Have some real technical HP/Torque questions. Confused lol.
#16
07-10-2007, 02:20 PM
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Originally Posted by logik05se
the slower speeds you ar are getting when your shifting for youself could be do to a slight lag when you dont let the computer control it. I know most tip-tronics have this lag, even if its minimal its still there.
only if i shift before 7200 do i get a slower time/mph
#17
07-10-2007, 02:39 PM
Originally Posted by skaterbasist
Shifting too early, which 6300-6900 definately is way too early, will bring you down to a lower torque curve after your shift. Shifting too late will also bring you down to a lower torque point at the next shift.
Point of shifting is to shift into the next gear at the highest possible torque, which is why you want to shift at 7500 on the HR's.
Same goes for the Revup
The HP vs Torque debate is never an easy subject unless you really understand physics.
.
Point of shifting is to shift into the next gear at the highest possible torque, which is why you want to shift at 7500 on the HR's.
Same goes for the Revup
The HP vs Torque debate is never an easy subject unless you really understand physics.
.
well its a good thing i know these guys
#18
07-10-2007, 10:11 PM
Originally Posted by logik05se
Ok I pretty much thought I had a decent understand of HP and torque. Today I deceided to do some browsing and came across this site that gives you the equation to figure out hp and tq etc.
It says that torque = hp x 5252 / rpm and hp = torque x rpm/5252.
I was just pluggin in some numbers and some things just werent adding up to me. On the window sticker of my G it says 306hp@6800 and 268tq@4800.
If you do the equation on the site. 306x5252/6800 you get 236 lb/ft of torque. I dont quite understand this fully yet as its late and im pretty tired from work. If the G is supposidly only putting out 236 lb/ft at 6800 rpm then wouldnt you actually want to shift slightly before that?
Another thing that im curious about is take a car like say the honda s2000 which is 242 hp @ 7200. If you plug those numbers into the formula above it actually says that it produces 162 lb.ft of torque which if im not mistake is exactly what honda says, so why is the G so far off then?
My main reason for checking out this site was trying to figure out shift points and actually how to increase torque without F/I. I always just bring my car to redline if i wanted to acheive the fastest acceleration, but it seems like after reading this, thats not the case. If seems to me that by going off these calculations that higher in rpms you go past peek, the more the torque drops off, therefore hurting acceleration times. Am I wrong?
People with real knowledge on this subject please respond.
It says that torque = hp x 5252 / rpm and hp = torque x rpm/5252.
I was just pluggin in some numbers and some things just werent adding up to me. On the window sticker of my G it says 306hp@6800 and 268tq@4800.
If you do the equation on the site. 306x5252/6800 you get 236 lb/ft of torque. I dont quite understand this fully yet as its late and im pretty tired from work. If the G is supposidly only putting out 236 lb/ft at 6800 rpm then wouldnt you actually want to shift slightly before that?
Another thing that im curious about is take a car like say the honda s2000 which is 242 hp @ 7200. If you plug those numbers into the formula above it actually says that it produces 162 lb.ft of torque which if im not mistake is exactly what honda says, so why is the G so far off then?
My main reason for checking out this site was trying to figure out shift points and actually how to increase torque without F/I. I always just bring my car to redline if i wanted to acheive the fastest acceleration, but it seems like after reading this, thats not the case. If seems to me that by going off these calculations that higher in rpms you go past peek, the more the torque drops off, therefore hurting acceleration times. Am I wrong?
People with real knowledge on this subject please respond.
This topic usually gets out of hand but here I go:
When a car is given a rating, they only list the PEAK values of torque and horsepower. In your case, all you know is 306 HP at 6800 rpm and 268 at 4800 rpm. In reality, the car's engine has a varying torque curve depending on the rpm. As such, there is a horsepower curve as well. In your calculation, you found that at 6800 rpm, the car is putting out 236 lb-ft of torque - a value that is less than the PEAK of 268 at 4,800 rpm. Similarly, by knowing that the cars peak is 268 lb-ft of torque at 4,800 rpm, you can calculate that the horsepower at 4,800 rpm is 244, a value less than the PEAK of 306 at 6,800 rpm. Everything in between is not known by just knowing these values but you can already get a sense for the type of engine you have. The torque went from 268 at 4,800 rpm down to 236 at 6,800 rpm - not much of a drop at the high rpm. That "sustained" torque value is what gives you the high HP of 306 at 6,800 rpm.
Now, the reality:
1. The values listed are not power that makes it to the wheels - just engine power. There are losses on the way to the tires.
2. In between the engine and the tires is a transmission made up of multiple gears. The gears are used to allow the tires to operate at a different rotational speed than that of the engine. If your car had no gears, than the maximum the tires could spin is equal to the maximum that the engine spins. In that case a tire rotating at 7,500 rpm (the equivalent of your engine redline) would be equal to about 536 mph!! Obviosuly, the car would never reach that high of a speed. The equivalent 150 mph top speed would only have the engine operating at 2,000 rpm. This causes 2 problems:
a. Gas engines don't make much power at those low rpms.
b. Since the ratios are 1:1 (engine/tire), the torque that the engine makes is equal to the torque that gets to the tire. Well, 268 lb-ft of torque isn't enough to move the car very much. By adding gears, the ratio of engine speed to tire speed is changed to say 12:1 for 1st gear (gets you to 45 mph), then 8:1 for 2nd gear (gets you to 65 mph) and so on. Well, when you do that you are actually "multiplying" the torque sent to the wheels. So, in 1st gear in the above example, the tire sees 12x the engine torque. In 2nd gear the tires see 8x the engine torque and so on. So, you can see that in almost all cases, a car will make more power in the current gear over going into the next gear. The exception is certain cars with odd-ball torque curves (like that older 5.0 Mustang). The Mustang made 300 lb-ft of torque but only 225 HP. So, you can determine from that same initial formula, that the car made ALOT more torque in the low rpm range compared to the upper rpm. Just think - since the Mustang made the 225 HP at something like 4,800 rpm - that means that it was only making 246 lb-ft of torque at 4,800 rpm. So, you can see where the engine already has a big dropoff. That dropoff continues aggressively all the way to redline. The difference has to be alot for you to want to "short-shift" your car, because as you can see, 2nd gear has only 8x the engine torque versus 12x in 1st gear. I would say 99% of cars are better shifting at redline to take advantage of the torque multiplication.
Ok, not sure why I went through all of this....but if it helps - great.
#19
07-11-2007, 01:55 PM
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Originally Posted by muscarel
This topic usually gets out of hand but here I go:
When a car is given a rating, they only list the PEAK values of torque and horsepower. In your case, all you know is 306 HP at 6800 rpm and 268 at 4800 rpm. In reality, the car's engine has a varying torque curve depending on the rpm. As such, there is a horsepower curve as well. In your calculation, you found that at 6800 rpm, the car is putting out 236 lb-ft of torque - a value that is less than the PEAK of 268 at 4,800 rpm. Similarly, by knowing that the cars peak is 268 lb-ft of torque at 4,800 rpm, you can calculate that the horsepower at 4,800 rpm is 244, a value less than the PEAK of 306 at 6,800 rpm. Everything in between is not known by just knowing these values but you can already get a sense for the type of engine you have. The torque went from 268 at 4,800 rpm down to 236 at 6,800 rpm - not much of a drop at the high rpm. That "sustained" torque value is what gives you the high HP of 306 at 6,800 rpm.
Now, the reality:
1. The values listed are not power that makes it to the wheels - just engine power. There are losses on the way to the tires.
2. In between the engine and the tires is a transmission made up of multiple gears. The gears are used to allow the tires to operate at a different rotational speed than that of the engine. If your car had no gears, than the maximum the tires could spin is equal to the maximum that the engine spins. In that case a tire rotating at 7,500 rpm (the equivalent of your engine redline) would be equal to about 536 mph!! Obviosuly, the car would never reach that high of a speed. The equivalent 150 mph top speed would only have the engine operating at 2,000 rpm. This causes 2 problems:
a. Gas engines don't make much power at those low rpms.
b. Since the ratios are 1:1 (engine/tire), the torque that the engine makes is equal to the torque that gets to the tire. Well, 268 lb-ft of torque isn't enough to move the car very much. By adding gears, the ratio of engine speed to tire speed is changed to say 12:1 for 1st gear (gets you to 45 mph), then 8:1 for 2nd gear (gets you to 65 mph) and so on. Well, when you do that you are actually "multiplying" the torque sent to the wheels. So, in 1st gear in the above example, the tire sees 12x the engine torque. In 2nd gear the tires see 8x the engine torque and so on. So, you can see that in almost all cases, a car will make more power in the current gear over going into the next gear. The exception is certain cars with odd-ball torque curves (like that older 5.0 Mustang). The Mustang made 300 lb-ft of torque but only 225 HP. So, you can determine from that same initial formula, that the car made ALOT more torque in the low rpm range compared to the upper rpm. Just think - since the Mustang made the 225 HP at something like 4,800 rpm - that means that it was only making 246 lb-ft of torque at 4,800 rpm. So, you can see where the engine already has a big dropoff. That dropoff continues aggressively all the way to redline. The difference has to be alot for you to want to "short-shift" your car, because as you can see, 2nd gear has only 8x the engine torque versus 12x in 1st gear. I would say 99% of cars are better shifting at redline to take advantage of the torque multiplication.
Ok, not sure why I went through all of this....but if it helps - great.
When a car is given a rating, they only list the PEAK values of torque and horsepower. In your case, all you know is 306 HP at 6800 rpm and 268 at 4800 rpm. In reality, the car's engine has a varying torque curve depending on the rpm. As such, there is a horsepower curve as well. In your calculation, you found that at 6800 rpm, the car is putting out 236 lb-ft of torque - a value that is less than the PEAK of 268 at 4,800 rpm. Similarly, by knowing that the cars peak is 268 lb-ft of torque at 4,800 rpm, you can calculate that the horsepower at 4,800 rpm is 244, a value less than the PEAK of 306 at 6,800 rpm. Everything in between is not known by just knowing these values but you can already get a sense for the type of engine you have. The torque went from 268 at 4,800 rpm down to 236 at 6,800 rpm - not much of a drop at the high rpm. That "sustained" torque value is what gives you the high HP of 306 at 6,800 rpm.
Now, the reality:
1. The values listed are not power that makes it to the wheels - just engine power. There are losses on the way to the tires.
2. In between the engine and the tires is a transmission made up of multiple gears. The gears are used to allow the tires to operate at a different rotational speed than that of the engine. If your car had no gears, than the maximum the tires could spin is equal to the maximum that the engine spins. In that case a tire rotating at 7,500 rpm (the equivalent of your engine redline) would be equal to about 536 mph!! Obviosuly, the car would never reach that high of a speed. The equivalent 150 mph top speed would only have the engine operating at 2,000 rpm. This causes 2 problems:
a. Gas engines don't make much power at those low rpms.
b. Since the ratios are 1:1 (engine/tire), the torque that the engine makes is equal to the torque that gets to the tire. Well, 268 lb-ft of torque isn't enough to move the car very much. By adding gears, the ratio of engine speed to tire speed is changed to say 12:1 for 1st gear (gets you to 45 mph), then 8:1 for 2nd gear (gets you to 65 mph) and so on. Well, when you do that you are actually "multiplying" the torque sent to the wheels. So, in 1st gear in the above example, the tire sees 12x the engine torque. In 2nd gear the tires see 8x the engine torque and so on. So, you can see that in almost all cases, a car will make more power in the current gear over going into the next gear. The exception is certain cars with odd-ball torque curves (like that older 5.0 Mustang). The Mustang made 300 lb-ft of torque but only 225 HP. So, you can determine from that same initial formula, that the car made ALOT more torque in the low rpm range compared to the upper rpm. Just think - since the Mustang made the 225 HP at something like 4,800 rpm - that means that it was only making 246 lb-ft of torque at 4,800 rpm. So, you can see where the engine already has a big dropoff. That dropoff continues aggressively all the way to redline. The difference has to be alot for you to want to "short-shift" your car, because as you can see, 2nd gear has only 8x the engine torque versus 12x in 1st gear. I would say 99% of cars are better shifting at redline to take advantage of the torque multiplication.
Ok, not sure why I went through all of this....but if it helps - great.
Thank you for clearing things up.
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