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Spark plug gap for modified motor

blubuckaroo

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Jun 11, 2007
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My '77 has the factory Duraspark ignition system. The factory plug gap is .044".
However, the engine has had some mods...
9.7:1 compression
Edelbrock performer cam
Windsor Jr heads
Edelbrock intake and 4 bbl carb.

I understand that bigger gaps and higher energy came along to deal with the lean mixtures and lower compression that the factory engines came with from '75 to '77.

With these changes to my engine, should my gap still be the same?
 

jckkys

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Mar 15, 2012
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5,198
Higher compression does create more resistance, but the Duraspark II ignition should be able to overcome that and produce a good spark with .044" gap.
 

spap

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You can always get an aftermarket coil that could give you a hotter spark, if your concerned about the gap
 
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blubuckaroo

blubuckaroo

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Another issue...I've bought Accel shorty plugs to help clear the headers.
These plugs came gapped to .035". If I open the gap to .044", the ground electrode isn't parallel to the center electrode.
It's just a bit out of alignment. Is that important?

I've also sent these questions to the Summit and Accel tech lines, but they don't seem to care enough to send me an answer.
 

DirtDonk

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In the "old days" (about 20 years ago) Jacobs said that keeping the specific plugs gapped to what they're expected to be, in order to keep the side electrode as parallel as possible was actually quite important. That's why they would recommend different plugs in order to get some more appropriate to the gap you intended to run, rather than opening up the existing ones.
It wasn't a critical thing to have it slightly out, and certainly not the only parameter to adhere to, but not to be out by much either.
If they're limited to .035" then, I would probably not go over .040" or maybe even a little less.

That's just memory talking though. Been awhile since I read his book. Have a couple of copies around here somewhere though, if anyone is interested.%)

Paul
 

bronconut73

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In the "old days" (about 20 years ago) Jacobs said that keeping the specific plugs gapped to what they're expected to be, in order to keep the side electrode as parallel as possible was actually quite important. That's why they would recommend different plugs in order to get some more appropriate to the gap you intended to run, rather than opening up the existing ones.
It wasn't a critical thing to have it slightly out, and certainly not the only parameter to adhere to, but not to be out by much either.
If they're limited to .035" then, I would probably not go over .040" or maybe even a little less.

That's just memory talking though. Been awhile since I read his book. Have a couple of copies around here somewhere though, if anyone is interested.%)

Paul



That Jacobs stuff was pretty cool. I remember reading alot about that back in the day.
Don't see Jacobs stuff much anymore.
 

DirtDonk

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Yeah, they got bought out by Accel and buried. Kept the name up and running for a couple of years, but basically dropped it down a hole not long after that. Kept the cool ceramic plug wires as their own, but not much else.

It was cutting edge from the seventies to the mid-nineties. Loved my old setup.
Speaking of spark! I still like telling the story of my engine running a little funny once, so I knew something was wrong right away. Popped the hood and saw that I'd left my distributor cap OFF and hanging to the backside of the distributor.
The rotor was out there for all to see, spinning merrily away while sparks literally JUMPED A FIVE INCH GAP!!!!!! jumping over to the cap!
Even though not every spark hit the right contact point (which was why it was running rough) each one nailed a contact on the cap and the engine was running.
I laughed at my stupidity for leaving the cap off, but was glad I wasn't stupid enough to try to reattach it with sparks flying!
Turned off the key, popped the cap on, and drove off with the engine purring.
And never touched another distributor with bare hands again.

Or in the words of Mick Dundee: "That's not a spark... THIS is a spark!":cool:

Paul
 

bronconut73

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Paul,

I seem to recall a plug gapper that claimed to create the gap and keep the strap parallel.
Do you recall such a device that made such a claim.
 

DirtDonk

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Yep. I own two of them in fact.
It was a crimper type setup made by Jacobs and worked well. But if I remember, it was stated that it's only good for just so much change to the plug's designed-in shape. It helped to keep things parallel, but wasn't capable of making a silk purse out of a sow's ear. So to speak...;)
But maybe gold out of lead?;D

Paul
 

DirtDonk

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I think they're still available from different brands now. But a quick search didn't net any pics. I'll keep trying to find some, or take a couple of my own when I get the chance.

Paul
 
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blubuckaroo

blubuckaroo

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Messages
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In the "old days" (about 20 years ago) Jacobs said that keeping the specific plugs gapped to what they're expected to be, in order to keep the side electrode as parallel as possible was actually quite important. That's why they would recommend different plugs in order to get some more appropriate to the gap you intended to run, rather than opening up the existing ones.
It wasn't a critical thing to have it slightly out, and certainly not the only parameter to adhere to, but not to be out by much either.
If they're limited to .035" then, I would probably not go over .040" or maybe even a little less.

That's just memory talking though. Been awhile since I read his book. Have a couple of copies around here somewhere though, if anyone is interested.%)

Paul
Thanks for the help.
Those gapping pliers are pretty cool too.
I did a little research from an old Jacobs manual. The theory is, the voltage stacks up to a point when it can jump the gap. If the gap is too small, the arc can jump because there's enough voltage, but because there's not enough current available yet, the arc will be weak.
Also, the electrodes need to be parallel because the arc will form at the closest point in gap. That point will be burned off more quickly than if the whole surface was carrying the arc.
 

Broncobowsher

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Plug gap is a factor of ignition power and cylinder pressure. The more spark power, the wider the gap can be. The wider the gap the more air/fuel gets blasted with the spark. Too wide and the spark won't jump right.

Cylinder pressure has to do with the number of molecules of air and fuel are in that gap. The higher the pressure, the more molecules are squeezed into that gap. Put too many in there and the insulating factor will prevent the spark. Air is an insulator. Not a great one, but it is one. Being compressible you change the insulation based on pressure.

So back to your engine. You have a little more compression then stock, and a factory ignition. The added compression isn't that huge of a change. The stock spec allows for erosion of the tip as the plug ages. To be honest, the stock .044 gap will work just fine. If you are planning on leaving the plugs in for a very long time and lots of miles, tighten the gap a couple of thou, maybe .042"

Too much gap for the cylinder pressure is usually reserved for boosted applications. If you hear an engine breaking up and misfiring while under high boost it is usually called "blowing the spark out". The added cylinder pressure (talking double or more of original with double digit boost levels) is too much for the spark to jump.

In the low cylinder pressure smog motor era of the 70's and 80's you could see spark plug gaps of as high as .080". Huge gaps. But there wasn't enough cylinder pressure for that to be a problem. And the modern at the time electronic ignition had enough power to toss a spark through those kind of gaps. It did have a tendency to burn up the center electrode on the cap with the huge hits going through it continuously.
 
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blubuckaroo

blubuckaroo

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Thanks Broncobowsher,
Lots of good info and history here too.
My experience goes back to point systems. If you had an engine that misfired because of weak spark, closing the gap would usually help. My most recent experience with that was while hunting, my Trail 90 quit. Closing the gap got me back to camp.
At one time before electronic ignition systems, I put together a capacitive discharge kit from Radio Shack. That thing worked great. I passed it on from car to car till I forgot to take it off the last car I had it on. If I remember right, the instructions said to increase the spark plug gap by .005" when using the unit.
Now ignitions have come a long way and sometimes I feel left in the dust. ;)
 

jckkys

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Mar 15, 2012
Messages
5,198
The best part of Dr. Jacob's book was his step by step method to establish the optimum centrifugal and vacuum advance curves for your specific engine/application. Few understand how much free power under all driving conditions and added MPGs are available this way.
 
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