Carbureted Small-Block Chevelle (From Car Craft Magazine)

It used to be that tuning a hot street engine or race motor was one of those black arts that only those carburetor and ignition wizards seem to understand. When (and why) an engine needed more or less fuel was a gray area that only those guys who seemed to be plugged into the engine could figure out. Well gang, air-fuel ratio tuning just got a whole bunch easier. There’s nothing new about plugging in a sensor into the exhaust and reading the free oxygen level and establishing an air-fuel ratio based on the sensor’s voltage readout. The OEM’s have been doing that since the early 80s. But these sensors were designed to be accurate only around 14.7:1 air fuel ratio. Attempting to build a gauge that could accurately deliver air-fuel ratio around 12.5 to 13:1 was challenging and usually inaccurate. Several companies tried and the result was a slew of inexpensive lean-rich indicators that were only really accurate around 14.7:1. The rest of the time, the best these gauges could do was tell you that you were richer than 14.7 at wide-open throttle (WOT), which wasn’t very useful. These gauges were all built around what were called narrow-band exhaust gas oxygen (EGO) sensors. A few years later, companies like Bosch and NTK began building wide-band oxygen sensors that could accurately deliver dependable air-fuel ratio readings in useful ranges for the performance tuner. The problem was these sensors were verrry expensive. But with advanced technology and time, these prices have come down, making a handheld device for reading and data-logging air-fuel ratio finally affordable. Enter Innovate Motorsports with an air-fuel ratio meter for a mere $350. Innovate has created a slick, digital handheld air-fuel ratio meter that can also data-log the information that will give you immediate and accurate drive-time results that you can use to help tune your engine. The system uses a five-wire Bosch heated oxygen sensor that is intended for use only with unleaded fuels to create the air-fuel ratio readout. At this time, the system is capable of data-logging only the air-fuel ratio, but by the time you read this, Innovate should have released auxiliary components that will allow you to data-log several other inputs, including rpm, coolant or inlet air temperature, and pressures…

Best Power

We mentioned the stoichiometric air-fuel ratio (14.7:1) that is the ideal ratio for lowest emissions, but this isn’t the best ratio for power. It used to be that 12.5:1 was considered the best power ratio, but with improved combustion chambers and hotter ignition systems, the ideal now is around 12.8:1 to 13.2:1. This is roughly 13 parts of air to one part fuel. It’s what combustion engineers call an excess fuel ratio and is intended to ensure that all the air is used to support the combustion process. This is because air is the oxidizer in combustion. Too many enthusiasts think that adding additional fuel beyond the ideal to create a richer mixture will make more power. This doesn’t work because you can only burn the fuel when you have enough air to support combustion. That’s why engines make more power when you add a supercharger or nitrous you’re shoving more air in the cylinder so that you can burn more fuel. Regardless of the amount of air in the cylinder, it still requires a given ratio of fuel to burn. Add too much extra fuel, and power will decrease.

When it comes to fuel mileage and increased fuel efficiency, this ratio changes again. All new cars run at 14.7:1 air-fuel ratio at part throttle because this is the lowest emission point. But depending upon the engine, it’s possible to run an engine at leaner mixtures like 16:1 or more at part throttle to gain mileage. The difficulty with this is that driveability and throttle response suffers at these ratios. Engine response is lazy and stumbles are commonplace. Each engine will be different, but there is fuel mileage to be gained by fine-tuning your carburetor. Don’t be intimidated by these lean mixtures at part throttle. You won’t burn the engine up since it is making very little horsepower at part throttle cruise often less than 30 hp.

The Meter

Now that we know how this trick little machine works and the air-fuel ratio that we’re shooting for, it’s time to start experimenting with the Innovate tool. First of all, you will need a sensor bung location in the exhaust, preferably in or just downstream of the header collector. This way, the sensor will read the exhaust content from all four cylinders (assuming a V-8 engine). You could place a sensor bung in each side of a dual exhaust, but for most street engines, we’ll assume that the left and right banks of your engine will produce similar results. There are some simple guidelines in the instructions on how the sensor needs to be calibrated for the first time. After that, it can be mounted in the exhaust system.

Putting the LM-1 to Work

We welded a sensor bung that was included with the LM-1 into the exhaust system of a carbureted small-block Chevelle equipped with a TH700-R4 automatic overdrive so that we could test both part-throttle and WOT air-fuel ratio. The Chevelle had previously been tuned for WOT metering, and a quick blast up a freeway on-ramp produced an output ranging from 12.3:1 to 12.8:1 air-fuel ratio. This appeared to be a bit on the rich side for optimal power, and it might be worthwhile to jet down a size or two on the secondary side to see if we could make a little more. We’ll save that test for the dragstrip where we can evaluate the change based on trap speed. Even though most car crafters only want to talk about WOT power, we all drive street machines that operate over 90 percent of the time at part throttle. Given this situation, we used the LM-1 to test the air-fuel ratio at part throttle. Our test Chevelle was equipped with a Barry Grant 750-cfm Speed Demon carburetor that Tim Moore had spent a decent amount of time tuning for best idle mixture, but the part throttle results we saw were a bit of a surprise. At light throttle opening speeds of 35 to 40 mph up to freeway cruise in Overdrive, the carburetor delivered consistent air-fuel ratios of between 13.0:1 and 13.5:1 which was close to ideal. The engine runs a GM Performance Parts hydraulic roller Hot cam (218/228 degrees at 0.050), so even with this mild cam, the engine may not like a leaner ratio, but at least this gives us a starting point from which to do some tuning experiments in search of better mileage. One area that car crafters may not be aware of is that most carburetors at very light throttle openings are actually running on the idle circuit rather than the main metering circuit, even if the car is equipped with an Overdrive. In this case, we tried leaning out the idle mixture screws slightly to see if that would improve the part-throttle air-fuel ratio. This leaned out the steady-state air-fuel ratio but caused driveability problems, so we will have to try a different technique. The next best thing will be to data-log all this information during a complete dragstrip run so you can use a computer to plot the data in a spreadsheet format to study more closely what the engine is experiencing during a complete run. Then you can make changes based on the information the LM-1 delivers. What’s even better is that you don’t have to place the laptop on board the car during the run. You merely hit the record button and the LM-1 will capture up to 44 minutes of data-logging information with a sample rate of 12 times per second. We’ve done this with much more expensive equipment and discovered amazing things about carburetors on the dragstrip.

Conclusion

The beauty of the Innovate Technology LM-1 is that it is not only inexpensive, but completely portable and capable of delivering extremely accurate data that until now could only be obtained using very expensive test equipment. Now anyone can accurately measure air-fuel ratio and get a leg up on tuning any engine for both optimal power and better driveability.

For a complete version of this article, visit Car Craft