and other ignitions.

And, if you want to believe wives tales, then all the different HEI couls and n=modules are all different, when actually, they allowed the same, same, same, same, same....SAME on every rtest device I have ever tested them on, and I am in that business.

I have seen, and heard every story in the book on plug gaps, coils, modules, and they all have the same outcome, they all perform the same, and, the coil in cap FAILS, which murders modules, not the other way around.

I am surprised someone hasn't got a weekly comedy show on TV festurng each week's new whizz-Bang HEI break through...THAT ISN'T.
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post #13 of 33 (permalink) Old Aug 9th, 16, 10:24 PM
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Re: HEI spark plug gap

Have no idea what your rant is about however at the risk of offending an expert all modules and coils are not created equal.... Hotter coils DO have the tendency to fail quicker and especially when they have no air flow or oil. Cheaper electronics tend to fail more often. There is no exact idea gap for anything...too many variables. I could live with .040 -045 at 10.5-1 with a good distributor and sleep just fine.
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post #14 of 33 (permalink) Old Aug 9th, 16, 11:30 PM
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Re: HEI spark plug gap

Quote:
Originally Posted by jeff swisher View Post
No Idea that is newer stuff I run old stuff that came on factory vehicles.

All I can say is if you have a module then install it and test it out.

I tested a couple many years ago in the late 80's on a big Allen scope.. watched the spark graph You would have a big voltage spike around 45,000 volts at idle then as that spike was finished you would have 6 or so more small spikes each time a little smaller.. than Nothing during dwell time ..then Bam another 45,000.

Now as R's increased the peak voltage was getting less and less by 4500 rpm it was about 15,000 on some modules.

My single point distributor would have less initial but at 4500 RPM it would have more than the HEI.

I can't tell you part numbers as I never cared to look at them,, figured either it will perform or it will not.

Just like today some praise the MSD.. another guy gets the same thing and nothing but trouble from it.

I figured no 2 were the same..The circle track guys all talked up the white Module ,, which was from Pontiac 400 and some Oldsmobiles when lean burn was the way to go from factory and plug gaps from factory were in the .060" or more.

Took a good module to last in that environment.. so I used those and tighter gaps.

You know out of all those old factory GM modules I only had 1 quit on me.

I use to know the guy that designed that Allen test scope! His name was Mac. Smart man! Einstein smart!

When he left Allen he went to work on his own and designed a nice adapter box that you could use to test the new (at the time) DIS coil systems with that monster Allen scope, that came out in the 90's.

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post #15 of 33 (permalink) Old Aug 10th, 16, 8:41 AM
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Re: HEI spark plug gap

How much does it change the idle quality going from .035 to .045?
Is it really noticeable?

Jim

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