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    Understanding exhaust.

    Saw this old thread i made on another forum, figure i'll share my knowledge to anyone with exhaust questions. Sticky?

    I posted this in a Honda forum, so the references are to that.

    Since i'm just waking up and don't feel like moving yet, i'll spread some information about exhaust. I was reading through a thread here about "loss of backpressure" being bad and no one corrected him, thought i'd pass on my knowledge of exhaust, feel free to add on to it, i don't know too much about it, but i have a really good idea of what goes on in it.

    Let me clear up that backpressure is not wanted in any shape or form, if it were true that backpressure created low end torque, let's just weld the exhaust shut for a truck, right? That'd make the most low end. The truth is, what makes low end power is flow, a small exhaust will flow at it's greatest efficency at low RPMs, which, as you can guess, will make low end power. A large exhaust will flow bad in low RPMs and the car will feel "doggy", however, in high RPMs, it gets it's efficency and flows the best and makes it's power in that area. Okay, so, let's put a 3" exhaust for the most power in high RPMs, well, no, in most small 4cyls, they won't flow enough air to efficiently use this size exhaust. A general rule of thumb is 2"-2.25" for most N/A 4cyl applications, be the judge on your engine what you think would flow the best, a 2.2 with porting and a cam will probably want to move up to 2.5"...

    Now, getting into theory, because i'm not positive on what i'm saying below. Exhaust is full of Kinetic, Thermal, and Sound energy, when any of these are taken away, flow is lost. Since the engine isn't flowing much air at low RPMs, it needs a smaller exhaust to keep it's velocity (kinetic) and thermal energy up, when exhaust cools, the air particles move slower. A large exhaust does the same thing, but it just does what i said above more efficiently in high RPMs because it isn't being choked down by the small pipe.

    A quick experiement you can do is open your mouth at about the size of a pinhole and blow fairly hard, it's pretty hard, you can feel it backing up, but if you lightly blow, it becomes easier and pressure decreases. Open your mouth completely and blow, the same is true, but you can't blow enough air to actually make use of this large opening (i suggest you not try either :P). Now, open your mouth to a comfortable level (like blowing out cigarette smoke) and blow, comes out nicely, right? ;)

    If you're running a stainless steel exhaust, header wrapping your whole exhaust wouldn't be a bad idea to keep thermal energy up, i suggest not to on a regular pipe as header wrap holds moisture and will rot the exhaust quickly. Just keep in mind holding in heat can lead to cracking the exhaust, be prepared to weld cracks.

    Okay, maybe it was a bad idea typing something like this as soon as i woke up, so if there's any questions, or anything i need to correct, let me know!

    -----

    BMW reference

    Now, for the single vs dual exhaust debate, i can't provide much of my two cents on this topic. From my reading, the dual creates a wide power band, moving to a single gives high end power, and a little more of it compared to a dual. A dual system will lose it's thermal energy over a single as well, how much difference in power this will make, i do not know.

    I personally am running two true 2" duals and my powerband goes a little passed my stock revlimiter and compared to a stock 325is, i ran about even with it, very slow pull in the high end, but he had some extra weight, if i have a passenger, he pulls me pretty good. I never ran him before the exhaust, but i felt no change when i put exhaust on, just a slightly smoother pull.

    Conclusion: Changing stock exhaust is a waste of money on a stock motor.
    Last edited by O 16581 72452 5; 12-16-2006, 08:03 PM.

    #2
    well a 325is already comes with dual 1.75" exhaust pipes...plenty for 170hp.

    they don't pick up nearly the power like american v8's engines that move a lot of air but only come with 2.25" exhaust and a restrictive single in/out muffler.

    Comment


      #3
      Originally posted by ptownTSI View Post
      well a 325is already comes with dual 1.75" exhaust pipes...plenty for 170hp.

      they don't pick up nearly the power like american v8's engines that move a lot of air but only come with 2.25" exhaust and a restrictive single in/out muffler.
      Yeah, i tried header, dual 2", and dual glasspacks, i was very disappointed, huge waste of over $500.

      Comment


        #4
        now you need to take your engine apart, mill the block to raise compression, port the heads, flat top pistons w/ valve reliefs, coatings on the pistons surfaces, exhaust port, and valve surfaces to minimize heat absorbtion thereby allowing you to raise compression beyond standard. add to that a larger cam, thus giving you even greater compression raising ability.

        with all this you can run 11.5:1 on 93 octane pump gas if you keep the timing advance in check.

        that would put your exhaust to good use.

        Comment


          #5
          didnt know exhaust wrap allows moisture.. isnt it made to prevent it from moisture and rot?

          theres a spray you can use to protect it from all that to be on the safe side...
          1991 318is nv m50 : brilliantrot

          Comment


            #6
            Originally posted by ptownTSI View Post
            now you need to take your engine apart, mill the block to raise compression, port the heads, flat top pistons w/ valve reliefs, coatings on the pistons surfaces, exhaust port, and valve surfaces to minimize heat absorbtion thereby allowing you to raise compression beyond standard. add to that a larger cam, thus giving you even greater compression raising ability.

            with all this you can run 11.5:1 on 93 octane pump gas if you keep the timing advance in check.

            that would put your exhaust to good use.
            I was thinking a supercharger personally. :P

            Comment


              #7
              I compared the volume of a 2.5" ID pipe vs the stock twin 50mm system on my euro 635CSi, and the 2.5" ID actually flows more... For the money, a big single exhaust is a great idea, it costs less to build, takes less time, and is way easier for a DIYer.
              '88 528e /// '88 M5 /// '89 951 /// '98 E430 /// '02 M5

              Comment


                #8
                i get it now
                Originally posted by Hawt drift chick
                Yeah, i'm a good dude i swear, i just like to be balls to the wall.

                Comment


                  #9
                  Originally posted by JPL View Post
                  i get it now
                  do you? do you really?
                  My 2.9L Build!

                  Originally posted by Ernest Hemingway
                  There are only three sports: bullfighting, motor racing, and mountaineering; all the rest are merely games.

                  Comment


                    #10
                    Originally posted by euroshark View Post
                    I compared the volume of a 2.5" ID pipe vs the stock twin 50mm system on my euro 635CSi, and the 2.5" ID actually flows more... For the money, a big single exhaust is a great idea, it costs less to build, takes less time, and is way easier for a DIYer.
                    two 50mm pipes have same cross sectional area as a single 70mm (2.75") pipe so 2.5" is slightly smaller in theory.

                    I would think that the pipe size has little effect on the pressure drop compared to things such as the changes in volume of mufflers, tight radii and things like cats and other obstructions etc.

                    If you changed from a single 2" to single 2.5" but kept cat back and cat same i would expect next to no change to flow capacity but i have never done such tests to provide real results.
                    89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                    new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

                    Comment

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