But there must be an optimum length, probably related to engine displacement and desired rpm. This sounds more complex than 'slap a 1/2" spacer in it and go!'... come to think of it, probably related to cam timing too b/c the valve opening and closing would set up the pressure pulses. Kind of like expansion pipes on 2-stroke motorcycles, no? (just on the intake side, and with valves...)
I'm also confused by the claims of lower RPM gains because this article
Best Carb Spacer Cheap Horse Power - Popular Hot Rodding Magazine emphasizes the gains in the upper rev range (although there are some mid-range gains too) like Christ states, but I'm not an engine builder and I'm more than a bit rusty at all of this.
This leads me back to the use of a TB spacer as a thermal break rather than as a method of tuning the resonant frequency in the intake tract.