Today when we have data-logging, infra-red timing beams and laptop PC lap-timing that can measure 0.001 second changes, measuring performance improvements on the road seems too simple. Shouldn't all testing be carried out on the track like in the pic above? But it is impossible to humanly feel a performance improvement fifty times bigger than 0.001 seconds (ie 0.05 seconds) in the sprint to 100 km/h - so is it necessary to have such accurate timing? If you have access to equipment that can measure such small time increments that's great, but it's simply not necessary that timing systems have this degree of accuracy to be useful on road cars.
Stop Watch Testing
The humble stopwatch built into most digital wristwatches is vastly under-rated as a performance measuring device. With care, stopping and starting a hand-held stopwatch can be carried out with a consistency of far less than 1/10th of a second. And it should be noted that any performance mod that results in less than a tenth of a second gain isn't very successful!
Practise starting and stopping a stopwatch while you view the second hand on an analog clock or the digital seconds display on another watch. Time a number of 10 second increments on your stopwatch and see how close you actually get to 10.0 seconds. I have just stepped away from the keyboard to do this and have got the following results: 9.94, 10.10, 9.98, 9.92, 9.94 and 9.96 seconds. You can see that when the single figure furthest from the median is excluded, the timing is only 0.6 per cent inconsistent! Obviously, with the g-forces pushing you back in your seat and the speedo needle whipping around while the road races towards you, the in-car timing won't be this accurate, but using a digital stopwatch is far better than most people think.
Using a stopwatch to measure acceleration performance in this way does not require that the speedo be accurate. If you wish to compare your results with figures gained in other cars, then of course the speedo must be correct, but in the vast majority of cases you will be simply comparing the performance before and after making a modification. Unless you change the tyre diameter, final drive ratio or some other gearing aspect, the speedo accuracy won't change during this period.
The greatest variation in standing-start acceleration times is that caused by differing launches. A high-powered manual RWD car and any manual FWD or constant AWD car is very difficult to launch consistently. In both types of two wheel drive cars, wheelspin will occur if too many revs are used on launch, while a constant four wheel drive car will very easily bog down. For this reason, accurately measuring performance changes with these cars is better done from a rolling start. All cars with automatic transmissions lend themselves very well to consistent 0-100 km/h timing.
In any type of testing where a hand-held stopwatch is to be used, pick a flat, preferably deserted road. Don't attempt to perform any timing runs in urban areas - it's just too easy for a child to run out onto the road from a house, or a car to suddenly appear in front of you. Set the watch to zero (it helps if it beeps when started and stopped) and then rev the engine to the launch rpm. In an automatic car, load the engine with the brakes while applying some accelerator. Release the clutch (or brakes in an automatic car) as you press the stopwatch and then accelerate hard through the gears. Prepare to press the 'stop' button as the speedo needle sweeps around, hitting the button a fraction before you see the needle actually pass over the mark. It is safer if a passenger riding directly behind the driver operates the watch, but I have often timed cars in this way while alone.
If you are after the most accurate figures possible, make three or four runs in each direction. You will quickly see if the figures form a pattern or are simply all over the place. Don't confuse timing inaccuracies with the performance of the car changing during the testing; as the engine heats up, power (especially in a forced induction car) will often decline.
An example of stop watch testing is that carried out on a six cylinder EF Falcon. The automatic car recorded a 0-100 km/h time of 9.2 seconds in standard trim. With the airfilter removed, this improved to 9.1 seconds. The airfilter was then replaced and the bonnet 'popped' to the safety catch. This let more air get to the factory intake snorkel and the time stayed at 9.1 seconds even with the filter back in its box. The bonnet was then returned to its closed position and the intake snorkel to the airbox removed, allowing the engine to breathe hot air. The 0-100 time then lengthened considerably to 9.5 seconds. This Falcon engine is fitted with a dual length intake manifold that changes from long to short runners at a certain engine speed. For the final test it was permanently held in its short runner position, resulting in a slow 9.9 second 0-100.
Stopwatch testing will clearly show performance gains (or otherwise) from cold air intakes, extractors, exhausts, and chips.
Accelerometer Testing
Performance measuring accelerometers are available in two types - electronic and mechanical.
Electronic accelerometers are most often integrated into full digital performance computers, and AutoSpeed has already run a full article on one of these - the effective G-Tech device
["Hitting the G-Spot"]. But what if you don't have the dosh for an electronic accelerometer? A mechanical accelerometer is cheap and easy to obtain and use. Compared with an electronic accelerometer performance computer, it requires much more work from the user - but there are always going to be some trade-offs! A mechanical accelerometer measures acceleration only in 'g' units, not 0-100 km/h or quarter mile times.
A mechanical accelerometer can use either a vertical pendulum that is deflected across a scale by acceleration, or a tube shaped in a semi-circle in which a small ball bearing is moved. A US company called Analytical Performance some years ago produced one of the best of the latter type of accelerometers, which was called the G-Curve. Their accelerometer consisted of an engraved alloy plate into which was let a long curved glass tube. The tube was filled with a damping fluid and a small ball bearing was sealed inside. A very good handbook was also provided with the instrument.
Unfortunately it appears that Analytical Performance is no longer in business but a substitute accelerometer can easily be assembled. Boat and yatching supply companies sell clinometers that are designed to measure the angle of boat heel. One such clinometer is the 'Lev-O-Gage', which in construction is very similar to the G-Curve. However, because it is designed to measure heel angles, the scale is calibrated in degrees rather than g units. Like the G-Curve, the glass tube is filled with a damping fluid to prevent the ball overshooting.
Both accelerometers are attached to the car in the same way. To measure longitudinal acceleration, the instrument is mounted level and parallel with the direction the car is moving. This means that the accelerometer is often mounted on the passenger side window. The G-Curve came with suction caps to allow this to be easily done, but the Lev-O-Gage is designed to be fixed in place with double-sided tape and so a suitable bracket should be made and then equipped with suction caps (available from rubber supply shops).
So how does the accelerometer work? When the car accelerates, the ball climbs up one arm of the curved tube, showing how hard the car is accelerating. To convert the degrees reading of the clinometer to g readings, simply use a scientific calculator to find the tangent ("tan") of the number of degrees indicated. This means that if the car is accelerating hard enough to move the ball to the 20 degree marking, the acceleration is about 0.36g (tan 20 = 0.3639). However, note that converting the degree readings into g's isn't necessary for most testing, where you are only trying to see changes rather than measure absolute values.
Like the G-Tech Pro, the mechanical accelerometer must be accurately levelled before performance measurement can take place. The accelerometer will clearly show any road gradient and so levelling is best done on a flat road. If the road is not level (and many aren't), the instrument needs to be adjusted so that the error when the car is parked facing in opposite directions on the same spot is of the same amount but in different directions. For example, the clinometer might show +2°
with the car facing in one direction and -2°
with it facing in the other. If the road was level, the instrument would therefore show 0°
. Using a mechanical accelerometer requires a sharp-eyed assistant armed with a paper and pencil to record the data.
One example of the use that can be made of a mechanical accelerometer is the measurement of vehicle acceleration in a single gear. A gear is selected and the car driven at as low a speed as is possible in that gear. After warning your assistant that you are about to start the run, yell "now!" and quickly push the accelerator to the floor. Every 1000 rpm after the initial rpm figure yell "now!" Each time you yell, your assistant records the accelerometer reading. In many cars the acceleration will be too quick for the assistant to keep up, so on the first run do for example 2000, 4000 and 6000 rpm, and in the second run do 3000, 5000 and 7000 rpm.
The graph here shows an example of some 3rd gear acceleration runs done in this way on a turbo car. In the first run the car was tested with the standard exhaust and standard boost pressure. The vehicle was then modified with a large exhaust and increased boost. As can be seen, the improvement was spectacular! The low rpm performance remained the same in both configurations as the turbo was yet to begin boosting, but once the turbo started developing boost - wow! The peak acceleration in 3rd gear went from 0.21g to 0.34g, a 62 per cent increase! The acceleration also remained higher for much longer, with the car accelerating an amazing 67 per cent harder at the redline in modified form.
With this technique you can quickly see both how much any mods have changed acceleration and where in the rev range the power increases (or decreases!) have come. The plotted acceleration line will be exactly proportional to vehicle tractive effort - how hard the road is being pushed back by the tyres. Any improvement in acceleration means that more power is getting to the driving wheels. While the test mentioned above measured just changes in the boost and the exhaust, using an accelerometer to also test turbo swaps is a very valid way of assessing results - better than using a dyno, in fact. The change in low rpm response of one turbo swap can be seen here.
Another very effective use of the accelerometer is to plot the best gear change points. Instead of acceleration being plotted against engine speed as was described above, it is plotted against road speed. The acceleration in each gear is measured, with the numbers being jotted down each 5 or 10 km/h. Each gear is tested from as low as speed as possible to the maximum speed possible before the engine redline. Doing this results in the sort of diagram shown here. As can be seen, in this particular car the maximum possible acceleration occurs if all gears are held to the redline. This is because each time a gear change is made, acceleration drops markedly. The first gear change drops acceleration by a massive 47 per cent - it appears that this modified car could use either a taller first gear or a higher engine redline.
However, as you can see in this diagram, short-shifting will give quicker acceleration in some cars. Here a slightly early change from second to third gear (at about 77 km/h) will improve acceleration. This is because holding second gear to the red-line results in the car being slower than if the early change to third gear is made. Note that using an accelerometer in this way is far better than trying to calculate the best shift points using torque curves, gear ratios and so on.
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