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Plug gap on stock 302?

Blue71

Bronco Guru
Joined
Aug 27, 2001
Messages
5,144
I usually gap my spark plugs at .035 on my stock 302's. What do you guys gap yours? was wondering if this .035 is what I really need. I've just done it out of habit and I am open to change if need be.

Let me know.

Thanks Much,

Blue71
 

broncnaz

Bronco Guru
Joined
May 22, 2003
Messages
24,340
depends on what ignition your running. points .035 electronic .044 with either you can open them up a bit more but it usually doesnt make much of a differance in performance and the stock coil make not live long when opened to far.
 

RKM

Bronco Guru
Joined
Feb 1, 2004
Messages
1,077
Loc.
Huntingtown, MD
I would say no. However I don't have a ton of experiance with carbs. mostly I have found poor performance with the gap to tight and the same with it open to wide without a hot coil or MSD to push the spark across a bigger gap. Best performance is open it up as far as your ignition will let you to get the best performance.
 

recoiljunky

Bronco Guru
Joined
Jul 19, 2007
Messages
1,478
Loc.
Dothan, AL. USA
Not to hijack but...

...would you get any performance boost with Duraspark and a 45,000 volt coil like the one Wild Horses sells? Maybe it would allow you to open the gap further or will the Duraspark not support that many volts?
 

DonsBolt

Bronco Guru
Joined
May 5, 2005
Messages
5,249
Loc.
Chestnut Hill, Mass
I agree go at least .035


The only reason to close the gap would be if you are supercharged/turbocharged, and run a lot of boost.

When I go racing with my Lightning I close the gap on the plugs, as at over 20lbs of boost it sometimes blows out the spark, but for street driving I run a normal gap.
 

duganfife

Jr. Member
Joined
Jul 19, 2007
Messages
87
what is worse having your point gap and spark plug gap to close or to far apart?
 

Broncobowsher

Contributor
Total hack
Joined
Jun 4, 2002
Messages
33,193
Plug gap is a balance between available spark and cylinder pressure. The wider the gap, the bigger the spark the better it runs, more power, more economy. But there is a limit. Points ignition can only handle so much. .030 or so. That leaves a little room for the plugs to erode and still fire away. Once the gap gets too big the spark doesn't have enough to jump the gap.

Now most of us have seen a spark plug wire jump a gap an inch wide, why can't we run an inch of gap on the spark plugs? Cylinder pressure. A few hundred PSI of compression packs a bunch of molecules between those 2 eletrodes. That makes it real hard to jump that gap. So we make it smaller so it can jump under those high loads. Also why high compression/ supercharged/ turbo cars run a smaller gap. The spark can only jump through so many molecules of gas. The opposite of this is a neon light. It is only 12,000 volts or so, but it will arc through feet of tubing. The trick, good vacuum and just a little gas.

So the doggier the engine (lower cylinder pressure) and the hotter the spark (eletronic will toss more spark then points) the wider the plug gap can be and should be.

Start building a lot of cylinder pressure and you will need to close the gap on the plugs. As the blower crowd will say when running a lot of boost it is blowing the spark out. That is the blower pushing so much air that the spark can't jump the gap at the pressures created. Either a closer gap or a hotter spark (top fuel dragsters run a nasty magneto system to handle there pressures)

All that boils down to points gap around .030 and eletronic gap to about .045
The increase in gap is most of the performance gain in going eletronic.
As for a higher voltage coil, It won't make that big of a difference most of the time. What you really need is enough voltage to light the spark. Then a bunch of joules to keep the spark fat and happy

Joules, Oh another can of worms I opened here. 1 Joule is 1 watt over 1 second. A watt is a volts times amps. So volts x amps x seconds is joules. Upping just the voltage but sacrificing amps and time generally yields less spark energy as the used voltage at the plug is only as high as needed to fire the plug then drops to a sustained spark level before running out of energy and the spark going out. Too much gap and the spark will fall off and not empty the coils worth of energy into the plug. Too small and the arc doesn't present itself to as much flamable material as would be desired.

I need a beer
 

airman

Contributor
Bronco Guru
Joined
Nov 26, 2003
Messages
1,834
I need a beer

If you ever think you are finished working on your Bronco, just search for a post by Broncobowsher (nobody thinks it through like you) and you will find yourself drinking beer, getting out your leakdown tester and gapping a new set of plugs when all you really needed was the beer. I think I gapped mine at .040 and I have HEI with an advertised 50K volts. I think I'll change to bigger now. I love this place! My wife hates this place!
 

Swaayze

Jr. Member
Joined
Oct 9, 2003
Messages
136
Loc.
DFW, TX
Interesting. I just set my new plugs to 0.035 for my '73 but it has a Pertronix so I guess I should open 'em up a little (say 0.040 or ?)? Also wondering about the timing for a stock '73 302? I saw 6 BTDC somewhere but mine's set about 8 right now; worth adjusting?
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
43,179
Not to hijack but...

...would you get any performance boost with Duraspark and a 45,000 volt coil like the one Wild Horses sells? Maybe it would allow you to open the gap further or will the Duraspark not support that many volts?

Are you saying you alreay have a Duraspark system and just wonder about a better coil? Or are you wondering if a Duraspark and coil will improve over an old points setup?

If just the coil, then it depends on what you've got now. If an old stock coil, then probably yes, a new high-output coil might just improve your spark and therefore performance/economy.
No guarantees of course, but in my book it's almost always worth an ignition upgrade if you're running old original stuff.
Will you feel it with more HP? Maybe, but probably not. But it might run smoother and get a tiny bit better mpg. You never know.
And yes the Duraspark can handle lots of voltage. That's what the large diameter distributor cap is all about. The electronics, like the trigger and the module, shouldn't care one way or the other how much voltage your coil develops.

If you meant upgrade the whole setup to Duraspark. Hell yes!
Duraspark, HEI, Pertronix, ACCEL? Any of the above is better than points.
I happen to still like points (in their proper place), and can still gap 'em in nothing flat. But they'll never hold a candle to a good modern electronic trigger for spark output and reliability.
Oh, and the big thing? YOU NEVER HAVE TO ADJUST POINTS AGAIN.
Priceless.
Just changing to electronic even with the same coil will increase your coil's output voltage. It's the "speed" of the switch (points, magnetic, hall-effect, optical, etc.) that helps determine a great deal of how well a coil works.
And did I mention that you never have to clean/change/gap points again?

Hope that answered your question. At least sort of anyway.

Paul
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
43,179
what is worse having your point gap and spark plug gap to close or to far apart?

With point gap you don't really have much say in it's size. You're supposed to keep it within a certain operating range to keep the "dwell" (how long the coil builds up steam) within it's optimum range. Also too much might keep the points from closing at all. Too little might keep them from opening. There's a limit to what they can take.
So smaller OR larger gaps for points is not going to be good.

As for plugs, I think the other guys kind of laid it out.
Too small = not enough spark size to reliably light the fire in the cylinder.
Too large = takes more engery to jump the gap and easier to blow out. Thereby also causing unreliable firing. Both of which is what you feel as a "miss" and rough running, lower mpg, lower power. all of that happy crap.

Paul
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
43,179
Interesting. I just set my new plugs to 0.035 for my '73 but it has a Pertronix so I guess I should open 'em up a little (say 0.040 or ?)? Also wondering about the timing for a stock '73 302? I saw 6 BTDC somewhere but mine's set about 8 right now; worth adjusting?

Could be right. But it varied by year and model. So not all 302's are the same.
In fact, I found with my stock '71 that it would tolerate a bunch of extra timing. Might be the way I had it jetted or the amount of mechanical and vacuum advance, but I could run it up to 15 or 16 sometimes and it wouldn't ping.
I actually tried to get it to ping once and ended up pushing the advance so far that it would barely run. But it didn't ping!

So I would experiment with a degree or two more advance now and then. See what you get. Test it a bit at a time until you think it's running better or worse.
If worse, back it down a notch or two until you get back to where you felt it worked the best.
Mine always ran best all-around at about 12 BTDC.

Paul
 

Swaayze

Jr. Member
Joined
Oct 9, 2003
Messages
136
Loc.
DFW, TX
Yeah, that's the problem. All the references say to check the emissions sticker on the truck which is long gone. I'll play with it some, thanks.
 
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