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More back flow on exhaust

Is it a good idea to remove the stock muffler on my 03 Silverado 4x4 PU and put on a low resistance muffler to increase performance?

No. The increase in performance is so little that you probably won’t recognize it. If you like the sound, that’s different. And you will get a very slight increase in power.

Increase in power, but in the upper RPM range. Almost no change in pulling power.

I believe that, In the last decade or two, more engineering has gone into exhaust systems. There is a lot more to it than resistance per se. There are pressure wave activities to consider, though it might not be that important that far back in the system.

You will have a difficult time increasing performance by hacking up a factory system and substituting a part. One thing you could check out is if your truck had brethren that left the factory with duals and very few other differences. That is the case with my car and that change alone is worth 15 hp added to 215. With other vehicles the exhaust might not be the limiting factor for the volumetric efficiency.

You could consider replacing the whole system from the block back including some nice new headers replacing the manifolds. That won’t be cheap and I would not say it will help without dyno data to prove it.

There is an old hot-rodder’s adage, “don’t mistake noise for horsepower”.

A badly designed aftermarket exhaust can actually reduce power yet the owner will kid himself that he improved it.