Comparative testing is an area where a dyno really earns its worth. Direct back-to-back testing is the best way to evaluate which components are right for a given combo. Enough time spent doing this type of testing is a real education for any engine builder. The parts combination for this big Hemi at Speed-O-Motive was largely developed through dyno testing on their DTS system.
Hunting for Power
If the groundwork has been done in advance, the test can be an efficient way to find power. Normally, once the engine is started, it will be inspected for any signs of problems, such as abnormal gauge readings, or noises. If a problem is suspected, the engine should be quickly shut off to make repairs or corrections, if needed. The coolant flow will be confirmed, as will oil pressure, mixture, and ignition timing. If it is a new engine, a break-in cycle will be initiated, and the engine brought to operating temperature. If you have specific requirements on running or break-in, make sure to inform the operator in advance. For instance, if the engine contains a radical high-lift mechanical camshaft, it may have been assembled with light break-in springs, which will be swapped for higher rate units when the wear pattern is established. In this case, the engine might require keeping the engine speed below a certain upper limit until the springs are installed. Don't assume the operator will know your engine's specific requirements-tell him in advance. After the warm-up and run-in, the engine is often shut down to make some additional adjustments, such as the hot valve lash, before any power pulls are made.
If the engine is going to be run with an exhaust, it is worthwhile to tune it on the dyno with the same components that will be run in the vehicle. Some dyno installations have their own dedicated exhaust system and mufflers, which preclude the use of another system. Discuss your requirements with the shop.
Though the temptation is there to just let it rip to see what an engine will do, a more prudent approach is to initiate the testing with a series of static pulls to collect initial data. In a static test, the engine is brought up to wide-open throttle, while the dyno holds the load to a constant rpm. Normally, the first static pulls will be at the lower end of the operating range. The static pulls will provide data on the air/fuel ratio, and uncover calibration errors before a full dyno pull is made. If a problem such as too lean an air/fuel ratio or audible detonation are apparent during the static test, an experienced operator will quickly abort the test, and corrections will need to be made. If the readings look good, the static rpm level will be raised, making similar checks higher in the operating range. After confirming that all is well in the static pulls, it's time to establish a baseline.
A peek inside the DTS "tower" reveals the two starters built into the dyno. The self-contained starter system is a time-saver on setting up the engine on the dyno, since the vehicle's starter is not required, nor is a bellhousing.
In a sweep test, the dyno is set to test the engine over a specified rpm range and acceleration rate. Modern dynos, such as the DTS, will automatically control the load to allow the engine to accelerate through the rpm range as specified by the control settings on the console, while measuring torque and deriving power output. At the same time, the dyno will be gathering other data through various channels, depending how the dyno is equipped and instrumented. If the first sweep test is completed successfully, a baseline has been established, and the numbers and data from the baseline can be used to compare with results of subsequent tests. A DTS dyno can provide this information graphically or in detailed numerical charts.
Controlling the dyno is the servo valve. The DTS unit is custom designed and is controlled by a very fast-acting electronic motor that constantly adjusts and precisely opens and closes the valve during testing. This in turn controls the load applied by the absorber.
Once the baseline is established, changes can be made and the engine re-tested to determine if the change results in an improvement. Typically, the first changes will be related to the tune-up, including the mixture and timing, however, only one thing at a time should be changed. If the mixture is looking in the zone for what normally produces best power with similar engines, a timing loop may be the first tuning step. The conservative approach is to remove timing, which is always safer, and then re-test and compare the outcome to the baseline. If the power is improved, more timing may be removed, however if power declines, the next test may be to try more timing than the baseline setting. Using this procedure, the optimal timing can quickly be established. Similarly, the mixture can be addressed with a mixture loop. Again, the conservative approach is to start by trying a richer mixture to see if that direction improves output. If not, jetting is taken in the other direction, leaning the mixture and again zeroing-in on the optimal setting. It's not unusual to find substantial power improvements in the timing and jetting loops. If parts swapping is not part of the program with the tune dialed-in, the basic dyno test is completed. Additional tuning may be performed to dial-in the part throttle and ignition curve advance rate.
While simply running a completed combination for a number is definitely worthwhile, one of the greatest assets of dyno testing is the ability to perform comparative testing. Here we are talking about parts swapping, be it various headers, intake manifolds, carbs, or even more elaborate components such as cylinder heads, camshafts, or valvetrains. In comparative testing, you are only limited by your imagination, the available time, and the parts on hand. Again, it is important to discuss plans for additional testing with the dyno facility, to coordinate a testing plan. Sometimes certain parts will be optimized with the addition of further components, so the sequence of the changes is important. Here, the value of an experienced dyno shop adds value in the planning and execution of the testing. Whether you just purchased an engine, or if you're an aspiring builder or a seasoned pro, there is always something to be learned on the dyno.