he winners from the losers. The full value of dynamometer testing is only realized when it is used to this end. There are a number of myths attached to the science of engine testing. One is that the dyno will tell you what you need to do to make more power and/or will provide you with the skill to make informed changes. Only experience, study and repeat tests will do that. A dyno can be the single biggest investment you can make in confusion, because they have a nasty habit of showing your best theories to be terribly wrong.
Another myth is that a dyno makes an engine work harder than it ever will in the real world. While it is true that a test done fully loaded, at a constant rpm or a step test, will put the engine under a heavier load than if it is tested under acceleration, at full throttle, the cylinder pressures and other related factors are the same or very nearly the same, whether on the dyno or in the vehicle.
The engine doesn’t know what it is driving. The only difference between a constant rpm test, and an acceleration test, is that some of the engine’s developed power is used to accelerate the engine itself and the dyno power absorption unit.
On a chassis dyno, additional power is used to accelerate the rest of the driveline, the wheels and tires and the rollers. The engine still develops the power in the cylinders and applies it through the rods to the crankshaft (rotaries excepted).
The more power required to accelerate the engine and other components, the lower the dyno readings will be. The main issue here would be the duration of each test, and how that compares to how the engine is operated in the vehicle. With the computer-controlled dynos in use today, it is possible to come fairly close to real-world conditions.
Back To The Basics
One of the more common misconceptions about a dynamometer is that it measures horsepower. WRONG – it only measures TORQUE and RPM… horsepower must be calculated.
I have had experienced dyno owners/operators tell me they didn’t realize that their dyno doesn’t measure horsepower, because their dyno has a horsepower gauge. They didn’t understand that the computer in the console generates the read-out from the gauge. In its simplest form, an engine dyno would only have a torque arm, a scale to measure the load exerted by the arm, and a tachometer to measure rpm.
In physics, work (W) is defined as force (F) times distance (D) (W=F x D). Thus if you move a brick from one place to another, you have done some work. Horsepower is simply a measure of how fast you do work. How many bricks can you move in one second, one minute or one hour?
In the 17th century, James Watt defined one horsepower as moving 550 pounds one foot in one second, or one pound 550 feet in one second. This is supposedly how much work an average draft horse can do, and is expressed as 550 ft.-lbs. per second. If multiplied by the 60 seconds in a minute, the 550 ft.-lbs. per second becomes 33,000 ft.-lbs. per minute.
Notice that the definition of horsepower includes weight (a force), distance and time. When we say that a machine makes one horsepower, we mean that it can do 550 ft.-lbs. of work every second that it is in operation. A two horsepower machine can work twice as fast as a one horsepower machine.
For our purposes, the force is usually expressed in units of torque, because the force is rotational rather than linear. Think of it in terms of cranking a bucket of water up a well. The torque is the effort needed to turn the crank handle. If you pull on the crank but don’t move the bucket, you have generated torque but no work and no horsepower. Horsepower is the result of lifting the weight of the bucket (Force or Torque) to the top of the well (Distance), and the Time it takes to do it.
If you raise the same weight bucket the same distance but do it in half the time (same torque, twice the rpm), you have generated twice the horsepower. If you double the weight of the bucket, but take the same time to get it to the top (twice the torque, same rpm), you have also generated twice the horsepower. Torque and rpm are necessary to develop horsepower.
Doing the Math
Example 1: If an engine develops 300 ft.-lbs. of torque at 3,000 rpm, how much horsepower is that? 300 (torque), times 3,000 (rpm), equals 900,000. Dividing 900,000 by the constant 5,252, equals 171.36 horsepower.
Example 2: 300 ft.-lbs. at 6,000 rpm. 300 x 6,000 = 1,800,000. Dividing 1,800,000 by 5,252 equals 342.73 horsepower. From this, you can see that developing the same torque at twice the rpm will result in twice the horsepower.
Note: 5,252 is a constant derived from converting the basic formula for horsepower from: Horsepower = Force x Distance to Horsepower = Torque x RPM Time in minutes x 33,000 = 5,252
The Dyno Cell
Good testing starts with a good test cell. In general, the room should be large enough to comfortably work on the engine or vehicle between runs. It should have an adequate supply of ventilation air (sufficient to refresh the room eight to 10 times per minute), induction air separate from the ventilation air, water for both the dyno and for cooling the engine, assuming it’s not air-cooled (I have been told of motorcycle dynos with more air blowing on the rider than on the engine – not the best situation) and fuel.
Good lighting is essential as well, and the dyno cell should not be used as a warehouse or storage room. As one engine specialist told me, “If you don’t want it in the engine, it shouldn’t be in the room.”
One often-overlooked item is exhaust leakage into the room. Few people appreciate the impact that exhaust gas recirculated into the induction system has on power. Is your induction air supply being drawn from within the room or is it separate?
Ideally, it should be sealed from the room, drawn from the shop (not outdoors) and be capable of supplying the engine’s needs without causing a pressure drop at the carburetor. I must confess, this part bit me in the past. Fortunately, it was so bad that the engine being tested wouldn’t make a full pull without choking to death; otherwise it might have gone unnoticed.
All of the major dyno manufacturers have information on the layout of a test cell, and their advice should be followed. Correction Factors and SAE Standards
When we test an engine, environmental differences have a significant impact on the actual power that the engine develops. On a daily basis, the barometric pressure, temperature and relative humidity all change, and when they change, they have an effect on the power that an engine develops. If you live at higher elevations or have taken trips into the Rockies, you have felt the loss of power that a car or truck experiences at these altitudes.
In Denver, an engine typically makes about 75% to 78% of the power that it would make on a similar day in Miami. So when I test an engine up here, how do I know if it’s competitive, and how do I relate a test done last week to one done today?
Well, I could take it to Miami and test it or I could wait until the conditions are exactly the same as they were during the last test. Or, I could just apply the appropriate correction factors to the raw power numbers, which would give me a standard of comparison.
The SAE (Society of Automotive Engineers) has standards that are used by automobile manufacturers to rate their engines. Although these standards change from time to time, it serves to give some credibility to the advertised horsepower you see in those television commercials all the time.
HP in the Aftermarket
In the aftermarket, most horsepower and torque numbers are corrected to conditions of 29.92 inches of mercury barometric pressure, 60° F temperature and dry air. In theory, a test done at 30.12 inches of mercury, 90° F and 30% relative humidity can be corrected to standard conditions and compared to another test done in another part of world at 23.50 inches of mercury, 75° F and 15% relative humidity. As long as both tests were done with accurately calibrated instruments, and with the same procedures, they should be within a couple of percent of each other.
In reality, if a shop is not concerned about comparisons to what goes on in other parts of the country, the leadership can establish its own standard atmosphere, and use that to generate the correction factor. Eliminating the basis of comparison to the rest of the world would certainly eliminate the emotion and ego factors seen in a lot of dyno results. Again, though, “you have to be able to handle the truth.” The correction factor is probably the most mis-used and abused element effecting horsepower figures, because it is so easily skewed to “enhance” the results.
Well, that may be a lot to digest in one article, so we’ll continue this discussion in the July issue. Next month in Part Two of this two-part article, we’ll address dyno safety issues, proper calibration and test cell differences.
