This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
What performance 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, you can make a few runs on the dyno and actually measure the horsepower 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 of 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 you can compare dyno runs. The dyno works by placing a load on the car, similar to air friction as you drive 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.
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, derived from an early English standard:
Horsepower = torque x rpm/5252
This translates into a power relationship 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 values for power and wondered what they meant. European documentation often gives power numbers in kilowatts. For reference, one horsepower equals 0.746 kilowatts. BMW'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 about the same.
You may also have 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 an engine management system (Project 23) to match the power output correctly. In other words, if you find that your engine is running lean at 4500 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.
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 modified six-cylinder M3 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 in the engine cuts off the ignition system.
An unfortunate downside to dyno tests is that they often cannot be compared to one another accurately. For one thing, large dynos cannot be calibrated easily. 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 to 10 percent differently than the same model at another location. When you consider the figure may become bigger when you include dynos from different manufacturers, the ability to accurately compare results becomes significantly less useful.
Another important issue with respect to dyno figures is that environmental conditions heavily influence the test. 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.
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 engine management systems like the TEC-3 (Project 23), you really need extensive dyno testing in order to determine what your optimum operating parameters are on the fuel ratio and ignition timing maps. The factory used the same type of procedure to optimize and program the Motronic factory chips the stock engine management system uses.
In order to gain the most horsepower from your engine, you need to perform several dyno runs while varying different engine parameters (timing, mixture, advance curve, etc.). Don't forget the scientific method: Only change one variable at a time, or you won't know which change made the difference. 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 that knowing you are extracting the maximum horsepower possible can give.
Since dyno testing is performed using rollers under your car's drive wheels, some forces will reduce the power 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 E36 BMW, typical driveline loss estimates are often about 15 percent, although modifications to the chassis can raise or lower that value. Through a complicated process of calculations computed by the dynamometer, you can determine 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 on where you want optimum performance, you can install taller or shorter gears into any of the five speeds on your transmission. The results of a dyno test will give you specific horsepower numbers for each rpm range and allow you to tailor your transmission gearing to suit your desires.
This software is what I call the poor man's dyno. It plugs into your BMW's OBD-II port and estimates power and torque based upon a variety of factors. The AutoEnginuity software that you use to monitor OBD-II functions also has a very good dyno emulator built in. With preprogrammed profiles for almost all BMWs, it has proven itself to be extremely accurate in predicting engine performance. (See Project 29 for more details.)
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This photo shows Chris Bethel's 1995 M3 on a rolling dynamometer. Engine modifications include large bore throttle body, cold air intake, Jim Conforti chip, Stromung exhaust, lightweight aluminum flywheel, 3.2L M3 clutch package, Schrick 256/264 camshafts, and 24 lb. Ford Motorsport fuel injectors. For this test, the car was driven slowly through its full rpm range on the dyno while carefully recording all of the applicable data. Fans and air temperature/humidity measurement devices ensured that the environment remained constant between dyno runs.
Shown here is a typical dyno graph for the car in Photo 1. The graph shows rear wheel peak horsepower of about 242 hp. The rating for this engine in its stock configuration is 215 at the rear wheels. Note that per the relationship between torque and horsepower, they are equal when the rpm has reached 5252.