BLiNK Motorsport have carried out research into the different types of Dynamometers available and a brief description of these is given below:
Inertia Type Dynamometer
Inertia dynamometers are essentially large free-spinning barrels, that work in conjunction with a computer to precisely measure horsepower and create horsepower to rpm charts. The barrels are spun as the vehicle accelerates, and the time that it takes to go from a set range of speeds determines the horsepower of the vehicle.
They are extremely simple but also very accurate as they rely on mathematical calculation only. The biggest disadvantage is they are not very good for ‘engine mapping’ as they can not hold a specific load. Further to this, because the ‘rollers’ have inertia they tend to smooth out any problems during the acceleration run. They can only be used for full throttle (WOT) acceleration pulls.
Similar to the above but the drums or drum are braked. This brake can be either hydraulic or electrical. Attached to the brake is a load cell that measures the force being applied to the brake and the vehicle torque can be calculated. From this the vehicle horsepower is derived. The braked dynamometer has several advantages over the inertia – it can hold a set speed or a set load and is ideal for mapping a vehicles engine management.
The biggest disadvantage with this type of dynamometer is the tyre to roller interface – it introduces an unknown variable that causes a rolling resistance and a tyre slippage point.
These dynamometers are used to calculate a flywheel horsepower by carrying out a ‘coast down’ test which measures the rolling resistance and transmission losses to give a flywheel torque/power figure.
Hub Mounted Dynamometer
These are mounted directly to the vehicles hubs and therefore have no tyre to roller interface. They do not have any inertia but are fully controlled from a control box to very accurately control their speed or load. They are excellent for vehicle mapping and for seeing minute changes in power output at the hubs but, because they have no inertia they can not accurately determine torque/power output at the engine flywheel. These are the type of dynamometers used by the major motoring manufacturers.
In view of the above we had to make a decision as to which type to go for!
The Inertia Type dyno was ruled out very quickly – it is good for measuring power gains at full throttle but would not allow us to do any useful engine mapping/diagnostics work – we felt it really didn’t fit in with our ethos!
The Braked Dynamometer seemed perfect in everyway, however when we came to examine the systems we found shortfalls for our use. The biggest downside for our use was the tyre to roller interface – this introduced too many variables for our liking – the tyre pressures, tyre compounds, the way the vehicle is strapped onto the dyno all make a difference to the power readings. Racing Mazda MX5’s, where a 3 hp difference is seen as significant we didn’t feel this type of dynamometer offered the repeatability we were after. In the UK we have a fixation with flywheel Horsepower figures yet we nearly always measure these indirectly on a chassis dynamometer and rely on a ‘fudge factor’ the operator chooses to use. Users of the braked dynamometers will argue that a ‘coast down test’ will eliminate the tyre and transmission variables but we are not convinced. A vehicles transmission (helical gears) is designed to transmit engine torque from the engine to the driven wheels, when a coast down test is carried out it transmits inertia through the wheels back towards the engine – this does not give accurate transmission losses; comparable yes but not accurate!
How is a Dynapack different?
The first and most obvious difference is the elimination of the tyre to roller interface on a conventional roller dyno. The Dynapack eliminates this variable by using a hub adapter that provides a direct coupling to our Power Absorption Units. There can be no tyre slip, no rolling resistance, and no chance of the vehicle coming off of the dyno at high speeds. Notice that we call this a variable. Sometimes it may be a problem area, other times it may not. tyre temperature, pressure, traction, etc, are all variables that can change – not only from run to run, but during the run as well. Throw an unknown variable like this into the equation and your data has now become subject to a potentially high margin of error. It is obviously better if these variables could be eliminated – which is exactly what we have done. There are other associated problems with the roller method as well. Take tie-down straps for example. Most roller dynos use ratcheting tie-down straps to attempt to hold the vehicle in position while being tested. If the straps are cinched down tightly, the tyre has become loaded even further, in an unpredictable manner. While this may be good for enhancing traction, it changes the rolling resistance of the tyre – skewing the data further. Since these tie-down straps aren’t perfect, the vehicle squirms around on the rollers – dramatically changing the tyre drag during the run. If the vehicle is tested in two different sessions, the straps can’t be set exactly the same way twice in a row. Again, the data will be inconsistent. We have heard of cases where the ratcheting tie-down straps were loosened by two clicks and the measured power increased by ten horsepower. What if the straps stretch – either from run to run, or during the run itself? Wouldn’t it be great if all of these problems could disappear? With a Dynapack, they were never there in the first place.
Another major difference is the effect of inertia. Street wheels and tyres spinning at high RPM have a large amount of inertia. A large steel drum spinning at the same ground speed has much more inertia. What you end up with is a giant, heavy flywheel attached to your engine. The inertia is such, that just trying to accelerate the mass of the roller is a substantial load for the engine. That is the principle that some roller dynos (or “inertia dynos” as they are also called) operate on. Accelerate a known mass to a measured speed over a given time and it can be calculated to equal a certain amount of power. There is nothing wrong with this theory, but like many theories, its application in the real world can be troublesome. How do you think your measurements will be effected by being subjected to this large heavy flywheel phenomenon? Will small fluctuations be noticeable? In a word, no. The flywheel effect tends to take small rapid variations and smooth them right out – as energy that should be going into the dyno is being wasted trying to accelerate a large lump of steel. This is great if you want your power curve to look like a smooth pretty line, but it doesn’t give you much insight into what is really occurring. What if you eliminated the flywheel effect? While nothing that has a spinning mass has “no” inertia, when compared the total mass of the wheels, tyres, rollers, and other associated hardware of a roller dyno, the inertia of a Dynapack is practically zero. This allows us to precisely measure and display tiny rapid pulses and oddities that you may not have seen before. Now you have a window into areas that no roller dyno will allow you to see. Another benefit of having virtually zero inertia is the ability to change the rate of acceleration at will. In many situations, you may want to accelerate the vehicle at a different rate to simulate a specific condition. With a few simple keystrokes, we allow you to make the vehicle accelerate very quickly, slowly, or anywhere in between. Because of our lack of inertia and total control of the engine speed, we give you choices that none of our competitors can even dream of – and as you know, choices are good!
How does the Dynapack work?
The theory of operation and the implementation of that theory is actually fairly simple. It took several years and a lot of hard work however to make our dyno as simple as it is today. The hubs of the vehicle are directly attached to hydraulic pumps. We can apply a variable but precise load with all of the potential holding power that hydraulics possess. Simultaneously, we are monitoring pressures and measuring hub RPM, so we can determine the amount of work being performed. It sounds easy until you realize that all of these calculations are very complex and are happening very quickly. Add to this, all of the data logging functions and real-time full-color graphics that are also being calculated and you begin to realize that what appears to be simple is actually very complex. Being the best is never easy. Traditionally, most serious engine builders have thought that chassis dynamometers were inferior to the results you could obtain from a quality engine dyno. We have effectively attached engine dyno style load cells to the axles, so we now have the type of precision and repeatability normally associated with an engine dyno, but with the convenience and benefit of having the engine operate in its natural environment – which has enabled many of our customers to see better results than they were getting on their engine dyno.
Because we need a precise and powerful loading device, we use hydraulics. We do not use inertia (more on that later), we do not use eddy currents, air, or friction. Because of the incredible holding power hydraulics offer, we have TOTAL control of the axle speed. Literally. Want to hold a steady RPM under high power? We can hold an exact axle RPM (+/- one RPM) at any power level – all the way up to the full maximum rated torque capacity of the dyno, CONTINUOUSLY – for as long as you like. If the software allowed it, we could stop the engine within one revolution of the axle – even if the engine is at full throttle at its maximum torque level. Obviously you would not want to do this, and our software prevents it, but it does give you an idea of just how much power we have over the axle speed. Our dyno controls the car – not the other way around. We control the axle speed and rate of acceleration at all times. Because we aren’t limited by the capabilities of eddy current brakes and similar devices, we open up a whole new world of tuning possibilities. Times change and technology evolves. What was once “industry standard” is now yesterday’s technology. The Dynapack is truly the most technologically advanced chassis dynamometer in the world – at any price.
Our Dyno runs are repeatable to better than 0.3%. Other dyno manufacturers claim to be repeatable, but no other chassis manufacturer is even close to the level of repeatability we achieve. One large reason for this because we have eliminated the largest variable of all – the tyre to roller interface. Rubber tyres don’t hold traction against a steel roller very well. Add a year or so of use, and the rollers become polished by the tyres and traction decreases further. Some companies charge extra for special coatings on the rollers – which quickly wear off. When you have this variable link in your data chain, you cannot have guaranteed repeatability – PERIOD. Sure a roller dyno itself may be repeatable, but as soon as you put a car on it, all bets are off. Many people think that this slippage only occurs in high power situations, but we’ve seen it happen with 250HP Hondas – ask some of the import tuners who have had guys sitting on the hood and fenders trying to get the tyres to hook up. With the Dynapack, we use a direct mechanical coupling to make absolutely sure that there is no loss, no slippage, and no inconsistencies in this area. We have virtually no inertia to mask small details and we use hydraulics for the ultimate in sensitivity and precision. The Dynapack is absolutely the most consistent and repeatable chassis dyno in the world.
We can reliably measure minute differences not seen on other machines.
Some examples include:
.010″ change in spark plug gap
Differences between various lubricants
The alternator load when the headlights are turned on (in real time as well)
A single step fuel jet change
Different spark plugs