Z, Fuel Ratio, sensors and all that stuff

Air : Fuel  Ratio Overview

Air and fuel are ideally mixed at a mass ratio of 14.7:1 (14.7 parts air to 1 part gasoline where parts are in kgs or lbs). 

This ratio is called the "stoichiometric" ratio which roughly translates to chemically ideal ratio in which a  burn of both air and fuel components are completely consumed. 

It is similar to the ratio for water H2O in which two parts Hydrogen combine with one part Oxygen.

If the gasoline to air ratio is not managed within the engine, side effects and problems can occur. Some side effects are good (cooler combustion temp, better mpg, more power), others are bad (melted pistons, poor mpg, bad emissions). Thus it is desirable for the engine to run slightly excess gasoline (rich) at times and slightly less gasoline (lean) at other times but for the most part, keeping the ration near 14.7 to 1 is the ideal

Running Lean

For an automobile, having a ratio where there is "extra air" (say 20:1) results in the mixture being "lean" or "thinned of gas". The good aspect of running lean is that there is ample air to mix with the gasoline to ensure nearly all of it gets burned however, there is still extra air kicking around that contributes to heat. This results in gasoline savings (better mpg) but the side effect is a very hot combustion chamber which can melt valves and pistons. Hot combustion chambers can also pre-ignite the air-fuel charge before the spark plug fires. This is called detonation and is more common with high compression engines and turbocharged engines. Another side effect is slightly less power when accelerating.

Running slightly lean and maintaining acceptable combustion temperatures is great for cruising at a constant speed down a highway to conserve fuel. In fact cars are designed to do this. The rule of thumb ratio for this is 15.5:1

Running Rich

Having a ratio where there is "extra fuel" (say 7.35:1 aka 14.7:2) results in the mixture being "rich" or "laden with gas". This results in greater gas consumption but the side effect is a cooler combustion chamber, and inefficient burn as there is not enough air to burn all of the fuel. The result is a sooty combustion and poor gas mileage. In fact if there is too much fuel, the chamber and plugs can be covered in soot in just a couple of minutes and the spark plugs become so fouled that the plugs will not fire and the car won't start. Another by-product of running rich is that the un-burnt gasoline can ignite in the hot manifold/exhaust and cause them to glow orange. Emissions are also increased and a black smoke can usually be seen coming out the exhaust and it may smell like nail polish.

Running slightly rich is great for accelerating and at high rpms as it ensures that nearly all of the air briefly sucked into the combustion chamber is used. (If you analyze engines, you realize the limiting factor to power is how much air the engine can pull into the chamber with each intake cycle. If you can burn all of this air, you can get maximum power). For turbo cars and high compression cars, the extra gas also has a nice side effect of cooling the mixture and preventing heat triggered igniting of the air&fuel (aka detonation) before the spark ignites it. The rule of thumb for richness and maximum power is 12:5 to 1 at maximum HP point.



Problems causing a Lean mixture

Fuel delivery problems

Air/Fuel management faults

Air/Fuel sensor configuration

  • Ripped downstream rubber boot in manifold
  • Disconnected hose to manifold
  • Poor engine seal or gasket
  • Incorrectly connected hose to manifold

Problems causing a Rich mixture

Fuel delivery problems 

Air/Fuel management faults

Air/Fuel sensor configuration


Common diagnostics to determine if you have an air/fuel problem.

Read Plugs

Compare power to a similar Z

Check fuel consumption

Common items to check

If it is low, check for:

  • clogged fuel filter
  • clogged inlet screen to fuel pump
  •  clogged tank pick-up
  • clogged fuel line
  • weak fuel pump
  • air leak in hose between tank and pump
  • faulty fuel pressure regulator

If it is high, check for:


Air Fuel Meter

An air fuel meter can measure air fuel ratio. There are two types of sensors: narrow band and wide band. The wideband is most stable and works the best but it is significantly more expensive. A narrow band kit can be purchased for ~ $100. A junkyard is a great source of sensors (aka O2 sensor).

Exhaust Gas Temperature Gauge (aka Pyrometer)

An EGT meter can measure temperature of exhaust gas. The temperature increases as the mixture leans. This can cause thermal stress, detonation or even component melting. 
This information is most useful during wide open throttle (WOT) engine operation. At WOT, values between 850C (1562F) and 900C (1652F) are often considered ideal; temperatures below 800C (1472F) are considered too rich; temperatures above 925C (1697F) and approaching 1000C (1832F) are considered dangerously lean and can result in excessive engine detonation (knock) and possibly burnt or melted components (valves, spark plugs, pistons, rings). The placement of the EGT probe affects the measurement (the farther the probe is from the head exhaust port the cooler the temperature can be) as does the amount of ignition timing advance (more advance can mean cooler temperatures). Be aware that some EGT gauges only read to
871C (1600F) 



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