3 inch would be over kill for a N/A Ecotec...you'll loose too much gas velocity when the exhaust gases exit the extractor collector to fill the extra volume of the tail pipe. This is **** house for a N/A motor because they rely on the exitting exhaust gas velocity of the exhaust system to help provide extra vacuum for extracting the next cylinder to open its exhaust valve and to help pull in the fresh charge of intake mix when both valves are open on the cam overlap. On the other hand you dont want to go to small either as it will take longer for the same volume of gas to move through the pipe. But when it's too big you'll loose intake cylinder filling which results in less bang, which is why over exhausted N/A cars feel gutless.
Supercharged cars on the other hand dont really need this effect as much as a N/A engine as the intake mix is forced into the cyclinder at higher pressure ensuring a complete fill.
hope that helps you make your decision !
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