hey mate - welcome to the s15oc
there is quite a bit of choice out there when it comes to s15 exhausts, you need to decide on a few things:
do you want a full stainless system or will you be happy with mild steel pipework and a stainless box? (beware, some exhausts like the 409 have mild steel baffles in the stainless rear box which rot from the inside out and rattle around - dont ask why they have done that
)
how much have you got to spend?
how loud/quiet do you want it?
how much power do you want to run?
it's worth going for a 3inch system as this will be the most free flowing and probably wont cost any more than a 2.5" system. there are the usual hks exhausts - hi power 409 (mild steel with stainless can) or silent hi power (full stainless and quieter). i run the silent hi power and its a good system that hasnt got any corrosion at all after a years use but has got louder over time. there are also the trust power extreme 2 which is meant to be very good, the apexi n1 evo and gt spec (also very good reputation but louder) and there are also a few more jap made ones and others from people like mongoose etc. but i wouldnt know about those - others will
.
you then have the s14/a systems (that can be made to fit quite easily) from people like apex/japspeed/ebay which are of similar design to the big brand exhausts and much cheaper/louder but most dont have a centre silencer and are made of a lower grade stainless and will/can spot and rust. one thing to note is that any system with a centre silencer on a lowered car will undoubtedly catch over bumps and you need to learn to avoid things like that or you risk damaging the exhaust all the way up to the turbo/manifold.
for a decat pipe (with an s15 system) you need an s13 3" decat, and the downpipe should either be s15 specific from someone like hks, or an s14 item from any manufacturer matched to a turbo elbow meant for the same model. i found that having a custom downpipe made and fitting stiff poly mounts for the exhaust (from dbpower) solved some of my ground clearance issues by lifting the entire system a good 15-20mm higher off the ground. it is important to maintain the mount that secures the exhaust to the gearbox mount so that the engine, box and exhaust move together, if the engine can move independent from the exhaust you risk cracking at the weakest point (usually around the turbo area) - downpipes with flexi sections are meant to help, but i found the flexi again made the exhaust lower to the ground and run without one with no problems.
it is important to use genuine nissan gaskets around the turbo area (manifold to head, manifold to turbo, turbo to elbow) from then on you can use the usual cheap gaskets with a bead of silicone and they will be fine - they havent failed me yet. you dont want to use the cheap ones around the turbo as if they fail, which they will, its a major pain to have to do them all again.
hope this is enough information to get you started, others will have different experience and info but what i have written is what i know to be true and works for me
best of luck - you came to the right place:nod:
