I have seen a lot of discussion, debate and difference of opinions on dyno's and figures lately so I thought I would post up what I would post up what I have found to be the best explaination of what it all means.
Written by Todd of Mainline Dynolog under the permission from Jason at LS1.com
I thought we might start a thread that outlines what are Dyno Correction Factors, how are they calculated, how they are displayed, and what effect they have on the overall Power reading that is displayed on a dyno.
Dyno Correction, or more accurately Atmospheric Correction, is used as means to ensure comparable results when vehicles are tested under different Atmospheric Conditions. This could mean differences from early morning to late afternoon, or from January to July (ie Summer to Winter). In a nutshell, more correction is applied in Hotter, more Humid conditions, and less is needed in Colder, Dryer conditions. Also, more correction is needed in conditions of Lower Barometric Pressure, and less in High Barometric conditions.
The Atmospheric Correction is calculated (in most cases) from Air Temp, Relative Humidity and Barometric Pressure. Unfortunately, there is not just one formula that is used world wide for Atmospheric Correction, and the differences between the varying formulas can vary the result by as much as 4%. This is why whenever a Power or Torque figure is quoted, it should be made clear what Correction Standard is used, and the amount of correction that has been applied is shown, in either a percentage or numerical method.
Some of the different Correction Standards that exist are:
SAE J607
SAE J1349
DIN 70020
ISO 1585
ECE
EEC
The SAE J607 standard is what we set our dyno’s to here in Australia, if we sell one in the US, it is set to SAE J1349. The SAE J1349 Standard is a newer standard than the SAE J607 Standard, and the J607 standard is one of the more generous standards, while the J1349 standard is one of the less generous standards (close to 4% difference between the 2). The main reason we use the J607 standard stems from the fact that in the early 90’s when DynoLog first started, the J607 standard had been in use for quite some time, and Engine Builders were comfortable with this standard, so their Engine or Chassis Dyno’s were setup with J607. Users of our Dyno’s can, if they so wish change the Correction Standard that is used on their Dyno’s, (there is 6 to choose from) and this would be then shown whenever a Report is Printed, so nothing is hidden from the Customer. This stops a person from using one standard on a before modification Test, and then using a more generous standard on the after modification Test, the same standard will be applied to both runs.
The way we display our Atmospheric Correction is shown on the following Report:
Notice the Correction Standard is CLEARLY displayed, and the AMOUNT of correction that has been applied is shown in plain English. A value of 2.38% means that 2.38% has been added to Measured Power, so if we measured 100RWKW, we would be adding 2.38RWKW, totalling 102.38RWKW. Atmos Correction can be displayed differently on other brands, you may come across a number such as 1.024, this would mean a correction factor of 2.4% has been applied, so whenever viewing a correction factor, be aware of the 2 differring methods to display the same function. Also be aware that negative Correction can, and will happen.
To show the varying levels of correction that can be applied due to differing Formulas, I entered the following 3 conditions into 5 different Standards Formulas:
25 Deg. C
1020 Mbar
50% Relative Humidity
SAE J607: 2.58% or 1.029
SAE J1349: – 1.70% or 0.983
DIN: 1.70% or 1.017
ECE: -1.73% or 0.983
EEC: -1.61% or 0.984
Some important facts about our Dyno’s, are that we have a Software Cap on the amount of correction that can be applied, and this is set by us the manufacturer at 10%, and cannot be changed by the user. From all of the installations we have performed, the highest correction I have seen applied was just over 9%, and this was in a concrete dyno cell in Bangkok, where the Humidity was in the high 90% range and the Ambient Temp was around 37Degrees Celcius. Also, on all but 2 of our systems, they all have Automatic Atmospheric Correction, this means the Dyno has an Electronic high accuracy Weather Station that is connected to the Dyno (the 2 that don’t have Automatic Correction just have no correction applied, full stop), and the Correction is continuously calculated and updated. The advantage of Automatic Atmospheric Correction is it takes the dyno operator out of the equation of being responsible for entering these variables, the risk with manually entering the Weather Conditions is the chance of accidentally entering a wrong value, or worst still, purposely entering values that could either make a vehicle look bad or good, depending on what they wanted to achieve.
The fact that over 9% can be calculated on our systems, does not mean anything is wrong, if a correction that high is shown, it simply means that the vehicle is being tested in conditions where the engine is far from being able to produce the numbers it would achieve in colder less humid conditions. The objective here is to be able compare the vehicle to itself at any time of the year, if ongoing modifications are being added, or in the case of a controlled racing formula, the vehicle is making the power it always should.
Another big difference with our system compared to some others, is while we do have an Inlet Air Temp Probe that the dyno operator can use to log Inlet Air Temp, IT IS NOT used in any way to calculate any form of correction. Once again, we shy away from allowing the operator to be able to influence the Power readings which can happen, either accidentally or on purpose, when an Inlet Air Temp probe is used in some way for correction. An example of the shortfalls of this style of correction is where recently a discrepancy of over 500RWHP was induced (possibly due to a faulty sensor, at a much publicised Dyno contest), and an Inlet Air Temp of over 200Deg C was recorded, resulting in a Correction Factor of over 40%.This means 1000RWHP suddenly becomes 1400RWHP!
You can see why on our system, such a discrepancy could not happen, due to the safeguards we have such as a Correction Limit of 10%, and also not using the Inlet Air Temp probe for correction.
If a user of one of our dyno’s was to have a failure of the Ambient Air Temp Sensor in their Automatic Weather Station that meant the Correction was calculated at over 10%, an Error Message is displayed warning the operator that the Correction exceeds 10%, and at this point, no Ramp Testing can be performed until the problem is repaired, or the user turns off the Correction altogether, thus allowing them to continue using the Dyno, albeit without any correction being applied. We believe that at all times the Dyno determines the Power output, not the operator, and you safegaurd the system so erroneous readings cannot happen.
If ever you are present where a Mainline DynoLog dyno is being used, the Correction Standard is ALWAYS displayed in both Testing Screens, ie our Speed Hold Screen, and our Ramp Testing Screen, we simply have nothing to hide. Those members attending the Dyno Day at Sams’s Performance this weekend I will point out where the Correction is Displayed in our Test Screens, it will also always be shown on the Printout.
The Correction is actually applied to the Torque, and Torque only, not AFR etc etc.
By applying the correction to the Torque, you are then applying the Correction to the Power as well, remember:
Power kW = (Torque (Nm) x RPM)/9549.3
or for the older folk:
Power HP = (Torque (ft/lbs) x RPM)/5252.1
or for the techno geeks:
Power (Watts) = Torque (NM) x Rotational Velocity (Radians per second)
Without Atmospheric Correction, you could see a variance of around 7% between winter and summer on your power figures. This is why is it important to correctly control it for comparative purposes.
07-10-2007, 02:17 AM
Now about exhaust pulse predictions...
Linear Mode
