this is from church's web site, and the dyno used, i'll let you decide for yourselves,
Church Automotive Testing
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How the Dynapack Works
A Dynapack is a chassis dyno that uses hydraulic loading to test and measure the output of an engine. A chassis dyno is a tool that measures engine output while the engine is still in the car (vs. an engine dyno, which measures output on a test stand without transmission, etc.).
Chassis dynos use one of two basic means to test engines. One is inertial loading, where a large mass of known inertia is accelerated by the test vehicle. This is a simple, but somewhat limited method for testing. The other method, used by Dynapack, involves a load control mechanism that places an operator controlled load on the engine using electrical (eddy current) or hydraulic (fluid pressure) systems. The advantage of a load control type dyno is the ability to simulate a wider variety of real world situations on the dyno. Additionally, because operating conditions can be fixed, hp changes are far easier to measure.
The final difference between a Dynapack and virtually all other chassis dynos on the market today is that the Dynapack eliminates the tire to "road" interface. By using a special hub adaptor that replaces the wheel and tire, the Dynapack eliminates wheel slip, alignment losses, tire inflation/wear issues and more. However, by eliminating the large mass (and attendant inertia) of the wheel and tire combination, the Dynapack does tend to read higher than comparable "roller" dynos.
How much higher a Dynapack reads is often the source of consternation and debate among enthusiasts wishing to compare numbers across different dyno types. Our first response is, "don't bother". We feel that the primary purpose of a chassis dyno is to measure differences from parts changes, tuning and the like. However, we also understand that bench racing is very common, and trying to ascertain where one stands versus a competitor is a valid pursuit. In light of this, we've undertaken a brief (and simplified) physics calculation to give people some ideas of how a Dynapack measurement will vary vs. the most common inertial dyno, the Dynojet 248C (let's give credit where credit is due, the Dynojet revolutionized the chassis dyno market and brought dyno availability to the masses).
There are two primary differences between the Dynojet and the Dynapack. The first is very clear when you see them. The Dynapack requires removing the drive wheels and tires from the test vehicle. The second is that the load time (the time it takes to accelerate the test vehicle over a specific rpm range) is operator controlled (and fixed if so desired) on the Dynapack. On the Dynojet, load time is controlled by the amount of hp produced, and by the gear ratio chosen by the operator. We will address both of these in our calculations. (all calcs will be done in metric terms and we will convert to more commonly used hp and lbs-ft at the end)
The first concept we need to understand is that of inertia. In particular, the inertia of a rotating mass. For this we will need to know something called "Moment of Inertia" or MoI as we shall call it. MoI uses the term kg-m^2. MoI is basically dependent upon the mass of the object, and how far that mass is distributed from the center of rotation. The higher the mass, or the further it is from the center of rotation, the higher the MoI.
In order to calculate the MoI of a wheel tire combo, we really need to measure the particular wheel and tire. However, we can easily make some reasonable estimates based upon what we do know of the wheel and tire.
Let's start with a couple of typical FWD wheel and tire sizes (since we had a couple lying around the shop to measure). First, a 17"x7" wheel with a 215/45/17 tire. The tire has a mass of 10.86 kg. The wheel has a mass of 8.5 kg.
We will approximate the MoI of the wheel by using a point mass model where the MoI (or I) = mr^2. To approximate our 17" wheel with spokes, we will use a 15" effective diameter as a rough estimate.
Thus, the MoI of the wheel is (8.5kg*(.1905m)^2)) = 0.3085 kg-m^2
We shall use the same equation to calculate the MoI of the tire, using the outside radius (24") less a small correction (1") since most of the mass of the tire is concentrated in the belt and tread surfaces.
Thus, the MoI of the tire is (10.86kg*(.2921m)^2)) = 0.9266 kg-m^2
Making our total MoI for a single wheel/tire = 1.2351 kg-m^2
Now that we know the MoI, we must determine the angular acceleration of the wheel/tire to calculate total torque required to accelerate the mass. On our Dynapack, for the typical street car, we use an acceleration rate of 500 rpm/sec (engine rpm) during ramp runs. With a 4:1 total gearing reduction (not unusual for a 4th gear run), our acceleration rate at the hubs is 125 rpm/sec. This equates to 2.0833 rev/sec^2, or 13.09 rad/sec^2.
When we multiply MoI by the acceleration rate in radians, we end up with a torque in newton-meters (kg-m^2/sec^2) = 1.2351 kg-m^2 * 13.09 rad/sec^2 = 16.168 N-m or 11.93 lbs-ft of torque. This is the torque required to accelerate a wheel with our calculated Moment of Inertia at the rate described. However, since most dynos return a torque calculated by measuring wheel torque and then dividing by gear ratio, the difference in dyno torque as printed on your dyno sheet would be 11.93 lbs-ft/4 = 2.98 lbs-ft. Finally, since this is per wheel, on a two wheel drive car, the total torque loss would be 5.96 lbs-ft.
Thus, the minimum expected difference between a Dynapack and a Dynojet (for this particular wheel/tire combo) would be 5.96 lbs-ft of torque (the Dynapack would read higher). If we were making peak power at 8000 rpm, we would expect the power difference to be approximately 5.96(8000/5252) = 9.08 hp. This is the difference solely attributable to inertia, and assumes that the acceleration rates on both dynos are the same.
These losses do not take into account rolling drag, tire scrub or tire flex (related to inflation pressure, vehicle weight and strap down tension). In testing on Dynojets, we have found that a change in camber of about 2 deg can result in a 5-6 hp change in measured power. However, these are extremely difficult to quantify - but be advised, they do exist on roller dynos, but not on the Dynapack (thus another source of differences).
As power increases for a given combo, the difference will grow due to the change in acceleration rate on the Dynojet (assuming the same gear is used for all tests). As the acceleration rate goes up, the total torque required to accelerate the wheels and tires will go up (remember, the torque calculation is MoI * acceleration - twice the acceleration rate requires twice the torque). But on the Dynapack the acceleration rate is kept the same (or at least should be), so losses remain largely the same. Furthermore, both dynos are subject to inertial losses accelerating the flywheel, transmission, etc. and the faster acceleration rate as hp climbs will show increasing losses on the fixed load Dynojet.
This is why we recommend people use a rough percentage adjustment to estimate flywheel hp on the Dynojet versus a rough fixed adjustment on the Dynapack. In our experience, a manual transmission FWD car will lose 20-25 hp to the hubs on the Dynapack. A RWD car will lose 25-30 hp and an AWD car about 35-40 hp (the FWD case has been verified on an engine dyno). In contrast, losses on the Dynojet will be in the 12-14% range for FWD and 14-16% for RWD (opinions vary).
Let's work through a comparative example for the same car on a Dynapack and Dynojet.
Car A produces 205 hp to the hubs on the Dynapack. This would equate to between 225-230 hp at the flywheel. The same car produces 195 hp to the rollers on a Dynojet. This would equate to 222-226 hp at the flywheel. Furthermore, the difference in power matches up very well with our calculated difference due to inertial losses. And interestingly enough, we have tested just such a car on our Dynapack and a Dynojet back to back with the same results (actually 196 vs. 206 hp).
Let us then assume that we modify Car A to produce 25 hp more at the flywheel (maybe a good head port and set of camshafts). The car would now produce almost 230 hub hp on the Dynapack, but on the Dynojet it would produce 217.5 hp. Same car, but the difference between the two dynos has grown from 10 to 12.5 hp, largely due to inertial effects.
Please keep in mind that these are rough approximations based upon some loose physics and real world experiences. But they should provide a better insight into the difference between loaded and inertial dynos, and hub vs. roller dynos, along with assisting the user in trying to compare results across the different systems.
Church Automotive Testing - 300 Quay Ave. Wilmington, CA 90744 310-518-4966
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by jetbtkng » Tue Jun 16, 2009 5:51 pm 
