The Basics of Internal Combustion The internal combustion engine is, in some respects, simple. As pistons move downward, they draw air into the combustion chamber. The fuel system combines this air with gasoline (or other combustible hydrocarbon-based petroleum products.) The piston compresses the mixture on the up-stroke. Then, at just the right moment, the ignition system ignites the compressed mixture that begins a chemical reaction between the fuel and air in which energy is released. The released energy sets the piston into motion – which sets the crankshaft into motion - which sets the flywheel into motion – which eventually sets the tires into motion. (For more details about the chemical reaction that occurs during the gasoline combustion process, please see April 1999 tech article, "Making More Power" .) Making Changes in Search of Power Most of the power-related products that are available to enthusiasts on the aftermarket are designed to increase the volume of the combustible mixture within the cylinders. This is often possible by reducing the restrictions to the motor’s natural inhaling and exhaling process. However, the plumbing system through which the motor inhales and exhales is elaborate. Air first must pass through an air cleaner - then through a passageway to the throttle body (or throttle-control mechanism) – through the throttle body - then into the intake manifold – into the head – past the valves - and into the cylinders. After ignition, the spent gases get pushed through the exhaust valves - back into the head – into the exhaust manifold – through the catalytic converter – through a long series of exhaust pipes – into the muffler –and finally into the atmosphere. Any and all of these pieces may be restricting the volume of potential airflow. And improving the flow in one area may create a more significant bottleneck to another area. So how is an enthusiast to know which restrictions are most relevant at various times throughout the modification process in order to make a tuning decision? To make more power, one must simply do one of two things: increase the volume of the combustible mixture within the cylinders or increase the compression ratio of the compressed mixture. Knowledge Through Testing Unfortunately, no one will ever completely understand the dynamics of airflow within a particular motor. But careful testing and collection of data about how a motor responds to changing inputs can create a picture – or model - as to how a motor behaves. Using this model, we can then make reasonable decisions that are likely to lead to increased performance. Respectable tuners - such as SPS – attempt to collect such data using dynamometers, racecars, and other tools in order to create a workable model and guide customers in an appropriate direction. But collecting accurate data that can be used with confidence to draw conclusions is very difficult. There are many conditions that can greatly affect the data output of a dynamometer test such as temperature, humidity, fuel quality, condition of the dyno equipment, etc. Eliminating these variables as contributing factors to the test results requires great skill and care – so much so that car companies (such as Saturn) pay professional engineers to conduct their tests under stringent conditions.
After trying ourselves on many occasions to use various chassis dynamometers, we learned first-hand that the meaning of the results is not always clear. They say that, "The dyno doesn’t lie". And certainly our attempts at conducting dyno tests produced lots of legitimate output data. But were we really measuring the effectiveness of a known modification, or were we only seeing the results of unknown, poorly isolated variables? As evidence of the latter, there were instances in which the same dynamometer would give us varying results even though nothing had changed on the car! This is certainly reason to question the validity of our data that had been generated on chassis dynamometers. But after several years of experimenting, we have finally learned the secret to conducting an accurate test: pay someone that knows what he’s doing. Some Things are Best Left for a Professional By matters of coincidence, we recently crossed paths with such a person by the name of Mark Womack. You may better know of Mark as the guy from Sunbelt Performance Engines that built the motors for the ICY Racing World Challenge T2 SC2s, prior to ICY Racing's unfortunate parting with Saturn Corp. In his spare time, Mark still builds racecar motors. And because of his "hobby" of building racecar engines, Mark maintains and operates a real engine dynamometer in his garage! (And you thought that your wife threw a fit about the stack of racing tires that you keep next to the lawnmower…)
Because of Mark’s exemplary reputation as an engine builder, we asked him to build the scR motorsports motor for the 1999 season. He agreed (for a price, of course) and convinced us to begin a series of tests in order to determine the baseline characteristics of the "stock" 1.9 liter DOHC Saturn engine. While doing these tests, he agreed to test some of the popular SPS parts too. Details of our Tests The results of these tests are posted below. But before you start jumping to conclusions, understand a few important points: - These tests were done on an engine dynamometer, not a chassis dynamometer. So they reflect output figures generated at the crankshaft, rather than the wheels. They do not reflect parasitic (frictional) losses within the drivetrain.
- Because development work is time-consuming, and because Mark’s expertise does not come cheaply, certain logical compromises and assumptions had to be made. We did NOT test each part in an otherwise stock configuration. Instead, Mark made an educated guess about the purchasing behavior of the "average" Saturn enthusiast and added parts accordingly. So each test builds upon the previous run. Admittedly, in the real world, you may get more or less power by using certain parts in different combinations. If you’re smart, you will not get hung-up on the details – look at the trends and overall behavior of the motor to determine what makes sense to do - and in what order.
- The "baseline" test included a K&N filter within the stock airbox. In previous tests, the K&N filter added 2-3 HP. Because of this, the comparable results of some other parts tested here (such as the Powerstack) may seem underestimated.
- Remember that the ultimate goal of these tests was to generate data for use in building a racecar engine, not to entertain or educate SPS customers and Saturn enthusiasts. As a result, you may sometimes find more or less information than you want. (For example, data collection starts at 4000 RPM. Why? Because data below 4000 RPM is useless to the racecar which only runs below 4000 RPM while it is waiting on the grid for the start.)
- In addition to torque and horsepower figures, Mark also collected data on oil temperature, water temperature, airflow rates, and fuel ratios. We posted this to satiate your appetite for data. Enjoy it, but don’t lose sleep over it.
- The points of maximum output and the point of the greatest gains are highlighted in red. Note that the greatest gain does not often occur at the same RPM as the maximum output.
The Results (Click on the item titles below to view the relevant test data.) |
