Q&A / Tech

Mailbag: Top 3 Ways to Cure a Pre-Ignition Condition

Q: I have a Ford truck with a 390 engine. I had the heads redone and installed a new cam and lifter set, true roller timing chain, valve springs, single plane Edelbrock intake, adjustable rocker arms, and headers. I use the truck to haul my 11 ½-foot camper.

When I reach highway speeds and keep the rpms steady, I hear a sound like a chain or marbles inside a coffee can. When I let up on the gas, the sound goes away, and when I hammer the gas hard, it goes away. I drove 1,000 miles with this condition. Is it the cam walking and hitting the timing cover? Could it be the lifters or valve springs?

A: Your sound does not seem to be caused by a mechanical problem. If it was lifters or valve springs, the sound would be constant, not just when you’re cruising. The problem sounds more like pre-ignition. There are several ways to cure pre-ignition:

  • Run higher octane fuel. Premium gas rated at 92 or 94 octane is best for an engine with a compression ratio between 9.25 and 10.25:1. Using premium can be expensive, especially in vehicles that use a lot of fuel.
  • Run the engine on the rich side. You can do this by using larger jets in the carburetor, or by running a colder range spark plug than the one you are using now.
  • Try playing with ignition timing. Most engines like to run more timing than stock specs call for. If you run too much initial timing, you will get pre-ignition; too little and you lose performance. We recommend starting initial timing at 10 degrees BTDC and adjust from there.

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3 Comments

  1. Ken Freund says:

    Pre-ignition is caused by things like glowing metal, such as a spark plug electrode or metal burr or glowing carbon in the combustion chamber. What you are describing is detonation, not pre-ignition. Detonation occurs due to excessive heat in the combustion chamber which ignites the fuel prematurely. It is caused by too much compression for the octane, or high temps due to a bad cooling system, etc. It can be helped in some cases by a richer mixture or by retarding the spark timing. Changing to a colder plug does NOT change the fuel mixture! I am a former technical service engineer for a major spark plug manufacturer.

  2. This is actually detonation the tail end of the fuel mixture (farthest away from the spark plug ) igniting after the spark plugs fired from excessive heat, high compression and wrong fuel, plus more reasons. The district ping is caused by the two explosions meeting each other and resonates through the block (fun fact you’re not hearing the explosion youre hearing the metal resonate )

    pre ignition makes no noise because the fuel ignites well befor the spark plug which is why its so dangerous

    My proof that this is true

    . Pre-ignition caused by a ignition source i.e overheated spark plug tip, carbon deposits in the combustion chamber and, a burned exhaust valve; all act as a glow plug to ignite the charge.

    pre-ignition. The charge enters the combustion chamber as the piston reaches BDC for intake; the piston next reverses direction and starts to compress the charge. Since the spark voltage requirements to light the charge increase in proportion with the amount of charge compression; almost anything can ignite the proper fuel/air mixture at BDC!! BDC or before is the easiest time to light that mixture. It becomes progressively more difficult as the pressure starts to build.

    A glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. It is very conceivable that if you have something glowing, like a spark plug tip or a carbon ember, it could ignite the charge while the piston is very early in the compression stoke. The result is understandable; for the entire compression stroke, or a great portion of it, the engine is trying to compress a hot mass of expanded gas. That obviously puts tremendous load on the engine and adds tremendous heat into its parts. Substantial damage occurs very quickly. You can’t hear it because there is no rapid pressure rise. This all occurs well before the spark plug fires.

    Remember, the spark plug ignites the mixture and a sharp pressure spike occurs after that, when the detonation occurs. That’s what you hear. With pre-ignition, the ignition of the charge happens far ahead of the spark plug firing, in my example, very, very far ahead of it when the compression stroke just starts. There is no very rapid pressure spike like with detonation. Instead, it is a tremendous amount of pressure which is present for a very long dwell time, i.e., the entire compression stroke. That’s what puts such large loads on the parts. There is no sharp pressure spike to resonate the block and the head to cause any noise. So you never hear it, the engine just blows up! That’s why pre-ignition is so insidious. It is hardly detectable before it occurs. When it occurs you only know about it after the fact. It causes a catastrophic failure very quickly because the heat and pressures are so intense.

    An engine can live with detonation occurring for considerable periods of time, relatively speaking. There are no engines that will live for any period of time when pre-ignition occurs

  3. Most people don’t know this, if the engine has a distrbutor from a engine that had an egr valve, there will be an additional 10′ or more timing at part throttle cruise built into the vacuum advance pot. This was done because adding the exhaust gas into the combustion slowed the burn rate.
    Most Ford vacuum pots are adjustable, use a 1/8″ allen wrench to adjust the spring tension in the pot to stop the detonation.

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