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DFCO is the cheapest way to inject air into the exhaust and remove any fuel delivered previously. In the old carb days closing the throttle quickly lead to a huge increase in unburned hydrocarbons since the vacuum skyrocketed and compression was too low to light off the fuel anyway.
That's why they used air injection, pumps lines and other stuff to break. With EHI, when you slam the trottle closed ony air is passing through the engine. To reduce the peak vacuum which could actually suck oil past good rings they use various types of devices to delay throttle closing to keep peak vacuum from spiking.
regards
Mech
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