Tuning your engine with the aid of an air/fuel meter can really help you dial in your carburetor. But what numbers should you aim for?

When people talk about the air/fuel ratio of their engine, do you get confused? Do they really know what they’re talking about, or did they just hear someone mention it in another conversation? Another term you might hear mentioned when talking about A/F (air/fuel) ratio is Stoich or Stoichiometric. Stoichiometric mixture is the working point that engines attempt to achieve during different load or cruise situations. Say for instance you want to set your “Stoich” to the proper setting while your car is cruising down the road at approximately 2,400 rpm. While that setting might help you get the best economy from your engine, put the engine under load or at wide open throttle for an extended period of time, and you’ll likely be so economical (lean) that you’ll be walking. In theory, a proper Stoichiometric mixture has just enough air to completely burn the available fuel. In actuality though, this “proper mix” is never completely achieved. This is due primarily because of the very short time available during an engine’s combustion cycle. Most of the actual combustion process is comple.ted in approximately 4 to 5 milliseconds at an engine speed of 6,000 rpm. This is the elapsed time from when the spark plug fires, until the actual burning of the fuel and air mixture is essentially complete.

Like we said, the best mixture or A/F ratio that gives you the best fuel economy is nowhere near ideal for maximum power. A widely-accepted proper A/F ratio number for achieving maximum power is with a Stoichiometric reading between 12.8:1 to 13.2:1. Keep in mind though, that number is for a non-oxygenated pump gas that in order to be efficiently burned, has an ideal ratio of 14.7:1. There is generally a particular ratio of air to fuel that results in all of the oxygen molecules combining chemically with all of the fuel molecules. This ratio is 14.7:1, meaning 14.7 parts of air for every 1 part of fuel. Running an engine at this particular air/fuel ratio helps to minimize emissions and maximize fuel economy, but again, this air/fuel ratio is generally not safe for an engine to run at high rpm and/or heavy engine-load conditions. Since most oxygen sensors produced are only meant to accurately measure a 14.7:1 air/fuel ratio, they shouldn’t be used in applications where much richer or leaner target air/fuel ratios are selected, such as that with fuels like E85, and alcohol. The wide-band oxygen sensor used with those fuels is capable of accurately measuring a much wider range of air/fuel ratios.

Using fuel injection makes achieving Stoich a lot easier than if you’re using a carburetor, as the electronics that control the actual injection requirements, can take into account, the inlet air temperature, the actual air/fuel ratio, engine coolant temperature, and/or a myriad of other readings to adjust the amount of fuel that is delivered.

A concern about reaching a proper Stoichiometric mixture is that it unfortunately burns very hot, and can damage engine components if the engine is placed under high load at this air/fuel ratio. Due to the high temperatures at this mixture, detonation right after maximum cylinder pressure is possible during high load conditions (detonation is often referred to as knocking or pinging). Detonation can cause serious engine damage as the uncontrolled burning of the air/fuel mix can create very high pressures in the cylinder. As a consequence, true Stoichiometric mixtures are only used under light load conditions. For acceleration and high load conditions, a richer mixture (lower air/fuel ratio) is used to produce cooler combustion products and thereby prevent detonation and overheating of the cylinder head.

What happens if you are putting the commonly available E10 mixture of pump gas in your Mopar? E10 is common at gas pumps in many parts of the country, and what about your race car that requires a high octane race fuel?

Installation is fairly simple, and only required a hot and ground wire to be connected in addition to the O2 sensor cables. We welded the O2 sensor bungs onto our dyno’s exhaust system to complete the installation.

We had our trusty 421-inch small block on our dyno from previous testing, so we filled the dyno’s fuel cell with VP110 race fuel to work with the engine’s 13.0:1 compression ratio. The carburetor is a Quick Fuel Q950. Previous tuning had left us with 88 jets front and rear, with no power valves, and number 30 main air bleeds. We set the fuel pressure at 7 psi and made a full sweep from 3,000 to 7,000 rpm. Our A/F ratio showed our tune to be very rich, with an average of 11.8:1. Reading the spark plugs showed us that the engine was still burning clean.