Best PowerWe 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 MeterNow 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 WorkWe 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.