For my 3.4l what is the torque and horsepower improvement with an aftermarket header? I have the Dansk mufflers. Does anyone have the actual dyno numbers? Manufacturers claim 10-15 hp but that seams awfully high considering the factory headers seam quite good.

 

You wont gain any power. Full Stop.

 

Headers will shift the power band a little, and make extra noise. Not sure about results on the dyno unless you've got other mods.

 

I'd recommend welding the outside runners and grinding down the inner welds to open the runner up instead of choking the gases as soon as they leave. The weld bead is atleast 1/4" thick inside. That is what I did with mine and added a thermal coating.