Why do cars lose low end torque with open exhaust systems?

[crazypete]

I have an offroad bassani x-pipe imminent and I am worried about losing all my bottom end umph! I'm pretty sure I got gyped on my torque converter cause she runs at very conservative maybe 1300 - 2000 rpms and starts pulling right at 800-1000. Therefore, I need low end umph until I get a real 2400 rpm converter. Teaches me to use stupid ebay.

Anyhow, I hear that cars lose all their low end torque on hpipes. Why and what can I do to get this back?

It's funny cause with the stock 4-cats back on, she feels very..... 'restricted' and toned down. I'm planning on being all clever and sawing the heatshields off the rear cats and welding them to the x-pipe to make it look like I have cats, you know, just in case the roar of my exhaust attracts unwanted attention, I can bluff my way through!