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This article was first published in 2003.
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Multimeters are these days incredibly low in price - a fully-featured one that can measure temperature, frequency and duty cycle (as well as the usual volts, ohms and amps) can be yours for AUD$70. That's dirt-cheap and the situation's even better than that, cos you can use the meter to improve your car's performance. Well, to collect the information that lets you make the right decisions about improving performance, anyway.
But first up, if you're unsure of how to use a multimeter to measure even the basics, have a read of this: "Using a Multimeter". It will give you an overview of what to look for when selecting a meter, and then tell you how to use it.
So how can all this make your car go faster?
Temperature
Nearly all good multimeters (and 'good' doesn't have to mean expensive) will allow you to measure temperature via a K-type thermocouple that can be plugged straight into the meter. Thermocouples are typically available as bead designs (very small and lightweight which allows their temp to change very rapidly, but fragile as well) or probe types (much more rugged but their temp changes more slowly). Often it's worth having one of each - bead designs are easy to slip under hose clamps, for example.
Exactly how you use the temp-sensing capability of the meter will depend on whether it has a 'max hold' function - if it has, you don't need to be actually reading the meter real-time to see how hot something got. Since generally in a car it's most often the max temp that was reached which is important, taking this approach works well. If the meter doesn't have a 'max hold' function then you'll need to position the meter so that it can be read real-time - for example, the meter can be stuck to the outside of the windscreen with masking tape (but we don't suggest that you leave it there permanently!).
One area where keeping the temp as low as possible yields definite power gains is the engine's intake air temp. In either a turbo or naturally aspirated car you can start off by measuring the temp of the air where it is drawn into the intake duct that leads to the airbox. (In cars fitted with aftermarket pod filters, just measure the temp around the pod.) This measurement can yield startling information, especially in urban stop/start driving. Temps as high as 70 degrees C can be recorded on 30-degree days - and that spells bad news for power, both because the intake air contains less oxygen per cubic metre and also because detonation is more likely.
Once you've measured the temp at the beginning of the system, you can move the sensor along the intake system. In a naturally aspirated car, measurements after the airbox and in the plenum chamber are the two to make next. On a turbo intercooled car, after the turbo (yikes! - can it really get that hot on boost?!) and then directly after the intercooler are pretty interesting. Follow that with a temp reading at the throttle body (so you can see how much the intercooled air gets re-heated on its way back to the engine) and then again, in the plenum chamber.
Depending on the temp results, you can make decisions on whether the air intake pick-up should be moved, shielded or ducted with cooler air, whether the airbox or pod filter should be heat-shielded, and whether the intake plumbing (eg the pipe from the airflow meter to the throttle or in turbo cars, from the intercooler to the throttle) should be wrapped in insulating material. You may even find the need for fitting insulating spacers to stop heat from the head getting into the intake manifold. If you can drop the intake air temp by 5 degrees C you will definitely feel it, while sometimes judicious insulating and/or moving of the intake (especially if it's already a badly modified car and you can undo some of the negatives) can reduce intake air temp by 40 degrees C or more!
Other pretty interesting areas to place the temp probe include on the sump (you don't want engine oil temps as low as possible, but at the same time anything regularly over about 120 degrees C is a worry), the auto trans fluid cooler lines (ditto re engine oil temps), and the coolant radiator - with the latter you'll then be able to see how the dash gauge is calibrated.
Voltage
There are two distinct ways in which you can use the multimeter's voltage measuring function to help you in performance. Firstly actual voltages, and second, voltage differences.
There are two important sensors that in most cars output a raw voltage: the airflow meter and the oxygen sensor. The airflow meter measures the mass of air being drawn into the engine. We're familiar with the concept that if the engine 'breathes better' then it's making more power - and in nearly all cases, that's the case. So if an increase in airflow into the engine means that your engine's going harder, a simple way to see if the mods are heading in the right direction is to measure airflow meter output voltage. (In some airflow meter designs, output frequency varies with airflow - in these cases, you just set the multimeter to measure frequency instead of voltage).
Obviously as you rev through the load range the airflow passing into the engine is also changing, so you'll need to pick a point - that could be a simple as the max voltage or frequency that's seen. (Again, if the meter has it, you can use the 'max hold' button.) So if before you put on the new exhaust the max airflow meter voltage that you ever saw was 4.4 volts, and then after the new zorst went on that rose to 4.6 volts, the engine's breathing more air and - almost certainly - making more power as well. Cheap, simple and easy to monitor!
However, this brings us to another point about measuring sensor voltages - especially airflow meter ones. What if you're watching the airflow meter output voltage in your highly modified car and it reaches a peak of 4.8 volts - at just 4000 rpm, full throttle? It doesn't budge from there as you rev harder and harder. What's happened is that your breathing modifications have been so successful that the airflow meter has reached its max output level - in this case, it's at that limit by only 4000 rpm. You really need a bigger airflow meter or some other major changes to the system, because you're likely to be right on the edge of engine death and destruction - in this situation at least do a dyno run and have the mixtures checked with an accurate meter. So in addition to measuring max output voltages, the multimeter can be used to see if the sensor is still working within its factory parameters.
Another important sensor to measure is the oxygen sensor. As most of you will know, the oxy sensor outputs a voltage between 0-1 (well, most do - some current designs change this range) which is related to air/fuel ratios. However the relationship isn't direct - the sensor 'switches' from a high to a low voltage around air/fuel ratios of 14.7:1. This area is a whole subject in itself - have a read of "The Technology of Oxygen Sensors" for more nitty-gritty detail. As you'll see from that article, measuring the output with a multimeter won't tell you what the mixtures are exactly - although it will tell you whether the air/fuel ratio is broadly rich or lean. The major problem with getting more accurate results is that the voltage varies a fair bit with exhaust gas temp as well.
However, there's no need to ditch the idea entirely: excellent information on the actual air/fuel ratios can still be obtained so long as you keep the testing conditions similar. For example, say the voltage output of the oxy sensor goes to 1020mV (ie 1.02V) at full throttle from 3500 rpm upwards. (Although the voltage doesn't change as revs further rise, in this case it doesn't mean that the sensor is outside of its operating conditions. Instead it shows that the mixtures have gone full rich and then stayed at that value - something that's wanted.) You do four test runs and the voltage always goes to this level - or very close to it. You then lift the boost (a bit more!) and you find that when tested in the same way, the voltage is now only 800mV. You can therefore say with some certainty that the mixtures are now noticeably leaner than before. In fact, lean enough for you to want to seek some accurate air/fuel ratio meter readings, eg on a dyno. On the other hand, if the oxy sensor voltage remains unchanging at this high level as you wind up the boost, you can be pretty confident that the air/fuel ratio has changed little or not at all.
And what was that about voltage drops that was mentioned earlier? This technique won't actually make your car faster, but it does give you information that's potentially useful in improving comfort and safety. Any device that draws lots of electrical power is likely to have a voltage drop occurring on the feed to it - that is, not enough current can get down the wire to fully supply it. If you measure the voltage at the battery and then compare this value with the voltage measured right at the device that's consuming lots of power (which could be a car sound amplifier, driving lights or headlights running uprated bulbs) then you can make an informed judgement about whether the wiring needs to be modified. If the voltage drop is more than about 1.5 volts, think about heavier duty wiring.
Duty Cycle
Remember how above we found that the airflow meter was maxed out - its output voltage didn't keep going up, even though the airflow into the engine was still rising? Well, a similar maxing out can also occur with injectors. The proportion of time that an injector is open for is called its duty cycle - open (and so flowing fuel) for half the time, it is said to have an injector duty cycle of 50 per cent. Open for three-quarters of the time equals a duty cycle of 75 per cent. Measuring duty cycle is as easy as setting the meter to that function and connecting the probes to the injector. Again, it's a natural for a peak hold button.
You'll find that max injector duty cycle occurs at peak power, peak rpm - in an unmodified car it's likely to be somewhere around 80 per cent. But if you've wound up the power you may well find that you're nearer to 100 per cent - and without lifting fuel pressure, that means the injectors are flat out. That is, they can't flow any more fuel even if your engine is crying out for it....
Conclusion
With multimeter prices so low and their functuality so high, they're a must-have for the toolbox.
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