Big plug gaps mean big power?

Big plug gaps mean big power?

Simple advice for beginners –

By Archie Bosman:  

 

Erik Brock of MSD

Erik Brock of MSD

“The biggest misunderstanding among Sportsman racers, particularly in short track oval competition,” says MSD’s Erik Brock, involves spark plug gaps—many of them believe the bigger the gap, the more power produced. But this is not the case.

“If it’s a mildly tuned engine it may not matter; the ignition system will usually do the job—especially if it’s ignited by a capable ignition box and coil—even though it has to work harder. But if it’s a race engine with compression ratios of 13 or 14 or 15:1, with 0.030in or 0.040in plug gaps, the ignition system is greatly disadvantaged. Generating a spark that’s compelled to jump a wide plug gap is inviting trouble. It’s even tougher if the engine is fueled by alcohol where larger volumes of fuel are involved,

“Often racers come to the MSD trailer with the same issue: the car won’t rev on the straightaway or it won’t come up to the rev limiter,” says Brock. “I ask them their compression ratio and their plug gap? Often they’ll tell me it’s 0.045 or 0.050in. Nine times out of 10 when they close the gap by 0.010in the car runs better. By closing the plug gap they reduce the load on the ignition coil. But this also reveals a weakness in the ignition system and at this point they realize they need a better coil.”

 

“Most engine builders know about plug gaps. Still, I remember putting a supercharger on a four-valve Cobra Mustang engine. When it was naturally aspirated it ran low compression with 0.046in plug gaps. But the blower generated 28lbs boost and beyond 5,200rpm the engine was spent—afflicted by interference similar to a rev limiter. I finally solved the problem when I closed the plug gaps to 0.020in.” – Chuck Lawrence

“Most engine builders know about plug gaps. Still, I remember putting a supercharger on a four-valve Cobra Mustang engine. When it was naturally aspirated it ran low compression with 0.046in plug gaps. But the blower generated 28lbs boost and beyond 5,200rpm the engine was spent—afflicted by interference similar to a rev limiter. I finally solved the problem when I closed the plug gaps to 0.020in.” – Chuck Lawrence

Grounding and welding

By nature, circle track cars get clobbered with mud and then blasted with water in the car wash. Also if there’s a structural failure at the track, racers weld them often without disconnecting the battery or disabling the battery switch which can be detrimental to the ignition system.

“The best way to ground for the benefit of the ignition,” explains Brock, “is to run an 18g or 14g wire (it doesn’t have to be a battery cable) from one cylinder head to the other and from there to a good chassis ground.

“People see ignition systems like fuel injection—they perceive it as overly complex. It isn’t. The most common troubles with circle track cars include using incorrect plug gaps and incorrect coils. Ignition isn’t magic, it’s basic.

General rules

“When you increase compression or cylinder pressure by adding boost or nitrous the plug gap has to be reduced accordingly. Turbo and blower cars running substantial boost on alcohol perform their best with plug gaps of around 0.019in.” On mildly tuned engines Brock recommends 0.030in to 0.035in.

Source

MSD
El Paso, Texas
(915) 856-2420
www.MSDignition.com

10 Comments

  1. Back in the 70’s I was IMSA racing at Daytona. MSD ignitions were the new thing. The talk was plug gaps of .060″. Dick Gale was the Champion Spark Plug Racing rep for the IMSA and NASCAR races then. I was skeptical and I asked Dick his opinion of the wide plug gaps. He looked at me and said, “If it goes to missing you will wish it was less!”
    I have stayed with plug gaps of .022″-.025″ ever since.
    One IMSA Championship and four Daytona 24 hour wins since.

    Racesports Performance Engines

    Reply
    • What plug gap would be best on a 10:1 compression engine with an MSD digital 6AL ignition box?

      Reply
      • It’s hard to say exactly, but .030″ is a good place to start, generally speaking.

        Reply
  2. I run a Dirt Modified with a 350 on 9.5:1 compression ratio and a 2bbl. We run E98 ethanol and try to turn around 7,000 rpm. We run an MSD Pro Billet distributor. What plug gap and heat range would you recommend?

    Reply
    • Terry,
      Erik Brock is now with HPL Oil, hence, no longer at MSD. However, I contacted him this morning regarding your question. Here is his response. “Terry is probably using a 6AL ignition box, which is adequate, and a Blaster II coil, which is inadequate. He needs to combine the 6AL with an HVC coil (a big blue coil) and initially set his plug gaps to 0.030in.” Hoping this helps

      Reply
      • I’m not using a 6AL box; it’s not allowed in Wissota B Mod. The reason I’m asking this question is we are having top-end issues, a high-end miss I guess is what you could call it, and we are running hot.

        Reply
        • Terry,
          I do have a couple of questions:
          Are you running an MSD HEI-type distributor (with the coil mounted on the cap)? If not, what coil are you using?
          What is your current plug gap?
          How do you have the cylinder heads grounded?
          Which ignition wires are you using?
          Are you running a 12 or 16 volt electrical system?

          Reply
          • It is MSD HEI Pro-Billet distributor coil on top. 0.025in plug gap. No rubber on the motor mounts, so I’m going to assume the heads are grounded to the chassis like everything else. Not sure on the model but the wires are MSD 8mm. 12 volt electrical supply

          • Terry,
            The HEI is limited to power output without the use of an external ignition box (which is not permitted) or increase battery voltage. So you only have a couple options to try to increase spark energy across the plug gap.
            The first thing I would do is try closing the plug gap an additional 0.005in and measure the results.
            Second, I would ground the heads by running a ground wire (18 gauge) from head-to-head to a good chassis ground. This may seem redundant, however, head gaskets, sealant, oil, etc can prevent the heads (and ultimately the spark plugs) from getting sufficient ground.
            Third, I would put a fresh set of ignition wires with low resistance (.50 ohms per foot or less) and good noise suppression. Either the AFIS 8.5mm or MSD 8.5mm wires are an excellent choice.
            Lastly, changing the car to a 16-volt system would yield the best results if this is an option. Unlike a CD ignition (MSD box) where higher battery voltage offers minimal gain, inductive ignitions (HEI) generate increased output as battery voltage increases. They also lose output as battery voltage drops.
            Hope this helps.

  3. Thanks Eric we’ll try your suggestions.

    Reply

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