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  1. #1
    Veteran Member Four Rings Euromike's Avatar
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    Does a 2.8 need back pressure in the exhaust?

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    I recently put a completely free flowing exhaust on my 2.8. 2.0 twin pipe down the center, through free flow cats, then through a Y pipe, and 2.25 piping thru a bullet resonator into a magnaflow 14815.

    I have two 2.8's and it seems like the one with the exhaust just isnt quite as spunky. Did a quick 0-60 test, where my other car gets 6.5 seconds, this car takes 8, even though it has the same mods, plus a freshly built engine....

    I was talking with my brother about it, and he says maybe it is lack of back pressure. I know the turbo guys dont need it, but what about us naturally aspirated guys? Could it rob me that much power?

    And how would i go about fixing this? i want to keep the exuast all stainless parts, I thought maybe a more restrictive resonator. where would i get a quality one?

    Thanks
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  2. #2
    Senior Member Three Rings Ldiaz12's Avatar
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    Are you really getting 6.5 with your 2.8?
    Laser Red A4 12v
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    Soon to come: PP IM, Bored MAF

  3. #3
    Veteran Member Four Rings Euromike's Avatar
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    Yeah, i really am. I counted myself so it might not be the exact number, but im getting around a 7 with it. My other car runs really really well. thats why i bought another v6 instead of a 1.8T
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  4. #4
    Veteran Member Four Rings Euromike's Avatar
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    Poking around google a bit, it seems like the b6 mustang guys who do custom exhaust end up in the same boat as me..

    http://www.allfordmustangs.com/forum...e-problem.html
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  5. #5
    Senior Member Three Rings Ldiaz12's Avatar
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    So your saying running highflow cats and a magnaflow helped out your times?
    Laser Red A4 12v
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  6. #6
    Veteran Member Four Rings Euromike's Avatar
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    no im saying my car with a full free flow exhaust gets worse times than my car with a stock exhaust, and im wondering if its related to the lack of backpressure in the custom exhaust
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  7. #7
    Senior Member Three Rings Ldiaz12's Avatar
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    Oh ok thats what i thought.....

    There was this guy at an Indy Auto Parts that told me that A car with no backpressure runs worse than stock
    Laser Red A4 12v
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  8. #8
    Veteran Member Four Rings Stone825's Avatar
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    Quote Originally Posted by SantorianA4 View Post
    Yeah, i really am. I counted myself so it might not be the exact number, but im getting around a 7 with it.
    Really... haha
    Audi A6 2.7T : 6MT : 88K Miles : GIAC Chip : 2.5" Catback : Hotchkis F+R Sways : AWE DTS

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  9. #9
    Veteran Member Four Rings Euromike's Avatar
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    you guys are welcome to come out and drive it! I have done it multiple times and made my girlfriend count once to make sure i wasnt off.

    is 7 seriously that good of a time? i thought it was pretty slow.. ive seen some of the heavy modded 1.8t guys getting low 5's
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  10. #10
    Veteran Member Four Rings Euromike's Avatar
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    And back to the backpressure problom, even if it was the issue, i have no clue how in the hell i would get the pressure back. I cant find a resonator out there that isnt designed with a straight thru core...
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  11. #11
    Senior Member Three Rings Ldiaz12's Avatar
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    Well 7 is about right i would say for a 2.8
    Laser Red A4 12v
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  12. #12
    Veteran Member Four Rings Euromike's Avatar
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    yeah, thats why my new car is driving me nuts. wheres my power!
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  13. #13
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    Simplest solution would be just putting the stock cats back on. No resonator will create as much back pressure as a cat.

    I was in the same situation as you, but with no cats at all, and i had no low end torque. I'm currently trying to go back to a completely stock exhaust with maybe a magnaflow muflfer on the end

  14. #14
    Veteran Member Four Rings Euromike's Avatar
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    Quote Originally Posted by lqm View Post
    Simplest solution would be just putting the stock cats back on. No resonator will create as much back pressure as a cat.

    I was in the same situation as you, but with no cats at all, and i had no low end torque. I'm currently trying to go back to a completely stock exhaust with maybe a magnaflow muflfer on the end
    wow, this was the answer i was looking for!!! So you noticed a huge low end torque difference?

    I dont have the stock system anymore, cats were melted, and i didnt see a reason to keep the resonator. im totally lost on what todo
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  15. #15
    Veteran Member Three Rings anmagro's Avatar
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    i havent noticed a down slope in power since my 2.5inch magnaflow non-res install. but when i took my muffler off (making it completly free flowing) its almost like the tq totally left the car. i had no tq at all without the muffler on...so, im guessing your correct with the new exhaust cutting the power.

    if i were to do it all over again i would probally stay with 2inch pipe. cherry bomb res. and a chamberd muffler at the end to keep my much needed back pressure.
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  16. #16
    Veteran Member Three Rings anmagro's Avatar
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    Also, no to thread jack...but what do you guys think about a side exit exhaust? Say that you keep everything about an exhaust system the same (same res. same muffler same pipe width) but you bunch it all together and cut down the length of the system, should i decrease power?
    Black On Black 98' Audi A4 2.8L 30V 5spd. Quattro "Esabell"


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  17. #17
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    Yeah there was very little power until maybe 2500 rpm. I've even heard of guys putting milltek cat-backs on and noticing a drop in power, which really aren't that much less restrictive then stock. You could try putting in a baffled resonator(they are impossible to find online, but most exhaust shops will have them).

  18. #18
    Veteran Member Four Rings somebody5788's Avatar
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    Quote Originally Posted by Ldiaz12 View Post
    Are you really getting 6.5 with your 2.8?
    6.5-7 on a stock clutch and no launch. Clutch was already slipping from a few attempted launches and had about 135,000 miles on it.



    That one isn't the best example cuz i shifted to 3rd just before 60. But 6.5 on a stock car is possible.

    With a nice launch and holding 2nd gear past 60 6 seconds is easily possible on a 30v
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  19. #19
    Veteran Member Three Rings black00A4's Avatar
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    Looks like i need new cats myself, would you guys recommend highflow cats? All I have is a magnaflow muffler right now with stock resonator.
    2000 Audi A4 2.8L Quattro Sport - Volcano Black

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  20. #20
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    this artical seems to have it right.. http://www.uucmotorwerks.com/html_pr...torquemyth.htm

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    Veteran Member Four Rings Euromike's Avatar
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    Looks like i hit the right idea then. So what do i do? stock cats back on arnt an option, they were melted when the shop took them off, plus they are gone now.

    Baffled res, you said most exhaust shops have them? do you think thats enough back pressure? i cant find one anywhere online. are the stock cats extremely restrictive? how much resistance do i have to get back?

    im really confused on what todo. help!
    Last edited by Euromike; 03-28-2010 at 10:29 PM.
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  22. #22
    Veteran Member Three Rings
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    being a NA then it will need an exhaust designed for the car to give the correct 'scavenging' from the cyls. in a NA car then fresh air/fuel mixture is not pushed in like a turbo so if you can get the exhaust to "suck" some of the exhaust out then you get more air/fuel in making a bigger bang (hence more power) this is different to back pressure although related. I would have thought a magnaflow exhaust would be designed properly though. at a guess I would say there is some other issue with the car not the exhaust but I could be wrong. how much do they differ in servicing/miles? do they have the same wheels, plugs etc? are they both manual?

  23. #23
    Active Member Two Rings
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    put a pair of aftermarket cats on, have the car retuned (hopefully, some extra advanced timing) for extra flow in (assuming you have done something with the intake), and extra flow out. You will most likely still be down on torque, even with a tune, but with the added hp at higher rpms from a more efficient engine, it should at least feel better than it does now.

  24. #24
    Established Member Two Rings
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    Yeh with my car I noticed a loss of torgue with custom piping and magnaflow rear muffler. I added a secondary muffler where the resonator use to be, also magnaflow. Feels a bit better. But in all honest, I wish I had my stock exhaust back on. With the resonator gone it felt and sounded good and wasn't too loud. Also when I had a b6 s4 I ran with no exhaust one day until I got my new one put on and it was a dog.

  25. #25
    Veteran Member Three Rings anmagro's Avatar
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    any chime in about the side exit exhaust?
    Black On Black 98' Audi A4 2.8L 30V 5spd. Quattro "Esabell"


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    Veteran Member Four Rings Euromike's Avatar
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    i dont really think side exhaust would look good or be productive, plus you would be going right over fuel tanks and such.. not a good idea

    I still have no clue how to restore the backpressure in my system though, I have no parts of the stock exhaust left. The piping size is the same though, if i can find a restrictive resonator to weld in i will...

    Im about to block off half of the muffler and see if it increases preformance.
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  27. #27
    Veteran Member Three Rings anmagro's Avatar
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    ^^^ Try and chamber'd muffler. It will increase backpressure rather than a straight threw muffler like a magnaflow. I will probally do this come better weather in my driveway.
    Black On Black 98' Audi A4 2.8L 30V 5spd. Quattro "Esabell"


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    Veteran Member Four Rings Euromike's Avatar
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    I love my magnaflow though, thats like the last peice of the whole system i would want to replace
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  29. #29
    Veteran Member Four Rings Euromike's Avatar
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    *************thought this was interesting********************


    Backpressure: The myth and why it's wrong.

    I. Introduction

    One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

    II. Some basic exhaust theory

    Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

    III. Backpressure and velocity

    Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

    The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

    Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

    IV. So how did this myth come to be?

    I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

    V. So why is exhaust velocity so important?

    The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

    VI. Conclusion.

    SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.
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  30. #30
    Veteran Member Four Rings Euromike's Avatar
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    *****************888and this guy goes on to argue with him****************

    Wrong. The aim of the exhaust system is to allow the catalytic converter and emission system to effectively process exhaust gas to reduce emissions. Efficiency is a secondary factor, otherwise damage to the converter will occur.


    Come along with me, visualizing the exhaust system and motor....

    1. Gas velocity through the exhaust pipe is not continuous. The person who wrote your guide seems to treat exhaust gas as a continuous flow, which is not so. They paid lip service to the idea of pulses but then modeled their explanation in terms of pipe diameter and not geometry of the system, which is a big factor in "tuning" it so that pulses do not interfere with one another or form resonances that MAY cause increases in backpressure.

    Exhaust gas "pulses". A pulse will contain a mixture of sine waves and thus different frequencies and thus will interfere with each freqs and pulses from other cylinders (and probably from previous pulses from the same cylinder - depending upon the transit time from the exhaust port to tail pipe.

    The pulses will have SOME inference with one another. They may act like waves going down a canal, either reinforcing or canceling one other to some degree. This is where the frequencies present in the pulses matter, some will interfere, others will not. The shape of the exhaust, both in terms of the length of pathways but also a given diameter of pipe, matter. In a continuous flow bigger is better, but that may not be the case with pulsatile waves.

    There will be turbulent flow through the interior of the pipe that will effect some viscous drag, as well as boundary layer conditions. This is dependent upon the diameter of the pipe though boundary layer conditions close to the side of the pipe are influenced by gas temperature and heat transfer, how much I'm uncertain.

    There be transference of heat between the gas pulses themselves.

    There is also mechanical transfer of vibration from the exhaust pipe to the gas and vice versa. Don't know how much of an effect that this will have on pressures and hence back pressure and flow rates.

    The rate of gas pulsation is dependent upon the RPM of the motor. My intuition is that there will be an optimal rate of pulsation for a given exhaust system, one that causes the least amount of back pressure. Might even lie in the motor's normal sphere of operation.

    We would have to model these effects to come up with a exhaust system shape and diameter of exhaust pipe.

    The plot thickens.....

    You have a catalytic converter which will alter the chemistry and thus the molality of the exhaust. That will alter back pressure to some degree. To make things even more interesting, you have two oxygen sensors which feedback into the ECU, which will then alter the chemistry of the exhaust, and this matters because fuel efficiency is a measure of average fuel consumption and not any particular singular combustion event. Chemistry is a big deal for fuel efficiency. There may other emission controls like EGR, which will be influenced by the ECU.

    We have to view the exhaust as a coupled system, starting at the exhaust port, with influence from the other three exhaust ports, the effects through the exhaust manifold, through the catalytic converter, resonators if any and the topology of the exhaust system itself. PLUS the ECU!!!

    I cannot assign weights to some of the effects, especially the catalytic converter, but I think it's safe to assume the follow effects go in this order of precedence... (however one must gather data rather than guess...)

    1. RPM, which will be a factor with along displacement to govern total gas volume. We can view at this first approximation a simple "volume of displacement times the frequency that the piston compresses on the exhaust stroke divided by 2 (four stroke engine, every turn a piston rise and fall). You will probably never get any more gas into the system than the motor can deliver - though that Catalytic converter could create pressure through its chemical action. Changes the enthalpy of the gas either more or less to some unknown degree.

    The frequency of the gas pulsations and their chemistry will affect resonant frequencies of the system and the interference of the gas pulsations.

    2. The ECU will govern the chemistry of the exhaust and its temperature. I think the ECU is where anyone ought to start modifications, if it's legal in your locale.

    3. The Yaris's VVT will have some effect upon valve timing, and hence the rate of dwell for the exhaust valve. Since the VVT is under the control of the ECU its effects will be fedback from the exhaust gas itself.

    Back pressure from the catalytic converter, which will be governed in part by its temperature and the chemistry of the mixture and changes to the mixture.

    I've just described backpressure, by the way, and not mileage. Mileage is a function of quantity of fuel per mile, which is dependent upon the efficiency of the motor, which will be moderated by the ECU.

    Where backpressure, inside of the combustion chamber, comes into play is how it influences the combustion process, and indirectly influences the following combustion (since the exhaust gas carries away heat from the combustion chamber and thus influences how the next mixture will behave and influence the ECU through the O2 Sensor). So there is a thermal history effect at play here.

    Unless you got a lab, lots of time and professionals to set it all up, I'd leave this problem to the Pros.
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  31. #31
    Veteran Member Three Rings anmagro's Avatar
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    dam thats alot to read...
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    Veteran Member Four Rings Seerlah's Avatar
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    I didn't feel like reading this whole thread, but having back pressure will increase low end torque, but be more restrictive up top. Having a free flowing exhaust is supposed to grant less low end torque, but grant more top end power.

    On the 1.8t, "some" back pressure is needed for internal wastegated turbocharger applications. If it's external wastegate, then a free flowing exhaust will help the turbo spool faster.
    I hate it when my car acts like a little bitch, treating me like the bitch

  33. #33
    Veteran Member Four Rings Euromike's Avatar
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    Anyone find me a chambered resonator? i gotta get my torque back! i have been searching the web high and low

    and another question, whats more restrictive? the cat or the resonator?
    Last edited by Euromike; 03-30-2010 at 09:16 AM.
    12' Tornado Red GTI DSG
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  34. #34
    Veteran Member Four Rings somebody5788's Avatar
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    Quote Originally Posted by Seerlah View Post
    I didn't feel like reading this whole thread, but having back pressure will increase low end torque, but be more restrictive up top. Having a free flowing exhaust is supposed to grant less low end torque, but grant more top end power.

    On the 1.8t, "some" back pressure is needed for internal wastegated turbocharger applications. If it's external wastegate, then a free flowing exhaust will help the turbo spool faster.
    I agree with the first part but I'm not sure about the 2nd part w/ the internal external waste gate application.

    I think the ideal exhaust is stock with just a muffler added on and then one of THESE for after 4500 RPM's hehe.
    -Nic

    2007 Nissan Titan - Hard wired Escort 8500 X50 | "Rigid" LED pod lights
    2006 Honda 919 - Dual Yoshimura slip on exhaust | Homelink | RAM X-Grip mount with charger
    1994 Jeep Grand Cherokee - OD Green | Rough Country Long arms | Rubicon Express 4.5" springs | Bilstein 5100's

    (Sold) 2001.5 Audi S4 - APR 93/100 | 710n's | EvoMS intake | RS4 clutch | Stern Snub | Stern motor mounts | GROM Bluetooth | DEI Remote Start

  35. #35
    Veteran Member Three Rings
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    Use a "Turbo" Muffler. They are chambered but also "high-flow". I had a DynoMax 2.5" straight through "muffler" on my 1.8 for a while and it sounded nice. The design was offset inlet to center outlet, but if you looked inside it was a straight diagonal perforated tube very similar to a "resonator". I swapped it out for a $25 Thrush "Super Turbo' muffler with the same inlet outlet design, but it was chambered. I did notice some subtle differences in the power band, but noticed HUGE changes in the sound. The car actually had a more "Euro" sound to it - if you can imagine that. It's tough to get that sound from a 4 cylinder if you ask me, and this muffler did it pretty well.

    This was on a car that had no actual "resonator". It was K03->ATP 3" Testpipe->2.5" pipe all the way back the rear axle. Originally the DynoMax muffler, then switched to the SuperTurbo. These are mufflers you can buy at Advanced Auto...

  36. #36
    Veteran Member Four Rings Seerlah's Avatar
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    Quote Originally Posted by somebody5788 View Post
    I agree with the first part but I'm not sure about the 2nd part w/ the internal external waste gate application.
    I learned this one day while snooping around the B6 A4 section. There was a nice little debate on it, but inevitably there were the reasons posted as to why the internal wastegate needed "some" back pressure while the external wastegated turbochargers didn't. I am not as technical as most others are, so I just pick up usefull information along the way and keep it moving.
    I hate it when my car acts like a little bitch, treating me like the bitch

  37. #37
    Veteran Member Four Rings Euromike's Avatar
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    what garage?
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    so im bringing this thread back from the dead with a simple question to think about.

    Im chipping the car in question. Do you think the remap of the fuel system and such which comes with the flash will make up for the loss of torque my open flow exhaust caused?
    12' Tornado Red GTI DSG
    07' 2500HD Silverado 4x4
    99' Laser Red B5 A4 QM SOLD :(

  38. #38
    Registered Member One Ring
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    No, because the curves on the chip will asume a properly matched exhaust

  39. #39
    Veteran Member Three Rings anmagro's Avatar
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    Quote Originally Posted by BigDanF4i View Post
    No, because the curves on the chip will asume a properly matched exhaust
    I agree. Which is exactly the reason why i will be adding a chamberd muffler before i get my newly SpeedTuning Chip'd ECU back. I would say for you to do the same.
    Black On Black 98' Audi A4 2.8L 30V 5spd. Quattro "Esabell"


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  40. #40
    Veteran Member Four Rings somebody5788's Avatar
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    Quote Originally Posted by Seerlah View Post
    I learned this one day while snooping around the B6 A4 section. There was a nice little debate on it, but inevitably there were the reasons posted as to why the internal wastegate needed "some" back pressure while the external wastegated turbochargers didn't. I am not as technical as most others are, so I just pick up usefull information along the way and keep it moving.
    I know on RX-7's the turbo can overboost if you don't port the waste gate or leave some back pressure but if a 1.8t overboosts when its tuned you really don't have to worry about it. Thats typically referred to as a spike on the 1.8t lol.
    -Nic

    2007 Nissan Titan - Hard wired Escort 8500 X50 | "Rigid" LED pod lights
    2006 Honda 919 - Dual Yoshimura slip on exhaust | Homelink | RAM X-Grip mount with charger
    1994 Jeep Grand Cherokee - OD Green | Rough Country Long arms | Rubicon Express 4.5" springs | Bilstein 5100's

    (Sold) 2001.5 Audi S4 - APR 93/100 | 710n's | EvoMS intake | RS4 clutch | Stern Snub | Stern motor mounts | GROM Bluetooth | DEI Remote Start

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