383 spark plug gap
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Why? A bigger gap takes more energy to jump that gap, and then the duration of the spark falls off faster since it was used up front to get the jump. Unless you have a really big magneto that makes power up into the "joules" ranges, why? GM went from .035 to .045 when they went to HEI, but it was more for emission requirements than performance.
Burning Brakes
Joined: Feb 2003
Posts: 1,128
Likes: 4
From: Upland Ca
You should be using a plug and gap that the head manufacturer recommends. GM heads, use stock gap, after market heads use what they say. 350 or 383 no diff. After a few miles you should read the plugs and make changes based on color, etc.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
up to a point their is some additional power gain derived from additional gap and indexing the plug. Every motor is dfifferent. With high powered ignitions and big wires it is no problem to have gaps bigger than .080.
the more practical limit using digital multispark boxes and the best distributers and coils is nearer to .055 Max
the more practical limit using digital multispark boxes and the best distributers and coils is nearer to .055 Max
Last edited by gkull; Aug 4, 2009 at 11:28 AM.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
With a high energy ignition, a slightly bigger spark plug gap can sometimes help to burn mixture quicker and better.
With my MSD-6AL and Accel U-groove spark plugs, I have good results with a gap between 0,045 and 0,055". The plugs are much cleaner compared to 0,035" gap.
But remember, the higher the gap, the higher the stress on your wires and coil... Mine are set at 0,045".
With my MSD-6AL and Accel U-groove spark plugs, I have good results with a gap between 0,045 and 0,055". The plugs are much cleaner compared to 0,035" gap.
But remember, the higher the gap, the higher the stress on your wires and coil... Mine are set at 0,045".
Last edited by 73StreetRace; Aug 4, 2009 at 11:25 AM.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
You need to find a happy medium which suits your ignition system and compression ratio. A slightly larger gap will minimize mis-fire at idle, but could produce a mis-fire at high engine speeds.
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Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
For performance with gas, alky, or nitro utilizing electronic ignitions or points, .035 gap on the plugs. Smog and basically stock engines that most of them are, whatever the manufacturer suggests...but it would be better overall at .035......
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I never thought it mattered too much but because of a faulty distributor I had when I fired the 427ci I fooled around with spark plug gap and there was a noticeable difference in power from .035" to .045-.050". This is on a pretty nasty solid roller motor though your results may vary 
