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AC-22 Accelerometer The Race Technology AC-22 Performance Meter is an accelerometer, similar to the Vericom or the G-Tech Pro. Like these other 2 devices it is able to measure the performance of your vehicle without any type of contact or connection to the vehicle under test. It does this by measuring the G-forces that the vehicle is experiencing every 10ms and using basic laws of physics it calculates distance, speed and horsepower. The AC-22 can measure G-forces, vehicle performance, and vehicle horsepower. It is a small box 12.5cm by 4cm by 6.5cm. It is powered internally by 2xAAs and has a 2 line, 16 character LCD display on the front panel. 3 buttons on the front panel control all the functions. The AC-22 is mounted in the vehicle using supplied Velcro strips. It can be mounted on any surface that is reasonably level. It does not need to be mounted absolutely level as it corrects for any deviation in the mounting position before you start. Velcro may sound like a poor solution for mounting the device but it actually works better than the suction-cup attachments that are typically used by other accelerometers. From the main menu you can select 'G-Force Measurements', 'Acceleration Timing', 'Configure' or 'Switch-Off'. The Configuration menu allows you to configure many aspects of operation of the AC-22. You can configure the speed range or distance range over which you wish measurements to be taken. You can configure how much information is displayed after each run. You can alter vehicle data so that the results are as accurate as possible. You can change the units between metric and imperial, or a combination of both. The G-force measurements can be configured to show the acceleration/braking G-force, the cornering G-force, or a combination of the two (known as the 'circle of friction'). All of these modes display the current and peak G in numerical form and as a bar graph. The AC-22 corrects for any error in the longitudinal G readings caused by vehicle squat, but does not do so for the lateral G. Therefore body roll in cornering will lead to an exaggerated reading of cornering power. Changing the 'tilt factor' on the configuration menu does the correction for vehicle squat. A softly sprung vehicle will squat down while accelerating, or pitch under braking, much more than a stiffly sprung vehicle. Without any correction the softly sprung vehicle will get slightly flattering results. There are suggested values for different vehicle types in the instruction manual. However to ensure the AC-22 is spot-on it is best to calibrate it against something else - Race Technology suggest a visit to a proper drag strip where you can calibrate the unit to get agreement with the printed time slip. This is not absolutely necessary and the suggested values will give quite accurate results. The acceleration timing is what most people will use this device for. You have the option to time acceleration over a distance range, a speed range, or to simulate a proper 1/4 mile drag strip. Your defined start and end point control both distance and speed timing. Typically this will be 0-60mph, 0-100mph, 0-60ft or any other range you desire. The Quarter Mile Timing is particularly useful. Here the unit automatically gives you all the data you would normally get at a proper drag strip i.e. 60ft time, 330ft time, 1/8 mile time and speed, 1/4 mile time and speed. In addition to this you can set the unit to include a 1ft rollout distance. Neither the Vericom or the G-Tech account for rollout and this is a significant problem in my view (rollout is the distance a vehicle moves at the drag strip before it activates the start line timer - it typically takes 0.3-0.4 seconds for this to happen). If you configure the unit to display 'interval' results you will see data for all of your intermediate points, like this:
This means that a huge amount of data can be gathered from just a few runs. It also means that you do not have to keep launching the car really hard if you are actually interested in through-the-gears acceleration, thus sparing your clutch. At each data point you can also see the G-force at that point and the rear wheel horsepower. In order for horsepower to be accurate the AC-22 needs to know the vehicle weight, the CdA and the rolling resistance. This data is commonly available for production cars. The Race Technology web site also has some Javascript tools that can be used to calculate CdA and rolling resistance based on coast-down tests. Weight of non-standard cars can be obtained to an acceptably accurate level from your nearest weighbridge, or better still, corner weight scales. Of course this is only necessary if you are interested in absolute numbers. If you just want to compare your car before and after changes you can use any values for these parameters. If the start speed is higher than the end speed the AC-22 will time braking performance. You simply start from rest and once you have exceeded the 'start' speed the AC-22 tells you to start braking. When you slow down to the 'start' speed it begins timing. OK, that's the theory, as ever the $64,000 question is, Does it Work ? I used my Westfield to find out. I headed off to Santa Pod where the accuracy could be established beyond doubt. After calibrating the tilt factor by making a few passes and adjusting the tilt until the numbers from the AC-22 agreed with the timing ticket from the strip I did a further 3 passes and got the following results:
I'll let you decide if that is good enough - the average error at all of the 12 timing points is within 1% of the official track timer i.e. typically a tenth at the finish and a few hundredths at the 60ft point. There is a slight discrepancy on speed but this is explained in the manual. At the drag strip the speeds are computed from the time taken to cover the 66ft preceding the distance marker while the AC-22 gives you the speeds AT the distance marker. In my car this takes about 0.5 seconds at the 660ft marker and 0.4 seconds at the 1320ft marker, explaining why the speed reported by the AC-22 is slightly higher than that given by the track timer. A friend of mine then tried the device in his car and got the following results:
To check the HP measurement I just did some 0-160kph passes using 4th gear for as much of the speed range as possible. I calculated the CdA and rolling resistance for the Westfield using a series of coastdown tests. I used 4th gear partly to reduce interial losses (the amount of energy used to speed up the rotating components, principally the clutch/flywheel) and partly because I was getting wheelspin in 3rd. The HP readings are very repeatable, so much so that the device can be used for tuning. The following graph shows the AC-22 figures compared to a rolling road and an engine dyno:
The shape of the curve is what is important here rather than the absolute numbers. This is because the engine dyno measures flywheel horsepower at a static rpm (no inertial losses), the rolling road measures wheel horsepower net of rolling resistance and inertial losses, while the AC-22 measures wheel horsepower including rolling resistance (and aerodynamic losses) but net of inertial losses. Plotting the power at every 10kph increment and converting this to engine RPM produced the graph from the AC-22. The rolling road data is incomplete, as it was impossible to prevent wheelspin as the engine approached peak torque. Summary: I think this is a great piece of kit. Whether it's messing about trying to do 0-60s or for keeping tabs on engine/car performance while modifying the car. I also have a G-Tech. The AC-22 is far more accurate, has many more functions, is much better made and only costs a small amount more. The thing to remember is that all this testing MUST be done on a level road, and it is best to take 2-way averages. If you are trying to make a proper comparison then you should really use the same piece of road each time. The only problem is when you get stopped by a Policeman whilst testing: 'Sir, do you know how fast you were travelling ?' 'Yes Officer, 109.4mph…' © Ian Crocker |
