Tune on the Run
By Jeff Smith – Photography by the Author
Information is the key to better performance. It used to be that if you had a tach, oil pressure, water temp, and voltmeter that was all you really needed. But things have changed.
Some may be content to live in blissful ignorance of carbureted air/fuel ratio, but all hot rodders are control freaks of the first order and in this new century need to know much more than their ancestors—especially if you’re driving a carbureted street engine. Give a modern hot rodder an affordable way to measure air/fuel ratio and most will jump at the chance.
Of course, air/fuel ratio meters are nothing new—they’ve been around for a long time. But the folks at Fuel Air Spark Technology (FAST) have come up with an even better mousetrap. The results are still the same, it’s just that now they can give you more performance for less money.
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Previous systems required a separate gauge that you had to mount in your dash somewhere. Often, this was the most difficult part of the installation. So, the FAST engineers decided to make it easier for any hot rodder who owns a smartphone— which is just about everyone. The new FAST wireless air/fuel ratio meter sends a Bluetooth signal from the meter’s control box to your phone, which gives you access to a stream of air/fuel ratio information that is updated almost instantaneously while you watch.
The new FAST system is available in either single- or dual-channel configurations and is shipped with all the necessary components. Since you don’t need to mount a gauge, the next hardest part is drilling a hole in the exhaust system and welding in the supplied mounting bung. To make things even more attractive, FAST has included a simple anti-theft device with the unit so that you can use your phone to disable either the ignition or the fuel pump relay to prevent unauthorized use of your car.
We decided to test this new system on our carbureted ’65 small-block, four-speed El Camino. The engine was fresh with less than 100 miles on it, and we wanted to make sure that the engine could run as lean as possible. This approach is best because if the engine idles or runs at part throttle in rich condition, this will tend to wash the oil off the cylinder walls, which not only hurts cylinder sealing but also increases wear. Mostly, we just wanted to know how well our 600-cfm Holley carb was performing.
Our exhaust already had an O2 bung, so that part was easy. We found a suitable location to mount the control box on the firewall and the wiring was equally simple since we didn’t bother with the security system. We were only interested in the O2 data stream we would be getting.
With everything installed we loaded up the free software in our phone and were surprised to see that it all worked the first time. The O2 sensor needs a few moments to warm up but when we started the engine, we were instantly met with a solid report of the idle air/fuel ratio of around 13.2:1. That seemed a little on the rich side, so we let the engine fully warm up and adjusted the idle mixture screws and were rewarded with a slightly leaner 13.5:1 number. Anything leaner than 13.5:1 and the engine lost vacuum and rpm. This was where our engine preferred to idle.
The next day we took the El Camino out for an extended cruise with the phone positioned so we could monitor the air/fuel ratio. In a couple of different situations, the signal was interrupted and we had to reload it in order to regain the signal. After some experimenting, we decided our power wires for the box were too close to the spark plug wires and after some rerouting both the spark plug wires and our box wiring, the problem disappeared.
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With a much cleaner and uninterrupted signal, we thought the cruise air/fuel ratio was a bit richer than it should be with readings in the low 13s. Our driveline combination included a rather tall 3.08:1 rear gear ratio with cruise rpm of around 2,800 to 3,000 rpm. This is enough load to open the throttle blades far enough that we often dipped into the main metering circuit on the carburetor instead of running on the idle circuit like most carbureted engines.
If you read the sidebar on page xx on “What is Lambda?,” it will help explain why this and most air/fuel ratio meters offer ratios in the Lambda format. But for now we will offer our results in regular gasoline air/fuel ratio terms. It’s important to note that air/fuel ratio meters like this don’t actually measure the ratio of air to fuel in the exhaust. They instead measure the free oxygen in the exhaust and use those levels to calculate the air/fuel ratio.
This is an important point because if there is an exhaust leak upstream of the sensor, it can be fooled into delivering a much leaner air/fuel ratio than what is really happening. That’s why it’s important to ensure that the header and collector flange gaskets are good and leak-free. If you can hear an exhausts leak (that annoying ticking sound), then it’s guaranteed that there is an exhaust leak that needs to be addressed.
Our exhaust system was solid with no leaks and after the short cruise we decided to reduce the primary jetting by two jet sizes just to see if that would make an appreciable difference. We did see a small improvement in cruise air/fuel ratio moving up to the high 13s, which we felt was fairly close to ideal.
Remember that the stoichiometric air/fuel ratio for normal pump gasoline isn’t 14.7:1 but instead 14.1:1. So readings like 13.8:1 are within 0.4:1 ratio of the chemically correct lean condition. This is the primary reason we installed this system on the car. Given that we have three other machines that need similar attention, we can easily switch between cars since we don’t have to bother with mounting a permanent gauge.
It should be obvious from this story that reliable feedback on modifying a carburetor makes the task of tuning so much simpler. In very short order we dialed in both the idle and part throttle numbers and even made a few wide-open-throttle runs to verify that the high-speed metering was also accurate, with readings between 12.5:1 and 12.9:1—all very safe.
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There are certainly old-school ways to verify metering by reading spark plugs and using dragstrip trap speeds to verify the direction of tuning. But you can get there a lot quicker when there are some instant feedback numbers flashing from your smartphone. Isn’t technology wonderful?
What is Lambda?
Lambda is the stoichiometric or chemically perfect air/fuel ratio for any fuel, such as gasoline, ethanol, methanol, or any blended fuel. Lambda at stoichiometric is always expressed as the number 1.
Hot rodders are used to hearing air/fuel ratios expressed in numbers like 12:1 or 15:1. But different fuels specify a different air/fuel ratio for its chemically correct stoichiometric ratio. As an example, straight gasoline’s stoichiometric number is 14.7:1. But add just 10 percent ethanol and the stoichiometric number changes to 14.1:1. Then if we look at E85, its stoichiometric number is a much different 9.8:1.
With all these different numbers, it would be very difficult to keep track of them all, especially if you start playing around with blended fuels like E30 or E50. A better way to keep track of all these numbers. Lambda is the key. With Lambda, the stoichiometric number is always 1 and lean numbers will be expressed as greater than one and richer ratios will be expressed as less than one.
The following chart offers Lambda equivalents for straight gasoline, E10 pump gas, and E85 but there are many others. Keep in mind that Lambda is always 1 for any fuel. This is what makes Lambda easier to use when mixing fuel such as gasoline and ethanol to create E50. It’s much easier to remember that 0.85 is close for the best rich power mixture than it is easier to remember than the actual ratio for E50 is 7.5:1. We made this little discovery when we multiplied 14.7×0.85 for straight gasoline and the answer is 12.5:1—a common rich power air/fuel ratio for straight gasoline.
Why would we bother to tell you all this? Let’s take today’s pump gas. All air/fuel ratio meters are designed to use 14.7:1 as stoichiometric for gasoline. But nearly all pump gasoline sold in the U.S. contains 10 percent ethanol. All you have to do is study this chart to see that stoichiometric for E10 (10 percent ethanol) has changed from 14.7:1 to a richer 14.1:1. You can make all this simple by knowing that a typical air/fuel ratio for E10 at max power will be 0.85 Lambda and a nice cruise air/fuel ratio might be 0.95 to 1.0. Those numbers will be the same for any fuel you use. Simple!
Air/Fuel Ratio
| Gasoline
|
Gasoline
E10 |
E85 | E50 | |||||
| Lambda | AFR | AFR | Lambda | AFR | Lambda | AFR | ||
| 0.58 | 8.5:1 | 8.2:1 | 0.58 | 5.7:1 | 0.36 | 3.5:1 | ||
| 0.61 | 9.0:1 | 8.6:1 | 0.61 | 6.0:1 | 0.40 | 4.0:1 | ||
| 0.65 | 9.5:1 | 9.1:1 | 0.65 | 6.4:1 | 0.46 | 4.5:1 | ||
| 0.68 | 10:1 | 9.6:1 | 0.68 | 6.7:1 | 0.51 | 5.0:1 | ||
| 0.71 | 10.5:1 | 10.0:1 | 0.71 | 6.9:1 | 0.56 | 5.5:1 | ||
| 0.75 | 11.0:1 | 10.5:1 | 0.75 | 7.2:1 | 0.61 | 6.0:1 | ||
| 0.78 | 11.5:1 | 11.0:1 | 0.78 | 7.6:1 | 0.66 | 6.5:1 | ||
| 0.82 | 12.0:1 | 11.5:1 | 0.82 | 8.0:1 | 0.71 | 7.0:1 | ||
| 0.85 | 12.5:1 | 12.0:1 | 0.85 | 8.3:1 | 0.76 | 7.5:1 | ||
| 0.88 | 13.0:1 | 12.4:1 | 0.88 | 8.6:1 | 0.81 | 8.0:1 | ||
| 0.92 | 13.5:1 | 13.0:1 | 0.92 | 9.0:1 | 0.86 | 8.5:1 | ||
| 0.95 | 14.0:1 | 13.4:1 | 0.95 | 9.3:1 | 0.91 | 9.0:1 | ||
| 0.99 | 14.5:1 | 13.9:1 | 0.99 | 9.7:1 | 0.96 | 9.5:1:1 | ||
| 1.00 | 14.7:1 | 14.1:1 | 1.00 | 9.8:1 | 1.00 | 9.85:1 | ||
| 1.02 | 15.0 | 14.4:1 | 1.02 | 10.0:1 | 1.01 | 10.1:1 | ||
| 1.05 | 15.5 | 14.8:1 | 1.05 | 10.3:1 | 1.06 | 10.5:1 | ||
| 1.09 | 16.0 | 15.4:1 | 1.09 | 10.7:1 | 1.11 | 11.0:1 | ||
| 1.12 | 16.5 | 15.8:1 | 1.12 | 11.0:1 | 1.17 | 11.5:1 | ||
| 1.16 | 17.0 | 16.3:1 | 1.16 | 11.4:1 | 1.22 | 12.5:1 | ||
| 1.19 | 17.5 | 16.8:1 | 1.19 | 11.7:1 | 1.27 | 12.5:1 | ||
| 1.22 | 18.0 | 17.2:1 | 1.22 | 11.9:1 | 1.33 | 13.0:1 | ||
| 1.26 | 18.5 | 17.7:1 | 1.26 | 12.3:1 | 1.37 | 13.5:1 | ||
| 1.29 | 19.0 | 18.2:1 | 1.29 | 12.6:1:1 | 1.42 | 14.0:1 |
Parts List
| Description | PN | Source |
| FAST wideband wireless, AFR meter, single | 170301 | Summit Racing |
| FAST wideband wireless, AFR meter, dual | 170302 | Summit Racing |
| Summit weld-in oxygen sensor bung | SUM-G2990 | Summit Racing |
| FAST weld-in oxygen sensor bung | FST-30168 | Summit Racing |
| FAST oxygen sensor plug | FST-30169 | Summit Racing |
Sources:
Fuel Air Spark Technology (FAST)
(877) 334-8355
fuelairspark.com
Summit Racing
(800) 230-3030
summitracing.com
Click on this issue’s cover to see the enhanced digital version of FAST’s Slick New Air/Fuel Ratio Wireless Meter.
