How Carburetors and EFI Work - Tech

Whew, that's pretty complicated! Here's the take-home message: the idle air/fuel mixture is fixed unless you modify the idle air bleeds or feed restrictions. The transition circuit is also a done deal for us non-gurus. About all you can easily change is the accelerator pump shot to smooth the off-idle hesitation.

An EFI system's idle/transition circuits are light years ahead in comparison, and a heck of a lot less mysterious. Street EFI systems use an Idle Air Control motor (IAC) to bleed air past the nearly closed throttle blades at idle. We're most familiar with FAST's EFI system after installing and tuning it on our Pontiac 455. The FAST system offers a few common-sense graphs to control the idle circuit. "Idle Speed vs. Coolant Temperature" is like a fast-idle cam that comes off as smoothly and as gradually as you desire-just move the green line with your computer's mouse-and tapers off when the engine reaches operating temperature. There's another table that tells the IAC how much initial air to bleed depending on the coolant temperature-here you're defining exactly how much to crack the throttle to get the engine started. The "Throttle Follower" graph keeps the IAC open at low throttle angles to keep the engine from stalling if you let off the gas in a hurry. For the ultimate in glass-smooth idle quality, the FAST system and others allow the computer to move the ignition timing a few degrees in either direction to really flatten out the idle quality. Sleeper fans rejoice; you may finally be able to disguise that big cam at idle.

The FAST system gives you just as much flexibility in acceleration enrichment. Smarter than any accelerator pump, the Acceleration Enrichment (AE) tables allow you to tune out any lean hesitation, anywhere the engine operates. Four AE graphs are at your disposal. We've only needed to play with one of them-"AE vs. MAP Rate of Change." The faster your load increases, the more additional fuel the engine needs. This could be brought on by a steep hill, an upshift, or just a violent stab of the "go" pedal.

By The Car Craft Archives

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