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.