03-05-2011, 09:49 PM
Everybody knows that a dynamometer (more commonly referred to as a "DYNO") is the tool that mechanics, engine tuners, and aftermarket parts developers use to show off their stuff and to try to validate claims. Dynos are great tools, but can easily be misleading. What most people don't know is that all dynos are not created equal, nor are results necessarily anywhere near accurate. There are a lot of factors involved, and it is extremely easy to get things wrong, or even in some cases to manipulate the data.
Dynamometers are excellent tools for tuning, and are the easiest way to determine what something is doing and whether you gain or lose power. They do require an operator that understands how they work, and what things can affect them. Otherwise, it's "garbage in - garbage out".
There is a ton of information out there, and I highly encourage you to spend time reading up, but for the purposes of general education, I will merely summarize things in a more simple form.
Dynos and more dynos
There are a number of manufacturers out there, and I won't attempt to list them all. There are basically 2 types of dynos commonly used. The first is an Inertial unit, and the other is an Eddy Current. Each has its own advantages and disadvantages, and it really depends on what you want from the unit as to whether or not it is the best for you.
Inertia Dyno
Inertia Dynos are essentially a large free spinning drum that works with a computer to precisely measure horsepower. They are very good for back to back tests, because they do not require any calibration. They just measure what is there. This is why they are most commonly used, and are very effective at the track for comparing cars. There is very little variance due to mechanical change.
However, these work under load only. This means that you must be at full throttle all the time. They cannot measure power at a steady state. This means it is very difficult to tune a car. You also cannot tune anything but wide open throttle.
The DynoJet 248 is this type.
Eddy Current
An Eddy Current Dyno applies a load to the car via a DC motor, and then measures what it takes to resist that load. This type of dyno has distinct advantage over the Inertia dyno in that it can measure steady state power, allowing tuning to be done at part throttle loads as well as full throttle. This type of dyne requires regular calibration though, which can lead to error if not done properly or frequently enough.
The SuperFlow / WynDyn is this type of dyno.
Atmospheric Conditions
Atmospheric conditions can dramatically affect the readings at the dyno. This is why any good dyno also has the ability to apply "Corrections Factors". This means that the readings are as a result of some math that corrects for the existing conditions. This presumes that the conditions are being measured. This is where manipulation can occur. There are a few ways to do that, and unscrupulous people or even poorly trained operators can really mess things up here. The measurements need to be very accurate. The better dynamometers do this for you. Some of the less expensive ones just have you to enter your own data.
Temperature
Ambient temperature can easily change things as much as 5% or more.
Locating the intake temperature sensor is critical. It is very important that the sensor be located where the intake gets it air, not just hanging out in the open. Failure to do that makes the temperature correction feature of the dyno completely useless.
This is an area where less than honest people can manipulate the readings by telling the dyne to crunch some numbers based on unrealistic factors. By locating the sensor in a hot spot, but allowing the intake to get its air from a cold one, will raise the numbers, because the dyno thinks it's hotter than it actually is, and adjust the numbers accordingly.
Barometric Pressure
Barometric pressure will also affect things, but can have an even larger effect on boosted engines where air density is everything. A low barometric pressure reading will lower the output reading. Basically there is not as much air to begin with, so the engine has less to work with. As an example, in the 300hp range, on a boosted engine, this can be a difference of 20hp just with a pressure drop from 29.9 to 29.2.
Humidity
Humidity will also change the readings, though has the least impact of the other factors, and is only about 1% loss going from 40% to 90% humidity.
Here is a link to a tool that will let you play around with these factors and see for yourself the changes:
http://www.csgnetwor...lhumhpcalc.html
Drivetrain Loss
This is an area that has the single biggest effect on the readings. Dynamometers typically used measure power at the wheels. There are engine dynos, but you have to remove the engine to use them. We won't concern ourselves with those here.
The dyno takes a reading of how much power it takes to spin it. Then you can either just use the Wheel Numbers, or you can calculate to the flywheel. Many dynos just use a plug in number for loss. This is NOT accurate. There is no fixed percentage number for any car that will accurately tell you what the loss is. There are a number of reasons for this. A few of them are:
What gear you are in
Tire pressure
Strap tension (as it adds friction)
Gearbox temperature
Wheel and tire mass
Further, a percentage is not going be accurate anyway. It is not a percentage. You will hear numbers like 15% or 17% loss. It is a fixed ratio like that. The drivetrain will have a set amount of loss, regardless of whether it is bolted to a 100hp engine or a 500hp engine. It can be measured, and is constant, as long as the conditions are constant. Typically cars with driveshafts (as opposed to those that have the engine bolted directly to the transmission) have about 30 to 40hp of drivetrain loss.
Parts developers and engine tuners love to use a fixed percentage though, because it can really inflate their numbers. It's not a horrible thing to use as a rough estimator within a close range to stock power, because it gives you a rough range that allows for condition variance, but it is nowhere near accurate when you start bumping things up, and should never be used to advertise engine power numbers.
The only way to accurately attempt to calculate engine power is to measure the drivetrain loss. Most dynos cannot do this. Some can.
Here is a link to a thread that has the content of an article about drivetrain loss:
http://www.968forums...loss-explained/
What does all of this mean?
To get any real data, you must test the same car, on the same dyno, and you must have controlled conditions of weather. The car must be warmed up to the same operating temperature at the engine and the drivetrain. It is very easy for the accumulation of factors to result in as much as a 20% difference. This makes it incredibly difficult to compare dyno runs from multiple sessions. It also means that you can never compare one car to another on a different day with any sense of true accuracy, even on an Inertia dyno. It doesn't matter how many times you run cars and get similar readings. All it means is that you got lucky with things in a similar range. That would be expected with most things on the same car. Modifications only make so much change and so readings will be reasonably similar, given reasonably constant conditions. Many parts manufacturers know this, and take advantage of it, but it doesn't make the data valid.
Another thing to consider is engine condition. A tired engine will not see the same output as a fresh one, and modifications will have the same proportionate results. There are some manufacturers that have been known to measure one tired car as their "baseline" and then measure a tight car with the new stuff. This inflates the numbers often by as much as the item added.
This is exactly why so many people get such varying readings, and why it is up to the consumer to do their due diligence in their research. For any advertising of numbers to be valid, they must have a measured and controlled baseline on the same car as the product being measured is tested on. The atmospheric conditions must be recorded, and the application of sensors must be consistent. The car must be warmed up to the same temperatures and all factors equal. It isn't cheap, and it is very time consuming, which is why most people don't do it. But you get what you pay for here. Garbage in - garbage out.
Dynamometers are excellent tools for tuning, and are the easiest way to determine what something is doing and whether you gain or lose power. They do require an operator that understands how they work, and what things can affect them. Otherwise, it's "garbage in - garbage out".
There is a ton of information out there, and I highly encourage you to spend time reading up, but for the purposes of general education, I will merely summarize things in a more simple form.
Dynos and more dynos
There are a number of manufacturers out there, and I won't attempt to list them all. There are basically 2 types of dynos commonly used. The first is an Inertial unit, and the other is an Eddy Current. Each has its own advantages and disadvantages, and it really depends on what you want from the unit as to whether or not it is the best for you.
Inertia Dyno
Inertia Dynos are essentially a large free spinning drum that works with a computer to precisely measure horsepower. They are very good for back to back tests, because they do not require any calibration. They just measure what is there. This is why they are most commonly used, and are very effective at the track for comparing cars. There is very little variance due to mechanical change.
However, these work under load only. This means that you must be at full throttle all the time. They cannot measure power at a steady state. This means it is very difficult to tune a car. You also cannot tune anything but wide open throttle.
The DynoJet 248 is this type.
Eddy Current
An Eddy Current Dyno applies a load to the car via a DC motor, and then measures what it takes to resist that load. This type of dyno has distinct advantage over the Inertia dyno in that it can measure steady state power, allowing tuning to be done at part throttle loads as well as full throttle. This type of dyne requires regular calibration though, which can lead to error if not done properly or frequently enough.
The SuperFlow / WynDyn is this type of dyno.
Atmospheric Conditions
Atmospheric conditions can dramatically affect the readings at the dyno. This is why any good dyno also has the ability to apply "Corrections Factors". This means that the readings are as a result of some math that corrects for the existing conditions. This presumes that the conditions are being measured. This is where manipulation can occur. There are a few ways to do that, and unscrupulous people or even poorly trained operators can really mess things up here. The measurements need to be very accurate. The better dynamometers do this for you. Some of the less expensive ones just have you to enter your own data.
Temperature
Ambient temperature can easily change things as much as 5% or more.
Locating the intake temperature sensor is critical. It is very important that the sensor be located where the intake gets it air, not just hanging out in the open. Failure to do that makes the temperature correction feature of the dyno completely useless.
This is an area where less than honest people can manipulate the readings by telling the dyne to crunch some numbers based on unrealistic factors. By locating the sensor in a hot spot, but allowing the intake to get its air from a cold one, will raise the numbers, because the dyno thinks it's hotter than it actually is, and adjust the numbers accordingly.
Barometric Pressure
Barometric pressure will also affect things, but can have an even larger effect on boosted engines where air density is everything. A low barometric pressure reading will lower the output reading. Basically there is not as much air to begin with, so the engine has less to work with. As an example, in the 300hp range, on a boosted engine, this can be a difference of 20hp just with a pressure drop from 29.9 to 29.2.
Humidity
Humidity will also change the readings, though has the least impact of the other factors, and is only about 1% loss going from 40% to 90% humidity.
Here is a link to a tool that will let you play around with these factors and see for yourself the changes:
http://www.csgnetwor...lhumhpcalc.html
Drivetrain Loss
This is an area that has the single biggest effect on the readings. Dynamometers typically used measure power at the wheels. There are engine dynos, but you have to remove the engine to use them. We won't concern ourselves with those here.
The dyno takes a reading of how much power it takes to spin it. Then you can either just use the Wheel Numbers, or you can calculate to the flywheel. Many dynos just use a plug in number for loss. This is NOT accurate. There is no fixed percentage number for any car that will accurately tell you what the loss is. There are a number of reasons for this. A few of them are:
What gear you are in
Tire pressure
Strap tension (as it adds friction)
Gearbox temperature
Wheel and tire mass
Further, a percentage is not going be accurate anyway. It is not a percentage. You will hear numbers like 15% or 17% loss. It is a fixed ratio like that. The drivetrain will have a set amount of loss, regardless of whether it is bolted to a 100hp engine or a 500hp engine. It can be measured, and is constant, as long as the conditions are constant. Typically cars with driveshafts (as opposed to those that have the engine bolted directly to the transmission) have about 30 to 40hp of drivetrain loss.
Parts developers and engine tuners love to use a fixed percentage though, because it can really inflate their numbers. It's not a horrible thing to use as a rough estimator within a close range to stock power, because it gives you a rough range that allows for condition variance, but it is nowhere near accurate when you start bumping things up, and should never be used to advertise engine power numbers.
The only way to accurately attempt to calculate engine power is to measure the drivetrain loss. Most dynos cannot do this. Some can.
Here is a link to a thread that has the content of an article about drivetrain loss:
http://www.968forums...loss-explained/
What does all of this mean?
To get any real data, you must test the same car, on the same dyno, and you must have controlled conditions of weather. The car must be warmed up to the same operating temperature at the engine and the drivetrain. It is very easy for the accumulation of factors to result in as much as a 20% difference. This makes it incredibly difficult to compare dyno runs from multiple sessions. It also means that you can never compare one car to another on a different day with any sense of true accuracy, even on an Inertia dyno. It doesn't matter how many times you run cars and get similar readings. All it means is that you got lucky with things in a similar range. That would be expected with most things on the same car. Modifications only make so much change and so readings will be reasonably similar, given reasonably constant conditions. Many parts manufacturers know this, and take advantage of it, but it doesn't make the data valid.
Another thing to consider is engine condition. A tired engine will not see the same output as a fresh one, and modifications will have the same proportionate results. There are some manufacturers that have been known to measure one tired car as their "baseline" and then measure a tight car with the new stuff. This inflates the numbers often by as much as the item added.
This is exactly why so many people get such varying readings, and why it is up to the consumer to do their due diligence in their research. For any advertising of numbers to be valid, they must have a measured and controlled baseline on the same car as the product being measured is tested on. The atmospheric conditions must be recorded, and the application of sensors must be consistent. The car must be warmed up to the same temperatures and all factors equal. It isn't cheap, and it is very time consuming, which is why most people don't do it. But you get what you pay for here. Garbage in - garbage out.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
(This post was last modified: 03-05-2011, 10:08 PM by flash.)
![[Image: biggrin.gif]](http://www.968forums.com/public/style_emoticons/default/biggrin.gif)
But then again, I use gig blasses.
![[Image: blink.gif]](http://www.968forums.com/public/style_emoticons/default/blink.gif)