The Dyno Correction Factors!

All our dyno runs come with the graphs you would expect to see, but unlike some companies who give you a bare minimum we actually offer full graphs of all the really important data like the dreaded correction factors.

You will get these graphs from us as standard:

• Power and torque at the flywheel.
• Power and torque at the wheels.
• Power and Air fuel ratio. (As AFR or Lambda - whatever you prefer)
• Power and tractive effort at the wheels on request.
• Anything you would like from OBD on request. (Inlet air temp after intercooler for example)

But you can also get these:

• Power and inlet air temp
• Power and ambient cell temps
• Power and barometric pressure
• Power and humidity
• Power and road wheel speed
• Power and run time

Why Bother? All I want is power isn’t it?

The last 6, whilst boring sounding to most people I realise, are actually the important sets of data that give you all you need to know to ensure that the figures are actually accurate and not just made up by spurious correction factors. With our graphs you can see not just a figure to cover the whole run, but the data as a graph, the same as power so you can see the inlet air temp right through the run at every plot point. To illustrate this, see our power and inlet air temp / Power and ambient air temp graphs below.

As you can see above - the engine is warming up the inlet air a little bit as it climbs the revs as you would expect from a 280whp 4 cylinder engine generating a lot of under bonnet heat, so you can see that the data we give you is real, live data. And as a point of interest the inlet air temps should never really be too far away from the ambients... say 10deg max difference. (Unless the air inlet really is in a terrible place)

Why does this data matter?

Well, as an example - a dyno operator can have an assistant move the air temp prove during a run to simulate more midrange torque, or higher peak power for example and some dyno graphs just give you a set figure from either data entered at the start of the day, or at the start of the run. If we moved the air temp probe near the exhaust manifold on the above run and saw 40 deg C, I would expect to be seeing the power at the wheels on that graph to increase to well over 300bhp! Our automatic weather station updates the main factors live at all times so that the data is constantly accurate and we ensure the inlet air probe is actually near the engines inlet, recording the real temperature f the air entering the engine. But without the graphs, how would you ever know we hadn't moved it?

What are these correction factors for?

The very term correction factor instantly makes people think that this is a complicated subject whereas in fact it is actually quite simple. If you look at the dyno printout you will find the number that relates to a correction factor that has been used. The reason we have a correction factor is because whenever we use a dyno we wish it to output the same result on the same engine every time we test it otherwise our figures from one day to the next will be completely incompatible meaning the figures are rather useless.

The problem... Variables.

The problem we have is this, let's say that we run the car up on a cool Friday evening with 10 deg C ambient temperatures and achieve 100 hp... we then lock the unit up and go home for our tea and come back in the morning to find it's a nice sunny Saturday and the ambient temperature is now 30 deg C. Fabulous… we make ourselves a nice cup of tea and proceed to run the car up once again and get it warm. On our power run we only make 94 bhp… what has happened? We didn't change anything on the engine!

Well the 6 hp loss is very simple to explain...

The 20 degree difference in air temperature has resulted in an air density change at the vehicles inlet that has cost us horsepower. Because of this simple fact dynos need an inlet air temperature correction factor that will correct all readings back to a known standard inlet air temperature. Using this correction factor, no matter what the inlet air temperature is, we should always receive 100 hp from this engine on this dyno.

But there is another small problem that needs correction too!

Most days there is also a difference in atmospheric pressure, so when the pressure is high your engine will make more power and conversely, when its low your engine will make less... So we use another formula that corrects the measured power for atmospheric pressure variations too! We now have correction for both inlet air temperature and air pressure in the dyno cell but then of course we have humidity to contend with, and guess what? Yes - that affects power too!

All this data has to be "accurately" measured and entered into the system before each run is made, if this is not done, the run cannot be accurate. Now how many times have you seen an operator do that? Its an absolute pain, so most never bother and just do it once a day (Or week/month/decade) but the reality is, these conditions change by the HOUR. This is why we invested in an automated system to ensure ours is updated EVERY RUN.

Just to add a curve ball, there is more than one correction standard, and they all give different results yet technically all are correct. The 3 most common are ATMC2, SAE-J1349 and DIN 70020. So as usual, nothing is simple. Our dyno can use any of them, but we opt for Dyno dynamics own ATMC2 unless the customer asks us to use a different one for some reason.

If you haven't fallen asleep already, you may be interested to read a little more about dynos in an article I wrote.
Available in my magazine published technical articles section. Here.