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We were asked by a customer to calculate the backpressure of an exhaust system we were supplying a muffler for. The system includes piping from the engine to our muffler, then from the muffler to the exterior of the building. I supplied the expected pressure drop of our muffler and the customer came back asking for backpressure instead of the pressure drop.

Is anyone able to explain the explicit difference between the backpressure and pressure drop? From my knowledge the pressure drop is the difference between 2 points (P1,P2) while the backpressure is the pressure acting against the system (P2). If they are venting this system to atmosphere would the backpressure just be atmospheric pressure?

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  • $\begingroup$ Both of those will change according to flow rate. As a case in point, we stuffed steel wool into a car's silencer to p*ss of a colleague (worked well - took him ages to work it out :) ). Engine starts and idles fine but as revs increase so does the gas flow rate then the steel wool restricts the flow so he could not get more than about 2k rpm... $\endgroup$
    – Solar Mike
    Aug 9, 2023 at 19:29
  • $\begingroup$ Yup, the flowrate is known. I can calculate pressure drops for any part of the system if I know the fittings/diameters/etc. Mainly just looking for the difference between the the two values (backpressure and pressure drop). and if my assumption is right about the backpressure of a system venting to atmosphere. $\endgroup$
    – JoshL
    Aug 9, 2023 at 19:32

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That's not how the game is played. They need to supply you with the required backpressures (and locations) of whatever you are responsible for. This can have a bearing on things like turbo setups and emissions compliance.

Every engine has a Technical Marketing Brochure that will list maximum backpressure. The target design point should be 1/2 this number. Pages 21 and 22 of this PDF explain backpressure and how to measure it. https://s7d2.scene7.com/is/content/Caterpillar/CM20160713-53120-15686

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  • $\begingroup$ Thank you for the link! $\endgroup$
    – JoshL
    Aug 10, 2023 at 19:55

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