The internals of cars are a new thing to me, so please excuse me while I try to apply my limited experience with engine-talk...
I'm getting that the idea of this mod wrt FE is to lower the resonance of the intake and shift the peak torque -- and simultaneously the island of minimum BSFC -- to lower RPMs. Is that right?
I guess maybe what I was asking was if a car already tuned for more low-end torque would benefit less from the same spacer than one tuned for more high-end torque. At least with given thickness of spacer, the car with the shorter intake would increase it's intake-length percentage-wise more than one that's built with a longer intake. If that's actually how it works (and I didn't completely munge that explanation), then it seems sort of mathematically obvious that a lower-tuned engine would benefit less from the same thickness of spacer.
And to go with that thought, a car tuned for high-end performance has the most interest in a retuning mod like this, to get the BSFC curve into an accessible range for a hypermiler's driving style. Yes? No? (wtf?)
Quote:
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Christ: "I believe, generally, that anything you can do to maximize the power output of your engine without increasing the fuel it consumes to do so will give you better FE."
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This intuitively-appealing statement keeps reappearing in different forms and on different forums. Doesn't it need to be qualified as to HOW you're maximizing your power? If you're maximizing your power linearly across-the-board, then I think this statement might be true. But it generally doesn't work that way, right? You might amplify your power in a certain rpm range much more than in others. And you may even retune the system in such a way as to create resonance peaks in one range that reduce power in other ranges.
So
any power increase isn't necessarily helpful. Hypermilers need efficiency increases in a very particular RPM range particular to the vehicle. If we can retune the system so that that particular range gets more power, then that should be helpful.
btw, I understand a little about "tuning" from the acoustical world. It occurs to me that little hacks like this can accidentally work to your advantage if you happen to disrupt the response curve in such a way as to make a little bump in the "right" part of the curve. You might end up with a bizarre profile with several low peaks, but if you only care about 1900 rpm or whatever, then it might just work for you. So I suppose as you said, this is something that you just hack at until something works, maybe even without a predictable pattern.