Air Fuel Ratio and the Toyota Powered Lotus

First the basic terms

AFR: Air Fuel Ratio

14.7: 14.7 parts air to 1 part gas equals stoichiometric (“stoich”) when working with unleaded gasoline. 

Closed Loop: This is when the engine uses input from the factory installed oxygen sensors (02 sensor) to achieve stoich. This is used during Low Load. It is very difficult to damage the motor from incorrect tuning, spark, or fuel during Low Load.

Open Loop: The ECU is running the motor fueling from a predetermined “fuel map” that references RPM and Load (among others). There is no sensor feedback to tell the ECU if the fuel map is commanding the right amount of fuel. This is used during High Load.

Narrow Band 02: (NB02) This is the type of sensor the Lotus ECU receives feedback from while in closed loop. It ONLY works for determining stoich and trimming the fuel to achieve stoich. It is not helpful for determining AFR outside of a narrow range, or band, around stoich.

Wide Band 02: (WB02) This is an 02 sensor that can accurately sense the AFR through a wide range of AFR values. Often times they can read AFRs between 9 and 16:1. For the purposes of all Toyota powered Lotus cars, the ECU only uses input from a narrow band 02. Any references to AFRs outside of stoich (14.7) should be assumed are coming from readings derived from an aftermarket WB02 with an aftermarket display showing those values. Do not replace the factory NB02 with a WB02 in hopes that the ECU will know what to do with WB02 data. Engine and or ECU damage will occur if you do this. 

WOT: Wide Open Throttle. This means exactly that, where the throttle is firmly planted to the floor without any transient throttle movement. This is High Load on the motor.

Transient: Throttle movement: This is when the throttle is moving through your input.

Idle: When the foot is off the throttle and the car is idling on its own. This is Low Load on the Motor.

Low Load: Exactly what it sounds like. The motor is not under stress to accelerate hard. It’s often in steady state cruise or idling when the load is low.

High Load: This is when the motor is being asked to “work" with an opening or open throttle. The load on the motor is high when accelerating. 

Primary 02: This is the NB02 sensor that is closest to the motor and prior to any catalysts. It is the ONLY sensor that provides feedback to the ECU for purposes of trimming the fueling to achieve the target stoich AFR. The V6 cars have a primary NB02 on each bank of cylinders. 

Secondary 02: This is the NB02 that is after the catalyst. It is not a relevant sensor for monitoring tune or running of the motor. It is only there to monitor the condition of the catalyst. The V6 cars have two of these. 

How to Measure your AFR?

You need a WB02. AEM, NGK, Innovative, Attometer, etc all sell WB02 gauges. We sell the AEM version. Forum experts and those who know will get into the weeds about the merits and accuracy of one brand or style vs the other. That said, a properly calibrated and relatively new WB02 from any of those companies can get reasonable data for tune and overall running verification. “Properly calibrated” means following the manufacturer’s calibration instructions. Some are automatic and some are not. “Relatively New” means not fouled from persistent track use, lead fuel, or long running time. 

For general tune and running verification, you’re interested in a single operating condition, which is steady state WOT.  You’re not interested in AFR at idle, as the ECU is forcing the idle to be at stoich through its closed loop operation using the primary NB02. It is common for a WB02 to not have great idle AFR data and will often show a lean indication at idle. This is due to a variety of reasons. Do not be alarmed if you observe a lean idle on your WB02 as it likely erroneous. You can verify your idle AFR with a scan tool and monitor the factory NB02 and the corresponding fuel trims. The idle AFRs are not a common problem and generally not a tuning issue due to the closed loop operation.

WB02s are inherently slow monitoring devices. Only an experienced operator is going to do a good job of interpreting part or transient throttle fueling with a WB02, therefore we are focused on WOT AFR monitoring in this discussion. 

To perform a WOT AFR test, and since we know the WB02 is a slow sensor, you should aim for the longest timeframe you can manage. A dyno using 4th or 5th gear is the safest way to do this test. The throttle should be fully depressed to WOT around 2500RPM and NOT move until at or near the rev limiter. Any transient throttle movement of the throttle will render the AFR data useless and a new test should be performed. You should record the AFR readout and the RPM together. In the absence of good logging equipment, a video of the tach and AFR display together can work reasonably well. If you must perform the test on the street, use a helper if you’re going to use the video method so that you can focus on driving the car safely. Be aware of the speed that you’ll reach in the higher gears. 3rd gear may be the highest gear you can safely use. If this is the case you should perform multiple pulls and average your findings. 

I’ve done my test, now what do the numbers mean?

The “right” AFR for best power and safety tends to be heavily debated. We will aim to provide some guidelines for the Toyota Motors in the Lotus with the most common setups. Turbos are not common on Lotus cars. While there is relevant information here for all cars, we will not spend time discussing aftermarket turbo AFRs. 

AFR Range: There’s a broad range where the motors will exist and be happy. It’s unreasonable to expect your car to always run with what you think are “perfect” AFRs. There’s a range that the cars can and should run in at WOT. There are many, many things that will impact operation of the motor from day to day.

Rich vs Lean. By definition, any number less than about 14.7 is Rich and any number higher than about 14.7 is Lean (assuming 14.7 is stoich). With that out of the way, about the only time we desire stoich is at idle and light throttle cruising (i.e. Low Load). You will be running LEANER than stoich (larger number) when decelerating and RICHER than stoich (smaller number) when accelerating. When decelerating, the ECU will command your injectors to completely shut off and will typically send the WB02 meter to its leanest reading or “AIR”. This is OK and expected. 

Idle and Cruise (aka part throttle). Remember the ECU is in closed loop at idle and is controlling the idle AFR. The WB02 info at idle is not something we’re interested in for this article.

If your car is stock with a stock tune, it is common for many manufacturers, including Lotus, to run extremely rich AFRs at WOT. We often see 10:1 on the V6 cars for instance on both boosted and NA variants. This is significantly over-fueled in both cases.

Boosted (supercharged) vs Naturally Aspirated (NA). The Boosted cars benefit from over-fueling more than the NA cars do. The amount of ideal over-fueling on the boosted cars in particular has a lot to do with the amount of boost the altitude of the car. Less boost requires less over-fueling for best and safest power.

At High Load, AFR will often be richer to help reduce the incidence of knock or detonation in the higher RPMs. Please take the time to read about detonation here. Over-fueling fuel tends to cool combustion and dilute detonation-prone end-gas, which is beneficial to safety and power production in those higher RPMs. As you look at your data, don’t be alarmed to see the AFR in the last half of the RPM range to be a full point or two richer than in the first half. Use this knowledge and the below references to interpret your data.

 Model Specific References (**see note below)

Evora NA: At sea level, tuned correctly, they run well with AFRs in the 12.2 to 13.1 range at redline. Richer than that may cost some power and will hurt fuel milage, but is not detrimental to the motor. We suggest a retune if leaner.

Evora S / Non-IC’d V6 Exige: At sea level, tuned correctly, they run well 11.8-12.5 at redline. However, as altitude goes up, the boost drops and the AFRs settle in to look more like the NA cars (leaner). Richer than that may cost some power and will hurt fuel milage, but is not detrimental to the motor. We suggest a retune if leaner.

Elise/Exige 2ZZ-GE NA: At sea level, tuned correctly, they run well 12.5-13.2 at redline. Richer than that may cost some power and will hurt fuel milage, but is not detrimental to the motor. We suggest a retune if leaner.

Exige with M62 and 3.0” to stock Pulley: At sea level, tuned correctly, they run well 11.5-12.2 at redline. However, as altitude goes up, the boost drops and the AFRs settle in to look more like the NA cars (leaner). Richer than that may cost some power and will hurt fuel milage, but is not detrimental to the motor. We suggest a retune if leaner.

REV300 and REV400 Kits: At sea level, tuned correctly, they run well 11.2-12.0 at redline. However, as altitude goes up, the boost drops and the AFRs settle in to look more like the NA cars (leaner). Richer than that may cost some power and will hurt fuel milage, but is not detrimental to the motor. We suggest a retune if leaner. 

REVX (25 PSI kit): At sea level, tuned correctly, they run well 10.8-11.7 at redline. However, as altitude goes up, the boost drops and the AFRs settle in to look more like the NA cars (leaner). Richer than that may cost some power and will hurt fuel milage, but is not detrimental to the motor. We suggest a retune if leaner.

 **Note: The above references assume "pump gas" with no more than 10% Ethanol. "Race Gas" and Oxygenated fuels may require less or more fuel, respectively, for best operation. 

We hope this information is helpful!

The Team at BOE