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Tech:HOW THEY WORK
By Bob Bergeron
1. WHY OWN A DYNO?
Every racer understands that, without horsepower, that kart goes nowhere. As dynamometers are the only tool specifically designed to measure engine horsepower, it's no surprise that top racers want their own. This article examines things to consider before selecting and using this expensive tool.
Like most test equipment, a dynamometer (or dyno for short) helps isolate and quantify a particular parameter (in this case the engine's power output) from overall vehicle performance. Why do you need to do that? Racers (that don't dyno) often rationalize "I only test on the track ...where it counts"! They infer that power output is good if lap times are low. But, that fails to isolate the contribution of a sharp driver from a strong engine! Want a doctor that, instead of checking blood pressure with instruments, determines patients are ok if they survive between visits?
Many hop-up modifications only help at high rpm, actually reducing power down low. Even with days of track testing you might condemn some new high rpm pipe unless you test a bunch of sprocket changes too. What if you need to match the fuel mixture too? Add up those exponentially increasing combinations, and thoroughly track testing stretches to years! Dynamometer owners get pointed in the right direction with just a couple of 20-second "pulls".
Using a dynamometer also helps you avoid discounting "insignificant" 1-% gains from modifications. Just because you can't "feel" a single 1-% power increase does not mean you want to forego ten such tricks! Combining small improvements is how pros win trophies.
2. WHAT DO I NEED TO DYNO?
I’ll assume you are a serious engine builder and want to start in-house dynamometer testing. What do you need? First, to measure engine torque, your dynamometer system must provide a load. Automotive engineers refer to this loading device as an absorber or a "brake" (since early dynamometer absorbers used a drum and band brake to load the engine). Absorbers do not actually absorb the power. Rather, they convert it to another form of energy, like heating water or air.
Currently there are several commercially available absorber choices for kart engines. Professional engineers, with Fortune 500 budgets, often use electric DC generators with computer controlled field excitation to load and regulate their engines. The engine's power is typically dissipated as heat in the armature area or wired to remote heating elements. If the test engine's operating rpm is low enough, it can be directly coupled to the armature with a short driveshaft. 6,000+ rpm kart engines will need a gear reduction drive to match them to these low rpm generators.
⇐ This neat little DYNOmite Kart brake has horsepower capabilities well in excess of the huge eddy current brake it's next to!
The main advantage of electric generator systems is that they can be readjusted anywhere from zero load to full load in microseconds. This allows the engineer to regulate engine speed within a couple of rpm (even while changing throttle settings). Unfortunately, the cost of an adequate capacity generator, excitation controller, and support hardware run into the tens of thousands of dollars. Then you still need to buy the data-acquisition system. If your kart engine runs at high rpm, you need the required gear reduction. Reduction transmissions add still more, cost, complexity, and parasitic drag.
The DC generator dynamometer has another shortcoming. It has too high a polar moment of inertia. That's a fancy way of saying that the generator's armature feels like a giant flywheel to the tiny kart engine. High inertia means a lot of horsepower is required to accelerate the armature. Likewise, a lot of stored horsepower will be returned when dropping down in rpm. This really skews the test data whenever rpm is changing. So, while generator dynamometers are great for steady state control, they are nowhere for testing rapid acceleration transient conditions.
MAX – How Eddy-Current Absorbers Work - Animated look at how eddy-current absorbers create load (MAX narrated).
Eddy current brakes are similar in operational characteristics to electric DC generator absorbers. The main difference is that the eddy current brake does not actually generate electricity. Rather, you use an electrical power supply to charge its electromagnetic coils. The brake's input shaft spins a metallic rotor inside that resulting magnetic field. When the dyno operator increases the current supply to the coils, the rotor shaft becomes harder for the test engine to turn. Like the DC generator, an eddy current brake's advantage is its lightning-fast response to the controlling computer's loading instructions. Unfortunately they also come at the DC generator dynamometer’s hefty cost.
These eddy current brakes dissipate the engine's power as heat input to the rotor. This rotor must be cooled or it will eventually melt. Air-cooled eddy current brakes have cooling fins on a big iron rotor, making them look like automotive disk brake rotors. These big rotors have too much flywheel mass though, and dominate the rotating inertia of a typical kart dynamometer installation.
Water cooled eddy current brakes are available that have significantly lower rotating inertia (at least compared to air-cooled eddy current and DC generator systems). Unfortunately, the cooling system adds complexity, making the price tag even harder to swallow. Still, if you have a $50,000+ dynamometer budget, give them a look.
Before you get frightened away by these high priced status symbols, let's examine lower cost absorbers. The simplest and earliest form of brakes were just that, brakes. A rotating drum with a friction brake pad was used to apply drag at the engine's output shaft. These looked like old truck brakes. To measure torque, some sort of calibrated scale linkage was inserted at the brake pad anchor points to display the applied drag load. Problems with friction brakes included much difficulty in accurately regulating the load and brake pad cooling.
A more controllable load device is the hydraulic oil pump. These are occasionally seen on low rpm, moderate horsepower engine dynamometers. A positive displacement oil pump acts as the brake, and an adjustable oil discharge orifice valve sets the load. They can have a lower inertia than the DC generator and eddy current units if the pump is small, but sometimes required gear reduction units and coupling adapters push it back up. Like many absorbers, the oil pump units convert a test engine's power into a fluid's temperature rise. Since the oil can't be just freely discharged, a cooling system (typically an oil to water heat exchanger) must be used to keep the oil's temperature within safe limits.
MAX – How Water Brakes Work (page) - Animated look at how water-brake absorbers create load (MAX narrated).
When low cost, low inertia, high rpm limits, and race engine horsepower capacity are all requirements, the most prevalent choice for an absorber is the water brake. These have been the favorite of professional automotive engine builders for decades. Water brakes are another form of hydraulic pump absorber. These pumps typically have one or more vaned rotors spinning in between pocketed stator housings. Load is controlled by varying the level of water in the brake with adjustable inlet and/or outlet orifices. Raising this water level increases the rotational drag of the pump's rotor, applying more resistance to the engine turning it. Interestingly the water brake is, by design, a very inefficient pump. It uses up your engine's horsepower output by making "instant hot water"! Since the discharged hot water is clean, it can either be allowed to just run off, or it can be air cooled and recirculated.
The power capacity vs. size of water brakes is startling. The 8 pound water brake in the photo on page 15 handles over 65 continuous Hp at 12,000 rpm! By comparison the 300 pound eddy current brake shown next to it has the same continuous power rating and is only good to 7,000 rpm. It is no wonder that water brakes are virtually the only choice for testing 2,000+ horsepower drag car engines. Modern water brakes like the one pictured a low enough weight and inertia that they can be directly mounted on the kart engine's output shaft. Direct mounting eliminates the inertia and parasitic drag of driveshafts, u-joints, pillow block bearing, etc.
All of the above absorbers can be controlled manually by the operator (with a simple knob), or under computer control. Manual valve water brake load control is not as responsive as the electric DC generator or eddy current controls but, with good electronic servo valve controls, you can close the gap a lot.
3. FLYWHEEL ENERGY ISSUES
In discussing the pros and cons of various absorbers I keep mentioning problems with high inertia. To illustrate just how much power flywheel energy can mysteriously "absorbed" let's "build" a crude, dirt-cheap dynamometer with no brake at all! This will be an "inertia dynamometer" because the engine's power output will go into "winding up" a heavy flywheel.
This example uses a flywheel that is large, in relationship to the engine, so accelerating the combination from idle to peak rpm takes several seconds. A fast data-acquisition system logs the time periods and rpm changes. From that information we calculate the torque and horsepower the engine supplied to accelerate that known flywheel mass. The formula for determining the torque is:
Torque = JM * rpm per second / 9.551
where JM represents the Polar Moment of Inertia of our inertia dyno's flywheel.
If we don't know the Polar moment of Inertia for the flywheel (and our flywheel has a constant thickness cross-section) we can calculate it with the formula:
JM = (W * r ^2) / 32.16 / 2
where W represents the flywheel weight in pounds and r is its radius in feet.
Once you have the torque, it is easy to calculate the horsepower with the standard formula:
Hp = Torque * rpm / 5252
Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.
Say our example uses a 10-pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test. That's all here is to it. Unfortunately, inertial dynamometers alone are useless for doing the steady state testing needed for methodical development of porting, pipes, etc. You can not adjust the load to hold the engine at a given rpm point, it must always be accelerating. Still, inertial testing is handy for working out acceleration and drivability problems.
The real reason for the above math exercise is to illustrate how much power it took to accelerate that small flywheel. If you buy an absorber with a polar moment of inertia in the same rage as our flywheel example above, don't expect to perform sweep acceleration testing. Even accelerating at just 200 rpm per second would consume 10-% of our sample engine’s power! Fortunately, high end computerized data-acquisition systems provide composition algorithms to back out the effects of absorber (and crank-train) inertia from acceleration data. On a high inertia dynamometer, compensation is required even for fairly low rate sweep testing.
4. MEASURING POWER
Assuming you settle on a nice low inertia brake to load the engine's torque output, how do you measure that torque? Some DC generator and eddy current dyno's use in-line rotary-torque transducers because they measure engine torque before the influence of the high inertia rotor! However, the rotary transducer alone may add $3,000 to $10,000 onto the cost of your data-acquisition system. Luckily, the low inertia of a water brake makes a rotary transducer unnecessary.
To get torque data without a rotary transducer, the brake's outer housing must be mounted free floating (i.e. in trunion bearings). Housing rotation is then prevented with a form of "torque arm" protruding radialy from the housing. Some stationary support linkage holds the end of the arm. The arm is called a torque arm because it "feels" 100% of the engine torque trying to rotate the loaded brake. Inserted somewhere in this anti-rotation torque arm linkage is a calibrated scale or "load cell transducer". This transducer converts any applied force into a usable torque signal that it supplies to a gauge or data-acquisition unit.
Beware that, some oil pump "dyno's" skip the expense of a load cell and try to use discharge oil pressure (usually in conjunction with a look-up chart) as a crude estimation of power output. This is unsuitable for performance engine testing. No matter what type of absorber you select, get a transducer which can directly and accurately measure torque, not "guesstimate" it.
An electronic display or data-acquisition system expects to interface with an electrical strain gauge bridge load cell. This type load cell has a metal cross section with a hairline electronic wire grid glued to its surface. As this cross section is compressed, tensioned, or bent (depending on the linkage and load cell design) the attached wire grid is likewise deformed. The almost infinitesimal deformation of the wire grid changes its electrical resistance some tiny amount. The electronic circuit acts like an ohmmeter to read the resistance change, only it is calibrated in pound-feet. This same principle is used in everything from $500,000 dynamometers to $19.95 digital bathroom scales.
Calibrating the torque display for accuracy is usually straightforward. Typically a certified weight is hung off the end of the horizontal torque arm while you observe the torque display. Multiply the distance from the center of the brake out to where you hung the weight, and it must match the pounds-feet of torque displayed. If the reading is off, the data-acquisition system will provide some means to recalibrate it for the deviation.
Once you have a system that is accurately measuring running torque, you only need a calibrated tachometer to calculate horsepower. Horsepower specifies the rate at which your engine is capable of producing a given level of torque (see the earlier horsepower formula).
5. LOGGING THE DATA
On old-fashioned dynamometers, an observer must record the simultaneous tachometer and torque gauge readings with a pencil and paper. Today, most dynamometers replace the observer’s notes with computerized data-acquisition electronics. You would not believe how often everyone watching a test gets so excited by the noise and thrill that no one records the data! Or worse, the readings are "rounded up" by the biased engine builder. A good computerized data-acquisition system should be considered mandatory for any real testing, period. Fortunately, today it is possible to get recording, control, and playback capabilities in a $2,000 hand held package that years ago would have cost the price of a house and filled a small room.
A suitable computerized data-acquisition system should have a fast sampling rate, especially for testing 4-stroke, single cylinder engines. By fast I mean at least 100 samples, of all sensor channels, per second (100Hz). A 200Hz logging rate is a bit better still. Why? Understand that, between sparkplug firings there is a measurable drop in the instantaneous crankshaft torque and rpm. The crankshaft gets accelerated in the moments after combustion, then begins to slow until almost two revolutions later the plug fires again. You can't feel these rapid highs and lows when driving around the track (with all that vehicle inertia), but the dynamometer will!
If you sample at only 50Hz, that’s only a single torque and rpm sample every other revolution (at 6,000 rpm)! Periodically, a series of samples will fall in synch with the firings of the plugs, while at other times sampling will fall in synch with the lower power compression strokes. By using a fast acquisition system to read each firing cycle multiple times, enough data is captured to average out this phenomenon. The illustrations elsewhere in this article show the same data with and without dampening and averaging. While experienced dyno operators see the same power curve in both graphs, inexperienced operator's expect that smooth "publication-quality" line.
The ability of the acquisition system to average and dampen the data is mandatory for other reasons. At 200Hz you're getting 2,000 lines of data for even a ten-second dyno pull. Who wants to always wade through 40-pages of data for a few second run? Averaging both eliminates transient "noise" and produces more practical half-page printout.
6. BELLS AND WHISTLES
A computer that only logs horsepower, torque, rpm, and time may be all your testing requires. It will certainly put you several notches ahead of those without in-house dynamometers. But, for more advanced engine development there is much more you'll want to capture.
Weather data, meaning air temperature, barometric pressure, and humidity is something that needs to be noted for each dyno test session. As you are aware, lower barometric pressures, higher air temperatures and humidity will lower an engines power output (and vice versa). Without doing atmospheric correction, data taken under other conditions can not be directly compared. Dynamometers often come with the atmospheric correction tables found in many engineering handbooks. These tables have factors for the various weather conditions, which you multiply against your observed torque data. "Corrected" data is a closer estimate of what the engine would have produced had it been tested under, for example, "standard" atmospheric conditions. Good data-acquisition software should allow entering or recording these conditions and automatically calculate the correct data.
Exhaust and cylinder head temperature thermocouples, identical to what you may already be running on the track, are good to have. They provide a safety check and insight into what is happening inside the engine. Monitoring the EGT readings is a nice security blanket when you start leaning her out! On air-cooled engines, special sparkplug thermocouples are equally important. Some dyno software even lets you program safety limits that will shut down the test if things get to warm!
Block mounted thermistors let you monitor temperature variables that might inadvertently influence engine power. For getting repeatable test data you want to test at consistent temperatures. Thermistors data also lets you check the engine's sensitivity to cooling system alterations
. Airflow metering turns the dyno and data-acquisition system into a dynamic flow bench. Small turbine type transducers are available that simply clamp onto the carburetor inlet like an air cleaner. With the Static Cubic Foot per Minute numbers you can sort out combustion efficiency improvements from mass airflow gains. The software should combine the airflow info with horsepower data and provide a Brake Specific Air Consumption number. Having BSAC data let's you compare your engine’s efficiency with published dyno data from others. Such comparisons help guide you to areas where improvements are most likely to be had.
Like airflow turbines, a fuel flow turbine provides instantaneous fuel consumption and Brake Specific Fuel Consumption numbers. I like having BSFC numbers along with thermocouple temperatures to help me isolate fuel mixture issues from those induced by spark timing, etc. This add-on pays for itself in shortened test sessions many times over. Combined with airflow data, software can even track the engine's real-time air fuel ratio. Keep in mind though that the Briggs engine takes some carburetor/tank retrofitting to allow reading fuel flow.
Another computerized data-acquisition software feature, one that buyers may not think of until after running the system, is automatic triggering of data logging. Just as observers often fail to note gauge readings, busy dyno operators forget to toggle the data record button at the start and finish of important tests! It's frustrating pushing the print button and getting nothing, or, ending up with hundreds of pages of engine idling data! Better systems allow setting rpm and horsepower trigger points which, once exceeded, automatically start logging. Similar algorithms should control the end of logging. This feature really makes a dyno operator's life easier.
For long-term investment protection, make sure that your acquisition system can adapt to future applications. It should handle numerous types of ignition system rpm signals, have provisions for other than 1:1 gear ratios (you may dyno a bike someday), and it should handle a wide assortment of torque transducer types and ranges (when you start building Formula-1 engines)!
By selecting a portable electronics package you can double your investment value. Just add vehicle speed sensors, accelerometers, etc. and you have a professional on-board data-acquisition system. In fact, the DYNO-MAX for Windows dyno software goes so far as supporting Global Positioning Satellite mapping of the kart's location on the racecourse! I like using the same equipment in the cell and on the track because it makes comparing data much cleaner.
7. INSTALLATION CONSIDERATIONS
Once you take delivery of the dynamometer you still have to hook it up. That means plumbing it to a good water supply (unless you have only have an air-cooled absorber). Thermodynamic laws dictate that water-cooled absorbers (including eddy current and hydraulic pump units) require one gallon per minute for every 20 horsepower being loaded (assuming a temperature rise of 100 degrees Fahrenheit). Ideally the supply should maintain a steady pressure in the 20 to 40 psi range.
Most shop's municipal water supplies meet the requirements for kart engine testing. In fact, you probably can get enough right from a ¾" garden hose. However, if you do come up short on delivery, try replacing that restrictive garden hose sill-cock with a high flow ball valve. If you have a private well you may get wide pressure swings as the pump kicks on and off. If so, stabilize things with a ¾" pressure reducing valve, set to about 25 pounds per square inch. You can also use something like Land & Sea's neat little 2-stroke powered pump and a bucket of water to even dyno test remotely at the track!
Besides a water supply you need plenty of fresh air. Most dyno operators significantly under estimate the ventilation requirements for the room. It takes large area intake and outlet ducting combined with fairly large horsepower (3+) blower(s) to properly ventilate the room. This is especially true if you are attempting to just run your exhaust out into the raw air of the cell. Even if you run a good muffler a lot of noise will go out the vent system. Insulated fiberboard ductwork can be used to add sound dampening for the neighbors. If you do not have the bucks to build a properly ventilated dyno cell, it may be best to simply test outside on a breezy day.
If your absorber did not come with a stand and engine coupling, you’ll have to fabricate one that is rugged enough for the loads of testing. 1-½" square structural steel tubing with a 3/16" wall works well. The frame must also provide vibration isolation and dampening to protect the expensive torque transducer, dyno hardware, and engine itself. Brakes remotely coupled to the engine require driveshaft couplings that allow for some parallel and angular alignment errors that will occur. If you have a lightweight brake that directly couples to the engine, the job is much easier, but still make sure that you have adequate vibration dampening somewhere in the torque arm support system.
8. GETTING CONSISTENT RESULTS
No matter what type of dynamometer you select, controlling the test conditions is vital to getting usable data. It's not enough for the dynamometer equipment itself to be accurate; you have to know that the engine's output is not being skewed by improper dynamometer procedures. For example, if you fail to start all your tests from a standard, stable engine and head temperature, there's no way to tell which variable is responsible for any measured power differences.
Likewise, poor cell ventilation can allow exhaust gas to be inducted into the engine, drastically reducing its power. I've actually seen dyno operators, squinting from the pain of exhaust fumes, trying to figure out why the engine suddenly lost 50-% of its torque!
Torque data dampening and/or averaging is vital if you are using a kart engine with the fuel tank doing double duty as a giant carburetor float bowl. This design, while perfectly adequate for running lawn maintenance equipment, is not noted for precise control of air/fuel ratio. As the engine shakes, the fuel sloshes around in the huge tank, changing the head on the metering jet. It's best to keep the tank level consistent and near full to minimize this effect. Depending on your rulebook, more sophisticated cures can be implemented. Don't be turned off by problems like this, they are your opportunities! Top racers use their dynamometers to track down and plug these horsepower drains.
Even if you select a low inertia brake remember that the engine's moving components still have there own inertia. If you take readings while the engine is accelerating or decelerating, inertial energy is being subtracted or added, respectively, to what your gauges indicate. Disappointingly, unscrupulous dynamometer operators use inertia to display impressive flash power readings by suddenly cranking on the brake load. Obviously such "inertial energy augmented" numbers have nothing to do with the true horsepower capabilities of the engine. After you run a dynamometer for awhile, you can spot such shenanigans in other's printed dyno data. This is another reason engine builders get their own dynamometers.
The subject of inertial energy brings us back to the capabilities of the dynamometer itself. If you're manually controlling the brake with your wrist, you may be limited to steady state testing at discreet RPM steps. It can be virtually impossible to do a controlled low-rate sweep on some peaky race engines. Instead, settle for simply adjusting the load valve to a stable rpm test point, and collect enough data there to allow averaging out the inevitable small inertial and transient spike influences. Once you have collected this data, quickly move to the next desired rpm and repeat the process. By averaging enough data, this method produces very usable data for those on a budget.
If you have sprung for a system with a computerized load control, the rules change. In a typical installation a servo valve, under the data-acquisition computer's control, adjusts the load rather than the operator trying to do it manually. Water brakes equipped with computer servo load control routinely hold the engine within 1-% of target rpm. That is much better than you should expect to do manually. Computer load control allows programmable rate sweep testing and automated step testing (i.e. running the engine at each even 250 rpm for a few seconds of settling time and then automatically logging a couple of seconds data). In fact, with the additional electronic throttle control on top of the electronic load control you can actually program an entire racecourse simulation and sit back and watch the dyno run the show.
9. YOUR FIRST DYNO TEST
Assume you've selected an appropriate dynamometer and properly installed it in a well-designed test cell. How should tests be conducted? If this is your first experience operating a dyno, it’s best to start with a fairly mild engine. By that I mean an engine that isn't running with ultra-peaky porting, super high compression, or anything else that makes the engine finicky to run. Pull out some low-tech engine that's inherently reliable and which you don't mind running often at peak power (or over-revving occasionally).
Once you have that engine mounted, warm it to operating temperature. During the warm up, practice by applying light loads to the engine. This speeds warm up too. Next, gradually open up the throttle to full load while using the brake's control valve to regulate the rpm. Notice that it's actually the throttle that controls engine load, while the brake's "load" valve actually regulates rpm!
Once you are at wide-open throttle (which is where most of your testing will be done) leave the throttle there while you move between desired test rpm points with the brake's load valve. If you're collecting data with paper and pencil system, its time to kick one of those observers in the shin to remind him to start jotting things down. Those with electronic data-acquisition system may need to push the record button (a third hand helps). On a good computerized system, you can preset data collection parameters so that on future tests recording will start automatically based on the horsepower threshold points you preset.
Once you've stepped through each rpm point (holding each long enough to get meaningful data) simply back off the throttle while simultaneously unloading the brake so the engine returns to idle. Stop recording data, your first test is done.
If it did not go well, try again. Learning to run a manually controlled dynamometer is like beginning to ride a bicycle. Everyone thinks they will never "get it", or that the load valve, brake, etc. is defective. Actually, with practice, operators soon get to the point that it becomes a reflex action.
If you have an automatic servo valve, program the holding rpm and end test point before starting the engine. Then just bring the throttle to full, letting the servo hold the rpm for you. Push the test and let the computer do the rest.
10. EXAMINING THE DATA
On a purely manual recording system it's time to grab the calculator and extend those torque and rpm readings into horsepower numbers. If you've got a manual electronic data collection it’s time to playback or print out the data. On full-blown personal computer equipped dynamometers you'll usually want to name the new data file and probably enter any pertinent engine data or special notation's about the test run just completed. Many software packages allow you to enter virtually every parameter under the sun in predefined windows. That’s helpful so you don't forget to log something important, plus it's all in one database for you later on. If your system is not equipped with sensors that automatically capture the weather conditions you should note them now.
Choosing the best output report format for reviewing the dynamometer's data is important too. In cases where I will only get to see the data presented one way, I find it more useful to look at it plotted vs. time, rather than vs. rpm. Presented with fine enough resolution and/or appropriate averaging, a time printout helps one sort out valid power data from bogus flash readings. When examining the data, don't rely on information captured during periods of rapid rpm change. Instead, look for ranges (during the period of wide open throttle operation) where the engine maintains a steady rpm for a few consecutive seconds. When you examine the recorded data vs. time like this it will be easy to spot the ranges where you held the rpm steady enough that your torque data is valid, and not influenced by crank-train inertia.
Make sure you average the data too. Even numbers with a bit of inertial error can be averaged out to produce usable information. Computerized data-acquisition systems allow you to set the averaging and dampening periods set to suit the type of testing you are doing. For our near steady state pull example you would turn on about a second of dampening and about 1/10 second averaging.
If something is obviously wrong with your results, like the rpm appears off by a factor of two, you might have selected the improper tachometer pulse setting. Or, if horsepower is only a fraction of what it should be, was the throttle wide open during the test? First time operators have a habit of backing of the throttle, instead of cranking up the brake drag, when trying to regulate rpm. Don’t forget about the problem of exhaust getting back into the intake system. Then again, if power seems only a little low, welcome to horsepower reality. Be glad it's a clunker motor your friends are seeing, not that "mega-power" engine you’ve been exaggerating about!
Do a second pull, repeating the same procedures as the first test. Remember to bring the engine back to some consistent temperature first. Since we haven't made any changes, we're looking for repeatability, not a power increase. In fact, you are really testing the repeatability of yourself and the engine, since the dynamometer does not change between runs. Whenever it's feasible, especially when chasing small improvements, retest the engine in its baseline form. This extra reality check saves a lot of time in the long run.
11. GRADUATION DAY
Only after you acquire some skill as a dyno operator and can demonstrate repeatability should you move on to changing things in search of power. Just as you shouldn't start testing new engine modifications on the track if you haven't run consistent laps in weeks, it's just as pointless to do it on the dyno. Of course it almost goes without saying, make only one modification at a time!
You should try a few modifications on that "beater" motor to gain still more dynoing experience. Something like a higher compression cylinder head and/or thinner gasket combination is easy to test. You can also experiment with various combinations of spark advance and jetting. If you've equipped the dynamometer with exhaust temperature probes, etc., watch how they change as you add horsepower with modifications and run time.
If you have other instrumentation, practice with it too now. An engine that has strong airflow, the correct air/fuel ratio, and appropriate exhaust temperature, but which has than stellar horsepower output, points you towards things like a low compression ratio. Seeing too high exhaust temperatures while you are indicating a correct air/fuel ratio hints of late spark timing. Try watching airflow as you test a few different exhaust pipes. If that new whiz-bang pipe sends both airflow and power down, you will not likely bring it to life with tuning changes.
The beauty of having own dynamometer is it provides the opportunity to do the methodical testing everyone whishes they could. Get prepared to be surprised too. You'll be amazed how certain little things make an improvement while many over hyped tricks return nothing.