Im not exactly sure what a Dyno is but one of the obvious uses is to bench horsepower. From what I have seen in mags, dynos are hooked up to the wheels of the car whether they be FWD or RWD. Here are my questions:

1)How does a Dyno work (gauge power output)

2)How do you take Dyno readings on an engine only?

3)How accurate are Dyno readings from the 'actual' horsepower of a car? (Real question: are there any other ways to rate HP and are they more accurate?)

 

First a little history. The correct term is Dynamometer. It is used to measure torque and then HP can be calculated from that. The first dyno's used a fluid coupling similar to a torque converter and water pressure to apply load to the engine. New dyno's used electro magnets and coils very similar to electric motors/generators to apply load to the engine as well as spin the engine.

1)The early water dyno's measure water pressure and readings on a strain gauge attached to the shaft to determine the amount of torque the engine was producing. Using this torque reading and knowing engine RPM HP can be calculated using the following formula

Torque*RPM/5252

Electric dynos measure field current need to keep the rotor at a constant speed which is a function of engine torque. These dynos are the best because they can react very quickly and also motor the engine to measure friction and paracitic losses. The devices used to measure current are also much more accurate and do not depend on material properties that water brakes use. All Chassis dynos are electic.

2)Engine only dynos mount the engine on a "stand" which is several shaft mounted to isolators to reduce vibration. The flywheel is connected to a shaft that is connected to a rotor or water brake.

3)Chassis dynos can not measure engine HP as accurately as an engine dyno. There are driveline losses that are difficult to measure. A popular way to measure driveline drag is to run the vehicle to a certain speed and allow it to coast down in neutral to another set speed and measure the time it took. Driveline drag can then be calculated from that reading and is normally anywhere from 20-40 percent depending on the transmission tires rearend and other such things.

 

That is a prety decent explanation. I would add that an engine dyno, while it can accurately measure engine output with the engine in whatever trim it's installed on the dyno with, cannot do any better than give a rough guess what the engine is going to do when installed in a chassis with an exhaust system, air intake system, etc. A chassis dyno tells you how much power actually reaches the rear wheels, which is the thing that along with the car's weight will tell you how fast the car will go. No guesswork as to driveline losses or the effects of all those chassis-specific things you can't duplicate on the engine dyno, just real-world reality. They're 2 different tools with different purposes; the engine dyno lets you isolate the engine from external influences and measure it by itself, where the chassis dyno is much more useful for measuring a whole car.

------------------
"So many Mustangs, so little time..."
ICON Motorsports