|
Statements regarding the advantages of backpressure may have had some merit in the good ol' days. Take a "performance" small block for example. To get high power for ad campaigns, there had to be considerable overlap between the intake and exhaust valves at high rpm. With a short time to fill and exhaust the cylinders, both valves were open simultaneously for part of the crank rotation. This was okay for high rpm because the intake charge didn't have time to migrate out the exhaust. But at low rpm, there is time, so part of the intake charge would go out without being burnt, hence lowered mpgs and power/torque. If one reduced the rate at which the exhaust left however, via a restrictive exhaust, that effect was reduced. Add in the total carb-chaos caused by low-speed valve overlap and the tendency for car makers was to mount a fairly restrictive exhaust to avoid stalling and other uncivilized behaviors at low speeds.
Gearheads, of course, would put on cherry bombs, reduce exhaust restriction and get much more top end power. The result? Loping at idle, missing/dying with light throttle application, etc.
All this goes away when there's no valve overlap. In the 1980's, emission considerations required little or no overlap but the top end power went away. Now, with VVT, we can have both, no overlap at low speeds, and a measured amount at the higher rpms. With it, high-restriction exhausts have no advantage or use whatsoever. (Though of course, resonance effects must also be managed).
|