sparkplug ?

[BigBlueIron]

390, propane, boost, street driven.

Currently using an Autolite 124 plug, which seems to be working ok. But I want to use an extended or projected tip style. I'm finding the AP7FS and A6FS Can anybody make a recommendation besides these?

Also do I need a resister style plug? Only electronics are the ignition box, fans and lights. Or does it really matter for my application.

 I do prefer Autolite or NGK brands.

Mallory box and coil MSD wires.

[cjshaker]

With modern plug wires, I don't think a resistor plug would do anything except hinder spark. I don't run them on anything I have, and I have no issues with electrical interference. I can't speak about a projected tip, except I'd be leery of piston contact, or worse, creating a hot spot within the combustion chamber. Maybe that's not an issue with propane?

[BigBlueIron]

Thanks,

Current piston has gobs of room, propane is harder to ignite then gasoline which is compounded under boost. On the first setup I had terrible spark blow out even at low boost levels, tightened the gap up and that helped considerably. A hot spot maybe something to consider, just trying to get the plug into the chamber at least as far as an original 45 plug. The 124's are a recessed tip.

[machoneman]

Thanks,

Current piston has gobs of room, propane is harder to ignite then gasoline which is compounded under boost. On the first setup I had terrible spark blow out even at low boost levels, tightened the gap up and that helped considerably. A hot spot maybe something to consider, just trying to get the plug into the chamber at least as far as an original 45 plug. The 124's are a recessed tip.

Any MSD like spark box?

[BigBlueIron]

Yes, Mallory #685 and Mallory promaster square coil. Can't remember the part number. One of those big old ugly square coils.

[cjshaker]

If propane is so hard to ignite, then why use a 124, which is a much colder plug? I'm no expert on propane, but I do know it burns cleaner than gas, so deposits shouldn't be an issue.

[BigBlueIron]

With a gaseous fuel there is no latent heat of vaporization as with gasoline. It is already a vapor when indued. So higher cylinder temps. So contrary to gasoline a rich mixture raises temps even farther. Yes it does burn clean one major benefit, but its almost impossible to conventionally "read" a spark plug.

On the plus side it is a homogeneous mixture. So that fact helps with ignitability but the ignition window of air fuel mixture is narrower. Its amazing how low the idle can go once running lpg.

[Ford428CJ]

390, propane, boost, street driven.

Currently using an Autolite 124 plug, which seems to be working ok. But I want to use an extended or projected tip style. I'm finding the AP7FS and A6FS Can anybody make a recommendation besides these?

Also do I need a resister style plug? Only electronics are the ignition box, fans and lights. Or does it really matter for my application.

 I do prefer Autolite or NGK brands.

Mallory box and coil MSD wires.

 Yep! I have used NGK AP6FS through AP8FS (coldest). They work extremely well!!! Those are some of my first picks for plugs