Need Help on Custom headers

[Mid-East Performance]

Well I've created some custom headers for my ride here in Kuwait, and the biggest problem is that I've actually lose performance instead of gaining. What I am thinking is the fact that the garage had created the primaries a little too large then what they should've been. They are a little over 1.5".

 

The header design is not a Tri-Y which might've been best to do, but they are dual outlets into the exhaust, that finally get joined once it reaches the cat back.

 

Now the garage has said they will re-do them, but I would like some input into how they should design them first. Tri-Y or dual type manifold, size of tubing, length of primaries and even collectors! From scratch up...

 

Anyone with the experience, plz jump in and let me know thanks!!