We mounted a GTechPro Expandable Gauge System (EGS) tach on my orange Chevelle. This tach
Oxygen Sensor Stuff
Matt Kelley, Evansville, IN: Hey guys, thanks for a great magazine. I opted to get a cheap PC-based oscilloscope and an off-the-shelf narrow-band O2 to do some A/F tuning on my '65 Falcon since the oscilloscope would have other uses over a dedicated A/F monitor. I tried it out and it seems to work OK, but the response time for the O2 seems slow and the ratio seems to fall to super lean (0.2V) at cruise and even lower at idle. The Falcon has a 351W, Hooker full-length headers, an Edelbrock RPM cam, and a 650-cfm Holley on a Torker II intake all running through a T10 and some 3.50:1 gears. I have the O2 installed on the right bank as close to the collector as possible. Would the cam overlap produce the low voltage at idle? I'm probably not using the best O2, but would it cause the lazy readings? Any discussion on this or maybe a full article would be greatly appreciated.
Jeff Smith: Matt, you have an interesting idea to incorporate an oscilloscope with an oxygen sensor. The narrow-band sensors are just that-they will deliver an accurate voltage feedback for air/fuel ratio but only directly around stoichiometric or 14.7:1 air/fuel ratio (A/F). On either side of 14.7, the voltages are drastically different-roughly 0.85 for a rich mixture, while lean would be what you're seeing at roughly 0.2 volts or less. If you look at the illustration of the narrow-band output (see the accompanying graphs), the drastic voltage change becomes obvious.
A wide-band oxygen sensor delivers a much more linear voltage curve (the graph on the right side) that starts at very low voltage around 0.90 and extends through roughly 2.1 volts. These wide-band sensors use the reference voltage of 1.47 as stoichiometric (also called Lambda), which is 14.7:1 A/F. So voltage delivered from the sensor of 0.85 equates to 12.5:1 A/F (14.7 x 0.85 = 12.5). Given this simplified output, you can use a voltmeter to read the A/F on most wide-band oxygen sensors merely by moving the decimal point. So if the oxygen sensor output reads 1.25 volts, the A/F is 12.5:1.
This illustration shows the relative voltage output differences between a narrow-band sens
Concerning cam overlap, remember that oxygen sensors are aptly named since they don't measure the actual ratio of air to fuel. Instead, the sensor reads the amount of free oxygen in the exhaust and calculates an A/F ratio. This is why a misfiring engine will often read a leaner A/F ratio because the sensor is reading the free oxygen that is pumped through the engine when the cylinder did not fire. When we use a long-duration camshaft with additional overlap, both valves are open at the same time for a longer period of time. It is inevitable that exhaust gas and fresh inlet air will pass directly from the intake valve right out the exhaust. This free oxygen is read by the sensor as a lean A/F mixture when the truth is the A/F ratio is probably significantly richer. My experience with the Lester Scruggs 404ci LS engine in my orange Chevelle has the Innovate meter consistently delivering an A/F ratio at idle between 16:1 and 17:1 in gear with occasional spikes of 20:1. With the huge amount of overlap in the cam in this engine, I doubt the engine would even run at these A/F ratios.
As to your mention of response time-keep in mind that there may be two things going on here. One is the response time of the oscilloscope itself combined with that of the sensor. This is a complex subject that I'm not all that familiar with, but from what I've read, it seems that more expensive sensors tend to be much quicker than less expensive ones. The age of the sensor is also a critical factor. As the sensor ages, it will become slower and less responsive, which means the computer is always playing catch-up with the engine. This is one reason a new sensor can improve driveability.
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