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Seeking some input on this 991 build..

 
Old 11-25-2012, 10:28 PM
  #61  
simsgw
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Originally Posted by rpilot View Post
So that's what some good ol' boys down were trying to tell me once. I never tried to really understand how those old time noisemakers worked.

I kinda also feel a little dumb for it somehow not "clicking" in my head that the PSE switch was an analogy for that exhaust cutout cable pull. Guess I drank too much of the sound symposer kool-aid to think straight.

And honestly, given what you have been saying about the PSE, I really dont see how it meets any regulations in any market unless they skirt around the letter of the law without honoring the spirit thereof. Perhaps since the PSE is an option, it does not count??? I don't know.
I suspect that it slides past the rules for many jurisdictions because it isn't quite as crude as those old cut-outs. First, it only cuts one muffler out of the loop and we have three total. Ours were placed right after the collector pipes and we "cut out" the entire exhaust system from the manifold back. Second, PSE is subtle. It does nothing at all until some threshold rpm is reached. Or... this is not quite clear in the documents... it does seem to change the nature of the sound even at low rpm. The car burbles more as soon as I push the PSE enable button. But according to the docs, it doesn't allow increased decibels 'curbside' until you pass that pre-set rpm level. And of course by then the curb is a distant memory. Unless you hammer the car in first gear, you can drive for hours without exceeding 4000 rpm. I am doing it routinely during break-in.

Basically, I suppose that's how they slip it past the inspectors for different markets. The test scenarios don't quite push the engine hard enough to trigger the earbleed phase of PSE behavior. We can safely bet that Porsche engineers know every test scenario in every market from Berkeley to Beijing.

Gary
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Old 11-26-2012, 10:09 AM
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Originally Posted by simsgw View Post
I suspect that it slides past the rules for many jurisdictions because it isn't quite as crude as those old cut-outs. First, it only cuts one muffler out of the loop and we have three total. Ours were placed right after the collector pipes and we "cut out" the entire exhaust system from the manifold back. Second, PSE is subtle. It does nothing at all until some threshold rpm is reached. Or... this is not quite clear in the documents... it does seem to change the nature of the sound even at low rpm. The car burbles more as soon as I push the PSE enable button. But according to the docs, it doesn't allow increased decibels 'curbside' until you pass that pre-set rpm level. And of course by then the curb is a distant memory. Unless you hammer the car in first gear, you can drive for hours without exceeding 4000 rpm. I am doing it routinely during break-in.

Basically, I suppose that's how they slip it past the inspectors for different markets. The test scenarios don't quite push the engine hard enough to trigger the earbleed phase of PSE behavior. We can safely bet that Porsche engineers know every test scenario in every market from Berkeley to Beijing.

Gary
Given that a more open exhaust should lower back pressure, and the engine has its critical air/fuel/combustion paramters under tight dynamic control, I wonder if the factory "no change in power" claim may have something to do with avoiding multiple variations in regulatory testing certification across markets. You do go faster when the PSE is open don't you?
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Old 11-26-2012, 04:55 PM
  #63  
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Originally Posted by chuckbdc View Post
Given that a more open exhaust should lower back pressure, and the engine has its critical air/fuel/combustion paramters under tight dynamic control, I wonder if the factory "no change in power" claim may have something to do with avoiding multiple variations in regulatory testing certification across markets. You do go faster when the PSE is open don't you?
Well, I do. I'm not sure it's because the engine has more power though.

Truth be told, modern engines, and especially the Porsche engines, are designed to take advantage of harmonic pressures in the manifolds to forcibly extract gases from the cylinders. A completely open exhaust vents directly to the atmosphere, but I'm not sure... the transient pressures in the exhaust manifold might be lower than atmospheric at certain points in the cycle. Seems unlikely on the face of it, but possible. I'm not sure, because you'd be surprised how seldom we consider exhaust pressures in satellite design. But even if they can't arrange to open the exhaust valves onto pressures below atmospheric, I guarantee they are lower than the mean pressure in the manifolds. The pipe resonances are chosen with great cunning.

Changing the acoustics amounts to changing that dynamic pattern of pressures. I suspect the biggest goal of the PSE subsystem design was to make sure they didn't reduce the engine's performance. Something like
  • Make it as loud as we can externally per regulations;
  • Enhance the euphonious elements of the sound;
  • Don't screw up the dynamic pressures!
I'd say they succeeded with those first two. I'll report details on that last one after my first track day beyond break-in.

Gary
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Old 11-26-2012, 07:50 PM
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Originally Posted by simsgw View Post
Well, I do. I'm not sure it's because the engine has more power though.
  • Make it as loud as we can externally per regulations;
  • Enhance the euphonious elements of the sound;
  • Don't screw up the dynamic pressures!
I'd say they succeeded with those first two. I'll report details on that last one after my first track day beyond break-in.

Gary
Thats what we need. Same car, same driver, same course, two position switch. Should be an open and shut case !
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Old 11-26-2012, 10:38 PM
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Originally Posted by chuckbdc View Post
Thats what we need. Same car, same driver, same course, two position switch. Should be an open and shut case !
Well, it's a burden, but I suppose somebody has to do it.

In about a month, since I'm closing in on 1200 miles tonight, and San Diego Region has a DE day in January.

Gary
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