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Reading Your Car's Brain - Part 3

Using the Auterra OBD scan tool as an on-road dyno.

by Julian Edgar

Click on pics to view larger images


In the second part of this series ["Reading Your Car's Brain - Part 2"] we reviewed the data reading and logging abilities of the Auterra Dyno-Scan tool. It's an OBDII reader that works in conjunction with any Palm operating system handheld. However, in addition to being able to extract engine operating parameters and fault codes, the system can also be used as an on-road dyno. In this final part in the series we take a look at how effective this function is.

How It Works

Live information on engine revs and vehicle speed is constantly available through the OBDII port. When working in its dyno mode, this data is used by the Auterra software to assess how quickly the car is accelerating at different rpm. Together with information manually provided re the car's weight, gearing, and aerodynamic drag, the calculation of power and torque curves is possible. Additionally, acceleration curves, 0-60 mph and quarter mile data are also available.

This screen shows the parameters that have to be manually entered. These include:

  • Vehicle weight (complete with driver, and only able to be entered in pounds)
  • The total gear ratio in the gear in which the test is being run (eg in second gear, that gear's ratio multiplied by the final drive ratio)
  • Tyre diameter (only able to be entered in inches)
  • Weather parameters (temperature, humidity, pressure)
  • Altitude
  • Aero Drag (drag coefficient and frontal area in square feet)

The software is capable of helping in the calculation of some of these parameters - there are built-in tyre diameter and gear ratio calculators. The tyre diameter calculator requires the entry of the tyre size (eg 205/65 15) while the gear ratio calculator works it out as you drive along, based on road speed and engine rpm inputs. The aero drag data can be estimated, with help provided by an extensive listing of vehicle data downloadable from Auterra.

Note that all data within the dyno function of the Dyno-Scan is in imperial units.

On the Road

Performing a power/torque measurement run is straightforward. With the data entered and the files configured correctly, a screen appears that gives the driver a countdown to start. During this time the vehicle can be prepared for the run. For example, second gear can be used and the revs let drop as low as possible in that gear. At the end of the countdown the Palm beeps, showing that data is being recorded. The driver immediately uses full throttle and keeps his/her foot buried until the redline. Lifting off the throttle and slowing down automatically terminates the run.

Following the run, the power/torque graphs are immediately displayed. A marker can be moved around the graph to highlight the actual power/torque figures at various points.

The peak power and torque figures, and the rpm at which they were achieved, is also available on a separate screen.

We tested the system using a standard auto-trans Honda Accord Euro. This vehicle has a claimed power output of 140kW (188hp) at 6800 rpm and a peak torque of 223Nm (164ft-lb) at 4500 rpm. However, those figures are at the flywheel, so driveline losses need to be taken into account. And how much they are depends on a host of variables...

Click for larger image
The Dyno-Scan showed 163hp at 6900 rpm... and 429 ft-lbs of torque! However, the torque figure is the result of the torque converter doing its work - it multiplies torque and at 1700 rpm it's doing just that. Peak torque away from the torque converter multiplication spike was 148 ft-lbs at 5300 rpm.

So, a peak recorded power at the wheels of 163hp versus a claimed 188 at the flywheel - how does that stack up? Pretty well, actually. Despite many in Australia believing that a 25-30 per cent driveline loss occurs in all cars, that's largely the result of testing on just one type of dyno, one that has fairly small rollers and so plenty of tyre distortion. The 13 per cent driveline loss recorded here is likely to be much closer to the on-road truth. Note also that the measured peak power rpm is very close to the factory figure (6900 versus 6800 rpm). On the Dyno-Scan graph the jump in power that occurs as the VTEC changes over can be clearly seen - it's at 6000 rpm.

The torque reading is also pretty close - the curve is flat enough that there's more than (a measured) 130 ft-lbs from 4100 to 5700 rpm.

Click for larger image

Also available is an acceleration graph. On the Accord we ran 0-100 mph (0-160 km/h) and got this result. In many ways we think that this graph is probably the most useful of all the data. If you make a modification and the car accelerates faster, well and good! Note the little bit of wheelspin and action of the traction control near the beginning of the run.

Click for larger image
The acceleration figures broke down to 0-60 mph in 9.3 seconds and a ¼ mile time of 17.0 seconds (at 85.9 mph). Given that the air-conditioning was on and it was fairly warm, again these figures look fairly close to the money for this auto trans car.

Conclusion

We don't think that the primary purpose of the dyno function of the Auterra Dyno-Scan tool is to generate absolute numbers. Much more important is the usefulness of the system in making comparisons - in seeing which way modifications are going. Compared with using a stopwatch, the Dyno-Scan allows you to clearly see where power improvements (or losses) are being made within the rev range. For example, graphing the acceleration in second gear will quickly show you whether you're losing bottom-end torque - or if a new cold-air intake is effective in real on-road conditions.

Overall, the tool is an impressive bit of gear. Whether you simply want to see what the standard engine management is doing, read and cancel fault codes, or assess the results of modification (both with the dyno and scan functions), the Auterra Dyno-Scan is immensely useful.

www.auterraweb.com

The Auterra DynoScan tool was purchased for this review

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