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Old 06-07-2006, 03:01 PM   #1
FLiP
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Exhaust Conundrum

I've got no idea what to do anymore...

I went and got a second quote for a performance cat-back exhaust for my BA 1.8 hatch...

for those who didn't read my earlier thread my first quote was as follows:

2" mandrel, all stainless steel, resonator and straight through muffler = $460 fitted

my second quote was:

2.25" press bent, aluminised mild steel, resonator and straight through muffler = $260 fitted
(the guy who quoted this told me i'd be shooting myslef in the foot for further mods if i went with 2" and that i should get the same power out of this as the other one)

here are my questions to you...
1. Is stainless steel really necessary? (esp. if the mild steel is aluminised?)
2. Is mandrel bending really that much better than press (ie. will it sound or perform better?)

I'm leaning towards 2" mandrel but in mild steel to keep costs down.

Let me know what you think, esp if you have knowledge in the area.
Thanks guys.
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Old 06-07-2006, 03:07 PM   #2
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Quote:
Originally Posted by FLiP
I've got no idea what to do anymore...

I went and got a second quote for a performance cat-back exhaust for my BA 1.8 hatch...

for those who didn't read my earlier thread my first quote was as follows:

2" mandrel, all stainless steel, resonator and straight through muffler = $460 fitted

my second quote was:

2.25" press bent, aluminised mild steel, resonator and straight through muffler = $260 fitted
(the guy who quoted this told me i'd be shooting myslef in the foot for further mods if i went with 2" and that i should get the same power out of this as the other one)

here are my questions to you...
1. Is stainless steel really necessary? (esp. if the mild steel is aluminised?)
2. Is mandrel bending really that much better than press (ie. will it sound or perform better?)

I'm leaning towards 2" mandrel but in mild steel to keep costs down.

Let me know what you think, esp if you have knowledge in the area.
Thanks guys.
firstly, i'd say mandrel all the way. crush bends are nasty, and create restriction in the pipe.

Noone looks at the exhaust pipe, and with shiney muffler (i.e. stainless muff), it'll do the job fine. However it will be prone to rusting etc...so some sort of high temp coating should be looked at to protect it a little.

i find it funny that the guy is saying on one hand that 2in mandrel will shoot you in the foot, then on the other offering you a 2.25in crushbent system that'd probably be even worse....

mandrel 2.25in is the go reckon....
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Old 06-07-2006, 03:16 PM   #3
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yeh... i thought that the 2.25 crushed would be worse to... i wonder if he could tell by the look on my face...

maybe he doesn't have a mandrel bending machine thingo... if that's the case i hope he didn't do the exhaust on the maloo R8 that was sitting outside the shop

you reckon 2.25 mandrel? i really need some low end torque to get me going... i've been told 2" will do that really well once i add some extractors.
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Old 06-07-2006, 03:18 PM   #4
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mandrel 2.00 is the same as crush bent 2.25 inch.

2.00 mandrel would be good for minor mods but if you plan on building with more mods than just air filter + ecu then you'll need 2.25 or 2.50 inch.

to be honest, i'd get a 2.25 mild steel system while you have no mods and then upgrade later when you get into seroius mods.

no exhaust will give you low end torque, more likely to take it away, extractors and power mods will get you low end torque
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Old 06-07-2006, 03:44 PM   #5
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I think I'm gonna go with the 2" mandrel mild steel. I plan on having the CAI by the end of this year and extractors early next year. So there's not much point in screwing about doing a half arsed job with the little money i have just to go and change it again in 9 months. I'll do what's necessary for the near future, and if i feel the need for even more power down the track then i'll think about upgrading the exhaust then to accommodate it. (I probably will get that urge for more power... i know me too well)

i think it's a matter of preference as to which system you get... i doubt anyone has taken a stock BA and tried multiple exhausts on it and dyno'd each one... i know i wouldn't be bothered... but please let me know if you have results of such testing.

thanks guys... nice to know im heading down the right track.
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Old 06-07-2006, 03:46 PM   #6
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mandrel 2" will be more cost than crushed 2.25" al will be the same, mandrel in 2" is a waste of time. and i'd be checking if they even do that also.

and if you don't wanna be mucking around wioth exhaust or changing then get 2.25" mandrel from the start.
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Old 06-07-2006, 04:26 PM   #7
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yeh... 2.25 mandrel is good... i might need to change the cat when i do my extractors hey... i'd imagine they (the extractors that is) come in different sizes too... (forgive me, im learning)

it'd be ridiculous (not to mention pointless) doing 2.25" piping on both sides of that cat with only a 2" cat because the cat can only process so much... yes?

And my car would emit excess hydrogen sulfide! And smell bad! Nothing like a bit of rotten egg gas to pull the chicks
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Old 06-07-2006, 05:31 PM   #8
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Yeh - dont let the choice of the size of your exhaust be dictated by "wanting low end torque"...in a correctly designed exhaust system (and there are very few - and you do need to spend big money to get one), the catback will not make more bottom end power at all....all it can do is restrict it... any power increase you go for by getting an exhaust system with stock manifold is just a removal of restriction, but all the power is made in the manifold.... if you end up with a really awesome set of headers, you'll rue the day you ever went to a 2in system..because it just wont flow enough, and you'll be back to square one.....You'll get told alot of things by exhaust shops - many of them are outright untrue - or apply to a lifetime ago when knowledge on the subject was scarce... either the exhaust shop doesnt know, or its in their best interests to missinform so they can charge stupid amounts for "correctly" designed systems.

I'd say go for the 2.25, even if you do loose a bit of punch for now... you'll make it back with interest when you start getting more mods. From memory (and twiggy will correct me if i'm wrong) a 2.25in system can flow to around 200 flywheel horsepower before it starts becoming a restriction again.

If you want to know more, and get the technical side of things, check out stuff like the integra forums (lots of info there that applies to exhaust and header technology in general), and the burns stainless website - they are one of the few exhaust places that will supply the theory behind how a good system works - though they stop short on telling you how to correctly design one yourself =)

Good luck with it, and remember the golden rule - backpressure is ALWAYS a bad thing - if someone tells you its good, look at taking your business elsewhere =)
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Old 07-07-2006, 03:29 PM   #9
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i agree with this novel that was written. like i said before, 2.25" mandrel is best if you don't wanna do it again later.
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Old 07-07-2006, 03:57 PM   #10
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i just put a 2" press bent on mine today and it sounds good as for performance the bigger the pipe the more back pressure you will lose and will lose low end torque. thats why i went with the 2" press as its heaps better than stock and havent lost that low end torque. they recomended the 2" anyway so they wernt tryin to get me to buy the more exy sizes but i got them to use 2 1/4" pipe for all the bends so there is no restrictions in the system. not bad for $140 catback. i used my own cannon but.
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Old 07-07-2006, 04:19 PM   #11
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Old 07-07-2006, 05:45 PM   #12
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Quote:
Originally Posted by AstinaBoiV6
i just put a 2" press bent on mine today and it sounds good as for performance the bigger the pipe the more back pressure you will lose and will lose low end torque. thats why i went with the 2" press as its heaps better than stock and havent lost that low end torque. they recomended the 2" anyway so they wernt tryin to get me to buy the more exy sizes but i got them to use 2 1/4" pipe for all the bends so there is no restrictions in the system. not bad for $140 catback. i used my own cannon but.
Dude. Losing backpressure is a GOOD THING.

You "lose" power with a big pipe because of a loss of velocity. backpressure is never ever ever ever ever ever ever ever ever ever ever ever ever ever ever a good thing.

Ever
Period.

Crushbends ALWAYS add restrictions (especially on tighter bends...its probably not a big issue on say, a 45deg bend, but a 90deg or higher...you are folding the metal in half no matter what size you get), which create backpressure, which as I have stated is BAD. Always. You do not lose low end power due to a loss of backpressure. You lose it because of a loss of velocity. Velocity you can win back with interest with a decent set of headers and other power mods.
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Old 07-07-2006, 05:55 PM   #13
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Quote:
Originally Posted by LordWorm
You "lose" power with a big pipe because of a loss of velocity. backpressure is never ever ever ever ever ever ever ever ever ever ever ever ever ever ever a good thing.

Ever
Period.
Back pressure is needed to make the headers work properly. If it wasn't needed then all cars would come off the lot with 2.5" pipes, the price difference would be minimal.
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Old 07-07-2006, 06:32 PM   #14
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Originally Posted by Cosmo Dude
Back pressure is needed to make the headers work properly. If it wasn't needed then all cars would come off the lot with 2.5" pipes, the price difference would be minimal.
Wrong.

Headers work by a phenomenon called scavaging.

As the gas is expelled through the exhaust port at high speed and high temp, it is trailed by a pulse of vacuume pressure. When this pulse of vacuume hits the collector, it applies suction to the other pipes in the header, which, at a tuned RPM, suck the next lot of gas out the exhaust port.

Backpressure is a myth, cooked up in the dark age of old V8 hotrods. It is not a requirement for power or torque, and is a detriment for both.

Required reading:
http://performanceunlimited.com/illu...s/headers.html There are a few things in there that are not quite right..but its a fantastic primer
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Old 07-07-2006, 07:11 PM   #15
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Quote:
Originally Posted by LordWorm
Wrong.

Headers work by a phenomenon called scavaging.

As the gas is expelled through the exhaust port at high speed and high temp, it is trailed by a pulse of vacuume pressure. When this pulse of vacuume hits the collector, it applies suction to the other pipes in the header, which, at a tuned RPM, suck the next lot of gas out the exhaust port.

Backpressure is a myth, cooked up in the dark age of old V8 hotrods. It is not a requirement for power or torque, and is a detriment for both.

Required reading:
http://performanceunlimited.com/illu...s/headers.html There are a few things in there that are not quite right..but its a fantastic primer
ROTFLMAO
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Old 07-07-2006, 07:24 PM   #16
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Headers work by a phenomenon called scavaging.
That's the second time you've used the word phenomenon incorrectly. If it's scientifically explainable, it's not a phenomenon.
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Old 07-07-2006, 07:28 PM   #17
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well... ok.
by a PROCESS known as scavaging.

Read the literature. Talk to exhaust places that know what they are on about. Goto burnstainless website, check it all out. You'll discover that backpressure is evil. And should be done away with....and anyone who thinks its a good thing should be beaten to death sticks.

Some quoting, from the website supplied, incase people cant be bothered reading:

Quote:
This is a good topic, one that is hardly ever discussed in detail. Generally you never want a restriction when attempting to evacuate exhaust gasses. Think of how headers works... they are designed to scavenge (suck) exhaust from the cylinder. Now, why would you want to keep burned exhaust gasses in the cylinder? You would not ever want that, except for in very isolated and specific combinations to cover up a problem due to miss matching of internal components, which we will discuss later in this article.

Left over exhaust gasses are like a fire extinguisher... not only do they take up valuable room in the combustion chamber that new, fresh air/fuel can not occupy, but it also contaminants an equal portion of that fresh gas, rendering it useless as well. This is called a "residual fraction". Usually most naturally aspirated engines see about 7 to 12% Residual leftover exhaust gasses in the chamber, thus, this is why actual volumetric efficiency readings are seldom read over 88% uncorrected on the dyno.
basically, if you leave unburnt gas in the cylinder, your not getting hte most out of your explosions...backpressure forces gas back (lhence the name) towards the combustion chamber...with enough backpressure...you leave the gas in there, and can't create nearly as big as an explosion as if you had gotten most if not all of it out.

Of course, if Cosmo Dude wants to explain how backpressure makes headers work, he is more than welcome to do so. I wont be the only person around here debunking it.
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Old 07-07-2006, 07:32 PM   #18
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Originally Posted by Cosmo Dude
ROTFLMAO
Why?

Do you know anything about flow dynamics and how it relates to this application?

To say that backpressure is a good thing displays, without a doubt, that you do not.
Backpressure in a system with headers defeats the purpose the headers were put there in the first place. How can a restriction like that aid flow? In a correctly designed system, the size of the pipe after the merge point on the headers means squat, as long as it doesn't impede flow it can be as large as you like.
I'm not an exhaust genius, but even I can realise that a small pipe is NEVER going to flow better than a larger, less restricted pipe.

Oh, and the reason cars come from the factory with small pipes, is actually cost. A smaller pipe is the most cost effective way of reducing emissions/power levels to within the relevant standards.
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Old 07-07-2006, 07:35 PM   #19
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I don't need to read all that stuff, I had more USEABLE power (torque) when I had the bigger (and much noisier) axle-back on the car...
So in my case I had more power with 2.25" over 2" up until 5500rpm where the 2" develops a Ferrari like advantage, which is kinda useless in everyday driving...
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Old 07-07-2006, 07:41 PM   #20
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Did I say it aids flow? Maybe back pressure controls flow, keeping power where it can be used.
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