One of the benefits of the electronic age we live in is the trickle-down effect of technology. Chevy started using electronic engine controls in the early 80s, and 10 years later all production engines were using an oxygen sensor to create electronic feedback to the on-board computer. With universal use of heated oxygen sensors in late-model cars, these sensors are now inexpensive and available through any auto parts store.
This O2 sensor converts free oxygen levels in the exhaust stream into a voltage signal. Oxygen level is a great indicator of the air/fuel ratio. Low oxygen levels in the exhaust means the engine is running rich, while high oxygen levels indicate a lean condition. Given this technology, you can convert this signal into a tuning feedback system displayed as an air/fuel ratio. Several companies like Edelbrock, MSD, and others have created lean-rich indicators over the years, but these gauges only tell you if the engine is running leaner or richer than a 14.7:1 air/fuel ratio, since that is the ratio where the factory O2 sensor is designed to operate most accurately.
While the early versions of this air/fuel meter were promising, a better way to go would be with a meter that indicates a full range of air/fuel ratio from 12:1 to 17:1. Thats exactly what the Nordskog air/fuel mixture gauge does. The system is designed to use a standard heated oxygen sensor, which is located in the exhaust pipe at or near the header collector on one side of the exhaust system. This voltage signal is converted into a readout that is displayed either digitally or with a 270-degree analog-style gauge.
The hot rodders challenge has always been to tweak the typical four-barrel carburetor so it delivers the desired air/fuel ratio from idle to wide open throttle (WOT). We also expect the carburetor to deliver excellent throttle response, good fuel mileage, and big-time power. Because carburetors are employed in a limitless number of engine variations, carburetor manufacturers play it safe by configuring the carburetor to deliver roughly a 12:1 air/fuel ratio from idle all the way to WOT. A rich engine will run just fine and deliver decent power while avoiding overly lean conditions that could hurt the engine and cause driveability problems. Unfortunately, a 12:1 air/fuel ratio is also responsible for less-than-optimal fuel mileage, lazy throttle response, as well as poor emissions.
Without some feedback for air/fuel ratio, carburetor tuning is a difficult task since the tuner must guess whether the engine is running lean or rich. With experience, you can begin to differentiate a lean surge from an overly rich condition. However, using an air/fuel ratio gauge offers a far more quick and accurate assessment of whats happening inside the engine.
According to tuners that weve spoken to, even engines with fairly big cams will idle at 13.0:1 air/fuel ratio and might be able to handle as lean as 14:1 air/fuel ratio at part-throttle. Much of this is dependent upon the amount of camshaft valve overlap and mixture distribution in the intake manifold.
For milder engines, the part-throttle air/fuel ratio can support even leaner ratios. For example, >> at highway cruising speeds of 2,500 to 3,500 rpm, the engine is only required to make around 20 hp to push the car down the road. Closed-loop EFI cars cruise at 14.7:1 air/fuel ratio, which improves both emissions and fuel mileage. Once you determine the cruise air/fuel ratio by using the Nordskog gauge, you can then experiment by reducing the primary metering jets by a couple of jet numbers. This will lean the cruise air/fuel ratio to a ratio closer to 14.7:1.
Unfortunately, its not a safe bet to rely on this air/fuel gauge for accurate WOT air/fuel ratio tuning. The voltage signal at 12.5:1 to 13.0:1 is not nearly as accurate as it can be around 14.7:1 where the sensor is designed to operate. Therefore, the error factor is likely to be fairly high at best power ratios. For WOT jetting, its best to do that at the dragstrip where you can use trap speed to tune the car for best mph readings.
The important point here is that using this air/fuel ratio gauge will make it much easier to dial in your street engine for best part-throttle metering. This is one case where a little feedback is a good thing.