Air/fuel monitor - best location for 02 sensor

[cds72911]

Hi all,

I have an autometer air/fuel guage sitting in the garage that I never got around to installing in my old race car. I'd like to install it in my 911.

Since it is a '79 SC engine it does not have an 02 sensor mount, besides I am using an early exhaust setup (SSIs and '72 muffler) so even if it had one it would be gone.

So, what is the best place to install the O2 sensor fitting?

Thanks,
Charlie


PS. I am comfortable welding (gas/mig/tig) so installing a fitting is no big deal, although I'd prefer not to modify the SSIs if I can avoid it.

[stlrj]

A/F monitors may not be as accurate as you think, according to this information:

http://www.autospeed.com/C_articles/A_0618/P_1/article.html

Joe

[orbmedia]

This article says that using a common zirconia oxygen sensor (like used in most vehicle fuel injection systems) is not an accurate way to measure A/F ratio for engine tuning purposes. This makes sense because auto manufacturers did not design them for this purpose as a piece of test equipment. This is not to say that they do not serve their purpose as a component of a modern automotive fuel injection component.

There are special A/F meters that are used for this purpose and they are thousands of dollars. I assume they do not use the common zirconia oxygen sensor. Is this true?

[cds72911]

Interesting.

In any case, I'd like to install it to use as a general indicator of the tune of my carbs. Any idea where to install the single O2 sensor?

Since it is a dual in exhaust and I presume I want a "blended reading" from both banks of cylinders I am guessing it should be installed near the outlet of the exhaust. Does that sound right?

Thanks,
Charlie

[911pcars]

I haven't done this modification, but have planned such a setup, I would suggest brazing/welding O2 bungs on each collector where the three pipes merge into one. That means using two sensors. Use a DPST switch to toggle each sensor signal into a single gauge.

I've measured almost 400ºF surface temps. in this area of the exhaust system (regular steel, not SS) with my IR temp. sensor at regular operating temps. so this might be okay for sensor response. However, I'm not sure if the distance from the exhaust ports to this sensor location is optimum for the temps necessary for quick response. Using 3 or 4 wire sensors (w/built in heating element) might be the answer.

Perhaps EGT sensors instead of O2 sensors will yield more useful information.

Sherwood Lee
http://members.rennlist.org/911pcars

[Formerly Steve Wilkinson]

I put such a gauge in my modified '83 SC (PMO carburetors, SSIs, twin ignition, etc.) I put two O2 sensors in, each in the exact same location on each bank of SSIs, just aft of where the three individual pipes feed into one, and then led the wires to a selector switch next to the gauge and can back-and-forth between each bank of cylinders. If you put in just one sensor, what is it telling you? Nothing more than that one or teh other or perhaps both of your carburetors are out of tune.

I find the gauge--a typical $189 unit, to be highly sensitive and accurate. I wouldn't use it to do the carb tuning, but when I do the carb tuning using the traditional methods, the gauge will tell me, "Yes, you're right on."

The higher-quality switch that you can use as a selector, the better, since you're dealing here in tiny voltages. Milspec if possible.

Stephan

[Formerly Steve Wilkinson]

I put such a gauge in my modified '83 SC (PMO carburetors, SSIs, twin ignition, etc.) I put two O2 sensors in, each in the exact same location on each bank of SSIs, just aft of where the three individual pipes feed into one, and then led the wires to a selector switch next to the gauge and can back-and-forth between each bank of cylinders. If you put in just one sensor, what is it telling you? Nothing more than that one or the other or perhaps both of your carburetors are out of tune.

I find the gauge--a typical $189 unit, to be highly sensitive and accurate. I wouldn't use it to do the carb tuning, but when I do the carb tuning using the traditional methods, the gauge will tell me, "Yes, you're right on."

The higher-quality switch that you can use as a selector, the better, since you're dealing here in tiny voltages. Milspec if possible.

Stephan