We often get inquiries asking about the difference between dyno charts and why dyno’s themselves vary between shops, manufacturers, & tuning centers. The questions are many times based on charts & data that is shared across social media sites & various web forums. This, in itself opens the data to interpretation as many times we are not only comparing non-identical combinations, we are comparing different test standards & procedures without even taking some of the most important parts of the equation into consideration, which I’ll touch base on. With that being said, the following information should be helpful when comparing various dyno charts.
An inertia dyno itself such as a Dynojet chassis dyno actually has very little test variance dyno to dyno, however nearly every other variable or factor in is different, or as better defined as relative: considered in comparison or relation to something else.
The conditions, the subject being tested, the hardware, the dyno cell design, the specific test procedure, etc.. all of these difference are relative. An inertia dyno works on the laws of physics; those laws do not change, however all of the other factors involved are relative, and we haven’t even taken the actual tune into consideration yet, so let’s take a further look
- Conditions: the specific air conditions the machine was tested in: temperature, humidity, barometric pressure. This will change greatly and affects power output to a high degree. Conditions will change hour to hour, day by day. Also be sure to compare charts using the same correction factor as this can change the overall numbers by as much as 5%. It’s also extremely important to understand a correction factor corrects the data set to a specific standard, not the engine output.
- Tuning: Don’t just look at the peak numbers, look at the entire curve. Is the curve smooth or abrupt, are there dips or anomalies? Are they performing a quick test or are they actually tuning? An accurate, comprehensive tune is essential. Is the individual performing the tuning highly skilled with the tuning product and engine configuration he is working with?
- Gear: The gear the test is performed in will affect overall numbers. You want to be as close to 1:1 as possible, on a Harley Davidson this is 5th or 6th gear respectively. Runs performed in lower gears will have lower numbers due to loss from gear reduction.
- Air exchange: the motorcycle needs to have proper air exchange in the dyno cell. If there is not sufficient make up air & exhaust extraction it will affect engine output. An engine cannot breathe its own exhaust. An airflow & temperature controlled dyno cell will be much more consistent than an open air dyno and the room should be continually monitored for CO (carbon monoxide)
- Engine Temperature: the motorcycle needs to be tested at the optimum engine temperature, this will greatly affect output as both commanded Air/Fuel mixture and spark temp correction vary based by engine temp, both which affect power output. Dyno cooling fan placement is also critical as it affects intake air temp under static conditions, if the intake does not get proper air IAT can skyrocket under stationary conditions on the dyno. Engines like cool fresh air.
- Fuel, oil, & lubricants: The specific grade of gasoline, the specific oil, as well as lubricants in the transmission and primary. The temperature & viscosity of each of these fluids is a factor, and if fluid volume is overfilled it is a problem as well.
- Tire, Wheel & Driveline: The specific tire & wheel size and weight will affect numbers, heavier components will reduce power output. A good example is the OEM wheels on 2018-2019 Road King, Road Glide, Street Glide “S” model bikes, these wheels are much heavier than others and these bikes consistently have dyno numbers 2-3% lower than normal, that’s not much on a 80Hp stock bike, but 3% is almost 5Hp on a 150Hp build. Aftermarket transmissions with heavier gears, shafts, & larger bearings, a chain drive conversion (as opposed to belt), and soft sticky tires will also affect overall numbers.
- Serviceability: The overall condition of the components; condition of the air filter, injector flow, correct fuel pressure, the tension on the drive belt & primary chain, clutch slip, & tire pressure. Each of these factors into power output.
- last but not least, Bias: is the individual doing the testing setting you up for success, or are they setting you up for failure? Do they want to prove something right, or something wrong? The person doing the testing and tuning has to want to make the best of your combination. If they don’t want your combination to work they’re not going to make it work, it’s as simple as that. If they make excuses, claims, or conclusions before they even get started tuning it’s not a good sign. Sometimes it’s about hard feelings in business or about competition, sometimes it’s as simple as your numbers are better (or worse) than some guy on an internet forum. Yes, this industry is weird that way.
These are just primary examples, there are plenty others. Consider for a moment if any of these factors varies slightly for each test (which they will) variances in output come into play, now stack all of these variances together and the reasons for variances between dyno’s becomes more clear. That is why it is so important to keep anything you have control over as consistent as possible. In the end the most important thing to remember is that exceptional components & exceptional tuning will always yield the best results. You’ll find the top tuning & engine shops follow this same program.