Hirdle measured them, they're actually 1 5/8" tubes

 

Stock converter and 3:42s... just sayin...
Back on track i do believe the primaries are smaller but sypposedly the 6.2 version is 1 3/4... fwiw they really are a nice set of headers...

 

they are very nice, best you can get for the $

 

Where's the thread, where somebody had measures them with pictures...

 

the OBX's are the same price as the Pacesetters

 

A few guys have measured them and come out with 1 5/8

 

I measured mine (exactly 1-5/8" INSIDE DIAMETER) in my Ebay header thread, but Matt from Texas Speed replied about 15 pages in, that header primaries are measured from the outside????? No solid definitive answer anywhere that I could find....

but...
All I know is that they are "EBAY auction advertised" as 1-3/4" INSIDE DIAMETER, and that is 100% INCORRECT.

Inside Diameter: 1-5/8"
Collector Size: 2.5"
Y pipe: 2.5" pipes to 3" merged outlet

As for the "small" 2.5" collectors...if you run a single 3" I pipe (like stock-type), or 2.5" true duals, than what is the reasoning for a bigger collector anyway? I don't understand the logic of 3" collectors into 3" Y pipe, that "merges" into 3" single pipe? That's the setup probably 90% of us with a longtube header (with 3" collectors) V8 LSx powered vehicle has, and if you think about it, it doesn't really make sense. I'd bet that milder combos actually run better with the smaller collectors/smaller primaries since they have a better transition into the remainder of the exhaust, instead of the obvious bottleneck a 3" collector/3" Y into 3" single setup has. JMO...

 

I believe that it has to do with exhaust velocity and the transition from the primaries to the smaller exhaust pipes. Too small of a collector adds heat and a flow restriction...you don't want abrupt changes to flow. Too big of a collector will slow down exhaust velocity btw. My mild 5.3 has been doing just fine with my 2.5" collectors...hope that my planned h/c LQ9 won't mind them either.