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1970 Mustang Fastback
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Discussion Starter #1
Hi guys,

I need advice on determining what the spark plug gap should be on my car.
I've been reading around to look for clues, but have seen mixed answers from .032 - .035 and .040 - .050
So I'm a bit confused on what I should set the gap for my spark plugs to. I know it's not a one size fits all, but some clarity on what range I would be safe at would be nice.

I will be upgrading my ignition setup this weekend with
  • MSD Iridium Spark Plugs
  • Edelbrock Spark Plug Wires
  • MSD Blaster 2 Ignition Coil
  • MSD Pro-Billet Distributor
And here is the current engine build for reference
(302 V8)
  • Eagle 347 Stroker Kit w/ ESP
  • Comp Camshaft, Lifters, Rocker Arms, Push Rods
  • Crane Valve Springs, Retainers, Locks, Stem Seals
  • Bronze Valve Guides
  • Sealed Power Intake Valves
  • ARP Main Studs, Rocker Stud Set
  • Milled Cylinder Heads (Factory)
  • Cloyes Timing Set
  • Engine Rebuilt – Balanced, Ported, Polished
  • Holley 650 CFM Double Pumper
  • Holley Dual Plane Intake Manifold
  • Aluminized Headers
  • Flowmaster Dual Cat-back Exhaust
  • Pertronix Ignitor II, Breaker-less Ignition
  • Griffin Aluminum Radiator
  • Engine Chrome Dress Up Kit
  • Cobra Valve Covers
  • McLeod Super Street Pro Clutch
  • Hydraulic Clutch Conversion Kit
Any help would be most appreciated!
 

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Try .40 or .45, kind of middle of the road, take full advantage of the coil saturation for as much spark as possible for the most complete burn, and inspect your plugs til you get it all dialed in.
 
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.032"-.035" was the setting for stock points type ignition system. .045" is a very common setting for electronic type ignition systems.
 

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1970 Mustang Fastback
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Discussion Starter #4
Okay got it, thank you for the responses. I will play around with the gapping .040 - .045 to see what runs better.
 

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Ty, do you have the plugs yet ? Iridium spark plugs are a very expensive crutch for a weak ignition system. If you don't have, don't get them. If you already have them, throw them out. Get Autolite Copper plugs, and set the gap to at least .050. Tight gaps are also a crutch for weak ignitions. Your MSD is not weak. LSG
 

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1970 Mustang Fastback
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Discussion Starter #6
Ty, do you have the plugs yet ? Iridium spark plugs are a very expensive crutch for a weak ignition system. If you don't have, don't get them. If you already have them, throw them out. Get Autolite Copper plugs, and set the gap to at least .050. Tight gaps are also a crutch for weak ignitions. Your MSD is not weak. LSG
I'm a little confused about what you're saying. As the car is right now, it has Autolite spark plugs, not sure if they are the copper ones and stock distributor. But I plan to upgrade my ignition setup (parts listed in my first post), are you saying that running Iridium plugs will be detrimental to engine performance in the upgraded setup?
 

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Ty, I am saying that irridium is an expensive crutch that is only needed by a weak ignition. Keep the copper plugs. The Irridium is a waste of money if you are running an MSD. They won't cause a problem, they'll just lighten your wallet. LSG
 

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Ty, do you have the plugs yet ? Iridium spark plugs are a very expensive crutch for a weak ignition system. If you don't have, don't get them. If you already have them, throw them out. Get Autolite Copper plugs, and set the gap to at least .050. Tight gaps are also a crutch for weak ignitions. Your MSD is not weak. LSG
So, if you already have them, don't throw them out, just don't buy them again as the lesser expensive Autolite plugs work just as well for less money when you have a properly operating ignition system.

I set mine at .045 as I run a Pertronix Ignitor III w/flame thrower.
 

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1970 Mustang Fastback
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Discussion Starter #9
I get it now! Thank you for the clarification guys.
 

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I’ll regret to say I agree with LSG. I’d keep gapping wider until you see or feel any signs of cutting out under a aggressive load, detonation. Next wait to see when your mph drops then move them back about 5 thou. Watch your plugs. I like the autolite plugs that are light gray for tuning because you can read them.
 

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I’ll regret to say I agree with LSG.
Wow. Anyway, 0.045 is a good number for an improved ignition system. When your car is otherwise perfect and you have way too much time on your hands and are bored you might try bumping the gap up to 0.050.
 
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