8th Generation Honda Civic Forum - View Single Post - Everything You Wanted To Know About Car Measurements(HP, TQ, Gear Ratios etc.)

ryker · 12-14-2007, 07:35 AM

Comparing Two Cars

I think at this point, it is clear that the car's acceleration ability is related to its engine's power output. Now, let's compare two different cars with two very different engines. ( I apolgize for the racial slur of the below ) Also not that great of an example since Honda makes good engines that have flat torque bands. But it does show why a nice flat thick torque band is great.

Both vehicles will have the same curb weight, and peak horsepower figures. They will also have the same transmission, tire radius, and so on. In fact, the only difference between the two cars will be the engines. One car will be equipped with a 500hp turbocharged 4 cylinder engine, and the other will have a 500hp big-block V8. The 4 cylinder will be named Ricer, and the V8 will be named Redneck. The 4 cylinder is able to rev to 9000RPM and produce a fair bit of torque, while the V8 can rev to only 6000RPM, but produce a ton of torque. To keep the math very simple, the Redneck's engine idles at 600RPM, and the Ricer's idles at 900RPM.
Below are plots of the two fictitious engine's torque and horsepower curves.

Figure 1: Torque versus RPM for Redneck and Ricer. These are unrealistic curves which have been exaggerated to help illustrate certain concepts.

Figure 2: Horsepower versus RPM for Redneck and Ricer. This is calculated from the torque at each RPM.

Both engines produce a peak of 500hp, just like I promised. The V8 produces 500hp at 5000RPM, and 573tq at 4250RPM, while the I4 produces 500hp at 8000RPM, and 337tq at 7500RPM.
RicerRedneckDifferenceRev Range900 - 9000RPM600 - 6000RPM50% more for RicerPeak Torque337tq573tq70% more for RedneckPeak Power500hp500hpEqualBecause the V8 was revving lower, it needed to produce more torque than the I4 to reach the same peak power figure. Conversely, the I4 needed to rev higher than the V8 to produce the same power, because it offers up less torque.
The Power Band

A very important aspect of an engine's output is its power band. The power band is the rev range where the engine is producing an arbitrary percentage of its peak power figure. I will use 80%, which is 400hp or more on our two 500hp engines.

Figure 3: Power band comparison of both motors. Note that the Redneck's average power production (area under the curve) is higher, and that the peak power is the same, at 500hp.

The x-axis may seem unusual. Because the Redneck's engine revs from 600RPM to 6000RPM, while the Ricer's engine revs from 900RPM to 9000RPM, the power production cannot be compared directly with revs. The Ricer has a 50% greater rev range than the Redneck, so the Redneck's graph has to (or "gets to") be stretched by 50%. Showing the x-axis as a fraction of each engine's rev range helps to equal the comparison.
If this seems confusing, there is a separate page on comparing power curves which explains why the rev ranges can't be compared directly.
We will see later on that Figure 3 is a more accurate comparison of the two engines ability to accelerate a car than Figure 2.

Notice that while both engines have the same peak power figures, the Redneck's engine has a much wider 80% power band. This situation is a considerable advantage for the Redneck. Between the two cars, the one with the made-up V8 is going to be faster than the one with the made-up I4, because the V8 has a higher average power level throughout its rev range.
An exotic sportscar, such as a Lamborghini Murcielago, will have such a wide power band that it can accelerate very hard from almost any RPM. This means that it can do things like go from 0-60mph in one gear. This is why exotics have such impressive performance.

Force on the Road

Let's now look at how much force the Redneck and Ricer are putting to the road, which as we talked about earlier, is the force which accelerates the car. For simplicity, both drivers will race by rolling from 20mph, flooring it, and then shifting at their redlines in each gear. Top speed will be considered redline in top gear, because we are ignoring aerodynamic drag.
We'll start off by giving both of them a good old TH350 3-speed automatic transmission, and a 3.73:1 final drive (axle) ratio.
Drivetrain Layout

TH350 and 3.73 Axle Gears

Figure 4: Rear wheel force versus vehicle speed for Redneck and Ricer when using a TH350 transmission and 3.73 axle ratio. The steep vertical drops are the gear changes at redline. Gear changes take place instantaneously for simplification.

Notice that the Redneck has a considerable advantage over the Ricer in first gear, but then not so much in second or third gear. This is because when he shifts into 2nd gear, the transmission doesn't bring him back to idle, but to approximately 3600RPM instead. The Ricer's engine also stays in reasonably high revs after the first gear change, and we saw in Figure 2 that he has plenty of power at high revs. Also note that the Redneck had to shift into second before the Ricer, so his ability to accelerate between 60-65 and 100-115mph is about the same as the Ricer's.
Now, let's move into modern day by giving them both a Tremec T56 6-Speed close-ratio manual transmission.

Drivetrain Layout

Tremec T56 6-Speed and 3.73 Axle Gears

Figure 5: Rear wheel force versus vehicle speed for Redneck and Ricer when using a Tremec T56 transmission and 3.73 axle ratio. Note that the close ratio transmission has reduced the drops in power at each gear change for both motors, especially for the Redneck's.

At certain speeds, the Ricer has caught up slightly. The close-ratio 6-speed transmission helps keep his engine revving near his power peak, and that has helped narrow the gap. The small dips on the Ricer's graph show the effects of having a narrow power band.
The Ricer's acceleration in first gear is still very poor, but the Ricer has a trick up his sleeve. He is going to install a set of 5.67:1 gears in his axle without the Redneck knowing.

Drivetrain Layout

Tremec T56 6-Speed

3.73 Axle Gears for Redneck

5.67 Axle Gears for Ricer

Figure 6: Rear wheel force versus vehicle speed when using a Tremec T56 transmission and 3.73 axle ratio for the Redneck, and 5.67 for the Ricer. Note that both cars shift gears at about the same vehicle speeds as each other now.

The Ricer has pretty much completely caught up now, especially at speeds above 40mph. With those gears he put in, he has traded his higher revs for higher torque to the wheels. Now, for certain vehicle speeds, he can accelerate alongside the Redneck. (if he is in the correct gear)

The Redneck would also see benefit from putting in different axle gears. However, this "arms race" cannot go on for long, because as the wheel torque is increased, speed is traded away. It would be very embarrassing for the drivers if they crossed the finish line sitting at redline in 6th gear, not accelerating. The Ricer can use a lot more gear because of his extra revs. Conversely, the Redneck doesn't need them because he has so much flywheel torque that he doesn't need to multiply it as much. There is not a significant benefit to either driver in this regard.

If we fitted both cars with a Continuously Variable Transmission (CVT) that had an infinite ratio spread, which can hold both engines at their horsepower peaks, the acceleration of both cars would be identical at any vehicle speed.

Low-Speed Acceleration

Even after changing the rear axle gears, the Ricer's (but the 8thgen k20 and R18 have a nice fat torque band) car still could not match the Redneck's acceleration from a slow roll up to about 35-40mph. This shows that the benefits of a having a very wide power band are most significant in first gear, and is therefore an important part of tuning an engine for drag racing, where the cars start from rest.
That low-RPM "hole" can be reduced with a few tricks:
With a manual transmission, the car could be launched by revving the engine way up and then easing off the clutch pedal quickly.
With an automatic, a torque converter with a high stall speed could be installed, which allows the engine to be revved up without moving the car too much while holding the brakes on. A good explanation of torque converters and stall speed is beyond the scope of this article. Think of a high stall converter as behaving like a slippy clutch at low revs, and a fully engaged clutch at high revs.

Shift Points

When the Ricer and Redneck were racing, they were shifting gears at their engine's redlines. In many real-life cases, shifting gears earlier may be advantageous for acceleration. An engine with a power curve that begins to "fall off" at very high RPM should be shifted earlier, if doing so would help yield more power. Gear shift points should always be chosen in such a way that the engine is putting out the highest average power to the wheels. (key phrase is Too the wheels not the flywheel)

Driveability

Driveability is a subjective term used to describe the ability to "access" an engine's power. A naturally-aspirated engine (or any engine) with a wide power band will have very good driveability. (provided the wide powerband is also large enough to get the work done) Putting the pedal to the floor at any speed in any gear should yield reasonable acceleration. On the other hand, a car with a narrow power band would not be considered as "driveable". Passing cars while cruising on the highway often requires dropping a gear to bring the engine's revs up to access the power. This is one of the reasons that luxury cars often come with large, naturally-aspirated or supercharged engines, while small, turbocharged engines are not as common and often found in more "focused" sportscars where outstanding driveability is not expected or required.
Engines are sometimes described as being "torquey". This is slang for having "good driveability" or "a wide power band". "Streetability" is also somewhat synonymous.

High Torque Engines versus High Revving Engines

Torque is somewhat related to the displacement of the engine. Larger displacement engines are likely to be much bigger and heavier, making them unsuitable for certain types of vehicles. This is why many small race cars have engines with small displacement, high-revving, and sometimes equipped with forced induction to produce high horsepower. Also, race cars are often given limits on displacement, which means their only chance at producing a lot of power is to rev very high or use boost. A large engine may be able to produce power more reliably than a smaller one, but not necessarily. There are plenty of big, gutless engines that don't last.
Heavy vehicles are almost always equipped with large displacement engines because they require more low-RPM power to accelerate from rest (and very low speeds). As the weight of a vehicle goes up, the acceleration from rest becomes increasingly significant. A 650hp V12 from a Ferrari Enzo could in fact tow a loaded semi at high speeds, but it's unlikely that it would have enough power at very low RPM to get the semi moving in the first place. On the other hand, a huge diesel engine can produce all kinds of power at low RPM to help get the vehicle rolling.

Conclusion

Shaft power is the product of shaft speed and torque, and the speed and torque can be altered proportionally using gears. If we want to apply a lot of torque to a shaft that is rotating, a lot of power is needed. The more power the better. However, the power must be accessible from all vehicle speeds, which can only be accomplished by producing a lot of torque throughout the rev range, or by having a transmission with many gears. It is this fact that has spawned phrases like "Torque is King", or "Horsepower sells cars, but torque wins races", which can be misleading. Torque on its own isn't useful in accelerating a vehicle, because it is not at rest; it is moving. Therefore, power is what matters. Cars are often described by their power-to-weight ratio, not their torque-to-weight ratio.
The vehicle with the largest average acceleration is the one that has the largest average force going to the pavement through a wide range of speeds.
" Peak power sells cars. High average power wins races. "
A vehicle's peak torque and horsepower figures can only give a general idea of performance. The best way to make a good comparison between two vehicles is to go racing! (or do all of the math formulas above - not as FUN)

12-14-2007, 07:35 AM	#22 (permalink)
ryker Senior Member Join Date: Sep 2007 Posts: 3,673 iTrader: 1 / 100%	Comparing Two Cars I think at this point, it is clear that the car's acceleration ability is related to its engine's power output. Now, let's compare two different cars with two very different engines. ( I apolgize for the racial slur of the below ) Also not that great of an example since Honda makes good engines that have flat torque bands. But it does show why a nice flat thick torque band is great. Both vehicles will have the same curb weight, and peak horsepower figures. They will also have the same transmission, tire radius, and so on. In fact, the only difference between the two cars will be the engines. One car will be equipped with a 500hp turbocharged 4 cylinder engine, and the other will have a 500hp big-block V8. The 4 cylinder will be named Ricer, and the V8 will be named Redneck. The 4 cylinder is able to rev to 9000RPM and produce a fair bit of torque, while the V8 can rev to only 6000RPM, but produce a ton of torque. To keep the math very simple, the Redneck's engine idles at 600RPM, and the Ricer's idles at 900RPM. Below are plots of the two fictitious engine's torque and horsepower curves. Figure 1: Torque versus RPM for Redneck and Ricer. These are unrealistic curves which have been exaggerated to help illustrate certain concepts. Figure 2: Horsepower versus RPM for Redneck and Ricer. This is calculated from the torque at each RPM. Both engines produce a peak of 500hp, just like I promised. The V8 produces 500hp at 5000RPM, and 573tq at 4250RPM, while the I4 produces 500hp at 8000RPM, and 337tq at 7500RPM. RicerRedneckDifferenceRev Range900 - 9000RPM600 - 6000RPM50% more for RicerPeak Torque337tq573tq70% more for RedneckPeak Power500hp500hpEqualBecause the V8 was revving lower, it needed to produce more torque than the I4 to reach the same peak power figure. Conversely, the I4 needed to rev higher than the V8 to produce the same power, because it offers up less torque. The Power Band A very important aspect of an engine's output is its power band. The power band is the rev range where the engine is producing an arbitrary percentage of its peak power figure. I will use 80%, which is 400hp or more on our two 500hp engines. Figure 3: Power band comparison of both motors. Note that the Redneck's average power production (area under the curve) is higher, and that the peak power is the same, at 500hp. The x-axis may seem unusual. Because the Redneck's engine revs from 600RPM to 6000RPM, while the Ricer's engine revs from 900RPM to 9000RPM, the power production cannot be compared directly with revs. The Ricer has a 50% greater rev range than the Redneck, so the Redneck's graph has to (or "gets to") be stretched by 50%. Showing the x-axis as a fraction of each engine's rev range helps to equal the comparison. If this seems confusing, there is a separate page on comparing power curves which explains why the rev ranges can't be compared directly. We will see later on that Figure 3 is a more accurate comparison of the two engines ability to accelerate a car than Figure 2. Notice that while both engines have the same peak power figures, the Redneck's engine has a much wider 80% power band. This situation is a considerable advantage for the Redneck. Between the two cars, the one with the made-up V8 is going to be faster than the one with the made-up I4, because the V8 has a higher average power level throughout its rev range. An exotic sportscar, such as a Lamborghini Murcielago, will have such a wide power band that it can accelerate very hard from almost any RPM. This means that it can do things like go from 0-60mph in one gear. This is why exotics have such impressive performance. Force on the Road Let's now look at how much force the Redneck and Ricer are putting to the road, which as we talked about earlier, is the force which accelerates the car. For simplicity, both drivers will race by rolling from 20mph, flooring it, and then shifting at their redlines in each gear. Top speed will be considered redline in top gear, because we are ignoring aerodynamic drag. We'll start off by giving both of them a good old TH350 3-speed automatic transmission, and a 3.73:1 final drive (axle) ratio. Drivetrain Layout TH350 and 3.73 Axle Gears Figure 4: Rear wheel force versus vehicle speed for Redneck and Ricer when using a TH350 transmission and 3.73 axle ratio. The steep vertical drops are the gear changes at redline. Gear changes take place instantaneously for simplification. Notice that the Redneck has a considerable advantage over the Ricer in first gear, but then not so much in second or third gear. This is because when he shifts into 2nd gear, the transmission doesn't bring him back to idle, but to approximately 3600RPM instead. The Ricer's engine also stays in reasonably high revs after the first gear change, and we saw in Figure 2 that he has plenty of power at high revs. Also note that the Redneck had to shift into second before the Ricer, so his ability to accelerate between 60-65 and 100-115mph is about the same as the Ricer's. Now, let's move into modern day by giving them both a Tremec T56 6-Speed close-ratio manual transmission. Drivetrain Layout Tremec T56 6-Speed and 3.73 Axle Gears Figure 5: Rear wheel force versus vehicle speed for Redneck and Ricer when using a Tremec T56 transmission and 3.73 axle ratio. Note that the close ratio transmission has reduced the drops in power at each gear change for both motors, especially for the Redneck's. At certain speeds, the Ricer has caught up slightly. The close-ratio 6-speed transmission helps keep his engine revving near his power peak, and that has helped narrow the gap. The small dips on the Ricer's graph show the effects of having a narrow power band. The Ricer's acceleration in first gear is still very poor, but the Ricer has a trick up his sleeve. He is going to install a set of 5.67:1 gears in his axle without the Redneck knowing. Drivetrain Layout Tremec T56 6-Speed 3.73 Axle Gears for Redneck 5.67 Axle Gears for Ricer Figure 6: Rear wheel force versus vehicle speed when using a Tremec T56 transmission and 3.73 axle ratio for the Redneck, and 5.67 for the Ricer. Note that both cars shift gears at about the same vehicle speeds as each other now. The Ricer has pretty much completely caught up now, especially at speeds above 40mph. With those gears he put in, he has traded his higher revs for higher torque to the wheels. Now, for certain vehicle speeds, he can accelerate alongside the Redneck. (if he is in the correct gear) The Redneck would also see benefit from putting in different axle gears. However, this "arms race" cannot go on for long, because as the wheel torque is increased, speed is traded away. It would be very embarrassing for the drivers if they crossed the finish line sitting at redline in 6th gear, not accelerating. The Ricer can use a lot more gear because of his extra revs. Conversely, the Redneck doesn't need them because he has so much flywheel torque that he doesn't need to multiply it as much. There is not a significant benefit to either driver in this regard. If we fitted both cars with a Continuously Variable Transmission (CVT) that had an infinite ratio spread, which can hold both engines at their horsepower peaks, the acceleration of both cars would be identical at any vehicle speed. Low-Speed Acceleration Even after changing the rear axle gears, the Ricer's (but the 8thgen k20 and R18 have a nice fat torque band) car still could not match the Redneck's acceleration from a slow roll up to about 35-40mph. This shows that the benefits of a having a very wide power band are most significant in first gear, and is therefore an important part of tuning an engine for drag racing, where the cars start from rest. That low-RPM "hole" can be reduced with a few tricks: With a manual transmission, the car could be launched by revving the engine way up and then easing off the clutch pedal quickly. With an automatic, a torque converter with a high stall speed could be installed, which allows the engine to be revved up without moving the car too much while holding the brakes on. A good explanation of torque converters and stall speed is beyond the scope of this article. Think of a high stall converter as behaving like a slippy clutch at low revs, and a fully engaged clutch at high revs. Shift Points When the Ricer and Redneck were racing, they were shifting gears at their engine's redlines. In many real-life cases, shifting gears earlier may be advantageous for acceleration. An engine with a power curve that begins to "fall off" at very high RPM should be shifted earlier, if doing so would help yield more power. Gear shift points should always be chosen in such a way that the engine is putting out the highest average power to the wheels. (key phrase is Too the wheels not the flywheel) Driveability Driveability is a subjective term used to describe the ability to "access" an engine's power. A naturally-aspirated engine (or any engine) with a wide power band will have very good driveability. (provided the wide powerband is also large enough to get the work done) Putting the pedal to the floor at any speed in any gear should yield reasonable acceleration. On the other hand, a car with a narrow power band would not be considered as "driveable". Passing cars while cruising on the highway often requires dropping a gear to bring the engine's revs up to access the power. This is one of the reasons that luxury cars often come with large, naturally-aspirated or supercharged engines, while small, turbocharged engines are not as common and often found in more "focused" sportscars where outstanding driveability is not expected or required. Engines are sometimes described as being "torquey". This is slang for having "good driveability" or "a wide power band". "Streetability" is also somewhat synonymous. High Torque Engines versus High Revving Engines Torque is somewhat related to the displacement of the engine. Larger displacement engines are likely to be much bigger and heavier, making them unsuitable for certain types of vehicles. This is why many small race cars have engines with small displacement, high-revving, and sometimes equipped with forced induction to produce high horsepower. Also, race cars are often given limits on displacement, which means their only chance at producing a lot of power is to rev very high or use boost. A large engine may be able to produce power more reliably than a smaller one, but not necessarily. There are plenty of big, gutless engines that don't last. Heavy vehicles are almost always equipped with large displacement engines because they require more low-RPM power to accelerate from rest (and very low speeds). As the weight of a vehicle goes up, the acceleration from rest becomes increasingly significant. A 650hp V12 from a Ferrari Enzo could in fact tow a loaded semi at high speeds, but it's unlikely that it would have enough power at very low RPM to get the semi moving in the first place. On the other hand, a huge diesel engine can produce all kinds of power at low RPM to help get the vehicle rolling. Conclusion Shaft power is the product of shaft speed and torque, and the speed and torque can be altered proportionally using gears. If we want to apply a lot of torque to a shaft that is rotating, a lot of power is needed. The more power the better. However, the power must be accessible from all vehicle speeds, which can only be accomplished by producing a lot of torque throughout the rev range, or by having a transmission with many gears. It is this fact that has spawned phrases like "Torque is King", or "Horsepower sells cars, but torque wins races", which can be misleading. Torque on its own isn't useful in accelerating a vehicle, because it is not at rest; it is moving. Therefore, power is what matters. Cars are often described by their power-to-weight ratio, not their torque-to-weight ratio. The vehicle with the largest average acceleration is the one that has the largest average force going to the pavement through a wide range of speeds. " Peak power sells cars. High average power wins races. " A vehicle's peak torque and horsepower figures can only give a general idea of performance. The best way to make a good comparison between two vehicles is to go racing! (or do all of the math formulas above - not as FUN) Last edited by ryker; 12-14-2007 at 12:57 PM.