Car Modding, Car Tuning and the Science of Horsepower: Terms and Concepts
You insert your key slowly, one notch at a time. As the dash comes to life your skin tingles with rapid anticipation. The sports cloth bucket seat and five-point harness you just installed hug your body close. She’s ready for you! The soft glow of red interior lighting is like fire all around you and as you turn the key, the machinery of your hopes and dreams becomes a reality. Her heart shudders violently in its cage and you can feel the power all around you, under you, in you. With the slightest hint of fear and hesitation you reach for the short throw shifter. No, it’s ok. She trusts you. You know what you’re doing. With renewed confidence you wrap your hand firmly around the shifter and put it in gear. Oil pressure’s good… O2 sensors should be coming up to temp now. Everything seems right. You take a deep breath and turn your eyes to the road as you prepare to dump the clutch.
“Here we go!”
If you’re like me, just reading that paragraph was enough to make you want to drop what you’re doing and go for a drive. Doesn’t really matter if you’re driving a Grand Am GT, a Corvette or a Smart Car… ok it does matter. Stop reading this if you drive a Smart Car. For the most part though, it’s the thrill of the ride, the love for technology and the joy of improving on something to make it better, faster and totally yours, which motivates us car enthusiasts to do what we do.
If you’re new to the auto world, I envy you. It’s something like losing your virginity. The first time you mod your car and feel that extra power under your foot is like nothing else. However, being new, how do you begin? It’s all so overwhelming!
Relax. Let’s start with the basics. In order to do anything to your car you have to understand how it works. I won’t cover everything since you should have the most basic of knowledge to begin with if you are serious about this but there are a lot of concepts and terms that will help you down the road. Let’s get started, shall we?
Internal Combustion Engine Theory – Gas is made up mostly of iso-octane and heptane, sometimes with ethanol added as well. This fuel is only flammable in the presence of an oxidizer. Normal air contains 21% oxygen, which is the only oxidizing component. Air mixes with gas and is drawn into a cylindrical chamber with 1 moving boundary, known as a piston. The piston draws the combustible fluid into the cylinder, the entrance (valve) closes and the piston moves up, causing the volume of the chamber to decrease and the pressure to increase several-fold (usually anywhere from 9x to 13x). In accordance with the laws of thermodynamics, the piston (by virtue of the engine) must do work to the fluid in order to compress it. Compressing it increases combustion temperature, pressure and efficiency. Upon undergoing combustion, the fluid now has a higher temperature and pressure than it did a moment ago because it is chemically different and the reaction released energy known as “heat of combustion”. Combined with the inertia of the falling piston, the exhausted fluid pushes down on the piston, doing work on it. The work on the piston depends on the instantaneous pressure and change in piston displacement but it is much greater than the work the piston applied to the fluid earlier. It is this net change in work that supplies torque to the crankshaft via the piston and ultimately, makes the car go.
Horsepower – We’ve all heard this before. It’s a big marketing feature. But horsepower isn’t the be all and end all in performance. Far from it. My car with its pitiful 175 HP can beat other cars with 200 HP or more. Why is that? Well it’s because horsepower is a “product” of 2 other things, engine speed and torque. It’s a measure of “power”, energy per unit time. Since engine speed is based on the gearing in your car you can demand pretty much any engine speed within mechanical limits. A faster engine speed will yield a higher horsepower value for a fixed torque. The problem is, the torque isn’t fixed. Let’s get to that.
Torque – Torque is the FORCE exerted by the engine as a result of combustion heat and pressure. It is the source of the car’s power. You need to maintain a high level of torque throughout the entire RPM band to achieve high performance from your car. That’s the ticket. A nice broad torque band will yield a lot of HP, a lot of power. If you were to graph torque vs RPM (engine speed), you want to see a large AREA under the graph. With careful intake/exhaust tuning you can get a spike in torque at a high RPM, which will produce a high peak HP value but if the area under the torque curve is pathetic, your car will always be less powerful than another with a flatter torque response, reporting a lower peak HP so don’t think you can race and beat anyone whose car has less advertised HP than you.
Traction – Remember in school how you learned about friction? Well this aint friction. Friction is completely INDEPENDENT of the surface area between 2 surfaces. Traction isn’t. Traction is mostly a combination of the inherent properties of the rubber and road (the friction part) as well as the geometry and contact area. Since rubber can deform around imperfections in the ground, it “grabs” the ground. This makes wider tires better than narrow ones without changing the rubber because wider tires grab more of the road. That is, they have a larger “footprint”. Traction is very important because all the horsepower in the world won’t mean a thing if you spend 2 seconds spinning your tires at the starting line. Rubber temperature is also important, as optimal temperature allows better deformation, grip and sometimes “tack” as well. Super hot isn’t exactly desirable either.
Drivetrain – This can be further subdivided. First of all you’ve got front-wheel drive (power goes to the front axle only), rear-wheel drive, all-wheel drive and some other niche types I won’t get into. As a rule of thumb, FWD offers better steering control, RWD offers better launch characteristics in reasonable conditions and AWD offers a blend of benefits but requires greater complexity and usually some additional drivetrain losses. On top of that you can have an “open” differential or perhaps a “limited slip” differential. The differential allows power to go to 2 wheels but allows them to turn at different speeds. A limited slip diff “tries” not to let the wheels turn at different speeds, an advantage when you’re racing in a straight line, but if you turn the car, the diff will allow the wheels to turn differently, a good thing when you don’t want to lose control of your car during a turn. These configurations all affect how power is transferred to the ground and where the forces are exerted on the car. In your transmission you have several gears (unless it’s Continuous Variable Transmission) and each gear has a gear ratio (the ratio of mechanical advantage with respect to the engine) and gear length (the RPM range over which that gear operates). What you strive for is to get maximum force from the wheels to the road in every gear. In a given gear, force will fall off towards the end of that gear. You want to switch to the next gear only when the force exerted at the beginning of that next gear is higher than the force exerted in the current gear.
Induction System – Everything on your car involved in “breathing in” fresh air. It includes tubing, usually an airbox, a filter, a throttle body and throttle plate, an intake manifold, runners, heads and valves. Sensors include an intake air temperature sensor, a MAF sensor (see below), a MAP sensor (below), a throttle position sensor, and perhaps others. The induction system is a considerable restriction in your airflow so this is where most people start when they look to gain performance. Cutting everything off or making the tubing huge is tempting but, believe it or not, you can get better performance by leaving it on and just “tuning” it with correct geometry and materials.
Exhaust System – Everything on your car involved in expelling exhaust. This includes the headers (pipes that collect exhaust from the cylinders), exhaust piping, 1 or more catalytic converters, perhaps a crossover and a muffler or resonator. Sensors include O2 sensors and catalytic converter sensors. Again, as with the induction system, you can cut everything off for extra HP but you gain much more by carefully controlling the geometry for a “tuned pipe” effect. In fact, a bit of back pressure can increase torque. Besides, exhaust is one of the best features of the car when it comes to the sound it makes!
Fuel System – Consisting of a fuel pump, fuel filter, fuel rail (which carries the fuel), fuel pressure regulator and fuel injectors, this system controls the delivery of fuel to the cylinders. Any power adders need to take the fuel system into consideration. Let’s throw the spark plugs in here as well since they are somewhat related. Spark plugs have a temperature rating that tells you how hot the tips get during operation. The more power the engine produces with the same displacement, the hotter the plugs tend to get, which could lead to pre-ignition of the fuel so get colder plugs if you plan on adding serious power. Colder plugs are usually copper, as opposed to platinum or iridium and have a flatter tip surface.
Valvetrain – The cams, rockers, pushrods, valves, valve springs and other tiny parts make up your valvetrain. This is what controls the little doors that let the air/fuel in and the exhaust out. The cams, essentially long shafts with lobes on them, push these valves open at a specifically designated time, based on the lobe position, push them open to a certain depth, based on the height of the lobe and with a certain speed, based on the curvature of the lobe. Cam design has a large impact on flow characteristics of air/fuel and exhaust as well as performance.
AFR – Air/Fuel Ratio is the mixture of air to gas inside your engine. A “perfect” chemical mix is called “stoichiometric” or “stoich”. That ratio is 14.7:1 for gasoline. It’s not the same for other fuels though. The car has sensors that normally try to maintain that ratio but it’s not perfect. Thus small corrections are made to keep it as close as possible based on a plethora of environmental and performance criteria. A value lower than stoich is “rich” and means there’s more fuel than needed. A value higher than stoich is “lean” and means there’s less fuel than needed. All 3 conditions are important in their own right.
Spark/Timing Advance – Hopefully you understand that a spark plug ignites the gas in your engine cylinders. It does so at a precise moment, measured by the angle of crank rotation with respect to “top dead center”, the point when the piston is at the top of its rotation. Spark is measured in degrees prior to this point so 23.7 for example indicates that the spark plug is firing when the piston is 23.7 degrees prior to reaching its highest position in the cylinder. GENERALLY speaking, the more timing (the earlier the spark goes off), the higher the cylinder pressure you can get, the more power you can get from a given charge of fuel. This isn’t always the case though as will be explained later.
Timing Tables – Usually there are 3 of them. A “Good” table, a “Bad” table and a “Mean Best Torque Spark” table. The good one is the optimistic case, when high octane fuel is used. The bad one kicks in when low octane fuel is used or there’s some other problem causing lots of knock. The MBTS table is your target values for maximum torque. All tables are 2 dimensional. They depend on RPM on the x-axis and cylinder air mass on the y-axis. They are nothing more than excel tables though, with each cell of the table containing a spark advance angle. The trend/tendency is that spark will generally increase with RPM and decrease with air mass. This is just the trend though and does not always hold. That’s because the engine does not perform at the same efficiency everywhere. The pattern can and does reverse itself in places where the engine operates better or worse.
Knock and Knock Retard (KR) – Knock is an event which occurs when a sudden and violent rise in cylinder pressure tries to make the piston travel backwards. This event can range from almost harmless to “my engine blew up”. There are many causes. The mixture could be too lean. The spark could be too far advanced. The engine temperature could be too high. The load on the vehicle could be too high. Many, many reasons. Diagnosing knock is no trivial task. In the mean time, the car has a built-in failsafe to protect you from this. It’s called KR and it is a computer driven correction to your spark tables that removes timing (lowers the degrees) until the knock goes away. The knock itself is detected by a small piezoelectric microphone that analyzes the engine’s vibration. When it detects a knock pattern, it starts to trim timing until the knock goes away and then it slowly adds it back again, keeping the car in the safest condition possible, given the default values.
O2 – The oxygen level in your exhaust pipe is monitored by the O2 sensors and by the computer. It is a voltage divider that returns a low or high voltage indicating an excess or an absence of oxygen respectively. This tells the computer how to adjust the AFR described above. It is a closed loop system. In other words, the computer attempts to guess a good value for the mixture based on the MAF (below) and tries it out. The O2 sensors figure out whether the chosen mix was rich or lean based on oxygen content and FEED BACK the data to the computer, which adjusts the future AFR to compensate and so on and so forth.
Fuel Trim – Fuel Trim comes in 2 flavours, long term and short term. One is long lasting, the other is fast acting, kind of like allergy pills. The fuel trim is a numerical representation of how much the computer needs to compensate for a given fuel mixture. If the mixture is really rich for example, you’ll see a big negative number like -15. This means “Take away 15 units of gas... there’s too damn much!”. First this will show up in the short term trim and then you’ll see the long term trim lag behind and catch up, provided the short term trim stays where it is. Thus the long term trim “learns” from the short term one and the computer has a limited memory for this. The trims are in place to make day to day adjustments to the fuel tables because of things like the properties of the air (winter vs summer for example). It’s also noteworthy that the O2 sensors play a part in setting the trims.
MAF (Mass Air Flow) – The intake plumbing in your car (where the airbox is) contains a MAF sensor, which detects the MASS of air entering the car at any time. This is extremely important because it tells the car how much gas to inject. It’s usually measured in hertz (Hz) and is non-linear… and confusing. You have to find a conversion table to convert these values into something meaningful like grams/sec.
MAP (Manifold Absolute Pressure) – A “somewhat” redundant sensor, this is still quite important and it measures the absolute (above 0) pressure of air in the manifold, where the air gets split up into each cylinder. It is analogous to engine load, since more pressure usually means higher throttle and more acceleration.
PE (Power Enrichment) Mode – Remember AFR? Keeping it at stoich is great for fuel economy but when you floor it, this ratio is no good anymore. Why? First, the engine is under high load and this makes it hot. Hot engines are not good. They cause pre-ignition of the fuel and knock. To counter this, you need a coolant. Excess gas is actually a decent coolant because it cannot burn (for lack of oxygen) so it’s just extra liquid soaking up heat in the chamber, keeping the temps down. Chemists will tell you that anything other than stoich produces less power of combustion. This is true when you look at both air and fuel as changing quantities. However in an engine, you don’t have less air and more fuel. You have the SAME air and more fuel. A little extra gas doesn’t lose you anything and in fact you get more power up until a point. Tweaking the ratio used in PE mode is key when you modify anything related to the engine or fuel system. The other thing is that flooring the car makes the RPMs shoot up. High engine speeds mean everything has to occur much faster. The atomization of fuel through the fuel injectors is somewhat inefficient and proper atomization becomes more of an issue when there’s less time for it to occur. Adding more gas in PE mode ensures an excess of gas droplet surface area exposed to the air so more of it will get ignited. This compensates for the inefficiency of running under load.
AE (Acceleration Enrichment) – This is a table that commands extra fuel instantly when you press the accelerator. The idea is to rapidly anticipate a greater demand for gas. When you hit the accelerator quickly (as opposed to slowly) it changes the airflow into the engine very suddenly. This could cause a brief lean condition, which could damage the engine. The AE table takes sensor data from the accelerator to figure out how fast you pressed it (not how much) and to add extra gas right away for a short time, to meet up with the sudden burst of air about to enter the cylinders.
Injector Pulse Width – Fuel injectors are on/off devices. They have a constant pressure driving them and open for a variable amount of time during each pulse, depending on demand. This is also known as the duty cycle of the injector. When you start modding your car you may increase the capacity of the engine to take in air. Adding a supercharger will do this for example. The problem is that the duty cycle of an injector can only go to 100%, which means it’s always on (called “running static”) and never turns off during each pulse. Beyond that you don’t get any more gas, your engine leans out and you have an expensive explosion. You need to keep this in mind because some mods require you to get bigger injectors (which can flow more gas) and a fuel pressure regulator, which will supply greater pressure to those larger injectors so that their flow reaches maximum potential.
VE (Volumetric Efficiency) – This is tricky to explain. It’s another 2 dimensional table consisting of manifold pressure and RPM, similar to the spark tables. This time though, the cell values are not angles. They are efficiencies represented as decimals (1 being 100% efficiency). Imagine a cylinder cut in half. As the piston goes from top to bottom, the volume of air it sweeps out is the theoretical capacity of the cylinder. For most engines, most of the time, you don’t get all of the air that space could hold because the engine has to suck the air in and that creates a bit of a vacuum. Thus the air that enters the cylinder represents a slightly smaller volume than the geometry would suggest. This produces a volumetric efficiency of less than 1. Sometimes however, when you have a tuned induction/exhaust system or a forced induction system (supercharger, etc.), more air will enter the cylinder than it should normally hold and you get a VE greater than 1. This table is used as a sanity check, especially when the car is under high load, so that the computer knows how to adjust fuel and spark values to respond to anticipated cylinder pressures.
Open/Closed Loop – As stated earlier, the car is usually running at stoich due to a feedback loop involving the O2 sensors. This is called closed loop mode. Because of how fast things occur when you floor the engine, it’s hard to maintain closed loop so the computer shuts that off and relies more on lookup tables and the knock sensors. This is called open loop. Open loop is also triggered when PE mode is enabled. When you tune your car you can choose when this occurs, usually as a function of throttle response (ie. The pedal’s position is measured as a % of being fully floored)
Torque Management – I hate this garbage. It’s a set of computer algorithms that detect when you’ve overstressed the drivetrain. It’s meant mostly to protect the transmission... which is good, don’t get me wrong. It’s one of those things that promotes durability of the vehicle, which is to say you can usually tone it down or remove it without causing damage, in the short term. Before messing with this I highly recommend checking all of the specifications of your hardware to be sure that your car can handle it. The effects of torque management are to augment the fuel and spark settings, to intentionally rob the vehicle of power, when it thinks there’s too much.
An Application of Torque Management
Transmission Shift Time – This is the commanded time it takes to shift an automatic transmission from one gear to another. It’s just a command though and the transmission itself will try to obey but might not be able to if there’s too much load on the car. See below for an explanation.
Transmission Shift Pressure – If your car has an automatic transmission, the shifts take place by moving hydraulic fluid around through different channels, causing bands, plates and clutches to engage/disengage. The fluid pressure controls how aggressive these events are. Upping the pressure can prevent your transmission from slipping when engine torque is high. This becomes increasingly desirable as you add more power to your car or start reducing shift time, as it allows the tranny to “bite” into the next gear better.
Transmission Slip – Whether you have an auto or a manual transmission, there are friction components that make shifts occur. These are called clutches (bands and clutches in autos). When you try to shift gears, the clutches press against each other at different speeds and try to reach the same speed through frictional braking. If the clutches are not rated for the power of the car or they are worn, they will slip. All clutches slip actually but we’re talking excessive slip. Excessive slip causes slow, whining shifts and generates heat and debris in the transmission oil. This wears out the transmission, makes the car run terribly and could lead to failure. Make sure you have the right transmission components for the job.
EGR (Exhaust Gas Recirculation) – This is a part of your emissions system. It reduces NOX production by lowering cylinder temperatures necessary to produce these nitrogen species. It works by injecting exhaust back into the engine again. Heh.. hehe… hehehe. I turn it off at the track because I don’t want exhaust going back into my engine again. Honestly, it does not have a big impact on performance because it shuts itself off automatically when you floor it. However I still believe that response time is everything so I turn it off.
Torque Converter – This resembles a turbine inside and works by transferring power from the engine to the transmission by pumping a viscous fluid from one set of blades to a nearby set of blades. The viscosity of the fluid transfers shear force between the blades and makes the output side move in response to the input torque. In this way, you get torque transfer without any physical contact. That’s why autos are so smooth feeling. Most contain a clutch though that can lock to produce a mechanical coupling.
Torque Converter Clutch – In automatics, this thing removes the fluid coupling of an auto and locks the input and output together, essentially making it behave like a manual transmission. It only happens when you’re cruising at a stable speed and it improves fuel economy. It’s important to know about because when you’re cruising (and it’s locked) and you suddenly floor it, the computer has to tell the clutch to unlock because the clutch isn’t made to handle that much stress. This process is slow and may take a second. During that time the car bogs and then kicks in all at once. If you try to pass someone on the highway you’ll know what I mean. For performance applications it’s best to tune the torque converter to be off the whole time. This eliminates any bogging effect but your mileage will decrease so change the tables again when you hit the road home. If you feel like risking damage to your car you can intelligently enable it during full throttle, at the end of a gear, to extend the range of that gear for some extra torque at the top end but I wouldn’t recommend it.
WOT – Wide Open Throttle. This means the pedal is 100% down. Some special things happen in a car’s computer when this is detected.
PCM – Powertrain Control Module. It has some other names too but it’s basically a chunky, heavy, dirty, metal box containing a computer processor from the 80s. It’s crude but very reliable. It contains some limited memory and runs 512 Kb files that contain the entire operating system needed to operate the car as well as a user block that contains all of the tables and settings that can be modified. It generates all your trouble codes, interfaces with all your powertrain sensors and can turn into a paperweight if you don’t know what you’re doing when you try to program it. The OBDII port is normally somewhere under the steering column on the driver’s side. This port allows read and write functionality between the PCM and a computer (Serial/USB/Bluetooth), provided you buy the appropriate interface module.
Well, we’ve certainly covered a lot of material. Stay tuned for more and remember, I can’t guarantee 100% accuracy of the above material and I’m not responsible for anything you do, whether it was your own idea or influenced by this article. Be careful and have fun with your ride :)