Thread: Aerodynamics and Exhaust Piping

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  1. #1

    Default Aerodynamics and Exhaust Piping

    Alright all, bear with me, my mind is a complicated thing

    For the time being, I'm going to look into 2.5" High Flow Cats to go with the 2.5" factory tubing and maintain the deleted Resonator.

    In what is sure to be a long process in modifying my Viper, I got into thinking more about my exhaust system (Mopar Cat-Back is all), love the sound. I'd love to add headers in the future which I've come to find all have 3" collectors. To hopefully maintain at least the visual compliance with emissions, new headers would then lead to 3" High Flow Cats and associated piping. Where the "problem" lies is the 2.5" inlet of the Mopar Cat Back. I want to avoid swapping all the piping out. This got me into considering then how to integrate a 3" system to smoothly connect to a 2.5" system, which then got me thinking about Aerodynamics and Bernoulli"s Principle. My buddy's think that the constriction would cause the engine to work harder to force the same volume of air through a smaller pipe. But, air is a compressible fluid and as it draws into a constriction, Bernoulli tells us that the volume of air will accelerate as the air has to have the same volume on the other side of the constriction, hence the acceleration. So, my logic is that this setup will not cause much of an issue (maybe a small power loss is all, which I hope will be canceled out by the headers). If anyone has experience with anything like this, I'd love your input! Or if anyone has any observation I missed, let's hear it. Not sure how aerodynamic properties will apply to exhaust systems, haha.

    Until then, if this comes to fruition, I'll do what I can to get some dyno comparisons before/after as I'm very curious how this could pan out.

  2. #2
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    Default Re: Aerodynamics and Exhaust Piping

    You gotta go 3 inch all the way. It's the only way.

  3. #3

    Default Re: Aerodynamics and Exhaust Piping

    Quote Originally Posted by Free2go View Post
    You gotta go 3 inch all the way. It's the only way.
    ^^^ This. A lot of this. One thing that disappointed me when i did a search for aftermarket exhaust, ill be building my own tyvm you manufacturers who only make 2.5" systems... We dont have inspections or E-Check in KY anymore, so ill be leaving the cats off possibly.

    4 liters is way too much for a 2.5" system, even with true duals when youre talking high performance. For a 4.0 Jeep or Explorer, youd be fine and would probably hurt it going larger.

    When I built exhausts for the DSM crowd 15+ years ago, i used VRS Exhaust in Florida, and while they dont make a kit for the Viper (they would if someone in Florida took their car there...hint hint anyone...) but they sell 3" SS piping, mandrel bends, high flow cats, SS tips and Magnaflow mufflers (if you feel the need and they are excellent mufflers) as well. Im sure theres somewhere in CO/NM that does TiG welding so you wouldnt have to worry about rust on the seams.

    I know i "waltzed in here" and im a severely new guy and dont own a Viper yet, but one thing i know is how to tweak the intake and exhaust side of a motor. I used to port the exhaust side of DSM turbos and just from the porting, the results were .2-.3 in the qtr and 3-5 mph and all my customers used internal WG's and never had a problem with boost creep or flow, although some opted to upgrade to a 34mm WG flapper over the stock 29mm.

    What i do know about Vipers is this: they are really crippled from the factory HP wise. I mean if they werent, why does swapping the stock intake flex hoses to straight ones net you 10hp? That alone tells you how much these things like to process air Its not about how much air you can get into, or out of a motor, but how much you get THROUGH it that makes all the difference

    Now, to address your question: hot air wants to expand, not be compressed. You will see negative gains using a 3" into 2.5" as the air will want to flow backwards towards the heads, more or less cancelling the pulses.

  4. #4
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    Default Re: Aerodynamics and Exhaust Piping

    Quote Originally Posted by TalonTSi90 View Post
    ^^^ This. A lot of this. One thing that disappointed me when i did a search for aftermarket exhaust, ill be building my own tyvm you manufacturers who only make 2.5" systems... We dont have inspections or E-Check in KY anymore, so ill be leaving the cats off possibly.

    4 liters is way too much for a 2.5" system, even with true duals when youre talking high performance. For a 4.0 Jeep or Explorer, youd be fine and would probably hurt it going larger.

    When I built exhausts for the DSM crowd 15+ years ago, i used VRS Exhaust in Florida, and while they dont make a kit for the Viper (they would if someone in Florida took their car there...hint hint anyone...) but they sell 3" SS piping, mandrel bends, high flow cats, SS tips and Magnaflow mufflers (if you feel the need and they are excellent mufflers) as well. Im sure theres somewhere in CO/NM that does TiG welding so you wouldnt have to worry about rust on the seams.

    I know i "waltzed in here" and im a severely new guy and dont own a Viper yet, but one thing i know is how to tweak the intake and exhaust side of a motor. I used to port the exhaust side of DSM turbos and just from the porting, the results were .2-.3 in the qtr and 3-5 mph and all my customers used internal WG's and never had a problem with boost creep or flow, although some opted to upgrade to a 34mm WG flapper over the stock 29mm.

    What i do know about Vipers is this: they are really crippled from the factory HP wise. I mean if they werent, why does swapping the stock intake flex hoses to straight ones net you 10hp? That alone tells you how much these things like to process air Its not about how much air you can get into, or out of a motor, but how much you get THROUGH it that makes all the difference

    Now, to address your question: hot air wants to expand, not be compressed. You will see negative gains using a 3" into 2.5" as the air will want to flow backwards towards the heads, more or less cancelling the pulses.
    Yeah...what he said

  5. #5

    Default Re: Aerodynamics and Exhaust Piping

    Quote Originally Posted by Free2go View Post
    Yeah...what he said
    LOL .

  6. #6

    Default Re: Aerodynamics and Exhaust Piping

    Hahaha, right on guys...I had a feeling applying aviation aerodynamics into a car was going to be a vastly different concept

  7. #7

    Default Re: Aerodynamics and Exhaust Piping

    Quote Originally Posted by ViperDEN_Nick View Post
    Hahaha, right on guys...I had a feeling applying aviation aerodynamics into a car was going to be a vastly different concept
    Well, you can compress cold air using fluid dynamics (think ram air effect with a forward facing intake), but not hot air. At least, not in this situation, which is why when the ambient air is hotter, you lose power (more or less)

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    Default Re: Aerodynamics and Exhaust Piping

    I'm not sure it's necessarily a huge loss. You figure you're pumping into 5x 1 3/4" primary tubes that merge into a single 3" collector to begin with (granted not all pulses are traveling at once). I think people tend to go a little overboard with exhaust systems thinking they need 5" pipes all the way back to make sure there is zero restriction. I'm sure you might see a little benefit, but I've also seen 1000+ WHP cars with pretty much stock sized exhaust and they don't seem to have much problem. Since the exhaust on the Viper is so short I think size is probably even less of an issue as you don't have much pipe you have to pump through anyway. I know there are some generic rules of thumb for pipe size per horsepower, but to my mind at least if the engineers designed with with 2.5" from the factory, they were the ones doing all the calculations and determining what the car actually needed.

  9. #9

    Default Re: Aerodynamics and Exhaust Piping

    Quote Originally Posted by MoparMap View Post
    I know there are some generic rules of thumb for pipe size per horsepower, but to my mind at least if the engineers designed with with 2.5" from the factory, they were the ones doing all the calculations and determining what the car actually needed.
    So the V10 can only produce as much power as the factory made it and no more? You dont think the bean counters hadanything t do with it? Or that they just wanted conservative numbers for reliability? I understand what youre saying, but thats a bit naive As far as the 1000+ hp cars go, what power increase would they see if they thought about using a non-off the shelf system?

    I mean really, based on your assertion, a V8 Explorer should never have larger than an 1 3/4" single outlet exhaust because it will cause the motor to go nuclear because thats what the factory installed on it.

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    Default Re: Aerodynamics and Exhaust Piping

    I don't mean that it can't make more, but I would never worry about a factory installation and I know they put a lot of effort into choosing what they did. Since it is such a performance car I think what I was mostly getting at is I doubt they put in only as much as it needed. I'm guessing it flows pretty dang freely stock. If you start modifying other parts I wouldn't disagree that it might need an upgrade, but if the only thing you are swapping out is the exhaust I don't think you'd see much benefit. An engine is an air pump, if you're not putting any more into it than stock, you aren't going to start flowing much more out of it. I think that was more my intent.

  11. #11

    Default Re: Aerodynamics and Exhaust Piping

    True, if you're not adding air, putting a more free flowing exhaust will only increase noise. I believe Free and I were going on the assumption that a supercharger was in the mix and would require something substantial on the back end. And then I was lamenting that Corsa and Borla were both 2.5" for some ungodly price tag but wouldn't add to the performance (much) in a balls to the wall situation.

    Ironically, this is somewhat of a sore subject on bike forums, too many people believe you need to jet or compensate for the increased air flow from putting pipes on because now the bike pops from a lean condition. No, the bike always popped, you just couldn't hear it because of the factory pipes muting the noise. Once the exhaust valves close, no amount of wide open piping can induce more air to flow. Although in the case of extremely aggressive cam lobe overlap ot can, but if you're in that situation, not only do you want it, you know what you're doing and the result.

  12. #12
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    Default Re: Aerodynamics and Exhaust Piping

    Huh, I never thought about that with bike. I know mine pops a lot (96 Magna). It's a set of Cobra slip-ons with the end caps drilled out (bought the bike that way). It pops something fierce on a decel, though I kinda like it. I can tune it down some by adjusting the idle jet, but that messes with other stuff so I just learned to like it.

  13. #13

    Default Re: Aerodynamics and Exhaust Piping

    Quote Originally Posted by MoparMap View Post
    Huh, I never thought about that with bike. I know mine pops a lot (96 Magna). It's a set of Cobra slip-ons with the end caps drilled out (bought the bike that way). It pops something fierce on a decel, though I kinda like it. I can tune it down some by adjusting the idle jet, but that messes with other stuff so I just learned to like it.
    Yea on my old 01 750 Shadow i ran a year with only needing to adjust the idle mixture screws. What creates the popping on a lot of bikes (especially Hondas) is the pair valve system that injects air into the hot pipes to light off any extra fuel that happens to escape getting burnt. IMO thats an excuse for a poorly designed or poorly operating combustion chamber.

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    Default Re: Aerodynamics and Exhaust Piping

    Probably and emissions workaround too if I had to guess. They didn't have cats for the longest time on bikes are were pretty notoriously lean to make the cut from what I've heard at least.

  15. #15

    Default Re: Aerodynamics and Exhaust Piping

    Probably a good guess. But IMO it just shows a lack of proper tuning, but given the wide possibilities, I can see it being cost prohibitive.

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