Dp testing and results THIS IS LONG SUPER LONG
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. I have the jhm tune and my car is an automatic. Better times have been achieved with manual cars following the same performance formula
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 beit 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
Last edited by Justincredible; 12-07-2008 at 10:13 PM.
If you don’t have haters, then you’re not kicking enough ass.
JHM powered 12.2 On the JHM Tune With Launch Assist for the Automatic JHM Nitrous kit/JHM headers all this in a Automatic.
And lastly for the grammar police that are bound to step in
I came here from Germany in the 6th grade. I could not read or write a lick of English. I am working to get better. Thanks and sorry. I always try to edit my posts for the best grammar and easiest reading.
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12-07-2008 10:06 PM#2
Justincredible
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Re: Dp testing and results THIS IS LONG SUPER LONG
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.