I purchased this gauge from Summit Racing. The part number is ITC-TH7009 and the price is only $29.95! That sure beats the $150 kits that Summit also offers. You will have to also buy an O2 sensor, but you can pick up a single-wire O2 sensor at any auto parts store. I bought a Bosch universal O2 sensor, part number #11025, cost $46. So, you can make your own exhaust-gas analyser for $76!! Costs do vary for the O2 sensor; if you have a friendly parts place you can ask them for the cheapest unit.
The gauge is a simple one-wire hook-up to the sensor, with +12V and ground wires used for gauge illumination. You should power your gauge from a switched power supply, otherwise the gauge will be lit even when you turn your car off. Battery drain...
Goals:
Gotta have goals, especially when starting a project like this. I got in contact with Fred Sisson, the fellow who wrote up the article on a more focused look at SU tuning. He told me of his experiences with using the gauge, and what to expect from the tuning process. His goals, which I aim to follow, include:
Placement and Sensor Selection Tips:
The gauge does fluctuate a little when the pedal is pressed, but it is to be expected based on the mechanical nature of the intake system (and not nearly as much as I was lead to believe). I'm happy that it works as well as it does. A word of caution though: when selecting an O2 sensor, you may want to spend the extra money and buy a three-wire O2 sensor. This sensor allows for external power to heat the sensor up to operating temperature (exhaust heat does this in the single-wire O2 sensor I bought). When the O2 sensor doesn't get enough heat, it doesn't register on the gauge. So, sitting at idle and cruising through town at 45mph in fourth gear, the gauge goes cold. I'm not that concerned about it, especially the idle settings, but it can force you to run up through a gear or two to wake/warm the gauge up. Bummer:)
Placement of the single-wire O2 sensor in the exhaust system also has an effect on how well it performs. Due to the design of the stock separate-runner L-16 Datsun exhaust manifold, it is not possible to place the O2 sensor in the manifold and be able to get a combined reading from all four cylinders. The only way to do this is to place the O2 sensor after the 2-into-1 collector in your exhaust system. Since I had my collector extended to the transmission mount, it is only further from the heat of the exhaust ports. Jason Gray also has an O2 sensor installed in his motor, but he is using the 4-into-1 log manifold on a custom turbo setup. Jason plumbed his O2 sensor into the manifold itself, and it stays hot almost all of the time.
That said, I already have a dream needle: Leaner in the first stations and much richer nearer the tip. There's plenty of experimentation to try out, including stiffer dome springs which will provide more enrichment over the standard springs. I also have the M43 needles to try out. All to come, stay tuned.
It works!! I finally got the whole setup installed and just took my first drive. I've had plenty of time to think about the needles I'm using (M66) and how far they are from ideal for my engine combination. My first test drive confirms my thoughts. When adjusted for best power at WOT I get a cruise and idle that are excessively rich. Cruise conditions can be very lean (and should be for best mileage), yet I am on the rich side of the scale at normal freeway speeds.
I would highly recommend installing an O2 sensor and gauge similar to what I've done. I've already learned quite a few things about the operation of my SUs that I would never have known without being able to see the air/fuel ratio. I thought ATF was the perfect damper oil; for my engine combination, however, it is not ideal. I also wondered at Datsun's selection of such soft dome springs. With the gauge installed, I can test different dome springs to see what works better, and what the changes affect.
As it is, without the proper needle, I'm still running rich at cruise, which is horrible for fuel economy. On the plus side, I've mostly eliminated the sudden lean condition I was getting using stock dome springs and ATF in the dampers.
5/13/99
...I was recently syncronizing my carbs, running around town without air filters. This had a pretty large effect on the air/fuel ratio. With the 15wt oil in the dampers and the 2.2oz. springs I was back to experiencing lean mixtures with increases in throttle. The mixture would stabilize under cruise, but would go off-the-scale lean during any acceleration. So, my K&N filters (admittedly old and contaminated) provide a restriction to incoming air, and therefore a richening effect. If you had new filters, or ran without them, you could try 20wt oil in the dampers. This is, after all, what the Datsun Factory Manual recommends.
6/18/99
Well, when you get the dashpot oil and dome spring dialed in, the air/fuel gauge doesn't do much moving. Most of the time it is pretty steady (after the initial on-throttle blip). My engine is still running rich, however, so the gauge will come in handy in the near future.
7/15/01
After sanding a pair of needles down to give more fuel to the top end (and allowing me to lean the whole needle out as a result), the O2 sensor showed a very nice air/fuel mixture. Around down it was in the middle of the gauge, on the freeway it was off-the-scale-lean (great for economy), and best of all, when I dropped the hammer the mixture went very rich! Exactly as I'd hoped.
Without the O2 sensor, I'd never have known any of this information. I never would have known the mixture was off in the first place. I never would have known where it was off. Nor would I ever have known that I'd gotten the mixture a lot closer through sanding of the needles (and not fiddling with dome springs or damper oil). Clearly the O2 sensor is invaluable if you are doing any serious tuning or using anything other than a bone-stock L16/L18 motor (for which stock needles are fine).