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      03-13-2013, 11:59 PM   #1
M3takesNYC
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AA xpipe smaller pipes than OEM

I have an active xpipe/HFC with 2 resonators and my oem xpipe off the car and was comparing them. The AA has smaller diameter piping throughout the entire way. I didn't have a tape measure but used a string and marker. Need to find a ruler or measure but it appears the AA is 2.25inch piping and the OEM maybe 2.5 inch piping.

I am wondering if this is engineered to increase velocity or was it simply a matter of that small of a difference does not matter and 2.5 inch is much easier to get than the wier d2.75 inch bmw likes to use in other applications?

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      03-14-2013, 07:59 AM   #2
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The stock pipe may be double walled stainless in places so it looks fatter and the hydro forming may also change the shape in a way that makes it look bigger. As I understand the actual inside diameter is a standard metric size that is in the range of 2.5 inches. Probably 65mm but I have not cut mine up. Someone with test pipes could measure for you. After market midpipes tend to be 2.5 inches though Dinan says its pipe is 2.75.
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      03-14-2013, 09:02 AM   #3
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Quote:
Originally Posted by M3takesNYC View Post
I have an active xpipe/HFC with 2 resonators and my oem xpipe off the car and was comparing them. The AA has smaller diameter piping throughout the entire way. I didn't have a tape measure but used a string and marker. Need to find a ruler or measure but it appears the AA is 2.25inch piping and the OEM maybe 2.5 inch piping.

I am wondering if this is engineered to increase velocity or was it simply a matter of that small of a difference does not matter and 2.5 inch is much easier to get than the wier d2.75 inch bmw likes to use in other applications?

Our X-pipe is 2.5 inches measure from the outside of the pipe. The inside measurement is 2.3 inch.

We made a custom 3 inch x-pipe for a customers race car per his request and lost power. Bigger is not always better...
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      03-14-2013, 09:13 AM   #4
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I used to have one of these. Great product Andrew
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      03-14-2013, 10:18 AM   #5
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Quote:
Originally Posted by M3takesNYC View Post
I have an active xpipe/HFC with 2 resonators and my oem xpipe off the car and was comparing them. The AA has smaller diameter piping throughout the entire way. I didn't have a tape measure but used a string and marker. Need to find a ruler or measure but it appears the AA is 2.25inch piping and the OEM maybe 2.5 inch piping.

I am wondering if this is engineered to increase velocity or was it simply a matter of that small of a difference does not matter and 2.5 inch is much easier to get than the wier d2.75 inch bmw likes to use in other applications?
Are we talking ID or OD? Can have the same ID but different OD as they might different wall thickness.
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      03-14-2013, 10:24 AM   #6
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I used to have one of these. Great product Andrew
Thanks Chris
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      03-14-2013, 11:10 AM   #7
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Quote:
Originally Posted by Andrew@ActiveAutowerke View Post
Our X-pipe is 2.5 inches measure from the outside of the pipe. The inside measurement is 2.3 inch.

We made a custom 3 inch x-pipe for a customers race car per his request and lost power. Bigger is not always better...
Oh I realize bigger is not always better. Its just not often you see aftermarket with smaller diameter than stock which to me can make sense to increase gas velocity but I will have to measure the stock as I am pretty sure its 2.75 inches OD.

I guess inner could be the same but doubt wall thikness would make up for a quarter of an inch difference in OD

Andrew any further thoughts on the purpose of the piping being smaller than stock from an engineering point of view or why it was chosen? Or is it alot of trial and error when making an xpipe like this?
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      03-14-2013, 12:24 PM   #8
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Quote:
Originally Posted by M3takesNYC View Post
Oh I realize bigger is not always better. Its just not often you see aftermarket with smaller diameter than stock which to me can make sense to increase gas velocity but I will have to measure the stock as I am pretty sure its 2.75 inches OD.

I guess inner could be the same but doubt wall thikness would make up for a quarter of an inch difference in OD

Andrew any further thoughts on the purpose of the piping being smaller than stock from an engineering point of view or why it was chosen? Or is it alot of trial and error when making an xpipe like this?
Based on our previous dyno testing and the 2.5'' x-pipe is the best suited for the s65.
Like I said before a 3 inch pipe on the racer car made much less torque than the 2.5 that we currently sell. The size comparison between stock and our xpipe does not really line up do to the extreme restrictions of the cats.

We do however switch up to a 3 inch rear exhaust section for better flow.
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      03-14-2013, 02:13 PM   #9
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Hmm to me that does not make any sense. The rear exhaust piping is where the gas velocity is the slowest, especially if coming from a smaller diameter pipe. It essentially would "hit a wall" if you will and come to a halt in terms of velocity. Many tuners actually would make the rear exhaust piping smaller to actually help keep the velocity up from the inherently slower exhaust by the time it reaches there.

Not sure there is any logic in that at all or its way beyond anything I have ever understood about physics and fluid dynamics
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      03-14-2013, 02:22 PM   #10
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Quote:
Originally Posted by M3takesNYC View Post
Hmm to me that does not make any sense. The rear exhaust piping is where the gas velocity is the slowest, especially if coming from a smaller diameter pipe. It essentially would "hit a wall" if you will and come to a halt in terms of velocity. Many tuners actually would make the rear exhaust piping smaller to actually help keep the velocity up from the inherently slower exhaust by the time it reaches there.

Not sure there is any logic in that at all or its way beyond anything I have ever understood about physics and fluid dynamics

That would make sense if it were a straight pipe.

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      03-14-2013, 02:47 PM   #11
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As versed as you may be in Physics and fluid dynamics, you must understand the dynamics of the S65 to realize what Active has accomplished. Great product all around Andrew. 3 years later and still looks like new as well



Quote:
Originally Posted by M3takesNYC View Post
Hmm to me that does not make any sense. The rear exhaust piping is where the gas velocity is the slowest, especially if coming from a smaller diameter pipe. It essentially would "hit a wall" if you will and come to a halt in terms of velocity. Many tuners actually would make the rear exhaust piping smaller to actually help keep the velocity up from the inherently slower exhaust by the time it reaches there.

Not sure there is any logic in that at all or its way beyond anything I have ever understood about physics and fluid dynamics
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      03-14-2013, 03:19 PM   #12
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I trust them I was just looking more for an understanding. I am always the type to want to know "why" and not just accept it is what it is. Although I accept it in this case I am just curious.

RPI for example does the opposite on some of their other exhausts. They make the rear smaller diameter to admittedly keep the exhaust gas velocity up.

Who knows obviously these guys are pro's!
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      03-14-2013, 03:24 PM   #13
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Quote:
Originally Posted by M3takesNYC View Post
I trust them I was just looking more for an understanding. I am always the type to want to know "why" and not just accept it is what it is. Although I accept it in this case I am just curious.

RPI for example does the opposite on some of their other exhausts. They make the rear smaller diameter to admittedly keep the exhaust gas velocity up.

Who knows obviously these guys are pro's!
They also have short 90 degree bends into the mufflers.
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      03-14-2013, 04:05 PM   #14
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Andrew, can you PM me some info about your new 100 cel HFC. I believe I have the old 200 cel and would be curious to know the difference as well as pricing. Thanks in advance and I agree with Chris, great product!
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      03-14-2013, 07:25 PM   #15
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Quote:
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They also have short 90 degree bends into the mufflers.
Off topic but if you need a local demo car for sc and other freebies don't forget about me
Free stage 2 gen 3 sounds good
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      03-14-2013, 07:49 PM   #16
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Don't you have a supercharger?
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      03-14-2013, 07:50 PM   #17
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Quote:
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They also have short 90 degree bends into the mufflers.
I am not defending or promoting them just commenting that it makes sense. But again you don't offer any explanation as to how 3 inch piping makes sense at the end of an exhaust if 2.5 inch was plenty all the way down. Just does not make a lot of sense
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      03-14-2013, 10:14 PM   #18
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Quote:
Originally Posted by M3takesNYC View Post
Hmm to me that does not make any sense. The rear exhaust piping is where the gas velocity is the slowest, especially if coming from a smaller diameter pipe. It essentially would "hit a wall" if you will and come to a halt in terms of velocity. Many tuners actually would make the rear exhaust piping smaller to actually help keep the velocity up from the inherently slower exhaust by the time it reaches there.

Not sure there is any logic in that at all or its way beyond anything I have ever understood about physics and fluid dynamics
High velocities are more important near the cylinders where high velocity help scavenging by creating low pressure that "sucks" the exhaust out of the cylinder. This also creates unwanted back pressure because pressure loss in a pipe is a function of velocity. Near the muffler, high exhaust velocity isn't as important. In fact, the exhaust cools quite a bit and slows down (because it's volume is reduced when it cools) back there. Near the muffler, you simply want to reduce pressure loss/back pressure. Reducing the amount of bends and velocity (larger pipes) are both ways to reduce back pressure. The gases do slow down but there isn't "a wall", there is less restriction.
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      03-15-2013, 01:03 PM   #19
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High velocities are more important near the cylinders where high velocity help scavenging by creating low pressure that "sucks" the exhaust out of the cylinder. This also creates unwanted back pressure because pressure loss in a pipe is a function of velocity. Near the muffler, high exhaust velocity isn't as important. In fact, the exhaust cools quite a bit and slows down (because it's volume is reduced when it cools) back there. Near the muffler, you simply want to reduce pressure loss/back pressure. Reducing the amount of bends and velocity (larger pipes) are both ways to reduce back pressure. The gases do slow down but there isn't "a wall", there is less restriction.
Most of this is true but does not explain why you want gas that is already at its slowest point due to being cool, near the end, want to make it even slower and not efficiently exit the system as a smaller pipe (or atleast the same size as the rest of the exhaust) would do. If the whole thing was 3 inches then fine but 2.5 and than 3 at the end. Taking slow gases there and making them even less able to exit with slower velocity due to piping, being cool and being at the end of the system so has lost a lot of kinetic energy.

I was just looking for a reason which I don't think is unreasonable when people way 2k for an exhaust. Have we really gotten to the point we defend tuning companies just because of their name without a sensible explanation?

Its lacking those standards which allow companies to just sell products that lack any real quality or testing (not saying AA does this) but clearly you guys realize this happens all the time with unsubstantiated claims and people buying a crap load of products.

Was real simple, I don't doubt its a great AA system but can someone tell me an explanation that uses physics and makes sense to go from a smaller pipe throughout to a larger pipe at the end
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      03-15-2013, 01:24 PM   #20
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Why do race cars and NHRA drag cars end their exhaust at the header? From a small pipe instantly into the largest pipe possible, the atmosphere. Do you feel the velocity should continue to increase until it exits the tailpipe? If so , why?

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Most of this is true but does not explain why you want gas that is already at its slowest point due to being cool, near the end, want to make it even slower and not efficiently exit the system as a smaller pipe (or atleast the same size as the rest of the exhaust) would do. If the whole thing was 3 inches then fine but 2.5 and than 3 at the end. Taking slow gases there and making them even less able to exit with slower velocity due to piping, being cool and being at the end of the system so has lost a lot of kinetic energy.

I was just looking for a reason which I don't think is unreasonable when people way 2k for an exhaust. Have we really gotten to the point we defend tuning companies just because of their name without a sensible explanation?

Its lacking those standards which allow companies to just sell products that lack any real quality or testing (not saying AA does this) but clearly you guys realize this happens all the time with unsubstantiated claims and people buying a crap load of products.

Was real simple, I don't doubt its a great AA system but can someone tell me an explanation that uses physics and makes sense to go from a smaller pipe throughout to a larger pipe at the end
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      03-22-2013, 06:40 PM   #21
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Quote:
Originally Posted by M3takesNYC View Post
Most of this is true but does not explain why you want gas that is already at its slowest point due to being cool, near the end, want to make it even slower and not efficiently exit the system as a smaller pipe (or atleast the same size as the rest of the exhaust) would do. If the whole thing was 3 inches then fine but 2.5 and than 3 at the end. Taking slow gases there and making them even less able to exit with slower velocity due to piping, being cool and being at the end of the system so has lost a lot of kinetic energy.

I was just looking for a reason which I don't think is unreasonable when people way 2k for an exhaust. Have we really gotten to the point we defend tuning companies just because of their name without a sensible explanation?

Its lacking those standards which allow companies to just sell products that lack any real quality or testing (not saying AA does this) but clearly you guys realize this happens all the time with unsubstantiated claims and people buying a crap load of products.

Was real simple, I don't doubt its a great AA system but can someone tell me an explanation that uses physics and makes sense to go from a smaller pipe throughout to a larger pipe at the end
I did explain that, you are not thinking about it correctly. I'm not trying to be rude but I think you may not have a complete understanding of pressure and flow.

We both agree that you want to maximize flow through the engine right? To maximize flow, you want to decrease the back pressure that prevents the engine from pumping the most air that it can because reducing back pressure increases flow rate. You can reduce back pressure with smoother pipes, less bends, larger pipes/lower velocity. In this case, we are concerned with velocity. Pressure drop in a pipe is a function of velocity. Even though the flow is going slower through the bigger pipe, the flow rate will still higher because of the reduced back pressure.

I think it's reckless to accuse me of defending expensive exhausts without a sensible explanation. First of all, I am not defending them, I am trying to explain to you why you are thinking about it the wrong way. Secondly, I am an engineer and I deal with flow in pipes and ducts on a daily basis so I know a thing or two about this.

I realize that people on here defend parts just because of the name but, frankly, in this case, you are part of the problem. You are defaming AA's product and spreading false information without a clear understanding of what your criticisms are based on.
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      03-23-2013, 05:25 PM   #22
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You just argued why a bigger pipe would make sense. The whole point is I am saying the AA diameter is smaller. So if a bigger one like the OEM or every other xpipe flows more than why is AA using a smaller one?

Also there is a point where flow "capability" is much different than flow. If a pipe is too large where gas velocity is too slow or there is simply not enough power to produce gases that are that great or have enough velocity, than flow will actually be less with a larger pipe than a smaller one.

The smallest diameter pipe that allows enough total flow is what gives you the most efficient and most performance.

So this is why I assume AA goes with this. Considering the stock sizes and AA use the same pipe for 600 hp applications which I really wonder why they are not using a bigge rpipe, clearly our pipes have a much larger flow capability than is needed for the NA power it makes and thus a bit smaller of a pipe for NA applications to keep velocity up and actual flow even more with the smaller pipe.

So I actually do understand flow
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