Was curious how is a twin cam motor effected by spark plugs being gapped at .028 compared to what is called for which is .040?
Easier or harder to start? Better or worse on perfromance gas milage etc?
Thanks

 

Coming from an automotive background,I think smaller gaps are a step backwards for performance. As ignition systems got more powerful the gaps were opened up to expose fuel mix to larger spark.

 

Larger gap = higher voltage to initiate arc = "hotter spark" = marginally faster combustion in chamber = theoretically higher pressure in combustion chamber = theoretically more power. Theoretically speaking.

Can it be measured? Maybe.

Can loss of power with smaller gap be measured? Yes, but due to shrouding of spark.

Old Chrysler hemi's loved lots of spark gap and lots of advance. My experience.

 

Same with the shovelheads with the high domed pistons combustion chamber on the heads, with an aftermarket ignition system with hotter coils you increase the plug gap. Mine likes .040" instead of the stock .028"

On an evo or twinkie closing the gap works against you with the flat top pistons , leave it alone unless you've made big engine and compression upgrades and are race tuning.

 

Thanks guys this info does help and kind of makes sense. The motor is built up , its a 2002 superglide with a full Head Quarters 120 bulild including a 51 mm CV carb supertrapp exhaust , headquarters ignition ,10.5 :1 pistons etc basicly a full Head Quarters 120 bulild. Today I was pulling the plugs and realized gap was set to .028 . The heads have an electric decompression and 2 manual decompression s in each head but still I was finding it was exhausting on the non stock battery ,kind of a 3 in 1 chance of it starting. I talked to the previous owner/builder who is a good friend of mine about the plug gap but he wasn't real sure and said to try the .040 gapping.

 

Somebody may have been playing with the gap because of the increased compression. At 10.5-1 .040" should work fine, if that gives you a radical change not for the better decrease the gap .002" between tries till you find the happy spot.

 

Thanks , I'm mostly looking to get better starting without loosing the great performance.

 

You are fine with more gap especially with a hotter ignition. Sierra mo power to you,71charger 440six pak.

 

My shovels ran best with double-plugged heads and timed spark plugs. Coils were either 3 ohm or 6 ohm depending on the particular engine. Plug gap, like you say, worked best gapped .040".

 

Running dual plugs & single fire ignition on mine @ 11.7-1 compression, staying with 5ohm coils the 3's ate the plugs too quick. Had to retard the timing a hair but she likes the plug gap between .038 to .040" depending on brand.