Spark plug gaps

[Swinger 340]

You have it backwards as a rule of thumb. Normally more compression will tend to need a colder plug. More compression = more heat and in order to keep that heat in check, you will want a colder plug. But I always go by the "how it runs" and how the plugs look" deal, because there are exceptions to every rule.

Just like tires. You need to read them and then adjust. It's true what you say about the higher the compression the colder the plug. Hence, early 340's used N9Y champion with 10.5 to 1 compression, while the later 340's with its dropped compression around 9.5 use a N12Y or N14Y plug. The plugs wear can give you an idea on your compression ratio.

 

[go-fish]

@Bewy
I switched to NGK BKR6E at .060” gaps as suggested. 73 340, stock piston. Firecore 8mm wires, MSD RTR dust., Blaster 3 coil. QFT Slayer 750, 224/224 @ .050, 110 LSA, SFT.



AC Delco’s at .038”, 1,000 miles.

 

[Bewy]

I had the owner of the current engine I am building, buy new NGK plugs for it. They came pre-gapped at 060. The NGK part # had a suffix -15, which is the preset gap in mm [ 1.5 mm=060" ].

 

[go-fish]

I had the owner of the current engine I am building, buy new NGK plugs for it. They came pre-gapped at 060. The NGK part # had a suffix -15, which is the preset gap in mm [ 1.5 mm=060" ].

Man, mine were tight. Like .030”.
I have only idled it with the new NGK’s but I swear the exhaust doesn’t smell like I burned fuel nearly as much. Also, it idles a little smoother. This might just be placebo effect but I do believe there was less fuel smell in the exhaust.
Nice thing about these NGK’s is they are the 5/8” socket size. The big plugs work fine on headers and aluminum heads but the small plug provides more clearance with my 340 manifolds and heat shield.

 

[Bewy]

Go-fish,
Your findings do not surprise me at all. But you will get people on this forum & others that say big-ger plug gaps do nothing.....

 

[Swinger 340]

Go-fish,
Your findings do not surprise me at all. But you will get people on this forum & others that say big-ger plug gaps do nothing.....

As I have said before, it's the whole package that works best. A wider gap on the plugs, with a hotter coil should work best. Not just opening the gap. More gap, more fire to burn more fuel, but you need a hot spark. I run a Accel Super coil on my 340 van and give it a little wider gap. I'm sure there is better ways today, but It's been a 45-year set-up and had no issues.

 

[go-fish]

As I have said before, it's the whole package that works best. A wider gap on the plugs, with a hotter coil should work best. Not just opening the gap. More gap, more fire to burn more fuel, but you need a hot spark. I run a Accel Super coil on my 340 van and give it a little wider gap. I'm sure there is better ways today, but It's been a 45-year set-up and had no issues.

Sounds about right. I hesitated in my thinking (going wider gap) but as was stated, the Blaster II coil and the MSD distributor ALLOWS(?) for the facilitation of a wider gap and can form a stronger kernel of flame for a more complete burn. Perhaps with OEM spec ignition it won't facilitate that kernel.
I'll report back with pics of the NGK plugs and a longer evaluation of how it runs with the .060" gap after some miles.

 

[AJ/FormS]

My 11/1 Eddie-headed 367 runs Champion Coppers that were new in 99, and last I checked in 2004 were set around .045/050. They've never been out since, and I think they have about 100,000 miles on them.
The old girl revs to 7000 at least once, every time I take it out, which in my combo is 66mph in First-over. BadaBOOM!, lol.
The Coil is an old Accel Super, the one with the big square top, that was new in 99
The amp is a Jacobs, built-in to their opto-timer, that was new around 2002,
The system only makes one spark per event,
but the engine is as happy at 500 rpm as at 7500,
and this Ignition has NEVER failed.
I highly recommend this coil.

In that time, the engine has seen up tp 200psi cylinder pressure, three cams, three intakes, IDK how many carbs, and the TTIs have been on it from day-1,
Ok I thought about it, 5 carbs.
The Q has run very well at .028, also at .039, but is now at ~.032.

 

[fishmens67]

It's not the size of the kernel, it's the energy of the kernel at the proper time.
I prefer 28-30, works for me.

 

[Bewy]

I would NOT run more than a 045 plug gap...unless you are using an E core coil......& no ballast resistor.

Those old Mallory & Accel 'funny' looking coils were just dressed up canister coils, but the mugs fell for them. There is a reason you do not see canister coils on modern cars......
Those coils traded voltage for current, & it is current you want. It is the heat in the spark current that ignites the mixture....The voltage merely enables current flow.