I had 1 7/8 primary to 3" longtubes. kooks recommended these only for supercharged cars north of 500whp, but they are what the job I had sitting around a little dinged up for damn near cost. they did make more power than my bbk 1 5/8 x 2.5 but there are some significant crushes and restrictions and the bbk are not of the same type of construction (primary length, collector design, etc.). I could probably have picked up 3-4 more in the midrange with a set of kooks or arh 1 5/8 x 2.5 setup but I'm going a different direction with the car.
I seriously doubt it'd make much more difference than it did even at higher hp levels. we were having customers with stock saleen center exit exhausts in the mid 600rwhp gaining similar things by unbolting the exhaust for a run on the dyno. at those big numbers you've got such a strong motor/blower combo it can really shove it out pretty good. on a 653rwhp car we only gained 8rwhp by going from a high flow catted x to a non-catted x and it sounded like shit.
