Section 1: Theory Of Operation; Basic Engine Requirements - Edelbrock 1403-1413 Owner's Manual

Edelbrock's carburetor owner's manual

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1403-1413 Perf Series Carburetor Owner's Manual.qxd

BASIC ENGINE REQUIREMENTS

The spark-ignition 4-cycle engine burns a mixture of AIR and FUEL. The air is controlled by the

driver's operation of the throttle. The fuel is mixed with the incoming air by the carburetor. The Ratio

of AIR to FUEL is the AIR/FUEL Ratio (A/F). This is a ratio by WEIGHT; if 12 pounds of Air are combined

with 1 pound of Fuel the A/F is 12:1, or more commonly, A/F = 12.

Despite the enormous variety in engine designs, virtually all (spark-ignition 4-Cycle) engines have

very similar A/F Ratio requirements. For fully warmed-up engines, the range of A/F is:

A/F RATIO

CHARACTERISTICS

RICH BURN LIMIT: Combustion is weak/erratic.

6-9

EXTREMELY RICH: Black smoke and low power.

10-11

VERY RICH: Some supercharged engines run in this range at full power as a

means of controlling detonation.

12-13

RICH: Best power A/F: Un-supercharged WOT.

14-15

CHEMICALLY IDEAL: At 14.6 the A/F is at the theoretical ideal ratio with no

excess fuel or oxygen after combustion. Good A/F for part

throttle cruise and light to moderate acceleration.

16-17

LEAN: Best economy A/F ratio. Borderline for part throttle

drivability (worse than borderline if EGR is used).

18-19

VERY LEAN: Usual lean limit (Driveability).

20-25

LEAN BURN LIMIT: Varies with engine and system.

Even though engines will run anywhere between 5 and 25 A/F, the usual target values for an un-

supercharged engine are a fairly narrow range (Figure 1). A/F is about 12.5 for the WOT and 14.0-

15.5 at part-throttle cruise. An intermediate value of about 13.5-14.0 is usually used for mid-range

power (non-WOT acceleration).