choosing a header for my Z and how much whp I can get

Originally Posted by SS_Firehawk

Here is an excerpt from a thread about ART pipes and LTH's. Will give you an idea of how they work

ART Pipe with an aftermarket shorty header vs LTH's. Longer primaries improve scavenging and merge the exhaust together in pulses. Longer primaries generally improve torque over shorter ones. 1 3/4" diameter tubing generally improves torque over 1 7/8ths" Stepped headers use a combination of the two different sizes to improve low end torque and keep top end power. A tuned header means tuning the length of the primaries to the powerband of the engine. LTH's improve scavenging and assists in pulling old exhaust gasses out of the combustion chamber. The higher the rpm, the less critical this is, but is VERY important for making power and torque down low and below 4k rpm. Both the PPE LTH's and the F.I LTH exit at 2.5" and has the benefit of not being restricted to 2 1/4" when connecting to the stock header.

Shorty headers attempt to do the same in a smaller package. Primary length is limited, they will not be stepped, and can only improve flow to a certain degree. Attaching an ART pipe will improve performance... over the stock cats or maybe a standard test pipe, but it's still just a pipe that can't possibly do the same job as having LTH's doing. The only shorty/test pipe combo that came close to LTH's were the momentum headers and TP. But there is no direct comparison as gains made included VVEL tuning.