the au loves a bit of rear entry loving ok, lets face it, something that uglies gotta take all it can get lol

but strategy wise for the ecu, an au will run alot leaner than an ea at most times (not to say it's considered lean,just in comparison). I don't claim to know how or why i'm just saying, the exhaust seems to make the difference, it's not a fuelling issue.

Is sound somewhat relative to flow ? is it possible to have a free flowing exhaust that's quiet ? Much like this thread it's debatable.

might be easier in these terms ... wat dont i do when building an exhaust

My advice is for a system on a budget is just fit a Redback system from cat back. Be aware there are different configurations available for the tailpipe - ie. straight pipe, pipe with resonator (hotdog) and pipe with rear muffler. That equates to freakin LOUD to bit hoonish to quite decent volume. You can pick up the various systems cheap off eBay all the time - ie. budget $350 to $380 or so including freight. I gather they're easy to fit yourself.

Don't get bent out of shape about not having extractors (and 2.5 cat etc.) - they're nice to have but it's not like the difference will rock your world - without serious other stuff like at least a cam, you'll be concentrating to notice the improvement they will give you. Just the 2.5 cat back will let the engine spin up decently and not be completely choked at high revs.

If you do want to go that way then of course Pacemaker small primary ones (4490 or 4480 - I can never remember which) - or on a budget I personally like the look of Tiger Headers - cost is about $270 from Best Mufflers online (google to have a look at them). Problem then is having them fitted as you'll need to have an exhaust place do it - ie. cut and shut the cat to suit etc.

And if budget is a bit more relaxed then you can go the whole hog with a 2.5 cat as well - which should be something between $250 and $300 before fitting.

On eBay from time to time there are very decent looking complete systems from Headers back - including hiflow cat etc. for $900 ish

AUs do run a lot leaner then e series off the shelf. Its very apparent when you compare them on a gas bench, we use the "coda" system.
A tuned well kept AU at idle will be around 97% efficiant. Havnt seen a eseries that's any better and more in decent condition sit around 90-94%. Mines a 87 job haha.
On topic of extractors. When I got my pacemaker 4499s on I didn't notice a huge differance
, mostly because the installer broke my pcv valve and she ran rich as and stalled everywhere! d**k. But also now I think its cos my headgasket was f**k that I didn't notice a change. Becuse now its all fixed you can really tell the extractors are doing there job, revs out nice and smooth and power gains at mid to top end.