I snipped this link from a link provided from Summitracing
"A Dyno Named Eddy: Understanding Eddy Current Chassis Dynos
It was in our backyard and we didn’t even know it.
Up the road from Summit Racing Equipment is Mustang Dynamometer. The company makes, well, dynos—chassis dynos, engine dynos, transmission dynos, emissions dynos. With over 100,000 square feet of manufacturing space, Mustang Dynamometer is the largest company of its type in the country.
One of the company’s coolest machines is the MD-1750 chassis dyno, which we used to test the induction and exhaust work we did on our Impala SS. The MD-1750 is capable of handling vehicles making up to 1,750 horsepower. It’s an eddy current, or steady state dyno, and consists of a pair of air cooled, electro-magnetic brakes (Power Absorption Units in Mustang-speak), two 50 inch diameter rollers, and a computer-controlled load cell
The MD-1750 works pretty slick. During a dyno test, the brakes act as power absorbers against the rollers. The load cell measures the power absorbed by the rollers and calculates it as a torque value. The dyno’s software takes that torque value and combines it with measured roller speed (measured at the roller shaft via a Hall effect pickup) to calculate the force being applied to the vehicle’s drive wheels at any point in the test. This method allows the MD-1750 to hold a vehicle at a constant speed at any throttle position.
The dyno operator can also control the rate of vehicle acceleration by activating a portion of the brakes’ power absorption capacity on the rollers. This allows the vehicle’s air/fuel ratio, engine temperature, supercharger boost level, and other parameters to reach normal operating levels during a power sweep test. That provides cleaner, more useful data.
All that techno-babble means the MD-1750 can be used to test a vehicle under a steady load or controlled acceleration condition. You can measure drive wheel output at any speed or rpm, not just peak power at wideopen throttle. You can make tuning changes at a given speed or throttle point and see the results at the exact same point all day long.
Real World Results
As cool as the MD-1750 dyno is, Mustang Dynamometer’s MD-7000 Virtual Inertia software may be even more so. Want to measure power output from zero to 60 miles per hour? How about passing power at highway speeds or quarter-mile ET and top speed? Virtual Inertia can handle it.
The secret is what Mustang Dynamometer calls Vehicle Simulation. This software factors in a vehicle’s weight and aerodynamic characteristics, pulled from a preset table, to simulate "real world" driving conditions. The software tells the dyno how much of a load to put on the vehicle’s drive wheels, then collects data on horsepower, torque, speed, and shift points. The results you see on the printout are probably as close to real-world as is technologically possible to measure.
Eddy current/steady state dynos like the MD-1750 are becoming more common in the performance industry. As a result, comparisons to inertia-type dynos like the Dynojet units are inevitable. Inertia dynos don’t measure torque directly like an eddy current unit. Instead, they calculate torque based on the rate a vehicle accelerates a known mass—in this case, a large drum. Because it can’t hold a vehicle at a steady speed or rpm, an inertia dyno can only calculate power under acceleration.
That doesn’t mean an inertia dyno can’t provide valuable information. It’s ideal for getting results quickly when swapping intakes, headers, or other parts. In fact, all Mustang Dynamometer eddy current dynos have an inertia mode for just that purpose.
The problem comes when comparing results between the two types of dynos. Because they measure output so differently, it’s really comparing apples to oranges. To get accurate, repeatable results from dyno session to dyno session, you have to use the same style of machine. We’d even go so far to say that you should stick with the same dyno facility and operator—results can vary between dynos of the same type and even between the people pulling the lever.
Do ya dig all the cool stuff the Mustang Dynamometer MD-1750 can do? You can get more in-depth tech info on it and other Mustang units at www.mustangdyne.com
. If you ask nicely, Mustang Dynamometer will even provide a list of shops that utilize its dynos. Then you can enjoy the benefits of eddy current dyno tuning too."