What project book would be complete without a section on dynamometer testing? One of the neatest trips you can make is to your local dyno shop. For about $100 or so, you can make a few runs on the dyno and actually measure the horsepower that is generated by your engine. The whole process is somewhat complicated, with varying degrees of detail and accuracy, but for the sake of this section, we'll just cover the basics.
What is a dyno? Short for dynamometer, the dyno measures the horsepower output by your engine. There are two basic types of dynos, one that you bolt the engine up to and run, and one that measures horsepower at the rear wheels of your car. This is also called RWHP (rear wheel horsepower). Most modern dyno testing is performed on a rolling dyno that measures the power output at the wheels. You drive your car onto big rollers and accelerate at full throttle until you reach your rev limit. Then, you let the clutch out and let the rollers spin down freely. Large fans and environmental controls aim to keep the test environment at a steady state so that you can compare dyno runs. The dyno works by placing a load on the car, similar to how you would experience air friction as you were driving down the road at high speeds. By measuring this load, combined with the total rpm of the vehicle, a graph of the power output by the car can be derived.
Torque / Horsepower: The dyno actually measures the torque output by your rear wheels. Torque is a measurement of rotational force and is related to the overall power output by your engine. The horsepower output by your engine is equivalent to the following formula, which is derived from an early English standard:
Horsepower = Torque x rpm / 5252
This translates into a power relation that horsepower is defined as 33,000 ft-lb (force) per minute. This is also referred to as the horsepower definition as defined by the Society of Automotive Engineers (SAE horsepower).
You may have also seen other definitions for horsepower and wondered what they meant. European documentation often gives horsepower numbers in kilowatts. For reference, one horsepower equals 0.746 kilowatts. Porsche's ratings are often listed in the European standard of DIN HP or kilowatts (kW). One DIN horsepower is rated as the power required to raise 450,000 kilograms one centimeter in one minute (or about .73 kW). The values of SAE and DIN horsepower are very similar, with 1 SAE HP being equal to .98629 DIN HP. For all practical purposes, you can think of them as relatively the same.
You may have also heard the term brake horsepower (BHP). Brake horsepower is measured at the flywheel of the engine with no load from the chassis, without any electrical or mechanical accessories attached, under ideal fuel and timing conditions. In modern terms, the brake horsepower figure would be mostly associated with what is now called gross horsepower.
Air/Fuel Measurement: In addition to measuring output torque and rpm, some dynos can also monitor your air/fuel mixture. This will allow you to adjust the mixture tables on a custom engine map to correctly match the power output (see Pelican Technical Article: Updating Your DME with Performance Software for the Porsche 911 Carrera). In other words, if you find that your engine is running lean at 4,500 rpm, you can adjust the fuel injection mixture to richen it up and produce more ideal combustion. This translates to more horsepower output from the engine.
Dyno Results: The dyno will generate a graph of horsepower versus rpm for the engine being tested. With this graph, you will be able to determine the engine's peak horsepower and peak torque. The graph will also show you the peak horsepower output from the engine. On a six-cylinder 911 Carrera engine, this will typically be at the higher end of the rpm range, near 6,000 rpm. The engine will peak in horsepower and then fall off dramatically as the rev-limiter kicks in and the engine cuts off the ignition system.
Comparing Results: An unfortunate downside to dyno tests is that they often cannot be accurately compared to one another. Environmental conditions play a large part in these variances, as well as the fact that the large dynos cannot be easily calibrated. As a result, tests from the same dyno with the same car on different days may produce different results. Even the manufacturers of some dynamometers admit that their dyno at one location may test 5-10 percent differently than the same model at another location. When you consider that the figure may become bigger when you include dynos from different manufacturers, the ability to accurately compare results becomes significantly less useful.
An important issue to mention with respect to dyno figures is that the test is influenced heavily by environmental conditions. This includes, temperature, humidity, and altitude, to name a few. Since conditions may change from day to day, dyno runs that span multiple days may produce different results.
Engine Optimization: As previously mentioned, dyno testing can be very subjective. Other than bragging rights, pure dyno numbers are not very useful. The true benefit of the dyno test comes when you are able to use it to optimize your engine. Particularly with software ECU flashes (Pelican Technical Article: Updating Your DME with Performance Software for the Porsche 911 Carrera), you really need extensive dyno testing in order to determine what your optimum operating parameters should be on the fuel ratio and ignition timing maps. The factory used the same type of procedure to optimize and program the Bosch Motronic factory maps used in the stock engine management system.
In order to gain the most horsepower out of your engine, you need to perform several dyno runs while varying many different engine parameters (timing, mixture, advance curve, etc.). Only after carefully analyzing the data can you determine what the best values are for your engine management system map. Measuring the power output of the engine will allow you to optimize your engine and get the peace of mind knowing that you are extracting the maximum horsepower that you possibly can.
Driveline Losses: Since the dyno testing is performed using rollers on your car, there are going to be forces that are going to slow down and reduce the power in between the flywheel and the rear wheels. These driveline losses include friction from the transmission, losses from brake discs dragging slightly, and friction in the wheel bearings. On the Carrera, typical driveline losses are often estimated at about 15 percent, although modifications to the chassis can raise or lower that value. Through a complicated process of calculations that are computed by the dynamometer, you can calculate your driveline losses by counting the time it takes the dyno rollers to stop when you let out the clutch. Using these calculations, you can then estimate what your horsepower output is at the flywheel.
Transmission Gearing: One of the benefits of dyno testing is the ability to design your transmission ratios to meet the exact power characteristics of your engine. Depending upon where you want optimum performance, you can install taller or shorter gears into any of the five or six speeds on your transmission. The results of the dyno test will give you specific horsepower numbers for each rpm range and allow you to tailor your transmission gearing to suit your desires.
Software Dynos: This is what I call the poor man's dyno. It is software that plugs into your Carrera's OBD-II port and estimates power and torque based upon a variety of factors. The AutoEnginuity software that you can use to monitor OBD-II functions also has a very good dyno emulator built in. With pre-programmed profiles for the Carrera, it has proven itself to be extremely accurate in predicting engine performance. See Project 20 for details.
This photo shows a Porsche GT2RS that has been modified by Global Motorsports Group. The GMG upgrades on the car include suspension, wheels, intercoolers, a full exhaust system and software upgrades. Even though the GT2RS is two wheel drive, top of the line Dyno shops like Global Motorsports Group will have four wheel drive dyno (as shown), which can really come in handy if you are running a four wheel drive car.
Shown here is a dyno graph for three separate runs of the GT2RS fromPicture 1. The graph shows rear wheel peak horsepower of 483 and a peak torque of 525. Conditions are tightly controlled using fans and air temp/humidity measurement devices to ensure that the environment remains constant between dyno runs. Even with the tightest environmental controls there will be slight deviations in the results as shown by the graphs.