Quote:
Originally Posted by darcane
Looks like a pretty exhaustive study to me.
Sorry, I couldn't help myself. 
But other that the one magazine article, I see nothing that counters my assertion that back pressure is never desired. And I don't put much authority in magazine writers.
A German OEM parts supplier has been making functional prototypes for about 5 years years:
https://www.eberspaecher.com/en/prod...echnology.html
They market it to either silence exhaust or generate an exhaust note for electric/hybrids, but I don't know of any applications where it is actually used yet. |
Perhaps you'll allow be so inclined to indulge me:
*The dynamometer data demonstrated that in some cases,when back pressure was reduced,performance fell off,which is contrary to your assertion.
*Standing waves can occur within both the intake and exhaust manifolds.
*Both the intake and exhaust systems need to be tuned.
*The exhaust pressure pulse is a longitudinal wave comprised of alternating compressions and rarifactions.
*Whenever the pressure wave encounters a different medium or different flow section a reflection occurs.
*When a reflection occurs from a change in direction of propagation,a compression is reflected as a compression,and a rarifaction as a rarifaction.
*When a reflection occurs without change in direction of prpagation,a compression is reflected as a rarifaction,and a rarifaction as a compression.
*The violent blowdown generates the pulse,contained within the system and held to sonic velocity.
*With an open pipe,the gas expands into a larger volume or atmosphere,and the pulse is reflected from the open end of the pipe as a strong rarifaction.
*With proper tuning,this rarifaction will reach the exhaust valve during the scavenging event,aiding gas removal,and preventing reversion of exhaust into the combustion chamber/intake tract (depending upon valve overlap).
*The wave will dissipate as a function of viscous attrition.
*Same thing happens in the induction system.
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*As I mentioned,back pressure begins with the intentionally undersized exhaust valve which is limited by materials technology to it's ability to transfer heat away from itself,even with sodium-filled stems.
*Additional surface wall drag occurs in the primaries,collector,down-pipe,catalytic converter,mid-pipe,muffler,tailpipe,resonator (if present),and exhaust tip.
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*'Back pressure' is inherent in all exhaust systems.
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*To further complicate things,the engine might as well be variable volume as far as the exhaust is concerned,as the gas load handled by the exhaust will vary from near-zero at idle,to maximum under full load W.O.T..
*It's a tall order to expect a 'static' system to operate ideally under 'morphing' load.
*And the dynamometer data appears to bear this out.
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*For fuel economy we're interested in the unit air charge,which is directly associated with Mean Effective Pressure (MEP), maximum torque,and Minimum Specific Fuel Consumption which occurs near the max. torque.
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*Regardless of overall internal resistance,OEM exhaust systems are 'tuned.'
*They are a compromise which attempts to optimize flow under ALL loads.
*Ideally,they would be shape-shifting,as Yamaha's smart system attempts.
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*If torque is a criteria for fuel economy,and there is laboratory evidence that torque can be harmed by arbitrary back pressure reduction without re-tuning,then it would behoove us to be mindful about exhaust system modifications if we do not possess the technology to re-balance the system.
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*As Ford's David Hwang infers,you wouldn't want to 'increase' back pressure,however,we're stuck with some under the best circumstances.
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*'Performance' exhaust systems are targeted for enthusiasts which have installed high-flow intakes,cams,headers 'n such and are looking for quicker 1/8th-mile and 1/4-mile times or a 'sound.'
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*And a cautionary note about 'pressure' claims.When you consider Daniel Bournoulli's association between volume/velocity and pressure: one could claim that their muffler lowers 'pressure' at rpm.And that would be true of the local static pressure at the measuring site.What they may not be saying, is that there's a high local velocity at that location,and very high surface friction drag as well as turbulence which they say nothing about.
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*Most performance product advertising says zero to little about what they tested,how they tested,etc..Anecdotal testimonies flood the pages of advertising,with zero science to bolster their claims.No context.A true informational wasteland.
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*In the context of daily driven passenger cars and light trucks,and their association to fuel economy vs back pressure,I'm going to have to respectfully disagree with your assertion about any back pressure never being desired until you can present more of a case.
*I remain the student,but as they say,extraordinary claims require extraordinary evidence.