It's general knowledge here that if you want your ZLET (Im use with this engine so I'll use it as my example) to produce more power then one of the first things to do is replace the 1st cat with a downpipe and then maybe put a sports cat on and a cat-back system.
So ... with that in mind and also taking into consideration Courtenays website saying you need a full exhaust to get close to the 240bhp of CS1... I want to know one thing.
The Z20 LEH is basically a stronger Z20 LET yes? Forged pistons dont give more bhp than normal ones though they can HANDLE more, oil spray bars dont give out more bhp etc
So ... why can the Astra VXR or even the AstraVan 888 have 240bhp if it has a STANDARD exhaust system? (IE: 1st cat, normal main cat and a normal cat-back system with maybe a sportier backbox) yet if a GSI or Coupe Turbo etc wants 240bhp we need a full exhaust system... ?