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    Measuring Exhaust System Backpressure

    Most of us are aware that to obtain maximum performance from our engines, the exhaust system should be configured to provide minimum resistance to the gas flow. While the exhaust pipe diameter and other exhaust system components should be large enough to reduce backpressure to a minimal level, at the same time they must be small enough to create enough gas velocity to promote best possible exhaust gas scavenging from the cylinders. In a normally aspirated engine, these dual requirements ensure that a minimal amount of backpressure will be present even in the most efficient systems - a necessary evil. In a turbo engine, after the turbo exhaust, the larger the exhaust pipe diameter the better as the need for scavenging is not then a factor.

    Wouldn't it be useful if we could actually take backpressure measurements to determine what effect, if any, changing exhaust system components would have on this parameter? Thus, we could compare differences among exhaust pipe diameters, catalytic converters, mufflers and even differences between headers and the stock manifold to find the best setup to meet our needs.

    But how can we DIYers accomplish this hot, dirty, task easily, safely and economically? Once again, Julian Edgar of the excellent Australian AutoSpeed online car magazine comes to the rescue. In his article, in Issue 524, March 31st, 2009, "Ultimate DIY Automotive Modification Tool-Kit, Part 1", among several other topics he discusses how one can make exhaust system measurements of the kind dealt with here. For access, go to:
    http://autospeed.com:80/cms/A_111109/article.html

    To measure total system backpressure, it requires nothing more than temporarily removing the O2 sensor and screwing in a replacement fitting to which a hose is attached and connected to a turbo boost gauge. If extensive testing is planned, first connect a copper or aluminum tube to the fitting so that enough heat can be dissipated not to melt the rubber or plastic hose at the other end. Since the exhaust gas doesn't actually flow through the gauge, it won't be damaged from the heat. Any mechanical boost gauge reading from 15 to 30 PSI (1 to 2 bar) should be suitable.

    The Protege O2 sensor uses an M18 x 1.5 thread so it should be easy enough to cobble up a fitting from an equivalent bolt. Incidently, when removing and installing O2 sensors, make sure you use a thread chaser to clean the threads and prevent damage. My thread chaser is made by the Lisle Corporation, part #12230.

    It goes without saying that the pickup should be after the turbo when so equipped. Measurements should be taken under full throttle, full load conditions in all cases.

    Although Edgar doesn't deal with this aspect of the problem, keep in mind that from thermodynamic principles, exhaust backpressure is dependent on the gas temperature as well as on the mechanical layout of the system. Unless measurements are referenced to exhaust gas temperature, you can't have repeatability of results across testing sessions. The exhaust system must be up to normal operating temperature - but not more. This is one reason there is wide disagreement between exhaust backpressure as measured on an engine or chassis dyno, and backpressure measured on the road where there is significant cooling of the exhaust system.

    Consequently, backpressure measured in the way described in this article without taking into account gas temperature, whether stationary or rolling, will only show qualitative differences between components, not quantitative ones. That is, we know that muffler no. 1 is the same, better, or worse than muffler no. 2 according to our measurements, but not how much better in a repeatable, numeric sense. However, this method is more than good enough for our purposes.

    Edgar provides some backpressure figures on cars he's measured and you can check them out in his article. He concludes that all the usual approaches to lowering backpressure are valid, and really major reductions in exhaust backpressure of as much as 75% are possible on many cars (however, this figure may be suspect because of the lack of quantifiable data). The catalytic converter is seen as being the most restrictive component, followed by the main muffler. However, unless bends have a very tight angle, for eample 180 degrees, press-bending does not normally cause much measurable restriction. Using the next larger size of pipe and using press-bends is usually more cost-effective than buying mandrel bends.

    Backpressure measurements can also be taken of the individual components that make up the exhaust system, but in order to do so, holes would have to be drilled at different points to install pickups, which after the termination of testing would have to be welded closed.

    Happy Motoring!
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    #2
    i should try measuring mine sometime, the wideband o2 is for tuning only so pulling it out wont hurt anything... hmmm
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    Comment


      #3
      general rule of thumb on turbos is equal pressure in intake piping and exhuast manifold, on a good flowing turbo setup

      Comment


        #4
        great read! i enjoyed that.
        -Jack

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        Comment


          #5
          I'm glad you found it useful.

          Happy Motoring!
          02 DX Millenium Red - The Penultimate Driving Machine
          MP3 Strut Tower Bar kit; Cusco Front Lower Arm Tie Bar
          MSP Springs, Struts, Stabilizer Bars, Trailing Links, #3 Engine Mount
          Kartboy Stabilizer Bar Bushings; Nyloil Shifter Bushings; Red Line MT-90 Gear Oil
          MP3 Shifter, Knob and Aluminum Pedal Set
          Suvlights HD Wiring Harness; Osram Night Breaker H4 Bulbs; Exide Edge AGM Battery
          Summer: 5Zigen FN01R-C 16 x 7" Wheels; Yoko S.drive 205/45-16s
          Winter: Enkei OR52 16 x 7" Wheels; Falken Ziex ZE-912 205/45-16s
          Modified OEM Air Intake; Racing Beat Exhaust System; Techna-Fit SS Clutch Line
          Denso SKJ16CR-L11 Extended Tip Spark Plugs; Magnecor Wires
          Power Slot Front Brake Rotors; Techna-Fit SS Brake Lines; Hawk HPS Pads
          Red Line Synthetic Engine Oil; C/S Aluminum Oil Cap
          Cyberdyne Digital Gauges: Tach; Ambient Air Temp; Voltmeter

          Comment


            #6
            if IM pressure is higher than EM pressure power goes straight up.

            read maximum boost by corky bell. this is known as TIP ratio and can be quite easily calculated. this is how you make big power from small engines
            Last edited by vtec-this; 03-18-2010, 02:12 PM.

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