Make sure, if you change the exhaust, you also remove the CAT. Removing that alone should add a few bhp. AS DCD said that's the best place to start. After that it's up to you what you want to do with it which course you take. Do you want pure bhp, low-end grunt or straight top-line speed? Up to you.
The first main two tuning parts to change are the Air Filter and Exhaust purely because essentials of an engine is to take air in as quick and as much as possible (Air Filter) and also to get rid of the fumes as quick as ppossible thus Exhaust.. Like Fuggles said I would deffo get rid of the cat cos believe me it makes a huge difference mate
Whats the point of removing the cat when all you gain is a few extra hp, i would leave it on, there are other ways to gain power. Just get the biggest loudest exhaust you can . Or try a twin 3.5 inch system or something like that.
in aus 99% of all moded up cars still have their cat, even the 1/4 mile cars. I got no idea how restrictive they are here, but i know it aint worth removing for a bit extra horsepower, when proper cam timing and fuel/air ratios can cover for this.
When we were running exhaust tests on our dynomometer here (at work, SAAB tuning), the catalyst was sapping around 10 Bhp, although the nissan cat is probably better flowing than a SAAB cat and may not be as much of a drain.
But straight swap on the dyno with no other mods yeilded 10 Bhp from around 2000 rpm upwards, if I can find the exact dyno sheet I'll post the results.
I replaced my whole exhaust system with a Mine's one...and decided to go with their sports cat. If it does sap power its only around 2 hp or somethig.
As x]SPHYNX[x said there are other ways of getting more power. If you are really hungry for bhp and need those numbers then get rid of it....but its just so easy for me to pass emission tests without swapping things round that its totally worth it
you know I guess I never even questioned removal of the cat. although I am across the pond in the us. I just prefer the sound with out the cat, I dont have emissions here either. over here if we can get away with out it it is gone. but at the moment I have yet to have the privalge of working on a skyline, but soon. mabey they dont like not having one it all depends upon the car.