I just bought a 2006 Monte Carlo SS (new with only 64 miles) and without a whole lot of thought, had "Dave" at the local muffler shop fabricate me Flowmaster cat-back system.  NO it was not Dyno Tested or Tuned, that is my problem.

Ok, so a magnaflow system is DYNO TEST at $900 and to put it back to stock will cost $580.  What would you guys recommend?

Why?  My car sounds like it is misfiring at low rpms, and I lost my low end torque.  One person says, it will hurt my engine in the long run to leave it as it is, and another says, it will do nothing, and that I may have lost low end torque but made it up in high-end horsepower.

So am I hurting my engine right now?  Do I have time to get a magnaflow system or take it back to stock before I hurt my engine? or am I alright?  

If the engine is really misfiring, misfiring longterm is not good for the engine. But is it really misfiring or does it just sound like that because of the exhaust? When the engine seems to be misfiring, is the motor "bucking" at all?

If you've got the money, I'd say step up and put the Magnaflow system on. One thing to rememebr with exhaust systems and LS motors, LS engines are sensitive to the amount of backpressure they get from the exhaust system. Too much performance suffers, too little performance suffers.  

The mechanics at work say it is not misfiring, it is the sound of the exhaust, but that I have no backpressure so that I am losing low-end torque.

I told them the guys that did the work said it was suppose to be increasing HP, and they said yeah.. at the cost of torque and torque is what moves the car.

But they could not answer me if it was hurting the motor.  I don't want to cook a valve and they tell me my warranty will not cover it.  

If your car was misfiring you would know it. It would most likely shake the entire vehicle and feel like you ran over something (when in actuality you didn't). Have you gotten under and checked the exhaust tubing to if anything is pinched? Did the mechanic tamper with the catalytic converter? I have several friends with newer model GMs that have aftermarket exhaust and they haven't had any problems(yet). I would take it to the dealer, a mechanic or maybe even autozone and have them read the computer to see if any codes pop up. Who knows, it may be just an O2 sensor(hopefully).  

I'm a finance manager at a Chevrolet Dealership.  It was our mechanics that looked at it.  However, these are the same mechanics that had my wife's Malibu for 3-days trying to figure out what was wrong, because the car was not throwing any codes.  So I got mad and took it to my mechanic at the local repair shop and he opened the hood on my car and said, "It's the Mass Air Sensor".  Two seconds, he had the answer, our mechanics had it 3-days and put $550 worth of parts in it trying to figure it out.

Today, these New mechanics are lost if the computer is not giving them a code to work from.  

With the Displacement on Demand setup on the 5.3 liter engine (2006 and 2007 Monte SS and Impala SS) four of the cylinders shut down, when there is little to no load on the throttle.  The exhaust note changes after the change from 8 cylinders to 4. When you step on the gas, all eight cylinders are firing, until you lighten your foot on the gas pedal. Likewise, with a louder exhaust, the change is more noticeable.  In all truth your car will have a misfire sound. (Sound like a Subaru?) I surprised the mechanics did not see "Displacement on Demand" on the engine cover, or have never read the sales brochure.  Your instrument panel also displays four cylinder or eight cylinder mode in the speedometer cluster.  

1st let me welcome you to the wonderful world of the LS4. I myself have the Grand Prix GXP (the 5.3L).

Right now there is no damage just an anoying sound with loss of performance (power and mpg). You have already determinded that yes you do need to replace the exhaust. The Magnaflow is the best way to go from my reasearch and talking to owners... 10whp and 1.1+ mpg increases. But DONT PAY $900 for it. IF you look you can get it alot less. I'm not going to advertise another message board on here or any business... There are a few places that have highly involved in our cars (w-bodys). If you want to email me CAMyers720@yahoo.com I could help you out with things for your car.

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06 Sport Red Metallic Grand Prix GXP  

i dont think it will hurt anything. how many miles have you put on the after the exhaust ? the ecm may/will relearn and you will get some of your lowend back unless ya put 4 inch cat back pipes on.if they are too big have the shop cut the tail pipes and reweld smaller pipes on.i put SLP loud mouths on my 01 coupe and first time WOT it fell on its face but it relearned  

if the engine was to be misfiring you would not be able to tell for long because the computer would pull timing too fast the knock sensors are external on these engines because of the dod this means if the exhaust is hitting the body at any points or rattling too much the knock sensors might pull timing for no reason. i unbolted the whole back half of the exhaust and bolted a thrush turbo on and it dynoed and 228 the dyno chart was unusual and it was because it was pulling timing with the stock exhaust it dynoed 251.... the knock you think you are experiencing might sound a little like helicopter noise its just the DOD