[Christ]

Thermal break is partially true too, although most commercial spacers, AFAIK are aluminum...thermally conductive.

I'm not sure about the low-mid TQ, although adding length to runners does move the resonance effect (n/a turbocharging) down lower in the RPM range, which would create a peak effect of TQ being lower than the actual peak... and still move the actual peak lower in the range.

PS - Runner resonance length is calculated as "distance from valve to valve" the plenum counts as an equal distance for all the runners, since it provides an interference of sorts.

What this means: effectively, adding a TB spacer, you're increasing runner resonance length.

I also didn't specifically note that with TBI - the "plenum" isn't like the MPFI plenum at all.. TBI is more like carbs, in that the plenum is an area that can actually be equally divided between runners without giving more or less volume for each runner's resonance length.

This would definitely attribute better fuel mileage to the TBI/Carb crowd, due to the same idea behind using it for anything else.. longer runner - lower TQ.

Obviously, if you're making more power on the same fuel, you're running more efficiently.