xhaust 101. VERY LONG
With lots of new members I thought it would be great to get out some posts of very important conversations and things that have been found to help the S4. This one covers the exhaust on our cars and its importance. Some of the information is simplified and set to help make for easy reads and conversation. There are always more points to bring and more facts that can be added. This post is a good general out line and help to guide new members
This post is here to help clear the air on what exhaust size is optional for your motors efficiency, flow, power, and over all performance. Selecting the proper exhaust is very important depending on what your looking for. Even if your looking for just a little better sound you might want to consider some of the below points.
For the people who don’t like to read lengthy posts. The summery
1.The corner stone of your car is the exhaust. For both power and a healthy motor life.
2. Your exhaust system is what helps being in and pull out air from motor and thus is one of the largest components in making power.
3. if your going to put on a cheap exhaust smaller then 2.5" just save your money as anything smaller then 2.5" isn’t much better then stock in both flow and performance wise.
4. math wise we need fall into the requirements for a 2.5" exhaust on a n/a 4.2 S4
5. just throwing together a 2.5" exhaust isn’t enough there are lots of components that the exhaust needs to help give you the fill beit
6 going cheap and getting bargain basement low cost exhaust is going to come with cheap parts. Such as material and mufflers. Good mufflers are not cheap for a reason. a cheap exhaust with cheap mufflers can actually be very restrictive and not much better then stock.
7. Sound wise do your shopping. A well balanced 2.5" exhaust is not going to be any louder then a 2.25" exhaust
8 finial thoughts.
we know and have show over and over. Getting your Dp Precat removed and a 2.5" catback with X is a great place to start and your car can make great power. Going FULL 2.5 is going to open up even more power
finally if you want the king of kings for exhaust and power sound. Get the JHM Headers and a 2.5" catback with X. The result is RS4+ power and monster TQ. All cars with the JHM headers dominate on the dyno and track if properly tuned.
If you want to know why the conclusions above are true or find out more information please read below.
Two years ago this week I made my first post on exhaust and Dp testing. For what it was and the data I had It still stands as very true. so please feel free to read first before continuing on.
Dp testing
More Dp and exhaust testing with addition of headers
Performance primer for the 4.2
What your going to see and read in this post.
some back ground on my interaction and information gathering including my own results
information on how important your exhaust is for power
how to determine the proper size exhaust including the math
that just having a exhaust a certain size isn’t enough
Cross overs
not all mufflers are the same
material
Testing
A little common sense
some back ground on my interaction and information gathering including my own results
As part of the testing I contacted flowmaster and magnaflow. Two exhaust only companys, I felt that this is the best way to go. I know there are lots of great companys that make exhaust but with these two there the top of there field and they have a solid record.
They both took a good list of information about my car and came back with the same results. The best exhaust size for the car is 2.5 with a X and a solid muffler.
After thinking about it I wondered why and how they (flowmaster and magnaflow) came to that conclusion? I also wondered why An exhaust is more then just how big the exhaust is, but more on that later
Having a N/A car the most important thing you can do to the car is get the air in and out best you can. and due to the fact we have huge cams and an already decent intake track the first place to start is the exhaust.
While Ive sat back and watched most people struggle with performance or not strong gains at the track/dyno/street. Ive had great results, and so have the others that have followed the proper exhaust modification steps. In my Dp testing thread I made the statement “The best chip for your car is a good set of Dp’s and a properly sized and built exhaust” Boy is that true.
In My opinion and with my results. If your not putting 2.5" exhaust with a good X crossover and a good muffler. Your just paying lots of money for new pipes that sit under the car. Keep in mind a shinny exhaust is only nice when you can see it. once its under the car no one is going to see it. But your going to have to live with the sound and performance or lack there of if you go in the wrong direction
For the S4 we have found that. Performace wise even if you add a supercharger you can’t make great power with a 2.25" exhaust.
For the 4.2 it stacks up like this. If your going to add an exhaust go with 2.5" anything less seems to make very little difference unless your just looking for sound. Even still the larger exhaust helps keep exhaust temps down. Also look for an exhaust with a crossover as our cars benifit from a X crossover.
Performace wise. The list of exhaust stakes up like this.
Piggie Dp’s flow just as good as aftermarket Dp’s with the precat. Most people will tell you how they destroyed there aftermarket Precats as having the pre cats in are hard on the motor and not very helpfull on the exhaust.
From lowest to largest All using a 2.5" catback with X crossover will yeald this.
Piggies are good up to about 300whp
Catless piggies flow good but there only good for a few more hp
100% catless 2.5" Dp’s will take you to about 350whp and then your going to start to see the manifolds are the restriction
Headers have shown to make all performance parts perform better and they have been testing good up to about 500whp on our cars.
The top running and fastest 1/4 mile cars that are n/a use the 2.5" and either 2.5" catless Dp’s or headers. No n/a cars running 2.25" exhausts ever come close to the same times as the larger exhaust cars. There are actually several faster header and 2.5" catback cars then some Supercharged cars with a 2.25" exhaust
information on how important your exhaust is for power
Your exhaust is actually a very important component in how much air your motor can take in.
The exhaust system creates a draw on the intake port and if the exhaust is sized right it can create more draw then the piston will on its own. Having too small of an exhaust will effect this draw. With too small exhaust the draw effect results in less power and Efficiency of the motor.
Creating a better flowing exhaust will create more intake draw and allow the motor to make more power. This is why people modify there exhausts and most of the time make it bigger, while bigger isn’t always better its usually the first step. As automotive manufactures save money using smaller diameter pipes for cost and production line assembly. They also do there best to keep noise and emissions down to a minimum. This also comes from smaller pipes
Modifying the exhaust such as Adding headers and a proper sized catback can get a bigger intake port draw to over 500% more then what the piston will do on its own. This is why When people do mods they have found that the better the exhaust is, the bigger the gain will be for the parts they install, IE more power from a supercharger or more power from an intake manifold ETC… This has shown true with almost all mods and not just limited to NOS, superchargers or the JHM modified intake manifold.
So your exhaust is actually one of the major parts of the intake track. Its your exhaust that plays the major roll in how you can easily adjust your motors efficiency and power. What we have seen in the past is too many people modifying there Dp’s,chip,intake manifold ETC. only to choke it off with too small of a catback
how to determine the proper size exhaust including the math
Ive laughed over the years when I saw companys put out exhaust systems. SOME companys are selling you nothing more then pipes that make noise and not a performance part. The exhaust issue, This one seems to be just like the Supercharger debate. It really comes back to CFM. Cubic Feet (per)Minute. Its actually very simple. You need to know that your motor will put out over 7% more then it takes in (CFM)wise. After you figgure for thermal expansion and the addition of spent fuel it becomes a real issue to make sure your getting enough room for all that mas to get out of the motor and out of the exhaust.
When you look at it math wise you realize that it takes about 1.5CFM to = 1hp Per that range of HP PER RPM. It will also take about 2.2CFM flow capability out of the exhaust for each hp from the motor to result in a restriction free exhaust. The result of this math is us needing 2.5" or bigger. But more on that later.
We know that 115cfm per inch can flow through a straight pipe with no restriction. When the pipes are bent the CFM ability will drop. Keep in mind your exhaust is more then just straight pipes this is also something that is a big issue.
Here is how it adds up. These figures are close to what we would see. this is more for examples then any actual measured amounts. Some measurements are conservative others are biased off available calculations and facts given the information we have.
THE MATH:
OUR motor pulls in about 500CFM at max RPM.
CFM IN= 500
CFM out=500 NOT COUNTING FOR FUEL ADDITION AND THERMAL EXPANSION
you add 7% (its actually more but for the sake of the conversation) this is for post combustion additions to the air and thermal expansion.
Then you need to add a loss of 3-6% CFM for the needed bends to get over and around objects on the car. Thus helping push for larger dia pipes to over come this.
If we do the math for a N/A motor we need to know that our starting CFM of 500CFM will actually go up as we modify the car with say a JHM chip or the JHM intake manifold. The end result will be MUCH GREATER if you add a supercharger but for the sake of the conversation were talking about n/a
Math note. some of the figures are simplified for the sake of conversation and to help the end user understand the results better. Some figures such as flow needs are going to be much higher this part is for a standard explanation only. Actual CFM from pipe dia over all spend fuel allotment are larger or are going to require larger then what Im giving.
So 500CFM + the 7% extra for added spent fuel and thermal expansion comes to around 570CFM
If we use the math saying that 1" pipe can flow 115CFM before restriction the math would work out like this
(Stock)2.18" exhaust will flow. 2.18"=250CFM you have a duel exhaust so 250x2=500CFM before restriction starts
(Mod)…2.25" exhaust will flow. 2.25"=258CFM you have a duel exhaust so 258x2=517CFM before restriction starts
(MOD).2.50" exhaust will flow. 2.50"=288CFM you have a duel exhaust so 288x2=576CFM before restriction starts
Now you can see why so many people say to not waste your time on the 2.25" exhaust. its not much better then stock for power or flow. The 2.5" exhaust shows clear dominance and exceeds the need for where were at and leaves room to grow.
Ok so the math shows us that 2.5" covers our needs. Why not just go 2.75. This was talked about a little in my first Dp post. Talking about the 4.2 S4 motor with 7000 RPM puts us under the bar for a need of 2.75. You really need to look at the motor flow. You can see if you look on a dyno or measure your MAF readings that your car stops taking in positive air at around 6500.
Sit tight this is where it gets complacated for a second. This all comes back to CFM
If your motor is running 100% efficent it will take in and put out (less spent fuel) 4.2L in and 4.2 L out OR CFM 0.148321 ft³ The issue is our motors with the manifiods and exhaust we have isn’t bringing in 4.2l and putting out 4.2L its not quite 100% efficent. Unless we add headers our motors put out just enough to make us need 2.5" and not much more.
A RS4 motor is also 4.2L but its more efficent then our motor So they might bennifit from an exhaust bigger then 2.5" as there CFM and RPM is higher then us. They pull in more air and get more out of there 4.2 motor then we do. The RS4 has Try Y headers larger cams and a higher RPM then we do so they might need it.
If we add headers and an a 2.5" on our S4’s exhaust we will make about the same as the RS4 will. Less the fact they have more RPM then us. That exdra RPM and flow keeps the RS4 ahread of us and even with headers keeps us still in the window for the 2.5" exhaust size. The headers put us closer to the need for a larger exhaust but we still don’t have the extra rpm. so 2.5" is more then good enough for us n/a 4.2 S4’s
ok ok ok. so what does all this mean. Why don’t we just put a 2.75" exhaust on our cars. The issue is keeping just over our flow needs and requirements. The further we get away from our MAX flow needs of 570 CFM the more ificent the exhaust will be at doing its job. So the answer is your never want to be under your flow needs but you don’t want to be too far over your flow needs. Remember we still need the exhaust to help create a draw on the intake. If its too large its not going to do that
NOTE: The further the restriction is from the source (the heads) the larger the impact. While the stock Dp’s are just over 2.25 in some areas and slimmer in others its not as damaging to the restriction as it would be if this was back at the mufflers. This is further why a catback that is too small is going to damage power. This brings us to our next part.
that just having a exhaust a certain size isn’t enough
you can have a 2.5" exhaust and still not get good power out of it.
Even when you get the right size dia of the exhaust there is still miles of work to be done. In my motor tare down thread I got to spend some time at Roush Racing. They were doing exhaust testing and I was able to see how big of a deal bends were in the exhaust for loss of power. If you think about it nascar exhausts dump before the rear wheels to avoid the power loss of making bends to go over the rear wheels. Unnecessary bends are going to kill power
When you think about it if you have a 15deg bend here and a 15 deg bend there 30deg bend going to the catback there and with a 90deg bend over the rear axles and 90deg down from the axles with then a 90deg bend going to the mufflers you just built an exhaust that makes 1 complete circle before it exists the car. Remember the further away from the heads the bend is or the restriction is the bigger the impact there is on power.
This is one more reason why not all exhaust systems are going to be equal. When I had my exhaust made I had the pipes as straight as possible with the proper angles added where they were needed and this helped really add power to the car.
Crossover
TO add an important piece to this. The exhaust dia is important and so are the bends the other needed piece is a solid crossover.
The exhaust crossover it the part just between the down pipes and the mufflers. This is the piece that helps pull the exhaust from the Dp’s. The Crossover is one of the big parts of helping the exhaust balance and draw exhaust out of the motor while assisting the Dp’s. The crossover allows each bank of the exhaust to mix and help create a draw on the weaker side of the motor. This helps make power and balance the motor.
The crossover isn’t just great for power its a helpful tool to keep drone and exhaust noise under control
Our stock unit comes without a crossover. The older Audi’s did come with a crossover for better power. On current Audi’s they seem to be looking to save money by installing a little smaller exhaust then optional to help keep the flow tight and not need a crossover. By putting an exhaust too small on the motor they can reduce the need for a crossover they also have the ability to read how each side of the motor is flowing (we have wide ban 02’s) and they can reduce power to one side of the motor to help keep the motor balanced. Adding a bigger exhaust and a X will help stop the motor from reducing power due to unbalance flow wise
Really no need for conversation on how effective this is. There are over thousands of tests and stats showing this is an almost MUST for higher RPM high hp V8 motors. no all X crossovers are equal its important that there is a crossover in any exhaust you select for your S4. Looking at the crossover can be one of the things that will make one exhaust better then the other. But when your looking to upgrade the exhaust this is one of the things you want the exhaust to have
not all mufflers are the same
Mufflers and resonators… There name just spells restriction. While this isn’t completely true this is the furthest point from the motor and one of the biggest HP killers.
NOT ALL MUFFLERS ARE THE SAME. While selecting the proper size is the key, selecting the proper muffler is just as important and this is one more point where your going to have some exhaust perform better then others. You can have a nice straight 2.5" exhaust and a solid crossover and just kill it in the mufflers. While some would think having a quieter exhaust would mean more restriction this isn’t always the case.
There are several Types of mufflers. The older and cheaper the mufflers are the more they tend to really start to give lots of backpressure. This is bad. You don’t want muffler back pressure. Having pressure that far from the manifolds is bad for power.
There are several mufflers out there that can actually help drop backpressure.
You have the chambered mufflers
you have the stright threw glass pack mufflers
You have some mufflers with a crossover in them
Then you have the standard flow tube exchange muffler more like stock.
Each design has its plus and its minus. You can’t point to any one and say thats the one to go for as there are lots of factors to look at when it comes to what muffler to use. and with little to no data from muffler companys its even harder to pin down what one is going to flow best.
It is for this reason I always say stick with a exhaust that has been tested on the street to show performance. There are lots of changes that are made bu companys that make mufflers that tere not going to share with the general public.
My rule of thumb. Cheap exhaust systems are going to use cheap mufflers. Popular name exhausts don’t get that way by chance. When your paying 500$ for an exhaust in most cases you not even getting what your paying for. On a n/a car where the exhaust is so crutial think twice before going with a cheap brand exhaust because our going to save a few bucks. The money our saving is comming from using cheaper parts mufflers being one of those parts
When it comes to exhaust sound is a concern just know that in some cases your going to take performance away with sound. know that adding the biggest restriction in the furthest spot from the motor can have one of the largest impacts on the power of your car. So take that into consideration