Spark Plug Gap- what's your gap?

[Bundy]

After my spark plug debacle I was wondering what the proper gap would be on my motor so I emailed World Products to ask them what gap I should be running:

I have a Ford M6009-B50 short block with your windsor jr heads on my 68 Ford Bronco and I don't know if I should leave the accel 276S plug gap at .035 or go up to .040. Any input would be great- thanks.

They sent me a link to wikipedia %)
http://en.wikipedia.org/wiki/Spark_plug#Spark_plug_gap

What are you guys running? I think the accel 276S come gapped at .035 but I bought a single equivalent and it was gapped at .040.

Any input?

 

[pcf_mark]

What ignition are you running? With high energy ignition (duraspark/msd/crane) and quality cap, rotor and plugs I would go all the way .050. Let the flaming begin because a lot guys like tight gaps but if you are not running boost you can open them up and pick up power and idle stability.

 

[blubuckaroo]

World products wouldn't know because they don't know what type ignition system or compression ratio you have. I ran into the same issue and called Accel. They told me I needed to leave the gap at .035" with my Duraspark and 9.5 compression. I only bought one short plug to use on #2 cylinder and the and a set of eight regular length for the rest. All came out boxes at .035". You would have a rough time gapping them bigger anyway. The ground electrode would end up at an angle to the center. There is an advantage to a smaller gap when using a carbureter. It doesn't flood as easily if the carb boils over, making restarts easier.

 

[Viperwolf1]

With high energy ignition (duraspark/msd/crane) and quality cap, rotor and plugs I would go all the way .050.

Duraspark isn't high energy. Just maintenance free.

 

[Broncobowsher]

Duraspark isn't high energy. Just maintenance free.

Still higher energy then points.

As others have pointed out, it depends on the engine and the ignition system. The higher the cylinder pressure (compression ratio, cam timing, boost, spray) the small the gap can be and still get the needed spark. Increasing the spark energy (energy isn't just voltage either) you can run wider gaps. Wider gaps light mole molecules of air/fuel then a small gap. Wider gaps produce higher loads on the ignition system and have been known for burning out the center eletrode on the cap I have seen some recomended gaps with GM HEIs on low compression engines as high as .080" to get the most out of them. I have also seen boosted engines hammered down under .020" to try and keep the spark when under boost.

In short, smaller gaps are easier to spark but the spark does less.

A good reference is to try and find an engine with the same ignition system and similer compression ratio/power level and see what spark gap is recomended for that engine. Don't care what the plug number is, the plug probably wouldn't even fit. Don't care about that, only the gap.

If you read the MSD instructions (it has been many years since I installed one) I recall they recomend opening up the gap about .005" above whatever it should normally be as a starting point. Then adjust it around and see what works best for your engine.

 

[jw0747]

very simple... .035 if you have a stock or close to stock setup. up to .050 if you've added a high energy ignition such as MSD, Mallory, Accel or if you've converted to efi. Duraspark would be considered stock.

 

[Bundy]

I have a M6009-B50 shortblock with 9.0:1 comrpession and Windsor junior heads-I run a Mallory Comp9000 unilite breakerless distributor and I am pretty sure there is an aggressive cam in there but I don't know for sure. Right now I have left the Accel 276s at .035" but I think I might play around with going up to .040" or higher if you guys think it would make the truck run better.

 

[1966bajabronco]

I am running a built 302 with Mallory comp 9000 dizzy, a mallory high output offroad coil, an very aggressive cam, heavily worked heads and a .045 gap. I had it at .035, but it runs much better at .045.

The engine is carburated.

 

[Bundy]

I am running a built 302 with Mallory comp 9000 dizzy, a mallory high output offroad coil, an very aggressive cam, heavily worked heads and a .045 gap. I had it at .035, but it runs much better at .045.

The engine is carburated.

That's about what mine is although I replaced the Mallory coil with the MSD blaster awhile back... I'm wondering if it is producing enough spark?

either way I am going to up the gap and see how she runs ;D

 

[1966bajabronco]

good luck!

 

[ilovemaui]

0.054, efi

 

[blubuckaroo]

Big gap or little... if it sparks it's going to fire. JMUO.%)

 

[Bundy]

Big gap or little... if it sparks it's going to fire. JMUO.%)

You don't think it makes a difference?

 

[pcf_mark]

The wider the gap the more of the arc is exposed to the air/fuel mix to increase the chances of lighting the fire. Every engine and ignition system misfires regularly. You are just trying to increase the odds in your favor of lighting something. But go too far and the spark does not jump the gap but finds a different path to ground (like another wire, crack in the cap, intake bolt etc.)

Bigger cams help you out because they have lower dynamic compression ratio (regardless of what the box of pistons say) so you can open up the gap to help you light off the overly rich mixture at low speed idle.

 

[blubuckaroo]

The wider the gap the more of the arc is exposed to the air/fuel mix to increase the chances of lighting the fire. Every engine and ignition system misfires regularly. You are just trying to increase the odds in your favor of lighting something. But go too far and the spark does not jump the gap but finds a different path to ground (like another wire, crack in the cap, intake bolt etc.)

Bigger cams help you out because they have lower dynamic compression ratio (regardless of what the box of pistons say) so you can open up the gap to help you light off the overly rich mixture at low speed idle.

You would have to prove to me that my engine misfires at all.?:?
Also, in an overly rich mixture it will be more difficult to for a spark to jump a wider gap. As I understand it, the manufacturers went to higher voltage/wider gaps when leaner running, lower compression engines came out.

 

[Broncobowsher]

... As I understand it, the manufacturers went to higher voltage/wider gaps when leaner running, lower compression engines came out.

Lower compression is a key point there. Also the introduction to more powerful and lower maintance eletronic ignition that can toss a spark across a wider gap. The bigger spark is needed due to being leaner and needing to get more of the fuel to ignite and propagate, avoid the lean misfire.

MSD was originally developed as a lean burn ignition system but gains were found on ordanary and high performance engines running the ignition system as well. Ignition system upgrades are one of the few things that can improve emissions, fuel economy and power all at once.

 

[blubuckaroo]

Another factor to consider is elevation. My carb is jetted for low elevation but I hunt at 8k-9k feet where the car sees a lot of idling along on bumpy roads. I've found that if I tighten the plug gap a bit, it runs a lot better overall.