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Understanding Engine Performance

The Basics

When one talks about engine performance "horsepower" and "torque" are often the measurement. These two values are typically given at a certain RPM where they "peak". By taking the highest points for both horsepower and torque manufacturers can easily convey to the buyer the potential of their vehicles.

First there are some important things to understand. Power is merely a measurement of how much force times distance is done in a given amount of time. Torque is the actual force done. The RPMs take into account distance (rounds) and times (minutes). When one value increases at a certain point, so does the other value at that same point. The equations below represent the relationship between the two:

While ultimately it is torque, also known as rotational force, that moves the car, the horsepower peak value typically plays a more important roll in understanding how much acceleration can be had. Gears multiply torque at the expense of RPMs. This is a linear process. Thus if torque is to be doubled at the output, the RPMs will consequently be halved. Having a higher value for horsepower indicates that there is more force in relation to RPM. This is where gears play a key roll. Since we can trade away RPMs for added torque through gears, we can actually achieve greater force at the wheels to accelerate the vehicle. Basically speed is being traded for increased acceleration.

It is extremely important to understand that horsepower and torque are almost always given as points. Not curves (with the exception of dynographs). Many people focus soley on one number at only one single point on the RPM range. Obviously this is not ideal since an engine typically uses a wide range of engine speeds to move a vehicle. By supplying us with two peak values, as opposed to one, manufacturers allow us to look a little deeper into an engines performance. Also one should note that manufacturers supply horsepower and torque values measured at the engine, not the wheels where it matters. That is where drivetrain loss comes in.

The torque value is very important as it helps us to understand what kind of potential we shall have at other RPMs. A higher torque value at a lower RPM typically indicates more area under the curve. In an ideal situation there will be a maximum amount of area under the horsepower and torque curves for the given RPMs the car is used in. Thus a high value and low RPM peak for torque is best. This would indicate a long lasting curve with a higher overall value, leading to increased acceleration.

Inferring Total Performance

Rarely a manufacture provides a complete dyno sheet or similar print out that details the potential of their motors. Since we merely have two points, an approximation is the best we can hope for. So how does one decide which is more important: The horsepower or the torque value? This is a complex question with no real answer. It really depends. For cruising around a more attractive torque value would typically take presidence as it is a good indication of how driveable and streetable the car will be. For all out performance, people typically cut straight to the horsepower rating. Are they missing something? Of course!

Below are two dyno sheets. The first is for a Ford 5.0L V8 found in the 1987-1993 Mustang. It was rated 225hp@4200rpm and 300ft-lbs@3200rpm. The second is a 2002 Honda 2.0L I4 VTEC. It was rated 240hp@8300rpm and 153ft-lbs@7500rpm. The two engines posted smiliar power peaks of 200 on the dynometer.

NOTE: The actual peak horsepower of the Honda motor was 198. Due to size limitations, it was eliminated from the above bridged data sheet. All calculations are with the complete values.

Clearly the curves between the Ford and Honda motors differ greatly. The Ford motor curve appears to be more lofty and parabolic. It slowly rises to its peak and then slowly falls off. The Honda motor is more linear. It steadily and steeply rises to its peak, then prompty falls off. While both of these motors display approximately 200hp of peak power, under the curve they are very different. Below is a dynometer graph comparing the two on a common axis in their useable RPM ranges.

NOTE: The RPMs on the dyno graph are on the Ford motor's scale. The useable RPM range for maximum performance is between 3000-5000rpm for the Ford. This is a typical scale where exiting first at 5000rpm will put you around 3000rpm in second. The Honda has steeper gearing to take advantage of its higher RPM range. The powerband was thus expanded linearly. When refering to the Honda data points they are at 1.76 times that of the standard RPMs. Eg. 5000rpms would be 8800rpm for the Honda and 3000rpms would result in 5280rpms. The powerband from 5280-8800 is about right for the S2000. The torque has already been multiplied by the factor 1.76 for the entire range, hence why there are abnormally high torque values on the S2000 chart. Horsepower remains unchanged since gearing does not effect horsepower.

The bottom values are the total summations of the torque and horsepower for each car over the useable performance RPM range. Clearly even though both these motors displayed an initial 200 peak horsepower, there is a distinct difference in actual performance. The Honda performance curve is only 86.2% of the Ford's. This would indicate that the car would be significantly slower (almost 14%) despite having equal peak power.

Only at the very end of the RPM range does the Honda engine actually produce more power under the curve. If one were to look only at the peak horsepower, they would be missing alot. By understanding that the Ford makes twice as much torque at a significantly lower RPM than its power peak, it is easy to see why the Ford motor outperforms the Honda. The Honda has a low peak torque value and it occurs at an RPM very close to the peak power. This short spacing and low value indicate a poor area under the curve and consequently reduced performance for a given peak horsepower. If only horsepower is considered, up to 15%, or possibly more, of the performance image is lost. This leaves alot to be wanting.

Many people would suggest a flat torque curve to be the best for street driving as it feels linear and predictable. This is true, to an extent. A flat torque curve, if it is of insufficent value, is useless. Maximum torque at a low RPM is key for driveability. It allows good performance from the stop light and easy cruising on the freeway.

The Effect of Displacement on Performance

So why does the Honda engine have such a lower torque value than that of the Ford? The answer is displacement. More importantly how much horsepower is being made per liter. The Honda makes a very high 120hp/liter whereas the Ford makes only 45hp/liter. The Honda motor is more "high strung" than the Ford engine. It cannot generate as much torque down low since the engine has a relatively large operating range. The area of peak efficency did not grow with the size of the powerband. Performance suffers for this reason.

Turbochargers and superchargers both can compensate for the displacement disparity, to an extent. Superchargers of the positive displacement variety (rootes, scroll, screw, etc) seem to be the best at producing a large displacement torque curve. Unfortunately these consume more power and typically reduce fuel economy compared to a larger engine or other forced induction options. Turbochargers and centrifugal superchargers can usually extend the torque curve down the RPM range giving a nice boost for a given horsepower. Consequently, these also will suffer when too much horsepower per liter is being made. Turbochargers and centrifugal superchargers merely add another level of adjustability into the system. They can be setup to provide minimal peak horsepower boost with a large peak torque boost (smaller turbocharger) or visa versa (larger turbocharger). The torque deficit on the low end when turbocharging comes from the turbo "lag" as the unit needs to spool up into its efficency range. This can cause a car to become much more "peaky" (ie. larger desparity between the lowend output and the topend) if the horsepower per liter is taken to the extremes. For instance, race car drivers have stated that a cars gas pedal can become like an "on/off" switch where no power may be preset followed by a sudden and difficult to control whallop of torque.

An astute observer might also have noticed that the Honda motor only makes 200/240 of its peak horsepower when dynoed. The Ford motor makes 200/225 of its peak. This type of discrepancy is common. Rating methods have changed over time and dynometers typically do not read with the same accuracy depending on the manufacturer. There definately is power lost through the drivetrain though. Front wheel drive cars typically lose the least, followed by rear wheel drive cars, then all wheel drive cars. Automatics transmissions typically consume more power than manual transmissions. So why does all this matter? The higher the input RPM the greater the loss through the drivetrain. This is not a linear function either. Drivetrain loss from drag increases approximately with the square of speed since the gears are traveling through fluid. Thus the higher the peak RPM the less power will actually make it to the wheels where it matters. A engine of equal peak power might need a larger transmission to cope with the torque which may balance this out to an extent. For more about drivetrain loss look here.

Summation

1. For a given peak horsepower, a higher peak torque value can net up to 15% more useable performance.

2. Larger displacements generate greater performance for a given peak horsepower.

3. Motors that operate at higher RPMs lose more power through gearing than those that operate at lower RPM.




	Understanding Engine Performance The Basics When one talks about engine performance "horsepower" and "torque" are often the measurement. These two values are typically given at a certain RPM where they "peak". By taking the highest points for both horsepower and torque manufacturers can easily convey to the buyer the potential of their vehicles. First there are some important things to understand. Power is merely a measurement of how much force times distance is done in a given amount of time. Torque is the actual force done. The RPMs take into account distance (rounds) and times (minutes). When one value increases at a certain point, so does the other value at that same point. The equations below represent the relationship between the two: While ultimately it is torque, also known as rotational force, that moves the car, the horsepower peak value typically plays a more important roll in understanding how much acceleration can be had. Gears multiply torque at the expense of RPMs. This is a linear process. Thus if torque is to be doubled at the output, the RPMs will consequently be halved. Having a higher value for horsepower indicates that there is more force in relation to RPM. This is where gears play a key roll. Since we can trade away RPMs for added torque through gears, we can actually achieve greater force at the wheels to accelerate the vehicle. Basically speed is being traded for increased acceleration. It is extremely important to understand that horsepower and torque are almost always given as points. Not curves (with the exception of dynographs). Many people focus soley on one number at only one single point on the RPM range. Obviously this is not ideal since an engine typically uses a wide range of engine speeds to move a vehicle. By supplying us with two peak values, as opposed to one, manufacturers allow us to look a little deeper into an engines performance. Also one should note that manufacturers supply horsepower and torque values measured at the engine, not the wheels where it matters. That is where drivetrain loss comes in. The torque value is very important as it helps us to understand what kind of potential we shall have at other RPMs. A higher torque value at a lower RPM typically indicates more area under the curve. In an ideal situation there will be a maximum amount of area under the horsepower and torque curves for the given RPMs the car is used in. Thus a high value and low RPM peak for torque is best. This would indicate a long lasting curve with a higher overall value, leading to increased acceleration. Inferring Total Performance Rarely a manufacture provides a complete dyno sheet or similar print out that details the potential of their motors. Since we merely have two points, an approximation is the best we can hope for. So how does one decide which is more important: The horsepower or the torque value? This is a complex question with no real answer. It really depends. For cruising around a more attractive torque value would typically take presidence as it is a good indication of how driveable and streetable the car will be. For all out performance, people typically cut straight to the horsepower rating. Are they missing something? Of course! Below are two dyno sheets. The first is for a Ford 5.0L V8 found in the 1987-1993 Mustang. It was rated 225hp@4200rpm and 300ft-lbs@3200rpm. The second is a 2002 Honda 2.0L I4 VTEC. It was rated 240hp@8300rpm and 153ft-lbs@7500rpm. The two engines posted smiliar power peaks of 200 on the dynometer. NOTE: The actual peak horsepower of the Honda motor was 198. Due to size limitations, it was eliminated from the above bridged data sheet. All calculations are with the complete values. Clearly the curves between the Ford and Honda motors differ greatly. The Ford motor curve appears to be more lofty and parabolic. It slowly rises to its peak and then slowly falls off. The Honda motor is more linear. It steadily and steeply rises to its peak, then prompty falls off. While both of these motors display approximately 200hp of peak power, under the curve they are very different. Below is a dynometer graph comparing the two on a common axis in their useable RPM ranges. NOTE: The RPMs on the dyno graph are on the Ford motor's scale. The useable RPM range for maximum performance is between 3000-5000rpm for the Ford. This is a typical scale where exiting first at 5000rpm will put you around 3000rpm in second. The Honda has steeper gearing to take advantage of its higher RPM range. The powerband was thus expanded linearly. When refering to the Honda data points they are at 1.76 times that of the standard RPMs. Eg. 5000rpms would be 8800rpm for the Honda and 3000rpms would result in 5280rpms. The powerband from 5280-8800 is about right for the S2000. The torque has already been multiplied by the factor 1.76 for the entire range, hence why there are abnormally high torque values on the S2000 chart. Horsepower remains unchanged since gearing does not effect horsepower. The bottom values are the total summations of the torque and horsepower for each car over the useable performance RPM range. Clearly even though both these motors displayed an initial 200 peak horsepower, there is a distinct difference in actual performance. The Honda performance curve is only 86.2% of the Ford's. This would indicate that the car would be significantly slower (almost 14%) despite having equal peak power. Only at the very end of the RPM range does the Honda engine actually produce more power under the curve. If one were to look only at the peak horsepower, they would be missing alot. By understanding that the Ford makes twice as much torque at a significantly lower RPM than its power peak, it is easy to see why the Ford motor outperforms the Honda. The Honda has a low peak torque value and it occurs at an RPM very close to the peak power. This short spacing and low value indicate a poor area under the curve and consequently reduced performance for a given peak horsepower. If only horsepower is considered, up to 15%, or possibly more, of the performance image is lost. This leaves alot to be wanting. Many people would suggest a flat torque curve to be the best for street driving as it feels linear and predictable. This is true, to an extent. A flat torque curve, if it is of insufficent value, is useless. Maximum torque at a low RPM is key for driveability. It allows good performance from the stop light and easy cruising on the freeway. The Effect of Displacement on Performance So why does the Honda engine have such a lower torque value than that of the Ford? The answer is displacement. More importantly how much horsepower is being made per liter. The Honda makes a very high 120hp/liter whereas the Ford makes only 45hp/liter. The Honda motor is more "high strung" than the Ford engine. It cannot generate as much torque down low since the engine has a relatively large operating range. The area of peak efficency did not grow with the size of the powerband. Performance suffers for this reason. Turbochargers and superchargers both can compensate for the displacement disparity, to an extent. Superchargers of the positive displacement variety (rootes, scroll, screw, etc) seem to be the best at producing a large displacement torque curve. Unfortunately these consume more power and typically reduce fuel economy compared to a larger engine or other forced induction options. Turbochargers and centrifugal superchargers can usually extend the torque curve down the RPM range giving a nice boost for a given horsepower. Consequently, these also will suffer when too much horsepower per liter is being made. Turbochargers and centrifugal superchargers merely add another level of adjustability into the system. They can be setup to provide minimal peak horsepower boost with a large peak torque boost (smaller turbocharger) or visa versa (larger turbocharger). The torque deficit on the low end when turbocharging comes from the turbo "lag" as the unit needs to spool up into its efficency range. This can cause a car to become much more "peaky" (ie. larger desparity between the lowend output and the topend) if the horsepower per liter is taken to the extremes. For instance, race car drivers have stated that a cars gas pedal can become like an "on/off" switch where no power may be preset followed by a sudden and difficult to control whallop of torque. An astute observer might also have noticed that the Honda motor only makes 200/240 of its peak horsepower when dynoed. The Ford motor makes 200/225 of its peak. This type of discrepancy is common. Rating methods have changed over time and dynometers typically do not read with the same accuracy depending on the manufacturer. There definately is power lost through the drivetrain though. Front wheel drive cars typically lose the least, followed by rear wheel drive cars, then all wheel drive cars. Automatics transmissions typically consume more power than manual transmissions. So why does all this matter? The higher the input RPM the greater the loss through the drivetrain. This is not a linear function either. Drivetrain loss from drag increases approximately with the square of speed since the gears are traveling through fluid. Thus the higher the peak RPM the less power will actually make it to the wheels where it matters. A engine of equal peak power might need a larger transmission to cope with the torque which may balance this out to an extent. For more about drivetrain loss look here. Summation 1. For a given peak horsepower, a higher peak torque value can net up to 15% more useable performance. 2. Larger displacements generate greater performance for a given peak horsepower. 3. Motors that operate at higher RPMs lose more power through gearing than those that operate at lower RPM.

Chris Moris \| theautolounge@autolounge.net