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The Basics of Internal Combustion The internal combustion motor 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 sidebar, "A Closer Look at the Combustion Process".) | A Closer Look at the Combustion Process Internal combustion is an exothermic chemical reaction involving gasoline and oxygen. Gasoline is a hydrocarbon molecule consisting of hydrogen and carbon atoms (HC.)* Oxygen molecules are composed of two oxygen atoms (O2.) Once the hydrocarbon and oxygen molecules are mixed together within the combustion chamber (cylinder,) a chemical reaction known as oxidation (or burning) is started by a spark from the spark plug which breaks apart the atoms within the HC and O2 molecules. The "free" hydrogen, carbon, and oxygen atoms combine to form new molecules – namely water (H2O,) carbon dioxide (CO2,) and carbon monoxide (CO) - that escape as exhaust gases. The important reactions are: HC+HC+O2+O2=H2O+CO2+CO* The formation of the new molecules releases heat (hence exothermic reaction). The gases within the combustion chamber absorb the heat from the reaction and react by expanding within the cylinder. The expanding gases act upon the piston by pushing it downward and setting the piston into motion. The kinetic energy of the piston – less any energy lost to friction - is transferred through the rods, into the crankshaft, into the flywheel, into the transmission, into the axles, into the wheels, and into the tires. Finally, friction between the spinning tire and the road surface sets the vehicle into motion. A powerful motor is one that releases large amounts of energy by creating a large chemical reaction. If you want more power, the recipe is "simple": combine lots of oxygen with lots of hydrocarbons and ignite. *Note: This explanation for hydrocarbons is simplified and assumes that each hydrocarbon molecule includes only one hydrogen and one carbon atom. In reality, hydrocarbon molecules can be very complex, including several carbon and several hydrogen atoms. The basic logic of the reaction remains the same. For a detailed online explanation, please visit: http://faqs.cs.uu.nl/na-dir/autos/gasoline-faq/part1.html |
Making Changes in Search of Power | Why Does Compression Ratio Affect Power? Put simply, increasing the compression ratio increases the theoretical thermodynamic efficiency of the engine. This is governed by the equation: Efficiency=1-(1/compression ratio)Ù gamma-1
where gamma is the ratio of specific heats at constant pressure and constant volume of working fluid. Notice that compression ratio is the denominator of a fraction that is subtracted from 1 to determine efficiency. As compression ratio increases, the value of the fraction decreases, causing thermodynamic efficiency to increase. As compression ratio decreases, the value of the fraction increases, causing thermodynamic efficiency to decrease. |
To make more power, one must simply do one of two things: increase the volume of the combustiblemixture within the cylinders or increase the compression ratio of the compressed mixture. 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.  | | Path of Air & Exhaust Through the S-Series Engine |
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? 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. | Velocity vs. Volume Have you ever witnessed water as it goes down a drain and forms a whirlpool that forces the water down the drain at higher-than-normal speeds? This is what many call the vortex effect. Because of the shape of the drain and the pressure of the water, the draining water tends to accelerate and create a vortex that "forces" the water through the drain at a rate faster than from gravity alone. This same phenomenon can occur within the intake manifold of a motor. If conditions are right, incoming air can actually accelerate and rush into the motor at a rate greater than the motor can pull it in by itself under vacuum. This is what many tuners are referring to when they mention "velocity." And it is this effect that causes the first peak in the DOHC Saturn’s torque curve just below 3000 RPM. However, the conditions that promote the vortex, or low-RPM velocity, generally include narrow passages and small ports. But these features limit the motor’s ability to inhale greater "volumes" of air at higher engine speeds where the motor’s own pumping efficiency is greatest. For higher peak power, a tuner must open the ports and passages so that the hungry motor is able to inhale large volumes of air without restriction at high RPM operation. Hence the potential trade-off. Although some parts will have little effect upon the final velocity of incoming air as it enters the combustion chamber, altering the head ports, valves, camshafts, and intake manifolds all have a direct impact upon air velocity just before it enters the cylinders. Understanding the relationship between velocity and volume are critical to tuning the motor to work best for its intended purpose. |
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. We do not blame the dyno shops for this. Instead, we take full responsibility for being unable to conduct scientifically meaningful tests. 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 engine 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 motor. 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.
 | | Kevin’s SC2 on the Dynojet at Passen Motorsports |
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.
 | | A real engine dyno has many advantages. However, using it does require removal of the engine from the vehicle! |
- Remember that the ultimate goal of these tests was to generate data for use in building a racecar motor, 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 links below to view the relevant test data.) 1. Baseline vs. cat-back exhaust with 4-chamber muffler 2. Baseline vs. cat-back exhaust with straight-through muffler 3. Chambered muffler vs. straight-through muffler 4. K&N drop-in filter vs. Powerstack short ram intake 5. Stock exhaust manifold vs. Hotshot tri-Y header 6. Baseline vs. combined Powerstack, 2-in mandrel exhaust, straight-through muffler, and headers 7. Stock intake manifold vs. Extrude-Honed intake manifold 8. Baseline vs. using no muffler at all 9. Removal of catalytic converter Beware the Magic Pill – Performance or Promises? Basic human nature causes all of us to want "something for nothing". In the case of our Saturns, we want lots of power, great fuel economy, long life, high reliability, etc. And we don’t want to spend any extra money to get it! Unfortunately, this desire puts some enthusiasts in search of the elusive "magic pill". You see it all of the time in the aftermarket: big promises of increased performance that seem too good to be true. And while we should know better, we are sometimes blinded by our desire to believe in the magic pill. At SPS, we often hear frustrating statements such as, "I was told that this 2-inch pipe provides more power than that 2-inch pipe"? Now that you understand more about combustion, you should know that this statement can only be true if one of the pipes allows comparably more hydrocarbons and more oxygen to enter the combustion chamber. At SPS, we make a serious (and expensive) effort to provide real data on performance. We do not believe in inflated claims or wish to promote unrealistic expectations. We sincerely wish to educate our customers about how a modification works – and why it works. And we believe that educated customers are less likely to become victims of the search for the magic pill! The next time you see a product with big promises of power, ask yourself the following questions: 1. How does this product affect the energy of the exothermic reaction within the combustion chamber?
In order to work, it must either increase the number of oxygen and/or hydrocarbons entering into the reaction, increase thermodynamic efficiency of the reaction (through higher compression ratios), improve efficiency of the spark catalyst that starts the exothermic chemical reaction, or reduce parasitic (frictional) losses within the drivetrain. 2. What enables this product to perform better than a competing product?
In order to work better, it must offer larger flow capacity, higher compression ratios, or offer some tangible characteristic that allows it to deliver more of its function from question number 1. Is there something specific about this part that allows it to perform better – or am I getting more promises of the magic pill? 3. Are the benefits – and trade-offs - from this product consistent with my intended use of the vehicle?
4. Has the seller of the product tested it in a Saturn application? Does the seller have an understanding of how the product will affect all elements of the motor’s performance?
5. Am I buying proven performance or empty promises?
Lastly, take responsibility for being knowledgeable about your car and asking the right questions prior to a purchase. These simple steps will help you prevent the search for the magic pill. Next Month: Fuel Delivery in a Performance Motor - What is the "right" amount of fuel
- The meaning of Stoichiometric
- Power vs. fuel economy
- How does a Saturn control fuel delivery
- Open loop vs. closed loop
- When you need more fuel and how to get it
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; "Path of Air & Exhaust Through the S-Series Engine")
