Spark plug gap on carb vs. injected engines.

[navigator336]

My boat has 350 vortec engines with EST ignition and is carbureted. The manual says to gap the plugs at .030", while the TBI and full injected versions, using the same ignition system are gapped at .045". I called the factory and they confirmed this is correct, but the tech couldn't explain why other than, "the engineers say so". I'm guessing that the fuel to air mixture is a little richer with carbs and that slows the ionization of the air in the spark plug gap thus making it harder for the spark to jump the gap. Is this true or is there another reason? Also, does changing the plug gap by .015" effect ignition timing? If it does, I would assume it advances it. And finally, is a timing light triggered by the rise in voltage in the plug wire as it builds awaiting the gap jump, or when the plug actually begins it's fire? I know we're talking fractions of milliseconds here, but at high rpms, things are happening at fractions of milliseconds. <br /><br />Thanks for your input!

 

[Walt T]

Re: Spark plug gap on carb vs. injected engines.

Widening the gap makes for a hotter spark as it takes more voltage to jump a larger gap. This theoretically results in a better burn. The larger gaps originated in automotive applications to help burn all the fuel in the cylinder resulting in lower emissions. The carbureted automobiles generally had smaller gaps because the emissions requirements werent as stringent. Pre catalytic converter cars did not have to worry too much about unburned fuel going into the converter and wrecking it. With converters it becomes very important to control the amount of unburned fuel, hence the larger gap. The larger gap on your boat may be helping fuel economy and power by burning the fuel in the cylinder and not sending it out into the exhaust. My guess is that the marine manufacturers get their specifications from the engine builders. It really doesnt matter much in the marine environment, but it will once we start having converters in our engine rooms.