Dyno-testing is an important process for all racing engines. It allows proper engine break-in, the ability to identify and correct potential problems, and certify their work. The top engine builders insist on dyno testing as a final step and even independent builders without their own dyno facilities take their engines to a reputable shop for validation.
If the dyno is used correctly, it can become a very valuable tool. However, it’s important to remember that even though the engine dyno is a sophisticated data logger, it’s work is only as accurate as the data used to program it.
For efficiency, it requires accurate engine spec and atmospheric data input, and operation must be consistent. Because there are too many variables, you can’t compare power numbers from one dyno to another. Choose one dyno source and ensure it is competently operated, well maintained, and regularly calibrated. Make sure your instructions are followed by the operator and the data you need is recorded. Most dynos – when equipped with the proper sensors – can record every possible piece of data about your engine.
Accurate calibration and consistent testing procedures are vital. Because dynos often “seem” to arrive at consistent results, a complacency develops among operators, and they fail to verify proper calibration.
It’s easy for day-to-day dyno operators to rely on their “gut feeling” that they will know if something isn’t right. This often gives them a feeling of superiority because they are experts and you are not, so you won’t know the difference if the calibration is out.
While good dyno operators will give you help and insight into how your engine is performing, expert operators will ask you exactly what you want at every point. A dyno operator who isn’t willing to listen isn’t qualified to test your racing engine.
Never let the dyno operator pressure you. If his emphasis is on going home on time, chances are he won’t be too thorough about evaluating your engine. Developing a personal relationship with your dyno operator is a good idea so he understands exactly what you are looking for.
There is only a certain amount of data channels that can be displayed in the printouts or on the screen. If you want to specifically track something, find out if the operator has a properly calibrated sensor. Without mentioning dyno calibration itself, airflow turbines are normally never re-calibrated or checked so you should ask when the turbine was last calibrated and inspect the turbine itself.
If, for example, it’s had a few nitrous backfires or it’s covered with dirty duct tape, chances are it won’t provide accurate airflow measurements and what’s more, it’s probably been carelessly handled.
If the honeycomb air straightening elements inside the turbine are bent or even partially smashed, ask the operator about it. If the answer is a flippant “Don’t worry, it still flows ok”, you may want to reconsider. Inguqu the dyno operator if he “floats” his or your fuel before testing so the fuel specific gravity is input based on the ambient temperature.
It’s important that you don’t wear the engine out on the dyno. Base most of your initial testing at or below the torque peak, establishing both optimum spark and basic jetting. Advise the operator that you need a full printout, including all the raw uncorrected numbers. The essence is in the collected data channels, with the least important being the correction factor.
Correction factors provide only an estimate which means very little to most engine builders. The bigger your dyno correction factor, the more suspect the power numbers are. Uncorrected numbers are what the engine delivers for any prevailing conditions.
What About Correction Factors?
There are two basic dyno correction factors:
- The SAE (Society of Automotive Engineers) and
- STP (Standard Temperature and Pressure).
SAE numbers are considered the standard for most chassis dynos and OEMs use these for all there testing too. However most “magazine” testers and dyno shops sell STP numbers because these are roughly 4% higher than SAE numbers, but all dynos only collect raw uncorrected numbers.
No matter which correction factor is applied, it is only a percentage calculation that is based on contributing factors recorded by the sensors, or input by the operator. The dyno printout can be set to list either raw, STP or SAE numbers, plus the actual numerical multiplier. This means that you can see all the data and determine the “bogosity” of the correction factor for yourself. Experienced tuners use basic recorded data and uncorrected numbers.
It has been stated a time or two, “If all of your tuning decisions are factored on corrected numbers, then you should use the side of a barn.”
Corrected numbers establish the benchmark and permit an accurate analysis of BSFC numbers and fuel usage. If you’re selling or seeking numbers, the correction factor is your co-conspirator, however if you’re an engine builder wanting to validate your combination, you’ll find all you need to know from uncorrected numbers.
The chances are slim that your engine will ever run with the same conditions as the correction factor, so what’s the point? Magazine testers use corrected numbers because they are impressive editorially. Expert engine builders obtain all the information they need to know from the raw data, particularly about fuel flow, air fuel ratios, airflow, BMEP, BSFC, VE and the shape of the power and torque curve.
One of the critical factors is the consistency of how the raw numbers are obtained. If consistent environmental conditions are maintained by the operator, well maintained dynos are very repeatable tools. These conditions include running all the tests at the same oil and engine coolant temperature and with repeatable atmospheric conditions. Many operators will run tests at coolant temperatures below actual operating conditions because it’s easier to make big power, and they don’t have to be as careful. Experienced dyno operators will be able to pinpoint inconsistencies and whether they are due to an engine problem or the dyno. Thoughtful evaluation will ascertain whether a tuning or mechanical issue is the cause rather than the dyno.
In fact, repeatability is more important than accuracy. For example, if the dyno reports that you made 825 horsepower you have no way of knowing if that is an accurate number. However, if it repeats within one horsepower repeatedly, then you can accurately judge the effect of any changes you make. Even if it only makes 795 horsepower, if it repeats and shows you gains or losses from your original benchmark, the actual number is less critical than the degree of gain or loss. Also, what the instrumentation tells you regarding fuel flow, EGTs, air fuel ratios and other relevant factors can be considered with the changes made and the subsequent results.
The data filtering method that is used to smooth the curves on test graphs is another important factor. While from a distance, those smooth curves can look impressive, a closer look at the data would reveal several peaks and valleys in the overall curve. These are based on inconsistent fuel delivery, mixture quality, ignition scatter and other intermittent interference which all affect the quality of the measured power curve.
Dyno Testing: In a Nutshell
There should never be a goal in dyno testing to pursue a magical maximum power number. Instead, the primary goal is to identify the power band that the engine delivers and establish the overall health of the engine. Identifying the optimum fuel and spark curves that produce the power band is a key task of dyno testing.
The fattest possible power curve will occur naturally if the engine flow paths are dimensionally optimized according to recognized mathematical principles. It’s important to bear in mind that this is not a race between you and the dyno. You are using the dyno as a tool to verify your efforts to position and shape the power band to suit your racing requirements.
Full appraisal of the full range of data is imperative without just accepting dyno test results at face value. Many engine builders would call a few timing and jetting changes that raise the power a good thing, however they should be evaluating how well the engine matched the predetermined performance blueprint or if they are missing something.
If you have planned your engine successfully, the dyno will be your best friend in helping you establish the optimum spark curve. Generally, ignition timing will translate well from the dyno to the track, however the fuel curve will almost always differ due to different atmospheric influences and how the engine is loaded in actual competition. Use the dyno to establish your engine’s power band and map its characteristics. Then, you can track tune the engine to ensure maximum performance.