Spark plug gap overview
Thread Starter
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Stellar HDF Member
Joined: Apr 2009
Posts: 3,039
Likes: 11
From: Northern Ontario
Spark plug gap overview
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
Easier or harder to start? Better or worse on perfromance gas milage etc?
Thanks
Ultimate HDF Member
Joined: Jul 2014
Posts: 8,251
Likes: 3,990
From: Windwardside, Saba N.A.
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.
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.
Club Member
Joined: Mar 2009
Posts: 66,131
Likes: 51,461
From: Niles Canyon Ca.
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.
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.
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.
Thread Starter
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Stellar HDF Member
Joined: Apr 2009
Posts: 3,039
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From: Northern Ontario
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.
Club Member
Joined: Mar 2009
Posts: 66,131
Likes: 51,461
From: Niles Canyon Ca.
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.
Thread Starter
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Stellar HDF Member
Joined: Apr 2009
Posts: 3,039
Likes: 11
From: Northern Ontario
Thanks , I'm mostly looking to get better starting without loosing the great performance.
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Tourer
Joined: Jun 2014
Posts: 335
Likes: 48
From: VW Ohio
You are fine with more gap especially with a hotter ignition. Sierra mo power to you,71charger 440six pak.
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Ultimate HDF Member
Joined: Jul 2014
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From: Windwardside, Saba N.A.
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.
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.
Club Member
Joined: Mar 2009
Posts: 66,131
Likes: 51,461
From: Niles Canyon Ca.
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.