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Playing Instruments - Part 1

AutoSpeed's guide to the most useful automotive gauges and meters.

By Michael Knowling and Julian Edgar

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It's a sad fact that most car manufacturers are pretty skimpy when it comes to fitting instruments. You get an odometer, fuel level gauge, water temp gauge and - if you're lucky - a tacho to read off engine revs. Some vehicles score extra gauges for perhaps oil pressure, volts or even a crude boost level meter. And if your car does come equipped, there's fair chance that the gauges are of limited range, poorly numbered - or not really marked at all!

Like, how many times have you seen cars with temperature gauges that only have a scale simply marked "HOT" and "COLD"? Too bad if you really want to know the actual coolant temperature! So there is demand for aftermarket gauges - especially for cars running modified engines where it's ultra important to keep tabs on temperatures and pressures.

Gauge Design

Gauges fall into a number of basic design categories:

  • electrical
  • mechanical
  • digital
  • analog
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Electrical gauges use a special sender unit (ie a sensor) that measures the physical parameter and turns it into an electrical signal. The output can then be displayed either on a digital or analog gauge. An example of this type of set-up is the coolant temperature gauge in your car - it uses a sender and the gauge in the dash is an analog, electric gauge.

Another way of doing it is to transfer whatever you are measuring right up to the gauge itself. A mechanical oil pressure gauge, for example, has a line that feeds pressured oil into the gauge, which almost always uses an analog display. Mechanical gauges are cheaper than electric gauges, because a special sender unit isn't required. However, that's their only advantage - electric gauges are more flexible in mounting (you don't need to bring a hose or pipe into the cabin), safer (for the same reason), and often more accurate.

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The output of an electric gauge can be either analog (a pointer sweeping across a marked faceplate) or digital (eg LED or LCD numbers). In general, factors which change quickly (like engine rpm and boost) should use an analog display, while slowly-changing information (like battery voltage) is best shown digitally.

LCD displays tend to work better in cars than LED, because an LCD display is visible even in direct sunlight. Manufacturers usually make sure that any built-in LED displays are shaded from almost any sun angle, but it's much more difficult to recess displays in this way when adding them after the dash has already been designed. On the down side, not many LCD displays are back-lit - necessitating the use of external lighting. Best results will be achieved in lighting LCD displays if the light shines on the display from near to the angle from which you're viewing it. In other words, using tiny 12 volt lamps (sometimes best recessed into tubes) and aimed at the display from the front will give best night illumination. Another approach is to use red or green LEDs as the light source, allowing easy matching with the night illumination colour of the factory instruments.

A backlit, negative LCD display (like that used in the AutoSpeed intake temp/exhaust gas temp project covered later) is best of all. It's visible in all lighting conditions, including direct sunlight and total darkness.

Temperature Gauges

Measuring the operating temperature of different components around the car is one of the most important monitoring things that you can do. The range that needs to be covered depends on what you're actually measuring - here's a guide:

Engine coolant 40-120 degrees C
Engine oil 50-150 degrees C
Transmission oil 50-150 degrees C
Intake air (turbo engine) 20-120 degrees C
Exhaust gas 100-900 degrees C

Oil Temperature

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An oil temperature gauge can be used in any of three parts of a car - the engine, transmission or the diff. However, the most common application is to monitor engine oil temps.

All engines that are worked hard should have an oil temperature gauge fitted - this is particularly the case if the engine has a high specific power output (ie lotsa hp per litre) or if the car is driven at high speed for long periods. Oil temperatures gauges are almost always electric. The sensor can be incorporated into a new dipstick probe (300 or 500mm long probes available), or alternatively, into the sump by the use of a tapped plug. If you're lucky, a sensor plug with the same thread and diameter as the sump plug might be available; otherwise, the sump will need to be removed to have a fitting attached.

High-powered cars with automatic transmissions should always use a transmission temp read-out. The life of an auto trans drops incredibly quickly if the oil is overheated - and since trashing the oil isn't hard to do, the fitting of a gauge is vital. The sensor is normally mounted via a drilled and tapped fitting, so if you have your auto trans out for a rebuild, that's the time to have a sensor installed.

Water Temperature

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If you wish to replace your car's standard coolant gauge with something that reads real numbers, here's a gauge to do just that. VDO's coolant temp gauges reads from 40 to 120 degrees C, in increments of 5 degrees. The first third of the needle's sweep compresses the 40 to 80 degree range, leaving more room for the crucial zone of 80 and higher (you want to clearly see what's happening when your engine's near the limit). This electric water temperature gauge retails for $50, with a $14 sender kit also required.

Exhaust Gas Pyrometer (EGT Gauge)

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An exhaust gas temp (EGT) gauge is one instrument that cranks out some really serious numbers! By inserting a thermocouple into the path of the out-going exhaust gasses (usually just after the exhaust ports or behind the turbo) it is possible to obtain an indication of the activities of air-fuel mixtures and airflow. It's also a very spectacular gauge to watch when you're working a turbocar hard up a long hill - the temperatures simply sky-rocket to around 800-ish degrees C. Great to impress your mates!

High boost engines may even go higher than 800, so it's lucky this VDO gauge reaches all the way to 900 degrees C (in 50 degree markings). Three hundred dollars will buy you a VDO pyrometer kit like this, with the kit comprising an electric analog gauge and the special purpose pyro.

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If you want a backlit digital pyrometer, have a look at AutoSpeed's very own superb LCD exhaust gas temp meter at TempScreen: Part 2 - Installing the Exhaust Gas Temp Probe. Here you'll see one of the cheapest and most elegant ways to get an EGT into your car.

Cylinder Head Temperature (CHT)

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A CHT gauge is really only required in highly modified engines or those prone to having head problems. It's also the engine temperature gauge of choice on air-cooled engines (along with an oil temp gauge). The sensor for this gauge comprises a special thermocouple washer that is inserted under the spark plug - so the sensor can be difficult to fit to engines that use deeply-recessed plugs. This VDO 50-300 degrees C unit (marked every 50 degrees) can be bought as a complete kit (including sensor) for $165.

Intake Air Temperature

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An engine sucking in hot air isn't making the power it could be - and that's not good! One of the best ways to watch your engine's intake air temperatures is to buy a simple LCD meter that's normally used in household applications. K-Mart, Jaycar Electronics and other stores sell LCD thermometers which are designed for use in cars to measure the outside and inside temperatures. The 'outside' temp is measured with a remote probe, which can be turned to other uses - like measuring inlet air temps.

A typical range of one of these meters is from minus 20 to plus 70 degrees. A turbo car with decent intercooling should never have an inlet air temp higher than 70 degrees, and so these meters make excellent post-intercooler temp monitors. In both atmo and turbo cars, the effectiveness of the cold air induction pick-up can also be assessed. Dual display units, some with memory functions, are available from just $30.

If you take this approach, pick a meter which updates frequently. If the meter samples three times a second, trends in the temp can be seen at a quick glance. Some meters update only every 5 or 10 seconds, which is too slow. Most meters read to one decimal place, and this too is useful when the spotting of temp trends is needed. Note that the durability of cheap LCD thermometers varies widely - a model once stocked by K-Mart lasted on a car dash for years, while other models have become defective after only a few weeks. However, there's no way of doing it cheaper.

Like many other temperature measuring systems, the probe will need to be re-mounted to allow easy automotive fitting. Epoxy'ing the sensor into a brass fitting is the easiest way of mounting it - but take care to use high temperature adhesive if the sensor is to run most of the time over about 60 degrees.

Non-intercooled turbo and supercharged cars can have very high intake temps, so a higher rated system is required in this application. One of the best LED set-ups you can use is made by Carel, which can be viewed at TempScreen: Part 3 - Displaying the Temperatures. It retails for around $260 and has numerous features (including the ability to work as an EGT meter). Note that it uses a K-Type thermocouple as the input sensor, and this sensor costs extra.

Knowing intake temperatures in a turbocar, it's then possible to see what kind of improvement a new cold air intake has made, how boost pressure effects intake temperatures, compare intercooler efficiencies, and more. It's also interesting to observe how much slower a turbocar (such as a WRX) performs when the intercooler is heat-soaked and there's hot air being swallowed.


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Playing Instruments - Part 2

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