Taken from an M3 forum. Some of you might find this useful and may explain some of the strange results from the various rolling road days that we've been to!
The guide was written by a BMW tuner here in the UK who use a Dyno Dynamics Rolling Road. So its interesting that they put up this information about a DD road when they operate one themselves! Could be a nice accurate Dyno for us all to go to one day!
Basic Guide about Dyno Dynamics and it's Cheat MethodsThis is a basic guide, correction factors have been explained in a very basic way so everyone understands it. This basic guide has also been kept as short as possible so people don't lose interest in reading it.
PART I - Cheating by Correction Factors - Ambient Air Temperature, Barometric Pressure, Relative HumidityBefore we talk about this we need to understand what correction factors are and why they are present in dyno software.
It is surprising how many people think they are there just as a built in tool for operators to make the figures up. Far from it.
Correction factors are all about standardisation.
Dyno Dynamics Correction Factors:
Barometric Pressure = BP
Relative Humidity = RH
Ambient Air Temperature = AT
Inlet Air Temperature = IT
Some general facts:
- An engine will make less power at high altitudes than at low atitudes.
- An engine will make less power with higher air temperatures
- At higher altitudes the BP is of a lower value
- The reference point for these correction factors is set - please see SAE.
- Static correction factors are as good as not having them at all. There are many companies which input 1013mB all year round. This is pointless.
An engine will not make the same power from one day to another as weather conditions are changing. Without correction factors dyno readings from day to day are not comparible. Therefore the difference in an ECU tune tested on a day which is different to the day the base run was taken should be ignored if correction factors are not present.
A very well explained internet page about correction factors can be see on the below link
http://wahiduddin.net/calc/cf.htmCorrection factors allow a car to make the same power on a dyno no matter what the atmospheric conditions are. This can only work if the ACTUAL conditions are recorded at the time of the dyno test. When I mean time I literally mean on a minute to minute basis as room temps where dyno's are located can vastly change within a short space of time.
There are many Dyno Dynamics owners, like us, who test the same cars all year round keeping constant the fuel type used and keeping the cars in good health. We allow the Dyno Dynamics weather station to take the AT, BP and IT. We get almost exactly the same corrected WHP every time no matter if it's mid summer or winter.
So if we upgrade a part of the engine/ecu anytime during the year we will know it's effect.
So the correction factors are there to give us consistency and allow us to compare the results no matter when they were done. It works and works well.
The problem - dyno operators know how these correction factors work and they change them where they can to show larger increases on power output.
To illustrate this we will use some example:
Car: BMW E92 M3
Actual Conditions: BP=1013, RH=50% and AT=25 Degrees C
The Dyno Dynamics weather station reads exactly this and the car makes 340WHP.
If we change the BP to 980mB the software will add power because at a higher altitudes the engine will make less power. But because this is a false reading the car is now going to make more power.......more false power because the mechanical efficiency is being calculated wrong.
So by changing these readings the correction factor also changes and therefore the output changes. By changing the readings from actual or whatever they were on the previous run you can manufacture what ever output you want! It's that easy!
How do you make sure this isn't happening to you?Very simple.
1) On the later Dyno Dynamics machines ask operator to press key 'A' and make sure the weather station is set to the 'ON' position. As long as this stays on then you have nothing to worry about (other than inlet air temp which is discussed in the next section).
2) On the earlier machines the weather station is remote and the operators have to input the values themselves. Ask to see the 'BAR SCREEN' and also what the weather station is 'ACTUALLY' reading. Do this before every run. As long as what the weather station is recording = to what's entered, you have nothing to worry about.
3) If the operator chooses to keep his values static then just ask to see the BAR SCREEN before every run. As long as the values are the same your ok. This method of keeping the values static is a total waste of time as it's the equivalent to having no correction factors.
In case 2 please ask to see the weather station and make sure it's not got anything put ontop of it between runs.
Part II - Cheating/Accidental Over Correction by Inlet Air Temperature Probe (IT)The Dyno Dynamics software is as far as I know the only one which has a probe which is to be placed in the inlet tract to measure actual air intake temperature.
This is one correction factor which is open to huge abuse and also causes many innocent mistakes to be made from operators.
This probe reading gives us an additional correction factor - the way it works is simple, it takes the ambient temperature and adds back power based on the difference in inlet temp and ambient temps.
If the probe is placed inside the airbox and is measuring the temperature of the air (and not the sidewalls of the airbox!) then it's all good.
If the probe is placed elsewhere (radiator, exhaust manifold or anywhere hotter than the airbox) then your being played - SIMPLE.
How do you make sure this isn't happening to you?Just ask to see the position of the inlet probe and make sure no one changes it's position in between runs!
If your being handed a graph then ask to see the difference in AT and IT. If it's the same your ok. If there is a vast difference between AT and IT between the two runs then you should have doubt.
The reason I say you should have doubt and not actually make an accusation is because it's an area which can be overlooked so easily. The probe can move from measuring the air to the temperature of the sidewall of an airbox. The dyno operator may not notice this (they normally have lots of things to look at!) and overlook this over correction.
IT can be seen on the BAR SCREEN at all time. So can AT. You will see IT dropping as a cooling fan is powered on and as the engine ingests ambient air.
Part III - The Climbing Effect and StrappingHow hard a car is strapped down will have an effect on all dyno's including the Dyno Dynamics.
If a car is strapped down hard on a base run and then 'looser' on the one after a small-medium difference will be shown in the output.
The problem here is that a car may be totally non-moving on a dyno and this can be done without you ever knowing the strap tension. How are you going to know how hard a car is strapped if it's not moving around?? No way of telling!
The Solution is simple and we use it.Allow the car to mount the front roller completely leaving the rear one. Once the car has mounted the front roller, it doesn't matter where it sits on it because the reading will be the same.
So not scare the hell out of customers we just allow them to leave the rear roller by about an inch.
The overall benefit is that you achieve maximum grip and strap tension doesn't come into it. All the dyno operator has to do is hold the car back once it's moved forward a very small amount.
You totally cancel out the effects of strap tension!
With all of the above you should have no issues. There are no other hidden secrets other than the obvious ones:
- not pressing the throttle all the way down on the base line run (difficult to deal with!)
- Applying the brakes lightly on the base line run (Stand at the rear of the car if it's safe)
- Changing the ramp rate between runs - will be displayed on your print outs.
If anyone has any questions please do not hesitate to ask.
Just to add to this, being in Shootout mode doesn't mean anything. Some operators will tell you that everything is locked down in shootout mode. It's not.
The Dyno Dynamics machines are very easy to get caught on with the very basic guide above.
Other makes of dyno (Bosch, Dastek, Maha etc) can be cheated with even more ease and it's alot harder to actually catch operators out. We'll be aiming to give a guide on the other makes in the near future.
