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    The only true 3 inch downpipes?

    Made by BB fabrication: http://www.bbfabrication.info/

    Our Downpipes are crafted out of high grade 304 stainless steel .Each pipe is cut and fit by hand in to our tight tolerance jigs. Unlike other downpipes that tend to reduce theirs down to 2.5inch pipe much closer to the turbo, we decided to take ours and not reduce it until the flange. This helps eliminate the turbo lag, more power and better sound.

    BB Fabrication 3'' Downpipes Feature

    * 3'' all the way do the way to the cat-back flanges .

    *Hand crafted part by a professional who has many years creating custom fabrication work.

    *TIG welded and back purged. this allows the pipe to have a sanitary weld meaning 100% welded with no interference to the exhaust coming out of the motor.

    *Flanges are 100% CNC cut to give you the best product out there right now.

    *Each set comes with our top of the line Cometic gasket, grade 10 metric hardware, and reusable Nord-Lock washers.

    Click here to enlarge

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    Sticky, I believe the custom pipes you saw on N54tech.com are different than these. These taper to 2.5" at the flange to fit 2.5" mid-pipes. The custom ones you saw were one-off by BB Fab and are 3" all the way through to fit 3" mid-pipes that have not been created yet.

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    Those are beautiful, man, I'll be damned if someone tells me fab work isn't art work. Just look at those.
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    Click here to enlarge Originally Posted by SlicktopTTZ Click here to enlarge
    Sticky, I believe the custom pipes you saw on N54tech.com are different than these. These taper to 2.5" at the flange to fit 2.5" mid-pipes. The custom ones you saw were one-off by BB Fab and are 3" all the way through to fit 3" mid-pipes that have not been created yet.
    But these are the BB fab pipes, unless I'm mistaken?

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    BB Fab makes 3" downpipes that are very similar to ar Design in that they taper to 2.5" very close to the flange. The description you posted is for their production 3" downpipes, which taper to 2.5" at the flange. That picture is not of their normal production 3" down-pipes. The ones pictured here, that you saw recently, are a custom FULL 3" set by BB Fab for klipseracer, and he plans to have 3" mid-pipes made as well. Basically, all the current "Full 3inch" down-pipes on the market for the N54 are not really 3" down-pipes, just mostly 3" and then 2.5" by the end.
    Last edited by fundahl; 03-29-2010 at 03:27 PM.

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    Click here to enlarge Originally Posted by SlicktopTTZ Click here to enlarge
    BB Fab makes 3" downpipes that are very similar to ar Design in that they taper to 2.5" very close to the flange. The description you posted is for their production 3" downpipes. The ones you saw recently are a custom FULL 3" set by BB Fab for klipseracer, and he plans to have 3" mid-pipes made as well.
    I see, thanks for clearing it up! So the pic is indeed of the first true 3 inch pipes?

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    damn so if i get this i have to get a 3inch mid pipe too?

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    Click here to enlarge Originally Posted by greatest335i Click here to enlarge
    damn so if i get this i have to get a 3inch mid pipe too?
    Or have a step-down adapter flange welded on, but that would negate the purpose of buying the 3" to 2.5" tapered version in the first place.
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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    I see, thanks for clearing it up! So the pic is indeed of the first true 3 inch pipes?
    Haha, I know, this is confusing. Yes, that is a picture of the first truely 3" all the way down-pipes, and you will need 3" mid-pipes to bolt up.

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    Click here to enlarge Originally Posted by SlicktopTTZ Click here to enlarge
    Haha, I know, this is confusing. Yes, that is a picture of the first truely 3" all the way down-pipes, and you will need 3" mid-pipes to bolt up.
    Ok, makes sense, and BB fab is doing the mid pipes as well?

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    THe pipes pictured aren't even welded. They must just be tacked on the opposite side or placed that way on the table.

    I recall Mr. 5's comparo of the 2.5 vs 3" pipes and how it made about 7 whp on his car. I honestly think that is in the noise of the results and would challenge the conclusion of 3" vs 2.5". I am biased b/c I do have the cp-e DPs and they are 2.5" but they have cast belmouths and I'll bet outflow most other pipes on the market.

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    Click here to enlarge Originally Posted by SlicktopTTZ Click here to enlarge
    BB Fab makes 3" downpipes that are very similar to ar Design in that they taper to 2.5" very close to the flange. The description you posted is for their production 3" downpipes, which taper to 2.5" at the flange. That picture is not of their normal production 3" down-pipes. The ones pictured here, that you saw recently, are a custom FULL 3" set by BB Fab for klipseracer, and he plans to have 3" mid-pipes made as well. Basically, all the current "Full 3inch" down-pipes on the market for the N54 are not really 3" down-pipes, just mostly 3" and then 2.5" by the end.
    This is correct. These are made using the same high quality material as the ar/bb pipes. Yes, they are made by BB fab, they are currently as it sits a one-off product done by request and now anyone claiming true 3" is confusing the customer because these are in fact true 3". Just look at a set of bb or ar pipes, you'll see a 2.5" flange at the end, where as these have a 3" v-band.

    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    I see, thanks for clearing it up! So the pic is indeed of the first true 3 inch pipes?
    Yes sir Click here to enlarge

    Click here to enlarge Originally Posted by greatest335i Click here to enlarge
    damn so if i get this i have to get a 3inch mid pipe too?
    There is no point in buying these unless you are getting 3" mid pipes. Unless you want an extra 3" of the larger diameter and like throwing money away and probably a lot of blood and sweat installing them.

    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Ok, makes sense, and BB fab is doing the mid pipes as well?
    BB is in fact doing the mid pipes for me, correct. Cat delete and tapering to 2.5" right after where the cats normally reside just in time to recouple with a custom flange for aftermarket exhaust or a slip for stock/bmw performance exhaust.

    Click here to enlarge Originally Posted by boom Click here to enlarge
    THe pipes pictured aren't even welded. They must just be tacked on the opposite side or placed that way on the table.

    I recall Mr. 5's comparo of the 2.5 vs 3" pipes and how it made about 7 whp on his car. I honestly think that is in the noise of the results and would challenge the conclusion of 3" vs 2.5". I am biased b/c I do have the cp-e DPs and they are 2.5" but they have cast belmouths and I'll bet outflow most other pipes on the market.
    They are only tacked for the picture. Welding them is a simple process considering the skill Justin has proven to have and its sure to be some of the best work you can get for the dollar. I am a cp-e fan, but no matter how smooth those bellmouths are, I don't think they can outflow a larger diameter at high rpm's and high boost.

  13. #13
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    Here is an update on the midpipes:

    Click here to enlarge

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    klipse,

    isnt the equation for the flow of similar pipes, that you square the diameter and divide? 2.5*2.5/2=3.125. I've seen single 3" exhausts work on 600whp cars.

    two 3" pipes will be 4.5. That much flow may reduce torque substantially, or it may possibly be the best new mod! I just remember Abid telling me, even with upgraded CHRA's, that my AA exhaust and RR DP's will be fine, flow wise.
    Click here to enlarge
    2007 335i Coupe
    Mods: Check the Garage

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    Click here to enlarge Originally Posted by oddjob2021 Click here to enlarge
    klipse,

    isnt the equation for the flow of similar pipes, that you square the diameter and divide? 2.5*2.5/2=3.125. I've seen single 3" exhausts work on 600whp cars.

    two 3" pipes will be 4.5. That much flow may reduce torque substantially, or it may possibly be the best new mod! I just remember Abid telling me, even with upgraded CHRA's, that my AA exhaust and RR DP's will be fine, flow wise.

    Well, I'm 99% sure your math is correct. I remember Abid showing the same example. However, don't forget that the flow diameter is not continuous throughout with my exhaust. It flows from 3" down to 2.5" just like everybody elses 3" downpipes however that extension of larger piping is extended a few more feet. The length of the exhaust also plays a role here. This is my interpretation of how it works using nothing but common sense:

    Once the exhaust gasses are expelled from the turbines, they are extremely hot. As we all know hot molecules bounce around rapidly and require more space. As the temperature of those particles cool, the volume require shrinks. Yes, 3" single could flow well for a 600hp car, but there are so many factors in determining what is optimal for our cars. Flow requirements hinge on: How many cylinders? How much displacement? What RPM's were reached? All of these factors and surely more will play a part in determining optimal diameter for flow. Regardless, we've SEEN 7hp+ gains by just expanding larger than your 3" 600hp capability example by going to 4.5" diameter instead of 3.125 on the exhaust even if its for a very short time. I believe this is because the hot gasses require more room than typically necessary on paper in the first section of the exhaust system. This is poor math but if you take the average, 3.125 and 4.5 it comes to 3.81 as an average diameter of the exhaust. Take that number for what its worth.

    Once these exhaust gasses cool, they no longer require such large diameter and flow capacity. So if the pipe were to remain 3" its very likely that the gasses would shrink and then slow down and start to 'tumble' and swirl inside the exhaust pipes causing turbulence because there is more room then necessary. If the 'swirling' is a bad representation then the 'pulses' of exhaust will start to slow down and smack each other causing reduced torque. These exhaust gasses become a sort of dead weight with less velocity and now will need to be pushed out of the exhaust. With a 4" dump tube right off the turbo outlet, it doesn't matter how big it is, there is no dead weight to push because the gasses are released to the atmosphere and are not being restrained inside of a pipe with only one direction to exit. This is where my concern for diameter lies and this is the reason for reducing back to a 2.5" diameter just like stock, to retain velocity of the exhaust gasses and to prevent the 'dead weight' torque/power losses. Hopefully someone can come here and scientifically explain my logic.
    Last edited by klipseracer; 04-01-2010 at 02:54 PM.

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    Almost finished!!!

    Click here to enlarge

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    I'm not going to be coving this in detail anymore because I've been really busy and all that but I will make one or two more updates. Just want to say the rear(hardest) downpipe is fitted to the turbo housing, and there is absolutely no rubbing on anything. I read reports about having to remove a heat shield.... Not anymore I can promise that.

    This is the rear downpipe installed. Note where the opening in the vband clamp is at. For me, this is the G-Spot for downpipe installation. Its nearly impossible to get the clamp top wrap around the back unless the opening to the clamp is angled upward like this or more.
    Click here to enlarge

    This is the pipe coming down from the flange, as you can see there is clearance and no rubbing on metal shields or anything:
    Click here to enlarge

    I read a fitment thread about having to take off a heat shield and all that... This is the heat shield, as you can see the fitment doesn't even come close to rubbing anymore:
    Click here to enlarge

    And here is a shot right back to the midpipes, as you can see, all looks good Click here to enlarge
    Click here to enlarge

    Now, I'm off to install the other one.

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    In any case, I'm having an issue with the midpipes. They would have been nearly impossible to do perfectly without my car, and the transfer is too short. And by that, I mean the distance that they angle and move over more toward the center of the car is modeled off of the stock downpipes perfectly, however since the pipe is wider, and the outlets on the 3" downpipes are different, I will need to have the 'transfers' extended a couple of inches for the vBands to mate with the down pipes properly.

    Here is a vid with just the new downpipes on:


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    Sounds mean! Keep us updated.

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    thats just a downpipe exhaust right? if so sounds sick! lol i remember doing a tC turbo kit and at one point we had the same midpipe problem, we had to run open-downpipe to get it to an exhaust shop, my god that was loud lol.
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    2007 335i Coupe
    Mods: Check the Garage

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    Click here to enlarge Originally Posted by oddjob2021 Click here to enlarge
    thats just a downpipe exhaust right? if so sounds sick! lol i remember doing a tC turbo kit and at one point we had the same midpipe problem, we had to run open-downpipe to get it to an exhaust shop, my god that was loud lol.
    Yes, open downpipes. It gets quiet(er) when it warms up, but I just had it towed to a shop that does exhaust work. Hopefully it will be done tonight, at least with the mid pipes on. Then it will be my job to get the remainder exhaust on or decide to have the rest of the exhaust custom made.

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    My last update:

    The iPhone mic can not handle the high decibels and craps out but I will say its slightly barely louder than stock idling and inside the cabin. The turbos spool just putting around now, they are really loud. I got onto the on ramp of the 101 freeway and gunned it and it gave me a 'deep' seat of the pants sucking feeling that I definitely know wasn't there before. It felt like a really cold night, but at 9pm in arizona it was 82 degrees. The system works and sounds great for what it is in my opinion. I'm going back to this local shop to get the second half custom made so these awkward bends don't exist anymore. There was some more work that needed done for the pipes to fit, but its all worked out. Its no question that I've not only gained response but power. My turbos spool and the bov expels air barely by touching the throttle to maintain my speed not even accelerating.




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    Click here to enlarge Originally Posted by klipseracer Click here to enlarge
    Well, I'm 99% sure your math is correct. I remember Abid showing the same example. However, don't forget that the flow diameter is not continuous throughout with my exhaust. It flows from 3" down to 2.5" just like everybody elses 3" downpipes however that extension of larger piping is extended a few more feet. The length of the exhaust also plays a role here. This is my interpretation of how it works using nothing but common sense:

    Once the exhaust gasses are expelled from the turbines, they are extremely hot. As we all know hot molecules bounce around rapidly and require more space. As the temperature of those particles cool, the volume require shrinks. Yes, 3" single could flow well for a 600hp car, but there are so many factors in determining what is optimal for our cars. Flow requirements hinge on: How many cylinders? How much displacement? What RPM's were reached? All of these factors and surely more will play a part in determining optimal diameter for flow. Regardless, we've SEEN 7hp+ gains by just expanding larger than your 3" 600hp capability example by going to 4.5" diameter instead of 3.125 on the exhaust even if its for a very short time. I believe this is because the hot gasses require more room than typically necessary on paper in the first section of the exhaust system. This is poor math but if you take the average, 3.125 and 4.5 it comes to 3.81 as an average diameter of the exhaust. Take that number for what its worth.

    Once these exhaust gasses cool, they no longer require such large diameter and flow capacity. So if the pipe were to remain 3" its very likely that the gasses would shrink and then slow down and start to 'tumble' and swirl inside the exhaust pipes causing turbulence because there is more room then necessary. If the 'swirling' is a bad representation then the 'pulses' of exhaust will start to slow down and smack each other causing reduced torque. These exhaust gasses become a sort of dead weight with less velocity and now will need to be pushed out of the exhaust. With a 4" dump tube right off the turbo outlet, it doesn't matter how big it is, there is no dead weight to push because the gasses are released to the atmosphere and are not being restrained inside of a pipe with only one direction to exit. This is where my concern for diameter lies and this is the reason for reducing back to a 2.5" diameter just like stock, to retain velocity of the exhaust gasses and to prevent the 'dead weight' torque/power losses. Hopefully someone can come here and scientifically explain my logic.
    When the exhaust gases are expelled from the turbos, you don't want to go too big too quick since this will kill the exhaust gas energy. I am going to repost the information I wrote on E90post, so that everyone can get a better understanding of thermodynamics as it pertains to this subject matter. See the post below, especially the information in bold.

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    Just wanted to clear up some confusion here guys. Most aftermarket exhaust systems on the market for the 335/135 are dual 2.5". If you want to convert that to a single piping diameter that is equal in flow here is the formula: 2.5 X 2.5 X 2 = 12.5 Square root of 12.5 = 3.53 (Final Diameter)

    Hopefully the following information I provide will help people understand exhaust system theories and how they work.

    N/A cars utilize exhaust velocity, not backpressure, in the collector to help in scavenging other cylinders during the blowdown process. To get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector, aka the exhaust, which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. If you go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure goes through the roof, more than offsetting any gain made by scavenging. There is a happy medium here.

    On a turbo car you want the exhaust velocity to be high upstream of the turbine in the header. Most primaries of turbo headers are smaller diameter than those of an N/A car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Getting the boost up early is a much more effective way to increase torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead.

    When building a turboback exhaust you want the least backpressure possible. The rule of thumb is larger the better to the point of diminishing returns. The idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. A turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible with the available inlet pressure, which is what drives the compressor and makes boost. Less pressure downstream of the turbine what you're looking for. This approach minimizes the time-to-boost by maximizing boost response and will improve engine VE throughout the rpm band. As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter.

    Your best bet is to measure overall backpressure when coming up with a final exhaust design. For example if you have a turbo that has a pressure ratio of 2.0:1 with a small stock exhaust that produces 10psi of backpressure @ redline, the total backpressure upstream of the turbo is as follows: 14.5 + 10 X 2 = 49 = 34.5 psi total backpressure. In this case the turbine contributed 24.5 psi of backpressure. Now if you take the same turbo with a larger exhaust that drops backpressure to 4psi @ redline, the total backpressure upstream of the turbo using the same formula would be 22.5psi. That's a reduction of 12psi, while turbine contributes 12.5 psi to the backpressure. The reduction in turbine backpressure results in lower expansion ratios resulting in increased engine VE.

    So to answer everyones question, a complete dual 2.5" exhaust system is more than sufficient to support stock turbo max hp levels. Dual 2.5" into one 3" will choke the exhaust system down by about .5 inches, but may result in more overall mid-range hp and torque eventhough a loss at peak could or may occur.

    Our car with turbo upgrades has our 2.5" dp's into a single 3.5" exhaust, which are more than enough to support the 540whp. We originally tried going into a single 3" and lost 35whp with higher EGT's. The single 4" was tried as well, but we lost tons of mid-range hp and torque, with no gain in max power. The 3.5" was the best option for this application.

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