Thought you said you can bend pipe tho...
Buy a redback one save some $$$ and time

I have the ebay redback one it cost me 160 for the cat back and 50 for shipping costs I think, the local carline wanted 300 for a standard bolt on system or 400 for the 2.5 inch with all the mufflers and resonators to keep the sound down so the ebay one was cheapest fix to my standard one rusting out, but as I said it sounds drony as and needs another muffler or a resonator on the end

I've heard that from a few people about the ebay exhausts, they're cheap enough though.

They bend exhaust pipe at the exhaust shop... it depends on the bender. The ordinary fence post type pipe bender may not do the job. Also depends how far you need to bend, the greater the bend the more chance of crushing.

or even just a hotdog to settle it down a bit.

Have you been reading performance books from the 70/80's?

My ol' Morris Minor needs back pressure too!!

was the deal with backpressure someone fill me in ..do i need it. or is a free flowin exaust the way to go

Ive been taught its always g ood to free up a bit of back pressure, but if you go too big you lose down low in your power?

Back pressure should be a word that is blanked out like swear words (in fact actually more so).

Keeping it simple; having exhaust gases travel at a slower velocity is bad, as they become more dense as they cool down and will cool down more before reaching the end of the system, making it harder for the engine to push them out. However there is a point at which making gases travel faster is also harder work for the engine and as such will choke the engine.

The crux of the story is you have a trade off, and as power is proportional to the amount of fuel/air mixture travelling through the engine you need to match an exhaust system to your application. For the purposes of a falcon i6, a 2.5" exhaust is probably your best all round system, but a 3" is fine for a worked motor as it will have a better gas velocity at high rpm and won't 'choke' the engine. Of course as a result of increased flow capacity the velocity will be lower than a 2.5" system at low rpm (as in lower under the ideal velocity), so you will lose torque/power at lower rpm.

'Back pressure' is a shocking term that I can only fathom is used by those who don't understand the physics of an exhaust system. It makes me grind my teeth whenever I hear it. Unfortunately 99% of people who work at exhaust shops fall into this category.

In 1985 I had a 4.2L (253) V8 Commondung 83 VC SLE (was very nice car with all the fruit - think Fairmont Ghia).

It had performance issues - and when I got hosed down by a Ford Cortina one day I took it to a small workshop who had their own roller dyno - where we discovered fuel vapour spewing back out the carby at higher revs. Blocked exhaust.

I went to the local exhaust place asking about twin 2 or 2.25 systems and was told "2.5 single system is what yer want mate!"

After a couple of weeks saving up (was a Trades Assistant - had about $2 left each week after expenses and making car payment) I made a booking to get the zorst done on a Saturday morning. To save money and see how it sounded and felt I hacksawed the pipes off on each side on the Frinday arvo (was living 20 mins drive out of town) and took it for a strap around the back roads near home. It sounded tremendous - surprisingly quiet under acceleration but very noisy on decel. Front wheels damned near lifted off the road when I planted it and it felt at least 50 percent stronger across the board.

So after a very nerve wracking drive to the exhaust shop - including going past a police speed trap - and yes, altho they didn't flag me down at the stop (I wasn't speeding after all) - in the mirror two of them walked out on the road and stood watching me a for a few seconds - asked the shop about infact fitting 2.5 twin or at least single 3 inch...

WELL!!! You'd think I was trying to get together a gang to go rape nuns or something!!! - I copped the biggest flogging about too big and need backpressure bla bla bla - and drove away with single 2.5 system. It was definitely better than with the old blocked original system but was less improved from that than it was worse than when I had the pipes cut off. Read that again and think about it.


Being young I didn't go back to them or even try to get a bigger system on.

The kicker is that several years later I was back in the same shop getting something done on another car (was std - not performance car or work) and overhead a conversation between the manager and some guy with an SS Commondung who was asking about exhaust - the shop guy was saying how once upon a time they used to pretty much refuse to fit anything bigger than 2.5 single to even 5L V8's but then they had some young guy who INSISTED they fit 3inch - and much to their surprise it worked really well so for V8's they now recommend at least single 3 inch...

AAAAARRRGHH!!!

I would consider the statement a mis-use of term rather than a lack of concept... not that I claim to be an exhaust person...

like you said it's a medium. a certain pipe diameter will increase or decrease air speed respective of revs.

what causes excessive popping n gargling on back off ?

Usually unburnt fuel/air mixture. It becomes more apparent when you run a louder system. I found that after fitting headers/full exhaust my car seemed to do it more so than usual for a while and also stalled a couple of times (still runs rich, but not as bad). Can also be caused by running cams with more overlap.

The reason saying backpressure creates torque isn't correct is because you can probably design a system better than the stock which will perform better over the whole of the rev range if you wanted to. It would have less 'backpressure' than a standard system but can be more efficient (after all, the standard manifold is a pos). I remember a thread from a few years ago where some backpressure advocate was trying to argue stock manifold was the best for downlow torque and you couldn't beat it because it was resrictive at higher revs and therefore better

I never said back pressure creates torque, though I understand your point.

But that doesn't explain why my fairly lean running standard except exhaust AU pops and gargles twice as much as my over fueled cammed ea which used to smell like unburnt fuel sometimes and puff nice black rich smoke with essentially the same exhaust, except press bent.

or does it ? thats why I don't claim to know about exhausts, every one i've made has been a bit different.

Dunno, maybe the AU just doesn't like the way you put your d!ck in it's exhaust pipe?

In all seriousness though, what makes you think it is specifically the exhaust causing the popping? I'm not saying it isn't, just I would have been looking at other things first. Also if it hasn't been tuned I wouldn't have thought it would run lean? My AU runs rich as a mother f**k....I'm no mechanic by any stretch of the imagination lol.

Obviously.

Back pressure is a term that should only be used if someone knows what they are talking about. Ideally, an exhaust system needs to be as large as possible whilst still keeping exhaust gas velocity as high as practical.

I'm going to be building an exhaust for my XR8 that doesn't follow any conventional system built in Aus, simply because I need a new exhaust, and to prove a point about exhaust gas velocity vs power. Back pressure comes from a poor flowing exhaust setup.