Let me toss my (contrarian).02 in here. If you simply punch the precat material out, as someone here posted that there's not much gain, and I believe it. However, if you take a close look at the internal construction of the precats AND address the internal impediments to flow there are significant gains.
My precats were welded internally & externally, ported and then punched. Although I did not do any back to back (also added other parts at the time of install)there were significant gains and my total outlay was $50. As I recall, Terry Haines quoted gains of approx. 10HP.
The main problem is the stock Y pipe collector. This would be the first item I'd address. IMO, $600 for a set of headers is outrageous. MSDS HP claims on the website are obviously overstated. Everyone I know who has them, has complained about premature rusting. Now I know they offer an optional better coating, but it's an additional cost.
So there you have it, $50 vs. $600+, although you get a Y pipe too. Labour is the same either way.
You decide.
