Ok, I've built and installed a custom 4-2-1 stainless system which I love the performance and sound. The one thing I don't like is the back-firing when I down shift. How do I reomve the air suction valve system as I have read elsewhere that this greatly reduces the back firing.

Take 2 wine corks or say a trip to NAPA and find a rubber cone that is the inside diameter of the metal end of the PAIR chamber or the snout out of the front top part of the valve cover. You insert your tapered cork or rubber plug into that reed chamber, then reinstall stock hose over the snout again and dare you go.

Once that is snug and can clear the hose going back on, you just closed down the fresh air into the exhaust. This will not cause a code on the dash. It more or less will cycle as if. It may quiet the pop, but you now have a dirty exhaust port that will carbon up on the one side of the dual ports. It will look like the other carboned up port more or less.

Thanks Hub!
You said that it will "get dirty". Do I need to occasionally clean it out or anything?

Not really. The other port is burning normal basically. So it would be like your normal engine without the leaner cleaner port. Don't worry about the same effect. It's how fuel burns and leaves that matter behind, being you cannot create or destroy carbon.