Dynos

There are two tasks which a dyno can perform which are useful to tuning. All dynos can measure the engine output over a range of engine speeds (a ‘power run’), and some dynos can hold the engine speed and show output (same can do this in real time), possibly while the engine load is varied. Good dynos will have a high repeatability between runs and sessions on different days. As a guide the type of dyno which Hondata uses is accurate enough to show the drop in engine output when the headlights are switched on.

Types of dyno

There are two types of dyno – inertial and variable load. Inertial dynos accurately measure the speed of a fixed mass while it is accelerated by the engine. Variable load dynos adjust the load on the dyno until the engine output is balanced by the dyno resistance, and then measure the resulting torque. Some variable load dynos can be set up to simulate driving conditions. E.g. a drag run.  Inertial dynos can only be used to perform power runs.

Setting a rev range

Set the lower limit as low as you can, even if you’re not interested in this part of the engine output. The lower starting point for the run will mean that the engine temperature will equalized between runs at lower revs.
Make sure the higher limit is lower than the rev limiter and the point where the engine will self destruct. Normally the torque of an engine will reach a point where it starts to drop away quickly. Altering mixture, ignition settings and cam timing will not have much effect on the torque drop off, so the upper rev limit for the dyno run only really needs to be past the torque drop off point. For a standard VTEC engine this point is about 1000 rpm less than the stock redline, so 500 rpm less than the factory redline is usually suitable as the upper rpm range.

Performing a run

The main thing when performing a run is to minimize factors which reduce repeatability. Most dynos compensate for ambient temperature and pressure, but very few compensate for engine temperature. Engine temperate has a big effect on engine output, so it is a good idea to try and start each run with the same engine temperature. HondaLogger can be useful showing the engine temperature, but also trying to keep the same time interval between dyno runs helps. Ideally a cooling system cold water bleed would be used to keep the cooling system the same temperature.

Interpreting results

It is best to compare torque curves, because this will highlight differences better than comparing power curves will. When comparing runs it is important not to look at peak figures, rather it is necessary to look at the whole of the curve. The torque curve which will accelerate a vehicle the fastest is the one with the greatest area under the curve for the rev range of the engine, so this is a good method to compare torque curves. Some dynos will show the average power, but not many will for a certain rev range.

Often the first part of the dyno run will show a bigger variation than the middle and end of the run. This is due to the engine being a different temperature at the start of each run, but equalizing part way through the run. For this reason it is a good practice to start the dyno at lower revs than you are interested in.