Anyone who has lived on a healthy diet of off-road and performance magazines has gotten a pretty good taste of the dyno process, even if it's just served on the printed page. It looks pretty exciting: a fresh, bad-assed mill doing battle against the machine, all for the honor of putting up a power number to be proud of. So, maybe you've never even seen a real dyno test, much less put together an engine in your garage and brought it in for dyno testing. Still, you might dream of the day when you've got an engine worthy of the effort. Yeah, you might even be serious enough about performance to know you'll get there one day. But no one wants to go in green and come out feeling like a chump for not knowing the ins and outs. Read on, because here you'll find out all you need to know and more.
The reasons to test can be varied, depending upon the objective, be it an involved development program or a one-off test of a completed engine. The underlying objective is to get the most from an engine package. Building an engine without testing is somewhat like playing baseball, but neglecting to keep score. You'll likely have some fun in the process, but it's going to be impossible to evaluate the stats. A dyno test will provide a wealth of information that will sharpen both the engine's performance and the builder's skill. See "The Numbers" on page 51 for some of what the dyno can tell you.
Besides the detailed information on the numerous running parameters of the engine, the test will also allow the builder to find the optimal tuning setting for best power and/or economy. This can be as simple as finding the basic timing and jetting requirements, or it may be more involved, including detailed tuning of the fuel and spark curves to really dial in a combination. The dyno makes it possible, providing instant feedback to various changes. Running the engine on the dyno will also provide an opportunity for more basic adjustments and checks of the engine's operation. The cam and rings can be broken in, and the valves given a final adjustment on the dyno, while running the engine under load can ensure there are no mechanical problems or leaks before the engine is bolted into your truck. This alone can be worth the cost of a test.
The main job of an engine dyno is to measure the power output of an engine accurately and repeatedly. Pretty basic stuff, and we bet you are already hip to that tidbit. What you might not know is how it happens. First, the dyno system needs to have all of the equipment to run the engine, just as if it was under the hood of your truck. There's the framework and mounting system of the test stand to bolt the engine to. The dyno will also have the cooling, fuel, and exhaust systems to let it run. Typically, dyno installations will have provisions to provide the engine with fresh, clean air and to evacuate the test room, or cell, of the spent fumes. Often a dyno installation will also have a built-in ignition system, including an ignition box and coil, requiring the engine only to have a distributor in place. Finally, the dyno will have a 12-volt power supply to power any engine electronics and to start the engine. Some dynos have built-in starter systems, but others require a regular starter motor to be bolted to the engine or bellhousing.
That sums up what gets the engine running, but how does it measure power? The key feature of an engine dyno is the absorber, which is basically a water brake that is very much like a pump that is attached to the crank via an auxiliary driveshaft, or a drive-plate. The absorbers used in nearly all automotive engine dynos are water brakes, and their job is to provide a resistance or load on the engine power output. When the absorber is attached to the crank and filled with water, it can create a tremendous load. The load applied by the absorber is usually controlled by controlling the water flow at the outlet side. Older dynos were manually adjusted for load, where the operator would turn a wheel or operate a switch to control the water flow through the absorber and therefore the load. To run the engine through its test rpm range, the operator would work the wheel or manual switch to load the engine under wide-open-throttle until the engine is pulled down to the bottom rpm of the test range. Then the load is reduced, again by manual adjustment, lessening the load. As the absorber slowly let go, the engine would work up the rpm range.
In modern computerized dynos, the flow of water through the absorber, and therefore the load, is automatically controlled by an electronic servo. Unlike with a manual system, a servo load control, as used by SuperFlow, allows the acceleration rate of the engine to be very closely controlled. In fact, the rate of acceleration can be selected from the dyno control console. Typically we test engines at 600 rpm per second, so a test sweep from 3,000 to 6,000 rpm (a 3,000 rpm test range) takes 5 seconds (3,000/600).
OK, so the dyno needs to mount up and run the engine, and the absorber lets us load it and control it at full throttle for a dyno pull through its rpm range-but how does it know the power? A device hooked to the absorber actually measures the force applied to it by the engine. This piece of equipment is the strain gauge. The engine is trying to turn the absorber, and the strain gauge simply measures this turning force, and turning force is torque. Older dynos used a torque meter that showed the turning force in pound-feet right on a giant gauge, similar to the torque gauge on a dial-type Snap-On torque wrench. Modern engine dynos use electronic strain gauges or load cells that read the torque on the absorber and send that information to the dyno computer electronically. So now we know the torque!
If you know the torque and rpm, you can have the horsepower, using the formula: HP = (Torque x RPM)/5,252. That's where the horsepower number always comes from, a calculation based on torque and rpm. Torque is the only thing a dyno really measures, and the horsepower is just figured from there. Back in the really old days, dyno guys would read the torque on the torque-meter, and at the same time look at the rpm, and then number crunch to get the horsepower. A modern computerized dyno has advanced data acquisition capabilities, recording torque, rpm, and a wide range of other inputs simultaneously. It does the math to calculate the horsepower, and can crunch the numbers to provide a wealth of additional information, depending on how the dyno is equipped and configured.
So it looks like it's time to jump in and have an engine dyno tested. What steps can you take ahead of time before getting there? The first step is to find the right dyno shop to do the job. It is worthwhile to make some inquiries both at potential shops and within the local performance community to get an insight into the facility. One of the key reasons for considering a dyno test is that it can provide a roadmap to making more power, if you know how to find it. This can have less to do with equipment than with personnel. An experienced and knowledgeable dyno operator will add immeasurably to the value of a test, and the operator will usually be the point man when it comes to finding that "magic tune." Experience plays a key role here-remember you are paying for the operator's time as well as for the time on the machine. A skilled operator will be an asset in evaluating the data from the dyno, and making interpretations that can lead to improved performance. Just as importantly, a skilled operator will often detect and head-off problems before they develop into serious trouble, and help find faults if they occur. Choose the shop carefully.
Preparing for a dyno test should begin with a conversation with the dyno operator. It's important that everyone is clear on what the objectives and goals of the test will be, and that everything required is identified ahead of time. Besides discussing the basic engine combination, it is important to be clear about what additional testing you'd like to accomplish. You'll need to know what components in addition to the engine assembly will be required for testing, which will vary from shop to shop. You may need to supply a set of headers, though in some cases the shop will have dedicated dyno headers, often set up to accept exhaust gas temperature probes. The dyno may be equipped with an ignition system, or you may elect to run the system that will be used in the truck. It's important to discuss the requirements and capabilities of the shop to run ancillary equipment, such as nitrous, blowers, or a dry sump. Find out what equipment and supplies the shop has on hand before showing up on test day. Things to consider include:
* Pre-lube adapter
* Fuel lines, especially for multi carbs
* Motor mounts
* Ignition system
* Fuel pump
* Spark-plug wires
* Oil and filters
* Tuning parts (Jets, spark plugs, and so on)
* Special tools
Don't hesitate to ask the dyno shop to detail just what items you will be required to bring. Nothing will ruin a dyno day like getting there and then realizing the test can't be run because something sitting in your garage needs to be with you on the day. It pays to make a checklist and get prepared before test day comes around.
If you've gone through the prep and come to the dyno shop locked and loaded with all the gear needed-and an engine ready to run-you'll be way ahead of the game. This will save considerable time in the setup, giving more time for testing. Once the engine is mounted and the dyno connections are made, you'll soon be ready for fire-up, but not before some preliminary steps. The engine's lubrication system should be primed or prelubed, usually via a drill motor and an oil-pump drive adapter. The oil pressure should be confirmed. Ideally, the valve adjustments, if applicable, should have already been done. The engine should be static-timed, and the firing order and ignition system connections confirmed. A fuel system check should be made, ensuring the fuel pressure is set at the required level and that there are no fuel leaks. With a carburetor, check that the float level is at specifications and there is no flooding or fuel flow with the pump on and the engine off. Confirm cooling water in the engine, the dyno cooling tower, and that the dyno or engine's water pump is functional. Finally, connect a timing light to the engine, and have it standing by. All of these steps help ensure that the engine will fire immediately.
If everything is right, the ignition and fuel pumps will be switched on at the dyno control console, and the engine should fire after a few cranks of the starter. The oil pressure should be confirmed once the engine comes to life, and the dyno instruments checked for any immediate signs of problems. If the engine is new, a break-in cycle will be the first phase of testing. Some dynos have an automated cycle to vary the rpm and load for a break-in period. Back in the test cell, once the engine is fired the timing should be checked and adjusted to confirm it is in a safe range. If the dyno doesn't have an exhaust gas temperature (EGT) setup, check that all the cylinders are firing with a temperature gun reading on each cylinder's header tube. Look and listen for any problems, like fluid leaks, burning wires, or any type of scary death sounds. A mechanic's stethoscope is a good way to look for valvetrain noises or other brewing trouble. Shut down and repair or investigate if something isn't right. After the engine is idled down from the initial run-in, the carb or injection system's idle adjustments should be made.
If the engine is equipped with solid lifters, after the engine is shut-down following the initial run-in, the first step should be resetting the valve lash with the engine hot. Temperature changes the clearances, and a hot-lash setting is truer to the actual operating condition. If any of the valves have a significantly different or changed clearance at this point, diligently check that valve for any abnormality or problem.
Once the engine has gotten past the initial fire-up and break-in steps, we come to the tune. The basic power tune involves just two things-the air/fuel ratio and timing. A well-equipped dyno will have a wideband Lambda sensor to precisely measure the air/fuel ratio. For the initial power pulls, the timing should be set conservatively, based upon the total timing setting for the type of engine being tested. Typically 30 degrees total timing is a good starting point for most V-8 engines.
The first pulls should be static pulls, where the engine is simply loaded to a set rpm. The first static pull should be at the lower end of the engine's operating range. As the engine is pulled down, the air/fuel ratio should be monitored. If the ratio is outside of a safe range, the pull should be aborted, and corrections to the carb jetting or fuel injection should be made. Here is where the wideband Lambda sensor is really valuable, but an experienced dyno operator will also be able to judge the ratio by a combination of the brake specific fuel consumption, the power output, the fuel flow, and the dyno's analog air/fuel ratio calculation. A series of additional static pulls should follow, essentially spot-checking to make sure that the engine is in a safe air/fuel ratio zone at various points up the rpm range.
If everything looks good once the static pulls are complete, the next step is to get into the sweep tests and tuning loops. In a sweep, the dyno is set to test the engine over the desired rpm range, and the engine is loaded to pull through the range. The best strategy here is to start with a short sweep, ending below the anticipated peak power rpm. There's no reason to take it to redline on the first pull. The sweep tests should extend to a higher rpm each time until the peak power points are determined. The dyno operator will study the data from each pull and determine if any adjustments are required along the way. Running a tuning loop of the air/fuel ratio can be done to determine what ratio provides the best power curve. This involves changing the jetting or fuel map in an injection system and then trying another sweep test, comparing the power curve to the previous pulls'. Generally, the procedure is to try a richer ratio first, since it is safer, and then, judging by the change in the power curve, the decision is made whether to try an even richer air/fuel ratio or explore the leaner direction.
The best timing can also be found with loop testing, similar to that done with the air/fuel ratio testing. Here it is best to take a little timing out, try it, and judge by comparing the power curves whether that is the correct direction. If not, the timing can be advanced until the best power curve is found. It may take more than one go-around with the timing and fuel loops until the best combination of these two factors are reached, which represents the optimal full-throttle settings. The dyno operator should be on the alert for any signs of detonation or unusual readings of the dyno's recorded data. Other considerations that the dyno operator should look at include the fuel flow balance between the primary and secondary sides of a four-barrel carburetor, and the engine's air/fuel ratio at part-throttle, light-load, or cruise. Additional tuning of the advance curve can also be made at this time.
Besides taking your initial combination, optimizing it, and giving you a clear picture of the power curve, a dyno test provides the opportunity to try a variety of equipment in comparative testing. Here the dyno results can provide the basis for component selection, based on the results. You might wonder, is a 1 7/8-inch header an advantage over a 1 3/4-inch set? Is a 750-cfm carb the best, or would an 850 prove better? Single-plane or dual-plane intake? The dyno will help answer these kinds of questions for your particular engine combination and intended usage. Be aware that valid comparative testing is time consuming, both in the wrenching time and the additional tuning time that will likely be required. Things like air/fuel ratio requirements can change dramatically when major components, like headers or intakes, are swapped. For the tests to generate good data, the tune has to be optimized for the combination. It's also important that the test regime is closely controlled, since factors like coolant and oil temperatures or the test rpm range can skew the results.
Just be sure to discuss the testing ahead of time, so that the operator can provide some insight on what can be accomplished in the allotted time, and how the test will be structured to make the best use of that time. Don't expect to show up on test day and surprise the operator with 12 intake manifolds. Explain to the shop what you'd like to test, and work out a plan to get that testing accomplished. You'll need to consider what additional items will be required to get the parts swapping done, such as gasket and sealants, fasteners, or special tools the shop may or may not have. The communication here is vital to a successful day on the dyno. It's good to prepare a checklist on what you'll need to bring.
A modern engine dyno is a data-gathering animal, and it can record or calculate a mind-boggling quantity of data. You'd have to be an expert dyno dude to know it all, but there are some key bits of dyno data it pays to be aware of. Besides the obvious temperatures and pressures, here's a breakdown of the most important stuff on a dyno sheet to know, and what it means.
Airflow SCFM: A measurement of the air volume taken into the engine. Airflow usually means power, so this is a number to pay attention to.
Brake Mean Effective Pressure (BMEP): A calculation that assigns a theoretical average cylinder pressure required to achieve the recorded torque. Cylinder pressure creates torque, so a high BMEP shows the engine is generating torque effectively.
Brake Specific Air Consumption (BSAC): Similar to BSFC, this is an efficiency calculation derived from the measured quantity of air ingested by the engine and the power output, given in lb/hph.
Brake Specific Fuel Consumption (BSFC): A calculation of the engine's efficiency derived from the horsepower and fuel flow rate. Given in pounds of fuel per horsepower per hour (lb/hph) the "brake numbers" give a good indication of how well the engine is working at making power, and in some cases can give the dyno operator insight into whether the air/fuel ratio is in the zone. BSFC will be lowest at near peak torque, with a number in the low 0.4s or high 0.3s there showing excellent efficiency.
Corrected Horsepower: The horsepower is calculated from the recorded torque, using the equation: HP=(TQ x RPM)/5,252. Corrected horsepower simply means that corrected torque was used to calculate it.
Corrected Torque: This is the actual twisting force applied by the engine, given in pound-feet. The corrected number brings the raw torquemeter reading to what the reading would have been if the testing was done under a set standard temperature, atmospheric pressure, and humidity. Correcting the number to a standard eliminates the variables that always exist in atmospheric conditions.
Correction Factor: Shows the correction factor applied to the raw or measured torque data to derive the corrected numbers. The two most common correction factors are STP and SAE. The correction factor will vary with the ambient conditions. On a 110-degree day with steaming humidity and a very low barometer, the correction will be high to correct the power numbers to the standard conditions.
Exhaust Gas Temperature (EGT): Some dyno installations are equipped with thermocouples that will read the exhaust gas temperature, which the dyno can record. The EGT can indicate dangerously high temperatures that can cause engine damage and point to mixture distribution problems. Sometimes it gives some clue to the air/fuel ratio, though the EGT can vary with so many other factors, like valve timing, that it takes uncanny experience to usefully interpret what they are telling you. It is virtually obsolete for tuning the mixture since the advent of advanced Lambda sensor systems.
Fuel Flow (lb/hr): Most dyno installations include a fuel flow meter that measures the amount of fuel entering the engine. Fuel flow is measured in pounds of fuel flow per hour.
Lambda A/F Ratio: The wideband Lambda or O2 sensor is the greatest thing to happen in the dyno world since the invention of the strain gauge. A Lambda accurately provided a very precise reading of the Air/Fuel ratio in real-time. Before the Lambda, the "super-tune" involved lots of guesswork and trial and error, and unbelievable skill to get right. This instrument spells it out.
Observed or Measured Torque: The actual torque reading delivered to the dyno as measured by the strain gauge or load cell. This is the raw, uncorrected number and can vary widely with the same engine if it is tested during widely varying atmospheric pressure, temperature, or humidity.
RPM: This is always the first column on a dyno sheet, and one that most everyone already knows. The same as the reading on the tach in your truck, it tells you engine speed in how many revolutions per minute the crank is turning.
Volumetric Efficiency (VE): Given as a percentage, this is a calculation that determines the percentage of air actually ingested compared to the displacement volume of the engine. A VE of 100 percent means that the engine is pumping 100 percent of its displacement volume at the given rpm point referenced. VE levels of well over 100 percent can be achieved in very trick race engines.