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How To Maximize The Business Horsepower Available From A Dyno
In recent years, the amount of aftermarket dyno testing has expanded dramatically. This is the result of media attention to the process, the availability of relatively inexpensive dynamometer systems and the growing sophistication of the consumer. Unfortunately, there is also a lot of misinformation and misunderstanding surrounding dynamometer testing.
By Ken Weber
To many consumers, dynamometers (flow benches and CNC porting too) represent black art. They believe that dyno testing their engine will somehow magically transform it into a producer of unheard levels of power. Others just want to compare their engine to one they read about in a magazine. And still others are seeking bragging rights. It's only human nature to want to brag about what you have: bigger is always better.
In the majority of engine shops, a dynamometer is used to improve the products, verify the performance and quality of the engines produced, and in the case of many chassis dynos, troubleshoot drivability issues and emissions calibrations.
On one side of the counter, the consumer often has no understanding of dyno testing in general. If he thinks he does, and is looking for bragging rights or trying to compare his engine's performance to another, he usually doesn't understand that the numbers he gets may not be directly comparable to those recorded by another shop in another part of the world. This enthusiast generally has a fixation on peak numbers, without regard to area under the power curve, driveability, idle quality, emissions or a host of other issues that may make the subject engine completely unusable in his application.
Additionally, he may have no knowledge of the other engine, the procedures used to run the dyno test or the facility the testing was done in, all of which have a bearing on the corrected power levels.
On the shop side of the counter, some shops have a dyno to lend a degree of credibility to their operation, or simply to provide a service they can sell. To others, it's regarded as a costly nuisance. In many cases, neither of these shops have an appreciation for what the dyno could do for them if used correctly.
Unfortunately, a high percentage of professional engine builders who have and/or operate a dyno, do not understand how to properly test an engine. The result is often improper equipment calibration and inaccurate test data from sloppy test procedures.
Some shops even generate phony numbers to please the customer and/or make their product look better than it really is. But as an executive at one of the leading dyno manufacturers explained to me, "You have to be able to handle the truth, even when you just know that it makes more power than that."
The impact these shoddy practices have on the industry, is to remind us all that horsepower and torque numbers are highly suspect until the source is known.
For those who do understand, the dyno is a valuable tool that gathers data: nothing more, nothing less. What is done with the acquired data - the skills used in translating data into information, and the use of that information to evaluate the tested engine or component - is, as they say, what separates the 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 th
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 lbs. 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 dime. 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.
Lets do some 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 5252, 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 5252 equals 342.73 horsepower. From this, you can see that developing the same torque at twice the rpm will result in twice the horsepower.
5252 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 = 5252
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 8 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 it the dyno cell should not be used as a warehouse or storage room. As Bill Hancock of Arrow Racing Engines says, "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 lay out of a test cell, and their advice should be followed.
It's never very important until it happens to you. Do you just bolt that bell housing to the block or do you take the extra time to install the plate between the block and the scatter shield and install all the bolts around it? What happens to your dyno and the engine if the flywheel or drive plate explodes? See photo for results of an improper set-up.
If you're testing on a chassis dyno, make sure the tie downs are adequate and in good shape. How many of you stand in front of or beside the vehicle while it's running? How about containment for things like a broken drive shaft or exploded clutch? If you are in the driver's seat with your foot planted firmly on the accelerator pedal when the clutch explodes, it is very likely that you will lose your right foot at the very least.
If you have never experienced a clutch explosion, I can tell you from experience (twice), that if a proper scatter shield is not installed, it will probably total the vehicle, and it is unlikely that you will come out of it in one piece. There is an incredible amount of energy stored in that rotating mass. You have seen the burned out distorted vehicles in Iraq on the evening news; well, that's pretty much what you can expect after a high rpm clutch or flywheel explosion.
Do fuel or oil lines run in the vicinity of the harmonic balancer? Balancers are known to explode from time to time too. Do you have your fuel supply in the dyno room? DUH! Engine oil sprayed on hot headers will burn too. How will you handle it when a carb float sticks and floods the engine? If it is the rear bowl on a Holley, you may not see it until you hit the starter and ignite the fuel. Marshmallows anyone? Again, look at the photographs.
Do you have a couple of large fire extinguishers close at hand? Sprinkler system? Fire hose and supply of water? Ever consider a Halon system like the funny cars and NASCAR guy use, with the nozzles strategically mounted on the engine stand and around the room?
Really - this is important, get all the information you can from your dyno manufacturer, and have the local fire department give you their recommendations. If you think that's a pain, you haven't seen how much damage a bunch of firemen with fire axes do to your shop when they respond to a fire (not to mention the fire itself). If that's not enough, think about the guys that narrowly escaped death when the smoke from a dyno fire made it almost impossible to get out of the building. Again, check out the photos.
The list is long for potential problems, especially if you are running the dyno as a concession and don't know anything about the engine or vehicle you're testing. How will you handle these events?
Here are some tips from Arrow Racing's Bill Hancock's presentation "Uncommon Sense in Engine Development" from the 1997 AETC.
When you are plumbing your cell, run an extra 2? line from the dyno "in" pump to a gate valve located outside the room. Go to your local fire department and ask them for an old 2? or 2-1/2? practice hose and then buy a cheap nozzle. Connect the hose to the valve, and keep it coiled outside the room. In the event of a fire, you need to first put out the fire with conventional fire extinguishers, and then you can spray the engine with cold water to remove the heat and prevent a re-ignition of the fire. By the time you put out the fire, you probably will not have enough reserve fire extinguishers to do it a second time.
Avoid putting trenches or drains in your room, as they are a place for gasoline vapors to collect, creating a fire hazard, and they become a trap for oil, water, and other trash, and make sure that any electrical devices such as battery chargers, tools, etc, are explosion proof.
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 percent 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.
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 percent 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 percent 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.
The more closely each variable, such as air temperature and humidity, coolant temperature, oil temperature, fuel pressure and temperature, and ultimately, barometric pressure of the room, can be controlled, the more accurate the tests will be. The smaller the error desired, the more critical these items become.
If you're testing outdoors, control will be difficult to achieve. I don't think outdoor testing is the best approach, but on the other hand, the raw data should be pretty close to reality. Unfortunately, the more variables you monitor and control, the more expensive the initial set-up cost is, and the more time consuming it becomes to keep it all properly calibrated.
When the dyno is initially installed, and regularly afterwards, all systems should be calibrated. Some suppliers say that getting shops to properly understand calibration procedures is one of their most difficult tasks. When calibrating the torque scale, use weights of known value: those old barbell weights may not really be the weight they claim to be.
The torque scale should be checked at several places in the range you expect to use. You should have a good quality mercury barometer, corrected for temperature and gravity, as a standard to compare to your dyno's internal unit, if so equipped. You should verify all of the temperature sensors against a known standard.
And very importantly, your fuel supply should be qualified to supply the needed volume at the required pressure. Just filling a jug from the open-ended fuel hose doesn't get it. Put an adjustable valve on the end of the line with a pressure gauge ahead of it, point it into a jug, adjust the valve to give the required pressure, and then measure the flow.
Once this has all been done, you have a baseline to work with in the future, and by regularly verifying that all systems are in calibration, you will avoid the likelihood that sometime in the future you will be chasing gremlins in your engines that are really in your dyno. Remember, reliable, accurate, high quality instruments are not cheap. A good quality mercury barometer alone is $500 or more. For good information and sources for these items, check out the directory at the end of this article.
Calibrate. Calibrate. Calibrate. Not just the strain gauge or load cell, but all of the sensors as well. How do you compare test results if the conditions are not always the same? Correction factors cannot overcome sloppy technique.
In high school chemistry and physics labs, we're taught the importance of good scientific procedure. That is, controlling all of the elements of an experiment and allowing only the one you are testing be a variable. This means that the only thing that changes from one test series to another is the part, such as the camshaft or carburetor, you are evaluating, and everything else stays the same.
Something simple, like a ten-degree difference in oil temperature at the start of a run can make a readable difference in the test results.
This is so important in testing engines. The accuracy of the test depends on good scientific procedure. Of course, if you only use your dyno as an expensive run-in stand, then calibration is a moot point.
The same fuel should be used throughout, and it should be fresh. Not your spec fuel that has been sitting in the corner for a year. The fuel temperature is also a variable that needs to be controlled. The same brand, type and weight of oil (unless you're testing oil) should always be in the engine.
Atmospheric conditions should be monitored closely: barometer, air temperature, and wet & dry bulb temperatures for vapor pressure (humidity), as they directly relate to the correction factor used.
The engine should be properly broken in and tuned to optimize the combination you are going to record. It then needs to be re-tuned for the next combination, and so on. And it goes without saying, that each test should be done at the same acceleration rate, if that mode is used, because you cannot compare data from tests done at, say, a 300 rpm/second acceleration rate to those done at a 100 rpm/second acceleration rate.
On a chassis dyno, tire pressure is critical. If you are rerunning a vehicle on a chassis dyno that was run some time ago, does the vehicle even have the same tires it had the last time? At the very least, the tires should be the same type.
Same for the rest of the vehicle drive line, including gear ratio. All tests should be run with the transmission in the one-to-one ratio gear, usually fourth, not in overdrive (unless your testing for power losses in each gear). Transmission and differential temperatures should also be controlled to eliminate that variable.
A good rule of thumb to follow is to establish a new baseline whenever there has been significant time or equipment changes to an engine you previously tested.
The operator of the dyno should have a specific routine he uses for each run to minimize and control each variable to the greatest extent possible. The atmospheric conditions should be adjusted for each run, and several pulls (three for example) on the engine should be done for each combination being tested. Those results should be averaged to generate the fourth data set that will be used to evaluate the combination.
One mistake I run into fairly often is to tune the engine on the dyno and expect that to be the end of the issue. In reality, an engine cell may or may not match the atmospheric conditions outside, there is probably no ram air effect, and the engine is not experiencing the forces of cornering, acceleration, or braking, or in the case of an aircraft engine, probably not running upside down. Tuning the engine on the dyno serves to establish a baseline for further testing, and in subsequent testing, to optimize the effect of any changes to the engine. Once the dyno testing is completed, and the engine put into a vehicle, it must be re-tuned to maximize its performance in that environment.
When making changes to a computer-controlled engine, most OEM computers go through a learning mode after the change is made, so you have to either do enough runs for the power levels to stabilize, or if possible, take the vehicle out and drive it for a while.
Why Doesn't It Make As Much Power As..?
A typical scenario: Let's say that two engines are both 350 Chevrolet small blocks built to the same specs. Engine one was tested with an automatic transmission flywheel, while the second was tested with a fifty-pound flywheel and clutch. If both are tested fully loaded, where the rpm is held steady at each rpm for say 10 seconds before the reading is taken, then the difference in flywheel weight is unimportant. However, if they are both tested at say 400 rpm per second acceleration, then the second engine is using more of its developed horsepower to accelerate the heavier flywheel and clutch, and therefore will show less net horsepower output.
Let's also say that engine one had an electric water pump versus the usual belt-driven type on engine two. These two differences alone could account for a difference of twenty or more horsepower at peak power. In a chassis dyno, the difference between an automatic and a manual transmission can make 10 percent or more difference.
The difference in test cells - each installation's ability to control its own environment, the difference in operator technique, and indeed, the difference in brands of dynos - all influence the results of a dyno test.
Chassis dynos are particularly difficult to control in this area, because there are dynos that use the rollers plus an eddy-current brake and dynos that use only an inertia wheel. Add to this the difference in roller diameters and the weight of the rollers, the relative size/weight of the rollers to the size/weight of the vehicle, and the result can be a substantial difference in power readings for the same vehicle.
A chassis dyno with an eddy current controller to load the roller can achieve the same acceleration rate on each test, or the test can be done fully loaded with no acceleration. This is not possible on a straight inertia dyno. The horsepower figures could be entirely different depending on the method of the test.
According to Norm Brandes of Westech Automotive, "On an inertia wheel dyno you can gain 25 horsepower by changing the flywheel, drive shaft and wheels." This is because every component in the driveline has inertia including the wheels and tires, and when you change any of them, you change the amount of horsepower required to accelerate the whole mass. Unless you are doing fully loaded testing, there is no easy way to account for the differences.
Once you consider the mechanical differences of each brand of dyno, the configuration of the dyno cell, and the operator himself, then you have to consider the software packages. How many samples/second the software generates has an impact on the accuracy of the data it records. More samples, more accuracy.
The source and accuracy of the information used to calculate the correction factor is another issue. If your barometric pressure is taken from a local TV broadcast, it is already corrected for altitude, and so probably won't be the same as the actual pressure at your dyno cell. As an extreme example, in Denver (5,000 ft altitude), the actual barometric pressure is about 6 inches of mercury lower than the barometric pressure given in the weather reports, so the only way to have reliable barometric information is to have an on site mercury barometer.
Is a sling psychrometer used to determine vapor pressure (relative humidity) at the dyno location? Is the temperature verified with a standardized instrument? A small change in correction factor makes a substantial change in corrected power. Even the charts used to determine correction factor from barometer, thermometer, and psychrometer readings can be different.
For these reasons, unless you know all of the conditions that an engine was tested under, comparing figures for one engine at one location, to a different engine tested at a different location must be done with a degree of skepticism. It's also one reason it's not uncommon for a given engine to wax the butt of another that supposedly makes substantially more power. If comparisons are made on the same dyno, under carefully controlled conditions, then you have the basis for a good comparison.
In summary, the real value of dynamometer testing is to evaluate changes to engine or chassis components, to determine the best possible combination of those components to satisfy the intended use. It is also used for quality control to verify that an engine or chassis measures up to the desired standard of other engines of known specifications, such as is done in NASCAR engine shops. It streamlines this process by eliminating variables seen in road or track testing, and by closely controlling the other variables
The development process requires many changes, many dyno runs, and a lot of time, effort, and expense, to achieve the goals of the project. The mistake is often not doing enough testing, as opposed to too much.
All too often, the customer has a shop make one run on his engine, and tries to make a meaningful comparison with some other engine he heard about. He is trying to race dynos, without understanding that not everyone is playing the game by the same rules.
Dyno And Equipment Suppliers
DEPAC Dyno Systems
Dynamic Test Systems
Land & Sea Inc.
For additional information, Ken Weber recommends the following resources:
SAE Technical paper 2002-01-0887
This paper attempts to develop a method for comparing chassis dyno results from several aftermarket inertia dynos to the power numbers advertised by OE manufacturers. Some good information here if you operate a chassis dyno. Available from Society of Automotive Engineers: 877-606-7323 or www.sae.org
"How to Read a Dyno Sheet and Avoid Getting Scammed," by Marlan Davis and Jeff Smith. This article from the June 1998 issue of Car Craft magazine article takes each item on a typical dyno printout and explains how it relates to the test.
www.stahlheaders.com - Click on "Tech Data." Several articles related to dyno cells and dyno testing in general by Jere Stahl. Good down to earth advice and sources for instruments etc.