About Dyno Graphs

21 Oct 2018
by Damien Devillian

This article isn't about "How Dynos Work", or "Types of Dyno". This was written specifically for understanding aftermarket parts - how to read the graphs to decide which parts to choose. So it applies to only chassis dyno runs, with an emphasis on comparison.

All the details are dependant on one main factor - the manufacturers being honest. None of it will apply for made-up numbers and the later part of this article will address that.

Reading Dyno Graphs - The Key Points



As in most articles, from convenience of access, I will reference the aFe Power Dyno to explain how to read dyno graphs that showcase aftermarket parts.



Smoothing


Starting with the easiest, and possibly negligible variable - smoothing - merely varies the smoothness of the line to make it more readable and ranges from 0 - 5. A smoothness of 0, would show a jagged line of peaks and valleys, and a smoothness of 5 would show more rounded peaks and valleys.


It doesn't affect output values by any noticeable amount, but it's handy to ensure that both runs are carried out under the same setting.



Correction Factor (CF)


This small, unassuming detail of the dyno graph, could be the most important thing to note when you are comparing runs.


Correction Factors are meant to negate the need for taking environment variables as a consideration when reading dyno graphs. It takes the run-time environment values like temperature and air pressure, applies a formula then generates values that you would get under a predetermined condition.


What this means, is that taking a dyno reading on a hot day vs cold day, or sea level vs high altitude, should not make a difference. You should get the same readings no matter where you run the dyno. While this means that


The 3 common settings are - SAE*, STD and Uncorrected, with SAE being the more widely accepted correction factor when comparing aftermarket parts.


The SAE* correction factor takes your run-time data and calculates the values to display as your vehicle would perform under the following conditions:

  • Air Temperature: 25 degrees Celsius
  • Absolute Pressure: 1 Atmosphere (sea-level)
  • Relative Humidity: 0%

The STD correction factor also applies a calculation to keep the readings constant under different environment variables. These numbers are usually a small percentage points higher than the SAE correction factor.

An Uncorrected reading would not apply any calculations to you dyno values, and display them as-is. While this makes it more realistic, it also makes it difficult to accurately compare aftermarket parts.


In Singapore and other hot/tropical climates, using the correction factor could also make you think your vehicle is producing more power than it actually is in the real-world.

It is evident that with our higher than 25 degree Celsius temperatures and high humidity levels, our vehicles are going to have substantially lower real-world values.

To get real-world values for your vehicle, you would need to use the Uncorrected setting.


*While the SAE correction factor was determined by the Society of Automotive Engineers (SAE), it is very important to note that this is not to be confused with guidelines/publications set forward by SAE, including:

  • SAE J1349 - Engine Power Test Code.
  • SAE J1312 - Procedure for Mapping Engine Performance.

These publications set out very detailed testing procedures, with considerations of fuel flow rate, coolant temperature, oil type, and much more. Almost none of these are adhered to in aftermarket parts testing. Dyno Operators still have a lot of variables to play with to get different results.



Run Conditions


Now that I've explained why correction factors make mean Run Conditions are unnecessary, I'll go on to explain how they could still be helpful.


While correction factors create a neutral ground for comparing dyno-runs, the lack of real-world results makes it difficult for you to actually tell how your individual vehicle is performing. It is more important to know how your vehicle is actually performing in the weather conditions of your city.


By using an Uncorrected setting for your dyno runs, you can have a more realistic picture of your vehicle's performance, and you can use the Run Conditions to loosely gauge performance improvements in a real-world setting. If you are mathematically inclined, you can also use relatively simple equations to work out a more individual baseline for comparing runs.



Comparing Dyno Graphs


Now that we know what to look for when reading dyno graphs, let's go on to how to actually compare before/after dyno graphs to help decide what aftermarket parts to choose.



For the rest of this article, I will be using a simplified chart that represents a typical dyno graph. This would make it easier to compare various scenarios for a better explanation.



Product 1 vs Stock



Left - This is the most common type of dyno graphs used for aftermarket parts. Manufacturers use a snapshot of the best scenario, and that "+10hp" claim is used in all of the product descriptions.


Personally, I've always determined true performance improvements with an "Area Under The Graph" view.


By filling in the Area Under The Graph, it becomes more apparent that your aftermarket parts is only an improvement in the 5,000 - 7,000 RPM range. You can decide from your own driving style, how often you are in that range for that aftermarket part to actually make a difference.


This method of filling in the Area Under The Graph is still missing one key detail - the power loss in the lower RPM. For that, let's look at the next example.



Product 2 vs Product 1 vs Stock



Left - In this scenario, manufacturers of Product 1 would have in their description that their product has a higher power output to their competitors (Product 2).


Right - This time, instead of simply covering in the Area Under The Graph, the charts are stacked to prevent overlap, and then a computation was run to calculate the surface area percentage of each.


Here it becomes clear that, in fact, Product 2 has a larger Area Under The Graph - which equates to it having more of a performance improvement across the RPM range. So unless you manage to keep driving at the 5,000 - 7,000 RPM, the vehicle with Product 2 will outperform yours.



Limitations


Though it's obvious that Dyno Graphs can't evaluate all aftermarket parts, there are still limitations to those you think can be determined by dynos. With aftermarket parts that provide multi-faceted improvements, the reliance on Dyno Graphs alone can lead to making the incorrect choice. I'll talk about some of the more common ones here.



No Sense of Time/Duration


The lack of having any timing-related data means you can't evaluate aftermarket parts that have an added benefit of "better response" - including parts relating to Air Induction and Exhaust.


E.G. If an aftermarket part has the same power output as stock from 1,000 - 3,000RPM, but it gets from 1,000 - 3,000RPM quicker, should it not still be a performance part? If you only used the Dyno Graph to evaluate, this part would be deemed pointless. Yet on the road, the vehicle with this part would be faster than the stock.



No Forward Momentum


Being stationery, dynos do not replicate real-world conditions, and may not be correct for aftermarket parts that are affected by forward momentum.


One aspect that dynos can't factor is lateral G forces. These forces can have an effect on parts like fuel pumps, oil filters, etc - which have to work against the force pushing fluids (incl. air) backwards as the car moves forward.


Another aspect of forward momentum that is missed by a stationery dyno, is the airflow. This factor is incorrectly dismissed when a cooling fan is used. However, the primary purpose of a cooling fan is to prevent engine overheating by flowing over the radiators.


Having a cooling fan blowing at a constant 50km/h and directed towards the radiator, is not the same as driving at 50km/h, and this is because of pressure waves created when you are driving through air. Not to be confused with the aerodynamics that affect vehicle handling, pressure waves also affect the airflow around and through the vehicle.



No Long-Term Measurement


With dynos being run in bursts over a short period of time, you have no way of evaluating an aftermarket part's performance over time. Most notable of these would be any parts that would be affected by sustained heat.



Summary


Hopefully this article has helped you in understanding how to read Dyno Graphs, when evaluating aftermarket parts. More importantly, I hope it raises awareness of why not to depend solely on Dyno Graphs when making a decision.


I'll also leave you with this to think about. In the motorsports world, chassis dynos are almost never used. Engines are tuned and improved with the use of an engine dyno, with almost all other testing being done by driving around on a track.



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