# 2 or 3 inch exhaust?



## evil_audi_tt (Feb 10, 2013)

Im sure this is a debate for a better part of a decade as it always is for 4 cylinders..... what exhaust is best for a 180q? 2 or 3 inch as far as power gain and back pressure? Also the single v.s. dual.


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## hunTTsvegas (Aug 27, 2012)

evil_audi_tt said:


> Im sure this is a debate for a better part of a decade as it always is for 4 cylinders..... what exhaust is best for a 180q? 2 or 3 inch as far as power gain and back pressure? Also the single v.s. dual.


I would say it's all going to depend on specifics. Do you plan on maxing out the stock turbo or do you plan on going big turbo? Something to that effect is going to drive your decision on piping size.

Next, when you say single vs dual are you talking mufflers because to get technical, both FWD and Quattro cars are single mufflers exhaust systems (as you know quattros have dual TIPS). When I think of duals, I think a Y somewhere in the pipe with two individual mufflers. Although this could be done, with the amount of space available it would be easier to stick with a single muffler if you want to have a muffler at all. Some people don't even run one. 

Food for thought. :beer:


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## Marcus_Aurelius (Mar 1, 2012)

evil_audi_tt said:


> Im sure this is a debate for a better part of a decade as it always is *for 4 cylinders*..... what exhaust is best for a 180q? 2 or 3 inch as far as power gain and back pressure? Also the single v.s. dual.


You're forgetting a huge part of the system that would make your reasoning and this whole "piping size" and "back pressure" debate valid... *the turbo*. A turbo provides all the back pressure (and then add plenty of restriction in our case) to an engine, especially with a small 1.8l of displacement and choked up manifold designs. Basically (without getting into it and starting another Cold War), post turbo, no piping is the best piping if performance is the driving goal. So, to answer your question, 2" should not even in the discussion and 3" is OK until you start pushing things (keep in mind, to make it work and bearable, you'll have to add bends, cat converter, resonator, muffler etc. further adding pressure drop to what 3" can flow).


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## Marcus_Aurelius (Mar 1, 2012)

hunTTsvegas said:


> I would say it's all going to depend on specifics. Do you plan on maxing out the stock turbo or do you plan on going big turbo? Something to that effect is going to drive your decision on piping size.
> 
> Next, when you say single vs dual are you talking mufflers because to get technical, both FWD and Quattro cars are single mufflers exhaust systems (as you know quattros have dual TIPS). When I think of duals, I think a Y somewhere in the pipe with two individual mufflers. Although this could be done, with the amount of space available it would be easier to stick with a single muffler if you want to have a muffler at all. Some people don't even run one.
> 
> Food for thought. :beer:


Beaten to death!
http://forums.vwvortex.com/showthread.php?5841144-single-to-dual-tip-exhaust&highlight=Dual+exhaust


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## hunTTsvegas (Aug 27, 2012)

Marcus_Aurelius said:


> Beaten to death!
> http://forums.vwvortex.com/showthread.php?5841144-single-to-dual-tip-exhaust&highlight=Dual+exhaust


Wow. Thanks for the read!

I'm no engineer but doing something as simple as installing Commercial and Industrial HVAC systems (before joining the Army) taught me the importance of fluid dynamics and how something as small as a 45 degree elbow would increase the amount of HCFCs (Hydrochloroflourocarbons = Freon) needed to reach the optimal pressure in the system so that everything "played nice" together. I understand that not all fluids (air, liquid, gas, etc) operate the same way either. 

I did always enjoy science though. Now if I could learn to love math then I think I would be one dangerous southerner. Hahahaha! :beer::beer:


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## Forty-six and 2 (Jan 21, 2007)

I wouldn't consider anything less than 3". I have heard velocity problems can occur when going with too much larger without having the output to warrant it. The exhaust gases will end up cooling off before reaching the the exit. This cause a "wall" of "cold" gases creating back pressures.


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## esoxlucios (Sep 17, 2009)

Marcus_Aurelius said:


> Beaten to death!
> http://forums.vwvortex.com/showthread.php?5841144-single-to-dual-tip-exhaust&highlight=Dual+exhaust


Max, just read that thread. Very frustrating topic.:banghead: 

I express no opinion about single vs. dual (I replaced my 180 stock with the Blue Flame (formerly Forge) system long before I read any of these threads). 

But, back in the 90s, when I was really into motorcycles (BMW K1, BMW R90/6, heavily-modded Yamaha XJ750J), I used to subscribe to the Motorcyclist, and I loved reading Gordon Jennings' tech articles. He was a pithy, erudite columnist with a gift for lifting the "obnubilation" over topics like how to read spark plugs and how spark and valve timing work together. He died in 2000. 

One of Jennings' most memorable columns was his explanation of why some "back pressure" is needed. I'm not sure this is totally inapplicable to our cars, since exhaust gasses bypass the turbo in all situations other than boost.

The gist of your comments in the aforementioned thread seems to be that anything that impedes flow is a power-robbing hindrance. I don't know enough to agree or disagree. But, my take away from Jennings' explanations is that straight pipes on bikes (including 4 stroke) aren't ideal, and it seems logical this would apply to automotive combustion engines, too.

Although I don't have that column handy, Jennings recycled his contentions in his book, "The Two-Stroke Tuner's Handbook," and in other articles, such as this one, which is a great read: http://www.bridgestonemotorcycle.com/documents/do_you_really6.pdf. You'll especially love his disclaimer on the subject:



> I must add that I do not regard my own understanding of [exhaust scavenging] as being the Almighty's Revealed Truth. My theories are in reasonable coincidence with physical law . . . Those of you who hold different views on the subject (and experience tells me you cling to your views with a tenacity most people reserve for minor religions) are welcome to them.


I assume it's this concept of scavenging in our four cylinder motors that's behind the claims that larger downpipes may cause losses in low and mid-range torque.

Why I care: In my car, I've got a custom-made 2½ downpipe (coming out of the GT20RS connected to the APR inconel mani) that has extreme bends in it to clear the transfer case. Was made by a guy at Tim's Custom Exhaust (who's since been fired — quite a story to that). At the time, he told me it was non-sense to have a 3-inch, because the Garrett outlet was only 2½. But, subsequently, Chuck at Full Blown Motorsports looked at it and said he could easily make a 3" with less extreme bends and would be an improved design. Because I didn't want to spend even more $$ redoing it all over again for unclear benefits, I just wrapped the downpipe in fiberglass (it's also ceramic coated) and persuaded myself that this would be efficient at scavenging.


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## Forty-six and 2 (Jan 21, 2007)

While the need to retain some back pressure is necessary in an n/a situation, any addition back pressure post turbo will only hinder it's spool time. The turbo create the pressure needed for the motor during all driving conditions. That is why he mentioned that an open turbo is the best, not realistic, scenario. The goal, obviously, is to allow the turbo to spin as freely as possible. Zero back pressure is optimal, but unlikely to obtain while having a full system. Like I said you will run into a velocity issue when using too large of tubing without truly needing it. 3" is typically a good diameter pipe to use, and in some cases 3.5" will free the system up a little more, if the size of the turbo needs it to help reduce spool time. When using a cat and/or a restrictive muffler 3" would suffice in the majority of cases.


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## Marcus_Aurelius (Mar 1, 2012)

esoxlucios said:


> Max, just read that thread. Very frustrating topic.:banghead:
> 
> I express no opinion about single vs. dual (I replaced my 180 stock with the Blue Flame (formerly Forge) system long before I read any of these threads).
> 
> ...


 
You're mixing theories and applications a little bit (naturally aspirated vs turbocharged, 2 stroke vs 4 stroke, although they're all based on the same internal combustion principle). Simply put, "back pressure" and scanvanging effects are for na applications (although in reality, for na motors, it's the need for keeping velocity high that brings back pressure into the picture as a byproduct). 

I repeat, for *turbocharged engines* you want the least amount of restriction that you can have. Obviously, due to packaging, emission where it applies, and noise suppression, you have to make some concessions and compromise a bit. However, the physics remains, regardless of who wants to accept of adopt it, "NO BACK PRESSURE OR RESTRICTIONS POST TURBINE". 

You also seem to think that all exhaust pulses get to bypass the turbine wheel, whenever you're not in boost. This can't be further away from the truth. Actually, whenever there is a need to slow down the turbine wheel speed and bypass some pulses (which also happen in the upper revs while in boost), only a percentage of the pulses get to be diverted away and flow openly around the turbine. The reason for that is the size of the actual gates (more so on internally gated turbos, but a big restriction still, even in the best performing externally gated setup). So, even when bypassing the turbine wheel (although a substantial percentage still flow through the wheels), there is plenty of "back pressure" to go around and keep the motor happy. 

Finally, to touch on the piping diameter subject, we know that we want zero restriction after the turbine. Now, what piping size/ID is ideal to achieve this? It all depends on the application! On our 1.8L turbocharged application, 3" it is a safe bet on stock KKK sized turbos. But there's a catch, push things hard enough (high boost, low system restrictions, high pressure ratio, fuel with high exhaust byproducts like Ethanol, etc.) and the 3" rule of thumb might not even be enough. My car is an example of that, with 3" ID all the way through, very minimal bends, I still could use some more piping diameter based on pressure recorded near the atmospheric dump. At the end, an exhaust is tuned system, and with our turbo motor that have no use for restriction post turbine, it's better to be a bit bigger (unnecessary noise and maybe a bit of added weight), than too small (killing efficiency and power). 


Below is what one of the engineer at Garret posted on the same topic in a different forum opcorn: :


_
Turbo Exhaust Theory

The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on Impreza.net regarding exhaust design and exhaust theory: 



" This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems. 

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here. 

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end. 

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure. 

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range. 

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.” 

"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles. 

A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above. 

If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow. 

Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all. 

Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems. 
Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure. 

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.” 

"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however. 

There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge. 

As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.” 




"Here's a worked example (simplified) of how larger exhausts help turbo cars: 

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is: 

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure 

o here, the turbine contributed 19.6 psig of backpressure to the total. 

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case). 

So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from. 

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level. 

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would. " _


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## esoxlucios (Sep 17, 2009)

Lot of information here to consume — thank you! I still encourage you to look up Jennings' articles, because they are applicable to 4-stroke, and he talks in terms of "positive" and "negative" sonic pulses and reflections, rather than exhaust gas "velocity," and I would be interested in hearing your feedback on his assertions (not in dispute of what you're discussing here, just as an independent, academic concept)..

Although I am tempted to get this 2.5" downpipe redone (the exhaust is already 3") at the same time as when I put in the new GTX turbo (discussed in another thread), I still have a question: Why is it that I've read here and there that a downpipe too large can rob low and mid-range torque in our forced-induction motors?

I've talked to David at Nür Technik this morning about getting together an estimate for the downpipe and new turbo. If I'm putting out 331awhp on the APR Stage 3+ kit now, I wonder by how much it'll improve.

BTW, is this considered a thread hijack? This is all germane to whether to run 2.5" or 3", right? If it's hijacking, I apologize and will bow out.


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## Marcus_Aurelius (Mar 1, 2012)

esoxlucios said:


> Lot of information here to consume — thank you! I still encourage you to look up Jennings' articles, because they are applicable to 4-stroke, and he talks in terms of "positive" and "negative" sonic pulses and reflections, rather than exhaust gas "velocity," and I would be interested in hearing your feedback on his assertions (not in dispute of what you're discussing here, just as an independent, academic concept)..
> .


 I will definitely read through the article referred as time permits this weekend. I'm always down for a good technical reading. :thumbup: 



esoxlucios said:


> Although I am tempted to get this 2.5" downpipe redone (the exhaust is already 3") at the same time as when I put in the new GTX turbo (discussed in another thread), I still have a question: Why is it that I've read here and there that a downpipe too large can rob low and mid-range torque in our forced-induction motors?


 I wouldn't put too much into what you read "here an there", there is a lots of keyboard experts roaming the internet, so everything needs to be taken with a grain of salt (I'm not exempt). Everyone have opinions that may or may not reflect what really happens in practice; heck, the neatest theories (even when backed up with solid math) are sometimes full of wholes when put in practice. 

There is too much empirical data, dyno plots, and solid theory to follow random keyboard warriors that are likely spitting stuff they "heard" themselves, or simply rationalizing their own modification path and mistakes (the average enthusiasts tend to adopt anything that somewhat solidifies whatever investment they made, my advice to you is do not fall into that hole). There is a point of diminishing returns with Downpipe and exhaust tubing diameter in turbocharged application. However, past that point of diminishing returns, you won't gain anything more, but won't loose anything either - the point is remove all possible restrictions post-turbine, nothing more, nothing else. If 3" does the job, there is no need for 4", but it won't rob you off your power like a choked down, restrictive 2.5" would. 

To focus on your particular setup, you're about to run a turbo that can flow a lot more than what the existing downpipe can allow to escape without crating a blockage and possibly cause sonic pulse reversion in the valve overlap process (this was experienced first hand by the earlier adopter of the JBS manifold. Simply allowing improved flow at the collector blockage point, cured the issue). The concept is quite simple, although often overanalyzed or misunderstood, maybe a simplistic break down will help it sink in for many: 

1) Pressure ratio before and after the turbine wheel is what makes it spin and determines the dynamic behavior. High or elevated pressure before, and zero/negligible pressure after, will spin the turbine wheel stronger and faster (that's where the energy from a turbo is generated). Optimize this and you will gain spool and power (this is regardless of what's happening on the compressor side of things where most people unfortunately seem to fixate their attention). 

2) Pressure before the turbine wheel will always rise and be high (obviously unless there is a mismatched selection of turbo for the displacement) . This is due to the fact that the exhaust pulses have snake their way through manifold runners, a collector, and finally small space left in between the spinning fins of a wheel. This exhaust pulse journey will allow pressure to be built in the hotside, just like it purposely does so on the compressor/cold side of things. 

3) Pressure after the turbine wheel will generally be lower. So, unless you're seriously choking things down with an non-optimal discharge portion of the turbine housing (bad turbine to DP flange design), restrictive downpipe section, and other choke points downstream, you should have a lot less, and ideally *zero* pressure past that point. 

Apply these 3 simple points to tuning your discharge on any turbo engine, and you're on your way to an efficient hot side (obviously there are other things involved like wastegate efficiency, but that another misunderstood topic in itself). 

Take it from the guy that picked up all his knowledge (still very limited) from trying, testing, and making mistake in the field while racing: "don't do it to yourself, attempting to push a GTX (even a 2863 at low boost target) through 2.5" ID piping. The rest of the turbo-back diameter don't mean much if you have that choke point in the lead. :beer::beer: 

BTW, your questions are what brought life to the thread and discussion, so don't worry about the hijack aspect of your posts.


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## esoxlucios (Sep 17, 2009)

Finally got the 3" exhaust fabbed up by Nür Technik in Minnesota. Pictures are below. I also went from a GT2860RS to a GTX2860R, so it's hard to tell what changes to impute to which mod. Also, it's been since March since I've driven the car, and I've been driving my wife's bi-turbo X5 diesel, which has incredible torque and power delivery, so my ability to compare before/after is impaired. 

In general, there is noticeable lag (the LiquidTT shows the actual boost somewhat behind the requested boost), but has very strong midrange and top-end. I am getting only 3˚ of timing pull on all four cylinders on a hot day, whereas I used to get as much as 6˚ previously, suggesting that the turbo isn't getting out of its efficiency range as much as it did before.


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## Teeguzi (Jul 22, 2011)

Great read; thanks to everyone in this thread for enjoying and inputing information without all the normal flame throwing.

I've been approaching exhaust expectations using the Mean Free Path information I use on the vacuum systems I maintain. Basically it says that the more impacts (directional changes) a molecule makes on the way out (to pumping source) = increased pumping time to reaching absolute pressure. We measure in the millionths of an atmosphere.

Of course mechanical limitations always offer boundaries to design and efficiency.

Good read: http://en.wikipedia.org/wiki/Mean_free_path

In our cars; the more times gas molecules impact each other and/or exhaust components = less power due to restricted flow. As mentioned before the need for back pressure is more than satisfied by the giant restrictor in the circuit we call a turbo.

Cheers!


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## Tdi13golf (May 31, 2014)

Marcus_Aurelius said:


> Below is what one of the engineer at Garret posted on the same topic in a different forum opcorn: :
> 
> 
> _
> ...


Bumping old thread. A few members on the mk7 section claimed 3" pipe was proper for stage 3 big turbo 2.0 cars that are pushing 500hp.


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## Marcus_Aurelius (Mar 1, 2012)

Tdi13golf said:


> Bumping old thread. A few members on the mk7 section claimed 3" pipe was proper for stage 3 big turbo 2.0 cars that are pushing 500hp.


Don't know about that! 10ft of 3" piping with several bends is guaranteed to create some pressure with a 2L motor as full swing. The point where bigger is not better with turbo exhaust is when there is zero pressure built into the exhaust system post turbine. So if they can make that happen on 2L of displacement, then they have won the battle (sarcasm).


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