Aerohead,
I believe we are in violent agreement here, once you realize what I am actually saying and not what you think I am saying. Keep in mind that the pressure in the exhaust system varies along it's entire length and varies with time. My assertion is that it is never desired to increase back pressure at the exhaust valve at the time it opens.
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Originally Posted by aerohead
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).
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What you have just described is a negative pressure wave traveling back towards the valve, timed so that pressure is reduced just at the moment the valve opens to assist scavenging the cylinder.
You have just reduced the back pressure.
It is NEVER desired to increase back pressure at the exhaust valve while it is open.
Quote:
Originally Posted by aerohead
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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.
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Just because it is done intentionally, doesn't mean it is desired. It is also undersized due to geometric considerations: there is only so much space in the cylinder to put all the valves and leave them reasonable un-shrouded.
Quote:
Originally Posted by aerohead
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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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And I have never suggested arbitrarily modifying the exhaust.
For an exhaust system tuned to maximize torque at a high RPM, a restriction placed late in the exhaust system may be beneficial to low RPM performance. This is not because backpressure is increased, but because the restriction alters the timing of the negative pressure waves to
reduce backpressure at the valve and aid scavenging.
As I've said on the other thread related to this, a better solution for a dedicated economy car is to tune the exhaust for optimal performance at the lower RPMs where it will get the best fuel economy.
Quote:
Originally Posted by aerohead
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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.
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I believe the extraordinary claim is that you can improve performance by increasing the back pressure at the valve. I don't believe that this is what you are trying to say though.