OK....so I often get accused of being somewhat different. That's only cuz I'm the only insane one amongst us.
So to keep with tradition I'm gonna put in my 2 cents worth.
Many of us have an interest in track work. That implies wringing every last bit of power one can from their engines.
But.....some of us are not into that, we only want a good street car that will blow most of the showroom cars from the road. Now, this is where I get to be somewhat different.
With regard to the Gen111 and IV Chev/holden motors the stock cast manifold has proven to be more than sufficient for healthy street work or, as plums described, an unopened but well tuned engine.
The restriction with these engines in their parent cars are from the cats back. In other words most gains can be had in the parent cars by replacing the standard exhaust system with aftermarket cat back systems.
The problem is that these standard manifolds can be difficult to use in a cobra because they exhaust backward. However, if you swop left to right you'll find in many cases they will fit and will allow a nice curved J pipe to run under the car to the cats.
How can I justify my claims about the performance factor of these standard manifolds?. Go the Australian LS1 site....do a search. You'll find plenty of comment and tests that imply that the standard manifolds are great up to the point of installing a cam.
Now....It's back to the funny farm for me, to the bees and flowers and laughing birds and basket weavers who twiddle their thumbs and toes, etc. etc. etc.
Cheers Les