Dp testing and results THIS IS LONG SUPER LONG

[Justincredible]

I feel it is important for people to understand the testing and the results I have gotten performance wise (13.3 @ 107.3) were from LONG testing on all sorts of performance stuff from chips to full exhaust.

The Idea was to find the best Dp and exhaust for the money. I wanted to pit stock Dp's modified against the 4 times more expensive aftermarket Dp's offered from Audi performance shops. The Idea was simple the B5 guys are using stock modified Dp's (piggies) and going low 11's why are we spending over 3 times as much to get the same performance. So I decided to see if the extra money was worth any more performance

I have seen people from time to time mention they are looking for the best chip. The best chip for your car is a good set of Dp's and a properly sized and built exhaust for your performance goals.

Chips should be the last thing you think about for the B6/7 not the first thing. There IS TONS of power to be had in the exhaust if done proper. Thus me sharing these results for people to look over and come to there own conclusions.

I feel that my conclusion and results speak for them selves in the performance of my car


This first post dose not include the Dp testing results. That is in the post below so feel free to skip down to that.

This post is to help some of the guys understand some things about exhaust that they might not know or just need to be refreshed on.

This is not a thermal dynamics or fluid dynamics post meant to change the world. IT is a plain as day wording of why parts of my Dp conclusion are important. I am not going to fill the post with terms that will have some searching for the definitions just so I can sound smart. I am not here to try and step on a soap box and show off my Internet engineering degree or try to advance some on line adjective. So for those of you who are going to pick apart any thing because you have nothing better to do remember If you disagree that is fine. This is the best way I could explain things to help bring those up to speed that may need it.

On to some of the back ground information that is important to the testing below

The goal behind changing Dp's it to get more performance out of the motor. All the Dp's tested removed one of the cats from the Dp design. The thought to this is If we remove one of the cats from each Dp we eliminate some of the free flowing restriction on the motor.

The best exhaust for our motor is one that will Give you a pipe diameter wide enough so that there is the least level of back pressure possible whilst achieving the highest exhaust gas velocity.

What dose this mean?

This means you want a exhaust that will allow the air to move as fast as possible without creating turbulence or an obstruction in the flow. Turbulence is an uneven flow of gas that creates a uneven flow pattern that actually slows down and can obstruct the exhaust.

AS AN EXAMPLE:

We see obstructions or turbulence if you turn on your out side water spicket. If you don't have a hose on the end of the spicket you see the water coming out going in several different directions. This causes the water to actually slow down coming out of the spicket. If you put a hose on the spicket you can create a positive alignment of water flow where the water is all now WORKING together to move in a specified direction as quickly and efficiently as possible. There is JUST enough back pressure in the hose to keep all the water moving close enough to quickly move in the same direction. So basically proper alignment and back pressure will corral the water and the exhaust in the proper direction in a much faster more efficient fashion then without.

Why is it so important to move the exhaust as fast as possible?

Well exhaust comes out in pulses (you can see this put your hand over the tail pipe you will be able to feel the pulse) and the thought here is if you can move the first pulse fast enough you create a low pressure area between the first and second pulse. In doing so The low pressure behind the first pulse creates a vacuum to pull on the second pulse and so on with the third exhaust pulse and forth exhaust gas pulse ETC ETC. The faster you can move the pulses the stronger the draw or vacuum you can create between pulses and less power is lost because of improper exhaust flow from the motor.

OK so now that we know what we want from an exhaust why do we want aftermarket Dp's that remove one of the cats.

The basic idea of a catalytic converter is to clean the out going exhaust gases. But for a cat to work proper and strip the chemicals out of the exhaust gas the cat needs to be very hot. To get the cat hot you need to slow the exhaust passing threw it down long enough to bring and keep the temp of the cat up to operating temps. Doing so creates a restriction and adversely effects our wanted end result in the exhaust flow.

Aftermarket company's have found that you can still clean the air with 1 set of cats not 2 ( we have 2 cats per Dp and we have 2 Dp's. One Dp on driver side and one Dp passenger side) as we have on our car. So removing one of the cats from Each Dp will now be LESS restrictive and help us to get closer to our optimal exhaust gas flow range. Thus allowing the motor to make more power threw the added exhaust flow.

On a side note:

Most people have said that adding a cat back with out doing the Dp's results in almost no added power. The biggest changes that come from a cat back usually comes in the form of sound and not performance. This most likely would be from the fact that the cat back exhaust is not the biggest part of the exhaust restriction. HOW Ever the cat back will be more effective after a Dp swap due to the fact one of the biggest restrictions has been removed and in doing so the cat back would benefit from a less restrictive Dp system.

Even with the best set of Dp's you need to asses what your goals are and match the cat back to the Dp configuration. The Dp's are just one part of this equations. If you don't properly match your cat back to your Dp's then you stand to loose power. MANY have seen or "FELT" this. The REAL issue is what Dp's do you have and what it the best solution for that. H pipes are imporntnat and A X pipe is only needed if you go the open route. Then in some cases you don't want a X or H this is all biased off what your doing with the Dp's so lets talk about the Dp's

[Justincredible]

Before I start let me say that during this testing processes I was in contact with a few EXHAUST specific companies trying to make the best exhaust for my car. After the Dp testing was complete I was able to then ask some of my questions about the Dp's results to the bigger more experienced Exhaust companies. It just seemed stupid to not ask other important exhaust related questions while I was inquiring about the cat back

The results have been talked over with FLOWMASTER (who was kind enough to lay out a very precise exhaust system. They helped me piece by piece and the proper placement for each piece. I just had to have it built. THANKS TO JHM for that) not only did they lay it out and explain why say the X pipe had to be xxx amount of inches from the headers and the mufflers had to be xxx distance from the X. They also went on to say there needed to be xxx amount of pipe after the muffler before open air.

I discussed cat options and my results with Magnaflow as well. They offer aftermarket cats and until I talked with them I thought this might be a good way to go. They went on to explain that OEM cats are better then most people think.

I also contacted Hooker headers about the possibility of headers and continued help with some of my cat and exhaust questions. Making power from your exhaust starts with understanding your exhaust manifold. They were very helpful in understanding the impornants in the transition from header to exhaust

OK with the interest of the best performance gains for the $ I have tested chips, Dp's and cat back exhausts to find the best one for ME. This is what I found and the parts I tested

1 Milltek Dp's. I used Milltek because they represent the BASIC pre cat design used by the other companies that offer Dp's for our car. That design, removes the main cat and puts a aftermarket cat where the stock Pre cat was

All the Stock Dp modifications were done at JHM. I sent JHM my cores for the #2 test set and cause you can't replace the removed cats JHM needed to HELP supply the cores for the #3 test. Cost for this modification cost me 450$ that is the standard fee for Dp modification

2 Stock Dp's with main cat removed...this would be just like the Milltek design. But obviously using the stock pre cat and not a aftermarket pre cat

3 Stock Dp's with the pre cat removed and the main cat still intact. This completely removed the PRE cat. This is very popular with the B5 guys and has worked as the staple for them for years.

4 Stock Dp's no cats and test pipe installed where pre cat was located to make for a smooth transition (there is a explanation of this later)

All the above were on a stock cat back and no other changes

RESULTS

Any cat removal Definitely gave a gain in performance. There is just a point where you would have to be dead to not know the car is running better. But to keep the info fact biased I did acceleration logs in Vag com.

All of the Dp combinations gave a jump in the RPM/pr SEC or the motor was able to REV threw the rev range faster then stock.

Due to the fact that the Milltek is almost the same size dimensions to the stock Dp. The results didn't shock me. If you want Milltek performance you can get it for about 1/4th the price. Remove your main cat. Performance wise there was NO difference between the stock modified (2) and the Milltek....NONE

Now here is where it gets interesting. The biggest jump came from removing the PRE cat and using the main cat. The car really responded to this. The car made a much larger jump in the RPM/per Sec.

Lets look at why this happened

To me the results seemed to make since. Don't put a restriction (PRE CAT) so close to the header collector. Altho I believe removing the pre cat is the way to go. I only had my highly tuned butt dyno to go off of along with the RPM/LOGS from Vag com So During the cat back portion of the experiment I started asking the people at flowmaster and magaflow, hooker about there thoughts on this.

All of the companies I talked to said about the same thing concerning the pre cat and any restriction or exhaust disturbance

The collector (the very end piece of the exhaust manifold where all the runners meet) should typically have 12-24 inches before a large and abrupt change in system cross-sectional area. OR any restriction of any kind. Basically as far as performance is concerned you don't want your cat that close to the header collector.

So back to the testing results

Putting the precat with in a 1 foot of the collector was definitely a hamper on performance. BUT we need to also look at the fact that if we REMOVE the PRE Cat on stock Dp's we are Also causing a abrupt change in the system cross sectional area with in less then 1 foot area. This is The large empty space in the Dp where the PRE cat used to be.

So is the empty gap left behind when you remove the pre cat worse then the cat it self?

NO the cat is a restriction and only gets worse as you climb in RPM. The turbulence (if any) caused by the now open Pre cat space Will fill and regulate flow in the higher PRM. I found the spot for me to be Around 2200.

So Why don't the Aftermarket companies know this. Well they do...Some of them that is.
I talked to Danny and FI. He knew what I was talking about and even mentioned that he moves the cat in his system as far back as he can to try and help not rob the motor of the power. He went on to say that if he moved the cat back to the normal position of the Main cat That would be better for performance. But from a manufacturing and emissions stand point he found his cat placement to be a GREAT compromise.

So if performance companies know you loose power threw a Pre cat why do they still use it. Why not just use one big main cat like they did for the past 30 years

But the trend of the main cat came from A federal bill

Federal Bill and the birth of the pre cat

In 2000 a new clean air emissions act went into place that required even cleaner exhaust emissions. The answer to this by the majority of the auto industry was the PRE cat.

The precat is less then 4" from the header collector. In the pre precat era the cat was almost 2 feel way from the header. This has been tested and accepted by all the major exhaust companies as the proper distance for performance and clean air regulation.

The performance companies that make Dp's for the Audi use the precat and remove the main cat. The thought behind this is that you move the cat closer to the exhaust manifold you can have a smaller cat. This is true. With the cat so close to the exhaust manifold you can get the same result with a smaller pre cat that you would get with the bigger main cat. On top of this the after market companies can use a less restrictive cat then stock and still get clean emissions.

One down fall to this aftermarket pre cat is the extreme heat generated from being so close to the exhaust manifold can ruin the cat. What can happen is the cat it self starts to fail due to the extreme heat and in such becomes more restrictive then the stock unit.

Hooker and magnaflow insist that you want the cat back at least 2 feet. They claim this is and has always been the best distance for clean air and performance.

Still there has to be a reason why companies don't use the main cat and go with the pre cat.

There are several

Stock cats are mandated by law for there durability and content of ceramic and Platinum content along with other expensive metals that make OEM Cats EXTREMELY expensive and extremely durable. Aftermarket companies don't need to meet these requirements and generally don't come close..GENERALLY i'm sure there are some that do I didn't test all the aftermarket companies cats nor did I try to account for witch ones are used.

A direct quote from flowmaster "There is no need to make a aftermarket cat. We never have and I don't think we ever will. The OEM units are getting better and better. If we were to try and meet the same standards as the OEM units but better the flow. The price would be so outlandish no one would ever get one."

So if you don't want a set of 2000$ Dp's the pre cat is a cheap CLEAN way to put in a cat. When I mean cheap I don't mean cheap as in bad.

My conclusion
Im going with the experts on this one.You just can't go wrong with the removal of the pre cat. It works for the B5 guys the cars are still able to pas emissions. Power is IMHO better and it follows the performance cat placement rules that have been in place for years.

Until the 2000 law it was almost unheard of to put a cat closer then 2 feel away. you only saw this in few occasions and they did it in apps with generally forced induction. The old 4.2 Audi's didn't have a pre cat. With precats the exhaust gas gets almost trapped right at the header collector. The heat trapped there IMHO is not good. You want to get the exhaust out and AWAY from the motor. We have seen what happens to the B5 if you don't remove the pre cat. Not only that but every aftermarket Dp for Almost every other V8 car on the market doesn't use the pre cat design

This doesn't mean that all after market Dp's with pre cats are bad. There was a difference in removing the pre cat. But some companies like FI and APR move the pre cat back a little to help the exhaust to collect proper flow coming from the header. I didn't test this design. SO I can't say how much this may help.

Even still if you were to run stock Dp's and remove your main cats or use Dp's that don't move the pre cat. You are still making better flow then stock. There is still a big power difference there. To me the difference between pre and main cat was easy to decide. I just removed them both.

There are always going to be arguments for this or that. Pre cat or no pre cat. But remember this is not a argument these are JUST my findings. If you want to spend big money on shinny pipes that is up to you. But for 450$ or free your self. IMHO you are not getting your moneys worth for any of the current designs out there

If you ask me. The best chip for your car is a set of Dp's. There is no sense trying to get more power out of the motor without letting the power get out first.

[cabracco85]

Good write up justin, Thanks for your efforts to move this car forward.

[bsrpilot44]

Wow that's a serious write-up....guess my DP's were the right choice.

Well done bud.

-B

[beemercer]

Very well put together , great read.

[dannyj777]

damn...that was thorough. thank you.

[mioStile]

This writeup is awesome. But just to be clear, if you take out the stock pre-cat and leave in the main one, you most likely will not pass emissions, right?

[Justincredible]

This writeup is awesome. But just to be clear, if you take out the stock pre-cat and leave in the main one, you most likely will not pass emissions, right?

Sorry about that. No actually you can pass emissions either way. Its just that the precat design tends to destroy more cats.

Not only that but no other performance aftermarket company domestic or other wise puts the cat that close in there design.

BMW M5 has a pre cat from the factory They moved it down stream as well

http://www.gmpperformance.com/index....ail&PID=141681

http://www.gmpperformance.com/index....ail&PID=200185

http://www.americanmuscle.com/slp-x-...tted-2005.html

http://www.stainlessworks.net/cart/p...oducts_id=1611

http://www.johnnylightningperf.com/s...roducts_id=154

http://www.stainlessworks.net/cart/p...oducts_id=1161

It doesn't matter from BMW to Mercedes to mustang. I have not found ONE not ONE manufacture of aftermarket Exhaust system That doesn't follow the 12"-24" before the cat rule.

http://www.bestperformanceautoparts....SLP-30125.html

http://www.americanmuscle.com/slp-catted-x-gt500.html

http://www.edelbrock.com/automotive_...ofit/ls1.shtml

http://www.jbaheaders.com/feature_pages/04GTOnew.asp

http://www.stainlessworks.net/cart/p...oducts_id=1510

http://www.rpmoutlet.com/c5tran.htm

http://www.xmotorsports.com/bb-trifl...-cats-p-3.html

Look Im not trying to start a huge issue here. You can get performance with a pre cat and any of the current Dp's on the market.

But one raise your hand if you do have a Dp with the pre cat and you have had a failure I"VE seen several pre cat failure posts on here. What if your pre cat is clogged and you just don't know it and you are loosing power. Go ask the B5 guys how many cats they have fail. GO ask all the PIGGIE Dp fellas how many stock modified pipes had an issue. You not going to find many if you can find any at all

Other then the few Audi performance companys that seem to just be copying the same bad idea over and over. There is not one company out there that puts the cat that close. Trust me call any of the companys that make these Dp's and they will tell you. OH don't worry about this we have tested this and its not an issue. MMM ok well the funny thing is they are the only ones saying this and the only ones doing this. If it wasn't an issue why did my car pick up so much removing the pre cat.

This is the kind of info that the performance companys are going to shrug off as no big deal. Im trying to stop this "just throw them something and they will buy it attitude with the Audi performance companys" We saw it in the chips and now we see it in the Exhaust. Smoke and mirrors Clearly little to no testing and if think I may be harsh all the companys below have a pre cat style set up on one or more of the cars from the factory..(im sure there is more but to help keep this from being a novel)
Mercedes
Jaguar
Mustang
BMW
GM
HONDA
No after market companies for any of these cars did I find a Pre cat Dp design. They all moved the Cat back or if they did a header design it was 12"-24" away

Notice the links above are from different companys. The funny thing is THEY ALL Follow the same IDEAS and they ALL seem to claim more HP then what we are getting with the current Dp offerings. MMMMM WONDER WHY.

In the end there is good news. You want REAL performance you can spend 450$ for Piggies (vast modified Dp's) or you can modify your own Dp's (as A side note I tryed modifying a set of Dp's..you are best off spending the 450 no CEL and the best money spent over trying to do it your self) In the end you get more performance for 1/4th the price

[Howard Hughes]

That's one hell of a write up,... nice job.

[Captain Insano]

Awesome info as usual. You da man!!!

This info is like gold for us folks getting ready to do the exhaust thing. :-)

[Cargo8]

So you suggest just modifying stock exhaust system to remove the precats?

[Justincredible]

So you suggest just modifying stock exhaust system to remove the precats?

The cat back needs to get replaced the stock one is not going to sound good or help with the over all performance. The Dp's you don't need to replace. They are about the same dimensions as the aftermarket Dp's so size wise you are not loosing there. Just remove the pre cat BAM aftermarket Dp's but better. Hell you can copy the milltek Dp's just remove the main cat on your stock Dp's.

[dextrek]

Great write up.
I would like to see what is YOUR ideal set up on a diagram or a picture.
I understand the concept but I would like to see the comparison in a visual.

For example: Stock diagram vs. Mod 1 vs. Mod 2 and such...

I found a diagram for WRX...sorta similar setup as audi.


(except, I believe their is no piping between the Front Cat and the turbo. Correct me if I am wrong.)

[93hrdtptt]

excellent writeup!

this may be the first mod I'm gonna have to do lol

[91gl]

good stuff as always Justin. another thread that shuts up the people who who claimed this does the car more bad then good. the people that have not one clue what they are talking about when tuning a car and said that you actually lose power because this car is "tuned for backpressure" and that a free flowing exhaust hurts performance. i have waited a long time for someone like Justin to come along and it is a shame that I am leaving the scene right as soon as he came in. But stuff like this will keep me in here regardless what i drive. Kudos to you man


i had the same setup as written above. gutted milltek dp's. let me tell you this, the car ran like a bat out of hell with that setup and after driving it for a while with it like that, i forgot how good it actually is until i put my stock fullback exhaust on 2 weeks ago. if i could compare the feeling i would compare it to a chipped b5 s4 then flashing the car back to stock. the car is completely different. revs up soooo much slower, and feels like it has no will in the upper ranges. i had to learn all over again how to rev match because now i need to blip the throttle harder to make up for the slow revving. and i cant even tell you all how much i miss the sound. :(

[b00st]

91gl....you ran straight pipes. we said you needed some back pressure. going with none will hurt u. at least thats what i said as i recall your thread when you said you were going straight pipes and you confirmed whatever i posted in your thread and you put your system back on...which was better.



so to justin...i think what most are wondering is if you moved the cat to a different location + delete precat or just used stock DPs and deleted the precat.

[drew]

i thought the FI downpipes only have one cat on each of them, there is no precat right?

[91gl]

91gl....you ran straight pipes. we said you needed some back pressure. going with none will hurt u. at least thats what i said as i recall your thread when you said you were going straight pipes and you confirmed whatever i posted in your thread and you put your system back on...which was better.

the reason i put mufflers on was because it was too loud. i would not have made it half an hour without getting a ticket.

and the whole argument in that thread was that mufflers create back pressure and that without back pressure i will lose torque AND hp which is completely false. you lose torque but NOT hp. you gain shit loads. and backpressure is created by just the diameter of the exhaust piping, mufflers and cats are just restrictions. they create the unwanted back pressure by restricting flow of the exhaust.

[Justincredible]

so to justin...i think what most are wondering is if you moved the cat to a different location + delete precat or just used stock DPs and deleted the precat.

Sorry for any of the confusion.

Here is a real bad diagram
stock design -p----m--
milltek design -P-------- The P stands for pre cat
Piggie Design 2 -P--------
Piggie Design 3 ------m-- The M stands for Main cat and this represents 99% of all aftermarket performance exhaust designs out there.
My current design ---------

The Stock Dp's were never changed from there current design other then the removal of 1 cat each side for each test. The current design on my car not only removed all the cats but removed the cat can and replaced it with straight pipe. This was probably over kill but I thought what the hell

i thought the FI downpipes only have one cat on each of them, there is no precat right?

Yes and No. let me say this. There are some companies that move the pre cat back to help FI is one of them. This still used Pre cat design. It is not going to stop you from getting more power over 100% stock Dp's. These designs still work All the Dp's will help make more power then not doing anything at all. But you are not going to make as much power as you would with the Piggie design that uses the main cat and ditches the pre cat.

The FI design is more like this --P------ That pre cat is moved back but still no where near the 12"-24" range.
I talked to Danny at FI. He knew what I was talking about and even mentioned that he moves the cat in his system as far back as he can to try and help not rob the motor of the power. He went on to say that if he moved the cat back to the normal position of the Main cat That would be better for performance. But from a manufacturing and emissions stand point he found his cat placement to be a GREAT compromise.

[drew]

Sorry for any of the confusion.

Here is a real bad diagram
stock design -p----m--
milltek design -P-------- The P stands for pre cat
Piggie Design 2 -P--------
Piggie Design 3 ------m-- The M stands for Main cat and this represents 99% of all aftermarket performance exhaust designs out there.
My current design ---------

The Stock Dp's were never changed from there current design other then the removal of 1 cat each side for each test. The current design on my car not only removed all the cats but removed the cat can and replaced it with straight pipe. This was probably over kill but I thought what the hell


Yes and No. let me say this. There are some companies that move the pre cat back to help FI is one of them. This still used Pre cat design. It is not going to stop you from getting more power over 100% stock Dp's. These designs still work All the Dp's will help make more power then not doing anything at all. But you are not going to make as much power as you would with the Piggie design that uses the main cat and ditches the pre cat.

The FI design is more like this --P------ That pre cat is moved back but still no where near the 12"-24" range.
I talked to Danny at FI. He knew what I was talking about and even mentioned that he moves the cat in his system as far back as he can to try and help not rob the motor of the power. He went on to say that if he moved the cat back to the normal position of the Main cat That would be better for performance. But from a manufacturing and emissions stand point he found his cat placement to be a GREAT compromise.

hmm...so does anybody make dps with the cat as far downstream as possible?

or is then the best choice to just take stock dp and gut the precat? on the stock dp is the secondary cat as far downstream as possible?

[pre007]

Awesome discussion, well done Justin. I was never convinced chipping was worth the trouble. My question is would a resonated system paired with a cat-less DP setup still restrict the exhaust flow enough to rob it of peak performance? Are non-resonated systems paired with a cat-less DP my only option? LaBree appears to have some awesome options, along with FI. I really like the FI 8" resonated X-pipe catback, and want to pair it with a cat-less DP system.

[drew]

so vast performance just takes your stock downpipes and guts the precat for $500?

[Justincredible]

hmm...so does anybody make dps with the cat as far downstream as possible?

or is then the best choice to just take stock dp and gut the precat? on the stock dp is the secondary cat as far downstream as possible?

No one in the Audi performance community has a aftermarket Dp with the cat in the main cat position. Like i said this is kinda strange due to the fact that every other performance company for every other make and model removes the Pre cat and uses a main cat design.

Gutting the Pre cat is easier then it sounds. I tried to do this and after several hours I just had my friend send his Dp's to vast performance. I don't know for sure but I think you may need to relocate the Rear 02 as well. This along with the fact that gutting the Pre cat is a HUGE pain in the A$$ (not to mention toxic) is why IMHO it is worth the 450$ to pay Vast performance to do this. NO CEL and good performance.

As for the cat being as far down stream as possible. It is as far down as needed while still getting you performance and Clean emissions while not effecting the flow distance needed for the exhaust. Every Performance exhaust company that makes header back systems that incorporates a cat. Places the cat in the crucial area of 12-24" from the header. The OEM main cat falls into that area. As do All other OEM main cats on performance cars and other wise. This 12-24" has been the standard for performance and clean air for years. So I don't know what came over the people making the Dp's for the Audi performance crowd. Not ONE company I talked to said this was a good idea (keeping the Main cat)

[drew]

cool man...seems like the most logical choice (going with vast).

[Justincredible]

so vast performance just takes your stock downpipes and guts the precat for $500?

450$ and There is a little more to it then that but trust me you would spend that money with out even thinking about it after you try to do it yourself

[ZONAWILDCATS4]

Thanks for the info. There is so much crap on this forum its nice to see someone come up with an original thought...not regurgitating what someone else said. Props Justin!

[b00st]

the reason i put mufflers on was because it was too loud. i would not have made it half an hour without getting a ticket.

and the whole argument in that thread was that mufflers create back pressure and that without back pressure i will lose torque AND hp which is completely false. you lose torque but NOT hp. you gain shit loads. and backpressure is created by just the diameter of the exhaust piping, mufflers and cats are just restrictions. they create the unwanted back pressure by restricting flow of the exhaust.

well we knew that too.

we know mufflers create back pressure. well i know you would lose TQ as for HP also....not sure. i thought you said it didn't feel the same up top or down low. I know the TQ loss would be there.

and yes backpressure can be created by the bends in the piping and diameter size.

but as i recall you run gutted dps, plus straight pipes...so a true straight pipe from the header back. which is very very minimal in the back pressure dept. even with high flow exhaust systems they greatly reduce back pressure...and you came close to almost eliminating it with your setup.

i will agree that the DPs are the single most restrictive part of our whole design. and we have 4 cats....so even going down to 2 cats ( 1 in ea.) will increase flow....or going JUSTIN's route and having gutted dps with no cats or an empty dp pipe with a cb system has definitely reduced the back pressure in the system.

[b00st]

450$ and There is a little more to it then that but trust me you would spend that money with out even thinking about it after you try to do it yourself

yeah been there done that. pay the lousy $450. save yourself hours of frustration. time is money. i've tried to use a crowbar in there and all sorts of things. not on my car...but others who wanted to do the piggies themselves. even on the b5 s4s. PITA!!!!!

[drew]

justin, do you think in terms of volume of sound and rawness...would the vast dps be louder then FI dps and have a more agressive sound?

[91gl]

well we knew that too.

we know mufflers create back pressure. well i know you would lose TQ as for HP also....not sure. i thought you said it didn't feel the same up top or down low. I know the TQ loss would be there.

and yes backpressure can be created by the bends in the piping and diameter size.

but as i recall you run gutted dps, plus straight pipes...so a true straight pipe from the header back. which is very very minimal in the back pressure dept. even with high flow exhaust systems they greatly reduce back pressure...and you came close to almost eliminating it with your setup.

i will agree that the DPs are the single most restrictive part of our whole design. and we have 4 cats....so even going down to 2 cats ( 1 in ea.) will increase flow....or going JUSTIN's route and having gutted dps with no cats or an empty dp pipe with a cb system has definitely reduced the back pressure in the system.

yes, i had posted about the loss in torque. you could feel it but the top end was there like i knew it would be. if you remember, someone called me an idiot in that thread, like he knew exactley what would happen when he had no clue. i would have kept it a true straight pipe but it was so damn loud. i couldnt even focus on how my car felt because i could not believe how loud it was. open header loud.

[ZachPDX]

Justincredible - Thanks for the great post and the pm reply's! I'm trying to decide to pick up some used gutted milltek downpipes and spacers and bolt that into my stock exhaust untill I can afford something with a x pipe. Its gonna be awhile till I can afford the rest of the exhaust tho and Im worried about how it will sound in the meantime. Any input? Ive heard mixed reply's.

[EYE4SPEED]

IL has emissions testing. Are you able to pass those without any cats?

Also, when buying aftermarket DP's, is there only one pre-cat included or is the main cat included too?

I currently have a Milltek non-res (not sure if it has H or X pipe or ? pipe). From reading your post, I see two options to increase performance:

1. If I want Milltek DP's, get ones with gutted pre-cats?

2. If I want to stay with stock DP's, send it to VAST and get the pre-cats gutted or both pre-cats and main cats?

Thank you for the good write up, I look forward to implementing one of these two options soon.

[joeycuccaro]

Justin- Excellent write-up. I will definitely be taking your advise into consideration next spring.

[Captain Insano]

IL has emissions testing. Are you able to pass those without any cats?

Correction - parts of IL have emissions testing...

The part of IL I live in does not.

[beecha91]

Correction - parts of IL have emissions testing...

The part of IL I live in does not.

Ya I would like to know as well, if you can pass with no precat? I have emissions in my part of chicago.

Don-

[drew]

Ya I would like to know as well, if you can pass with no precat? I have emissions in my part of chicago.

Don-

i think you should pass. i could be wrong but i believe the precat has mostly to due with controlling increased emissions on a cold start...

[tatarin]

downpipes FTW !

[Justincredible]

Thanks for all the kind words guys the research took a long time. I know there have been a few questions I have not addressed feel free to re ask them or P.M. me

As for emissions. The Removal of the precat will not effect your ability to pass emissions. If you try to remove the pre cat your self you will need to make sure you relocate the secondary 02 bung to make sure you don't get a CEL. I have not gottent a Cel and my car passed all the Readiness Tests. As far as the car knows there is no cat issue and the Tune and all the fuel trims are unaffected

The end result with piggies is no pre cat but retaining the Main cat. You can still pass Ca emissions or IL. You have the clean benefit of the cat and the performance that comes with the proper location of the MAIN cat from the factory.

If you REMOVE ALL your cats you will need the X pipe other wise it is kinda over kill.
You don't need the extra back pressure If you have the Main cat you can get away with the H pipe. If you have the Pre cat you don't need either the H pipe or the X pipe as you already have too much back pressure

[b00st]

and there is the key ladies and gentlemen.

[550orm5]

great write-up! thanks for that