Making your Engine Pull Stumps
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.