Wednesday, June 6, 2012

Dyno Tech

Dyno Preparation Checklist:

  1. Have sufficient fuel in the tank
  2. Ensure your engine is free of all oil and water leaks
  3. Have sufficient engine oil, transmission oil, and coolant levels
  4. Have a good set of spark plugs installed in your engine
  5. Do not bring your vehicle to the dyno with a weak battery, failing alternator, or bad starter
  6. Do not bring your vehicle to the dyno with one or more check engine lights
  7. Ensure that you have a clean fuel filter and properly operating fuel system
  8. Set your tire pressure to the recommended setting
  9. Bring your vehicle to the dyno with a good set of tires
  10. Avoid bringing your vehicle to the dyno on racing slicks or sticky drag radials
  11. Bring your vehicle to the dyno properly aligned
  12. Ensure all vacuum hoses, coolant hoses, and pipes are tightly secured
  13. Bring a vehicle to the dyno with a healthy engine
  14. Tell the dyno operator what your goals are for the testing

Chassis Dynamometer Differences & Testing Goals:

Much debate revolves around measuring the power and torque levels of your vehicle and engine on a chassis dynamometer (dyno). One machine may give your measurements drastically higher than the next. My first objective is to explain some important differences amongst the most widely used dynos. However, my main objective is to discuss the importance of setting dyno testing goals.

Inertia vs Eddy Current Dynos

There are a variety of different types of chassis dynos; however, the two most common machines are either based on inertia or eddy current magnetic retarders.

Inertia dynos calculate power by measuring the power required to accelerate a drive roller. The drive roller has a known mass. There is no load that is moved, in turn there is no measurement of load. The most common inertia dynos in use are manufactured by DynoJet and Mustang Dyno.

Eddy current dynamometers have an electrically conductive core, shaft, or disc. It moves a across a magnetic field to measure resistance to movement. The eddy current magnetic absorber allows for quick load change rates. The best models enable the operator to conduct true steady-state and controlled rate operation. The most common eddy current dynos in use are manufactured by DynoDynamics and DynaPack.

Roller vs Hub Style Dynos

There are also two main types of design used in chassis dynos.

Most dynos consists of rollers mounted in a frame or in a pit in the floor. The vehicle is strapped to the frame or floor and driven on the rollers. The most popular manufacturers of roller style dynos are: DynoJet, Mustang Dyno, and DynoDynamics.

Another type of chassis dyno is one that is mounted to the hubs of the vehicle after the wheels have been removed. A popular brand is DynaPack.

It is obvious that a roller type dyno will deliver lower measurements of power and torque, since the drive train loss is carried further. This loss is increased the further we move from the engine's flywheel, to the transmission, to the axles, to the wheels, and finally to the tires. A dyno that measures at the hubs does not suffer from drive train losses at the wheels and tires. Therefore, it will measure higher power and torque numbers that are closer to those measured at the flywheel.

Correction Factors and Smoothing Factors

Dynamometer manufacturers also incorporate different features with regard to atmospheric correction and smoothing. DynoJet uses a smoothing factor to average out the bumps and spikes to compensate for the fact that the raw data from the DynoJet's acceleration sensors is very jagged and noisy. The scale runs from 0, least smoothing, to 5, which eliminates nearly all spikes and dips. You can imagine that the spikes and dips can significantly distort the mesured power and torque numbers. Atmospheric corrections are handled by incorporating dyno room temperature, humidity, and barometric pressure in to the dyno's software. With these variables, the dyno operator ensures that the measurements are equivalent during widely varying seasonal conditions.

Testing Goals

All types of chassis dynos share one common characteristic: they are tools to measure output levels and to prepare appropriate tunes for the intended purpose of the vehicle. Therefore, it is of lesser importance what type of kind of chassis dyno is used, than what the dyno professional does with the machine.

A good tuner will prepare your street vehicle with a consistent combination of air/fuel, ignition timing, and cam timing. The tuner's focus will be on safety and reliability, not power numbers. On the other hand, a good tuner will prepare a track oriented vehicle for the highest power and torque results that are necessary for the type of racing intended. A much more aggressive tune is justified in race vehicles, with a greater focus on power numbers. Regardless of what the intended purpose is of the vehicle, the tune should reflect a repeatable, lasting combination of air/fuel, ignition timing, and cam timing.

In conclusion, considering the large amount of differences from one dyno to another, it may seem difficult to determine the best approach when you want to put your vehicle on a chassis dyno. However, your decision will most likely depend on the proximity of the dyno shop, their rates, their customer service, and their availability.

Here are some useful tips for successive dyno testing sessions:

  1. Use the same dyno for all your testing
  2. Ask the dyno operator to use the same smoothing factor (DynoJet)
  3. Ask the dyno operator to use the same correction factor or method (all)
  4. Establish a baseline first, which can be a stock vehicle, a stock engine, or just the start of a new project on your already modified engine
  5. Compare your baseline results to the results of the next trip to the dyno and every subsequent one
  6. Ask the dyno operator to attempt to conduct successive dyno pulls within the same range of engine and intake temperatures
  7. Be consistent with gear selection, dyno pulls in different gears will produce different power and torque measurements
  8. Always check with the dyno operator how to properly prepare your vehicle for your visit

Context courtesy of Import Hot Rod Garage

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