It's not resistance, exactly - Saab 9000 Bulletin Board - Saabnet.com
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It's not resistance, exactly
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 17 May 2007 08:02:47 Share Post by Email
In Reply to: Re: Direct Ignition Cassette Question., R. Frist, Thu, 17 May 2007 07:18:38
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It's harder for a spark to jump across a gap as the pressure of the gas (air-fuel mixture) increases. It's not resistance, exactly. It's the very simple concept of Mean Free Path. A spark occurs when the electrical field acclerates molecules, which then crash into each other with enough force to create a plasma.
In a low pressure condition, the molecules are far apart. So they have a longer path to accelerate before they hit something. Going faster is more energy is easier to create that plasma. At higher pressure, the particles are all pushed together, so they don't have much space to accelerate - when they hit, they don't hit as hard, so the result isn't hot enough.

Sorry you asked, no?

Spark plug gap is important, as it is in the compressed gas of the cylinder. But around the DI is important, too. There is a spring in the rubber boot of the DI that connects the coil to the top of the spark plug. If it isn't seated right, there will be a little gap between the spring and the spark plug, or spring and the coil (or both). This is an additional air gap the spark must jump. Add in the increase in pressure under boost, and a gap at the top of the spark plug can also act like a too-wide gap on the plug.

Also, if there is dirt, grease, or moisture on the spark plugs or in the boot, that looks like a preferential path for the spark. The spark goes into the plug at low boost, but as boost increases, the spark takes the easy way out.

It's possible that your old DI is OK, but a spring didn't seat right. Check the top of the spark plugs for signs of burning (arcing), and pull off the DI's plastic cover and look for arcing on the coil-to-spring contacts. Also look for signs of arcing on the sides of the DI coils.

And yes, it could be a DI on the way out. But given the price, it's always nice to check the cheap stuff.

posted by 192.249....


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