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About The Vette Doctors Chassis Dyno (copied from The Vette Doctors Site):

Our Chassis Dyno
Our in-house Chassis Dyno is a Dynojet�  Research Model 248. This state-of-the-art dyno provides advanced tuning capabilities to measure, record, and diagnose performance problems quickly. It is used to perform before and after dynamometer testing on your vehicle to ensure complete customer satisfaction. The dyno provides the precise horsepower measurements a technician needs to make quick and accurate evaluations of engine and drive-train performance.

The Vette Doctors test all types of race cars, street rods, street machines and trucks on our computerized chassis dyno. Our stationary in-ground dyno allows quick vehicle mounting of rear and front wheel drive vehicles for testing.

About Dyno Testing
A dynamometer tests the power of an automotive engine or power train. It measures the wheel speed and torque. By applying the laws of physics to that data, power output can be calculated. A computer is attached to the dyno to perform the calculation and provide the results on the computer monitor and can be printed and saved to a data file.

The chassis dynamometer accommodates an entire vehicle and allows operation of its power train while the vehicle remains stationary. The car is securely attached to the dyno with tie-down devices the drive wheels sit on metal inertia drums.  The dynamometer design features 48" diameter, knurled, precision balanced drums. The 48" drums provide stable contact for the tires eliminating unnecessary slippage and tire deformation resulting in precise repeatable results every time. The drums are connected to the dyno measuring system to test drive wheel output. 

A combination of two laws of physics, force equals mass times acceleration and work equals force times distance, gives us this equation: W=m X a X d. "W" is the work (in pound-feet) the rear wheels are doing, "m" is mass equivalent (the drums), "a" is acceleration (increasing drive wheel speed) and "d" is distance (drum circumference). Once we have the work, we can find horsepower. 

Torque can be figured by multiplying the horsepower by a constant, 5252, then dividing that product by the speed at which the thrust force was measured. 

Un-Corrected vs. Corrected Horsepower & Torque
The dyno and its computer output the horsepower and torque numbers for your vehicle. There are really two sets of numbers, un-corrected and corrected.  Each dyno is subjected to different atmospheric conditions. To facilitate comparison, the "standard" is Society of Automotive Engineers (SAE) standard J1349. The dyno results are "corrected" the this standard in order to accurately compare the "Numbers". Dyno numbers corrected to SAE J1349 alter the data to make it seem as if it was taken when the atmospheric pressure was 29.23 in/hg., the temperature 77�F and the humidity zero. Automatic measurement of the barometric pressure and ambient air temperature together with humidity measurement ensure repeatability between tests.

Always ask what correction factor was used to calculate the data when judging various performance products on the basis of dyno test results.

Uses
Our unique dynamic testing capability allows a realistic simulation of street and track conditions, highlighting mechanical problems graphically as they would occur and eliminating unnecessary engine and power train wear in actual road use testing. All dyno runs are stored for future comparison and analysis.  The uses of a dyno are:

• To evaluate the effects of modifications
• To calibrate fuel injection systems and spark curves
• To diagnose engine problems, such as detonation or mismatches between various modifications
• To solve driveline noise and vibration troubles
• To evaluate the parasitic losses caused by different types of transmission and rear axle lubricants.