Spark plug gap

[Mayhem]

350 Chevy, 10.5:1, 292/501 cam, aluminum heads, HEI distributor.

What would you gap your plugs at?

I was told .045, they came out of the box at .035, the old set were at .040-.041. Someone today said they thought .035, but everything I have read says .045.

However, it was screaming with the .040 set, and runs like crap with the .045. I do not want to do this again more than once.

What say you?

[Salvatore]

that much difference from .040 to .045? gap them at .040 again and stop reading. Have fun!!!

[PxTx]

Are you running any type of CD ignition? You do use up spark energy to "Jump" the gap, that is why a Mag has to run a tight gap- like .020 so while cranking the spark will actually jump.

I've found on CD systems, a larger gap works but on conventional .035 is a good place to start.

[Mayhem]

That is exactly what I was thinking. I just didn't want to find out it would be even better at .035.
The header makes it too much of a royal pain to keep messing with them due to the #6 plug being right behind the double tubes.
You have to pull the header to get to it at all. Also, the last rear header bolt on that side is practically up against the firewall.
I am putting Allen head bolts when I do this again. Thanks for the help.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: [email protected]</div><div class="ubbcode-body">that much difference from .040 to .045? gap them at .040 again and stop reading. Have fun!!! </div></div>

[PxTx]

Closing the gap actually affects your ignition timing. Closer the gap the more &quot;advance&quot; it gives you.

[Mayhem]

Accel HEI Dist. &amp; &quot;super-coil&quot; that mounts on the top of the cap.

You can kind of see it in this pic-




<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pxtx</div><div class="ubbcode-body">Are you running any type of CD ignition? You do use up spark energy to &quot;Jump&quot; the gap, that is why a Mag has to run a tight gap- like .020 so while cranking the spark will actually jump.

I've found on CD systems, a larger gap works but on conventional .035 is a good place to start. </div></div>

[Mayhem]

That's what I thought. You can definitely tell when driving at constant speed, like 45mph. Timing has changed.
I just REALLY want the right gap, and then I can tweak the timing from there for peak performance.
Thanks Paul.

Has anyone ever used the MSD Adjustable timing control that mounts inside, so you can fine tune while driving, as opposed to bumping the distributor a little and checking responsiveness?

MSD Ignition Control

I prefer that fine tuning method, also for peak performance, or at least to know where it is in comparison to correct timing placement, for track setting.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pxtx</div><div class="ubbcode-body">Closing the gap actually affects your ignition timing. Closer the gap the more &quot;advance&quot; it gives you. </div></div>

[Salvatore]

What is the timing at when the plugs are .040? Is it a big difference? You are right, you only want to do plugs once especially if they are really hard to get to.

[Mayhem]

It is recommended to be at 12-14 BTDC, but I honestly don't remember what it was at now.
I don't think it's a noticeable difference, maybe 2 degrees. But when driving, it now has an occasional miss, and not near the power it did.
I can put them back to .040 like the previous set, but I am being told .035 and .045, and really want to know which is right.
I can figure out what works best, but that involves a couple more plug R&amp;R sessions that I REALLY don't want to do.

[miket1]

Use an adjustable timing light so you can set total advance with timing all in on the top end, maybe 3000 to 3500 rpm or so, whatever rpm the advance is all in, then you will know your correct total timing advance, using different plug gaps.