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Thread: Some Dyno Information

  1. #1
    UBFJ #454
    I found this article on the InterNet and thought it was worth sharing.
    Date: 05/18/2006
    Author: Brian Ebert
    Title: Dyno Testing - Facts and Myths
    Details: This is the first of several articles regarding the building and tuning of high performance automotive engines. Hopefully, it will help you to differentiate the facts from the fictions that are far too prevelant in our information rich society today (i.e. - the internet). As with anything in life, good knowledge is of utmost importance in making decisions about your high performance automobile and it is my goal to try help you in this area. Our first subject.... dyno's.
    A dyno (dynamometer) is a device for measuring the output of an engine. It is used for comparison TESTING..... I emphasize testing because far too many people have come to believe that a particular dyno output number is a goal in and of itself and then use them to compare with other numbers from other cars and dynamometers. Doing this generally leads to nothing but misconceptions and disappointment. The reason for this is in the procedure itself. Most of you have probably heard of the term "scientific method". It refers to the procedure of testing something by establishing a baseline in a controlled environment and then changing one thing at a time (to negate the effects of other influences) and noting the resulting of the change, or lack thereof. When we apply this to engine testing, it means we have to have control of all the things that can influence the power output of the engine being tested. This is no small task, as the list of things affecting an engine's power is quite large, such as - atmospheric conditions (temp., barometer, humidity), coolant temp, oil viscosity, oil temp, oil level in the oil pan, accessory friction losses (water pump, alternator, etc.), fuel type, fuel temp, and so on. When you run an engine in the car, you add a whole new set of variables such as trans type, trans oil type and temp, condition of u-joints, u-joint angles, rear end friction from ring and pinion, oil weight, oil temp and level, wieght of wheels and tires, tire inflation, tire rolling resistance and drag from the brakes. Therefore, trying to use power numbers from one shop's dyno and vehicle and comparing it to another shops dyno and vehicle without knowledge of all these variables is wasted effort. A dyno's true purpose is to allow an operator the ability to compare and optimize the POTENTIAL of a particular combination of parts. How that potential is used is a completely different matter (more on that in another article).
    The next thing to understand is the difference in the way power is measured. We express power in two different ways: torque and horsepower. Torque is the measurement of static FORCE. We commonly express it in lb/ft. It is determined by applying twisting force to a shaft and then measuring the linear force it creates tangental to a bar one foot long radiating from the center of the shaft. Horsepower is the measurement of WORK performed, meaning the application of force over a period of time. It is computed by the formula 550 lb/ft per second = 1 horsepower. In our automotive use, working with rotational torque numbers, we get the well known algebraic equation HP= (TQ x RPM)/5252. There are two common ways dyno's measure power. The first is by using a water brake (like used on a SuperFlow dyno) or electrical retarder (such as on a Mustang dyno) to apply a load to a running engine, and then measuring the force required to hold the housing of the brake or retarder stationary. This determines the torque output of the engine, and by filling in the variables in the HP equation (tq and the RPM of the engine at which it produces said tq), we compute the horsepower to engine produces. The second is by inertial load testing. An object of known inertial mass (such as the drum on a DynoJet 248) is accelerated by an engine under power. Through a mathematical formula, the rate of acceleration of the drum tells us how much horsepower the engine is producing. Then, by filling in the equation again, we can determine the engine's torque output. Note that since the load bearing dyno changes the load on the engine to control it's rate of acceleration and an inertial dyno's rate of acceleration changes with horsepower input, it is impossible to compare results from the two different methods of measurement without knowing the rotational mass and frictional losses of the crank, flywheel, transmission, tires, etc. One method of power measurement is not necessarily better or worse than the other, but they ARE different.
    The last thing I need to address may be the most important.... the variables. This is where you will really see a difference from dyno center to dyno center. As I stated earlier, controlling the variables is VERY important in engine testing. Let's face it, if you allow more than one thing to change from test to test, how can you tell what caused the change? Some variables are easy to monitor and control such as oil temps and levels, tire pressures, etc. One thing we can't control however, is the weather. It is constantly changing, even from minute to minute. Fortunately, we have a solution for this problem......IF it is used correctly! That solution is called the correction factor. It's purpose is to cancel out the changes in an engines power output based on atmospheric changes. Simply put, it is a mathematical formula that corrects the observed power output of an engine to what that engine WOULD produce given a standardized atmosphere. This way, a dyno operator can compare results taken during two or more different periods of time and different weather conditions, and if all other variables are kept the same, the corrected results should all be the same even though the daily results were different. To this end, two sets of standards are commonly used: Standard Correction and SAE standard J1349 rev. June '90. Standard correction uses 29.92 inches on mercury (Hg) at 60 degrees F with no humidity as the base atmosphere. The SAE standard uses 29.23 inches Hg at 77 degrees with no humidity. (Please note that these formulas are for naturally aspirated engines - forced induction requires a different formula although rarely actually used). Naturally, the engine corrected to the higher barometer and cooler temp will show more power - it doesn't really matter which correction factor is used, as long as the same one is always used when comparing results. This leads us to what I believe is the most overlooked factor in dyno facility configuration - proper ventillation. Since any changes in atmosphere can have drastic effects on power output (by changes in temp, humidity, baro, or contamination of intake air from exhaust gasses), a stable environment is critical in dyno testing. This point is emphasized by dyno mfgs. such as SuperFlow, Land & Sea, DTS, etc. in their recommendations for properly ventillating a dyno cell. It should be noted that these recommendations are for testing an engine on a stand AND in the car on a chassis dyno. Yet, while many engine dyno facilities have good ventillation systems, it is VERY rare to see a chassis dyno cell that is properly ventillated. What is proper ventillation? According to a Land & Sea article,, (and similar from SuperFlow), WITHOUT dealing with radiator, driveline and exhaust system heat, a dyno room's air should be changed at the rate of 2,000 cfm per 100 HP! That means that when testing an engine making 500 HP on a chassis dyno (WITH the radiator and exhaust system heat to deal with), the cell should be vented at a rate of approx. 15,000 cfm! A simple fan or two in front of the car does NOT accomplish this task. Since the weather readings for the correction are seldom taken directly in the path of the engines intake air, but somewhere else in the room, the only way to keep the readings accurate is to keep the entire room fresh by venting it at the massive rates just mentioned. Anything short of this diminishes the possibility of getting repeatable results. At our shop here in Minnesota, we have a 13,000 cfm fan in our current test cell, and will double that at our new shop this fall. This is why we can make back-to-back tests with no changes and have the same results within 1 HP. Remember, if you can't repeat within 5 HP, how can you be sure that timing change you just made was responsible for the 3 HP gain on the next test?
    To sum it all up, it is time and money well spent to find a properly equipped dyno facility and an experienced operator to get the most out of your combination. Also, try not to spend TOO much time worrying about other people's 'numbers' when you don't know all the facts about how they were derived. A 'number' does not necessarilly equate to a certain on-track performace. More on that in another article. As always, please feel free to contact us if you have any questions regarding this article or anything else about your vehicle.

  2. #2
    Very informative post. Alot of unknown info there..........I hope the posts on this subject in the future talk about maximum HP vs Maximum performance, and how one doesn't necessarily lead to the other..........MP

  3. #3
    Ray, I love posing a similar question to what you brought up. I like to ask people about the HP average over the RPM range that their motor sees. When we build a motor going in a power glide we try to design the engine to make its power over the wide RPM range it experiences behind that tranny. With the three speeds that we are running right now we can make the power a little more narrow being that the engine does not see as big of an RPM drop. Sure it is great if you engine makes 1200 horse power, but you sure do look silly when you get beat by the guy with less peak power, but better suited for the combination. So like you said Ray, it will be interesting to see if he talks about that.

  4. #4
    Very interesting, and educational, Thx.
    I had heard of correction calculations for dyno figures to standard conditions, but was unaware of what they were.
    Yes fc-pilot, I have come to think a LOT of people don't consider the purpose they build an engine for (it's expected opperating conditions) when they build one. Good (and very comon sense) examples from you there.

  5. #5
    UBFJ #454
    I like to ask people about the HP average over the RPM range that their motor sees. When we build a motor going in a power glide we try to design the engine to make its power over the wide RPM range it experiences behind that tranny.Paul
    My thinking is that the same logic should be used when designing & building a motor (be it N/A'd or Blown) for a boat ... Especially a High Performance Motor for a Jet Boat where the Hp Range and Impeller must be Matched (typically in the 5,500 to 7,500 RPM Range). Example: If you take a motor designed for asphalt that outputs its Tq & Hp in a higher RPM Range, say in the 7,500 to 9,500 + range, and put it into a Jet, your faced with dealing with the mismatch of the motor's output range vs. what an impeller/jet pump can handle ... some compensate for this by using a gear reduction box ... something I personally think is somewhat a Klugh due to the inefficiency's involved.
    Note, a V-Drive is somewhat more forgiving in terms of Tq/Hp output range as props need the higher RPM's and you have the gears in the V-Drive, the strut and the prop, etc. to 'Tune' with.
    For me, the Dyno is (for under 2,500 Hp motors ... most Dynos can't handle more than that) to break the motor in, check and verify, in a controlled environment, the motor's design (that it does what it was built for), establish a Baseline to 'Tune' from and, match drive components .......

  6. #6
    As per what FC-PILOt is relating to, the actual power curve is a very important factor. This is to USE the power you actually have. Match the power curve with your impellar or to match your power curve with your gearing, shifting, torque converter, etc. I see this a lot in the lower class dirt track engines. A guy wants to run an engine at 8000 rpm when his engine peak powered back at 5500. In one specific incident after engine testing and re-gearing the guy went out and won the rest of the races for the season.
    UBFJ#454 You are very correct in the importance of dyno cell configuration and air flow. You also may desire to read information available about cell design, air movement, etc. available from Jerry Stahl of Stahl Headers. Not only is the air flow critical, but the balancing of the air movement across the room and air pressure. Too much air being pulled out of the cell can cause a negative pressure in the cell. We have a Superflo dyno, but varied from their plan which pulls air out of the cell.
    We have a series of fans filling a "pegboard" wall that also includes air registers for directing air into the room. The pegboard wall also acts as an air diffuser. Then we have a fresh air fan providing air for the induction system. As you mentioned the more power an engine makes the more heat it makes as well as the engines injest more air. So, the more power, the more air is going to go into the engine.
    We also have one of the large exhaust fans to flow this air out the back. Another factor of the clean air, is you do not want any residual air from the exhaust or valve covers, etc. to go into the carb as this may polute the incoming air and affect the test. You need to exchange the air in the cell at several times per minute. A larger cell may require larger fan capacity. Just as in your cylinder head, the change in air density may require changes in fan capacity.
    As you mentioned dyno testing is a scientific experiment. Even with the correction controls, it may be good to test an engine at times with different weather conditions. There is no correction that can be exact for every air condition. (power wise... there is no substitute for good air) Also in a scientific experiment one attempts to keep as many variables as possible the same. We have ducted the air to our induction from our office. In other words, we can climatize the air to keep the temperature more consistent from run to run. The big thing you have to watch here is the change to humidity from climatized air. But, it helps dynoing, especially in the winter as dynoing with air below 60 degrees usually is a waste of real world time.
    I also agree with the repeatability of a dyno and the dyno operator. The dyno needs to repeat at below a .5% repeatability rate. A lot of the gains in a high efficiency engine are very small and we need that kind of repeatability and instrument resolution.
    Another area of concern is the exhaust size itself. The more power (heat and air) from the engine, the larger the exhaust system capacity required. I can't remember off the top of my head the exact sizing, but something like a pair of 6" exhaust for 400 to 600 hp, a couple of 8" exhaust for 800 hp and a pair 10" exhaust stacks for 1000 to 1200 hp.
    I definately agree with your references to procedures and temperatures. They really need to be kept within 5 to 10 degrees (water and oil) from one run to the other. This can be tricky at times. And to compare from one dyno to the other, one really needs baseline runs on each one. I believe there are two or three factors that come involved. First the type of dyno as some dynos record, correct and accelerate differently. Second, using the same style/type dyno one can see differences due to geographical and weather variations. Then the third... calibration! This is a little bit of a pun, but I have seen differences in what I call "engine builder" calibrations and "engine test station" calibrations. Some engine builders are selling NUMBERS. Now don't get me wrong, I like numbers, but I don't particularly care what they are as long as we attempt to get the best power and power curve we can.
    Two examples. The first seems to not be the norm. I tested an engine for a fellow that came from Reher-Morrison. When using the same parts, the same tuneup and the same test procedures and it happened to be about the same time of year and weather conditions. The test results between the two were within 5 foot lbs of torque and 5 HP from bottom to top. I was quite surprised and rather pleased to know the two test systems were that close and that RM sold real stuff.
    On the otherhand, there is an engine builder not too many miles from me that I test some of his customers engines on occasion. On a BBC that makes 800 - 900 HP in the 6500 to 7500 range, my results are about 60 HP less than his and my numbers generally correlate with the track times these guys see. The worst one I have seen was a local guy wanted the most HP for his $ and purchased a really nice small block off the East Coast. It is a longer story, but the bottom line he was looking for 100 more HP than his present two 600 HP 406's were making. He bought a 700 hp engine from this guy, put it in his car and ran it all year. It did not run any quicker... At the end of the year he brought it over to dyno and the engine peaked at 590. And then he sent the engine back and forth a couple of times. The builder did more work and picked up his power, but each time it was at more $ and my dyno still indicated 80 to 100 less than his sales numbers along with the performance more closely matched our test numbers.
    Bottom line, one needs to be careful when just picking numbers versus $'s from some add in a magazine or paper. Many times it goes back to you get what you paid for. Deal with reputable people that have customers and proven performance that you can be happy with.
    Good Luck!

  7. #7
    Anybody here using doing any Engine Cycle/Pressure Analysis with their dynos? If so, whose hardware/software are you using?

  8. #8
    UBFJ #454
    A number of engine R&D'ers use a device made by Kistler. For gasoline motors, Kistler manufactures what they call a 'measuring sparkplug' which is a plug with a pressure sensor built into it. They make the device in numerous different sizes, with various types of electrodes and a variety of heat ranges. Kistler also makes a 'measuring glowplug' for diesel motors.
    From what I've found so far most people are using National Instrument's LabVIEW Software (introduced some 20 years ago with the current version being 8.20) for data acqusition and analysis.
    Since engine cylinder pressure analysis & control/optimization is where the forefront of engine R&D is right now, I imagine both the senors and software are pretty pricey for the average Racer ... we'll see as I'm currently in the process now of finding detailed spec.'s and just how Pricey.

  9. #9
    Senior Member
    Join Date
    Mar 2010
    Power under the curve is what wins races. Each application of racing has an rpm range that one runs in. Making flat power in that rpm range makes a good engine and designates a good engine builder.
    I love dynos for figuring out issues and testing parts and theories. I hate dyno's used for marketing.

  10. #10
    Senior Member
    Join Date
    Mar 2010
    Anybody here using doing any Engine Cycle/Pressure Analysis with their dynos? If so, whose hardware/software are you using?
    French Grimes is developing software that will measure cylinder pressure in 1/2 degree increments of crankshaft rotation. He is after the perfect tune for the combustion engine.

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