Engine Power Diagram: Why linear hp but a huge dip in torque?
02-24-2007, 08:53 PM
#16
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Thread Starter
Great explanations. Thanks all! I have been shifting (and banging off) near the top everytime. Just wanted to make sure I was not only maximizing the entire progression through the gearbox but as well know that I am shift at proper points.
Thanks again!
Thanks again!
02-24-2007, 10:01 PM
#17
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Originally Posted by Derick Cooper
What you are really asking about is HP vs. Torque. While a lot of people think Torque is more important because HP is just a "mathematical calculation," the truth is that that mathematical calculation takes into account the gearing of the car.
Your car will have the highest rate of acceleration at it's torque peak in any particular gear, but if it had a continuously variable transmission, it would have the highest overall acceleration at the HP peak.
Here's how to judge the best shift point:
1) look at the hP curve
2) calculate whether the HP is higher in the lower gear or the next highest gear after taking into account the HP drop due to the drop in RPMs when you shift. As an example, let's say you drop 2000 RPMs between 2nd and 3rd. Is the HP higher at 8000RPms (2nd) or 6000 RPMs (3rd)?
3) stay in the gear with the higher HP for as long as possible (almost always take it to redline).
Even though the engine torque starts to fall off, the torque multiplication due to the lower gearing will give you more effective torque at the wheels in a lower gear. Usually, this more than compensates for the torque drop as RPms increase.
To measure the relative torque difference between gears, just divide by the RPM drop. In the example above, the 3rd gear overall torque multiplication would be 75% of second gear (6000/8000). In other words, the engine would have to produce 25% more torque at 6000 RPMs than at 8000 RPMs in order to justify using a higher gear before redline.
Most people mistakenly think Torque is king. In reality, RPMs (and HP) are where it's at. Look at F1, they don't rev to 15,000 plus RPMs just to sound good.
Stay in the lower gear and bang it off the rev limiter; that's what these cars were made to do.
Your car will have the highest rate of acceleration at it's torque peak in any particular gear, but if it had a continuously variable transmission, it would have the highest overall acceleration at the HP peak.
Here's how to judge the best shift point:
1) look at the hP curve
2) calculate whether the HP is higher in the lower gear or the next highest gear after taking into account the HP drop due to the drop in RPMs when you shift. As an example, let's say you drop 2000 RPMs between 2nd and 3rd. Is the HP higher at 8000RPms (2nd) or 6000 RPMs (3rd)?
3) stay in the gear with the higher HP for as long as possible (almost always take it to redline).
Even though the engine torque starts to fall off, the torque multiplication due to the lower gearing will give you more effective torque at the wheels in a lower gear. Usually, this more than compensates for the torque drop as RPms increase.
To measure the relative torque difference between gears, just divide by the RPM drop. In the example above, the 3rd gear overall torque multiplication would be 75% of second gear (6000/8000). In other words, the engine would have to produce 25% more torque at 6000 RPMs than at 8000 RPMs in order to justify using a higher gear before redline.
Most people mistakenly think Torque is king. In reality, RPMs (and HP) are where it's at. Look at F1, they don't rev to 15,000 plus RPMs just to sound good.
Stay in the lower gear and bang it off the rev limiter; that's what these cars were made to do.
Being the statisics professor that you are, only you could understand this explaination. Thank you Derick for that wonderful example.
Last edited by multi21; 02-26-2007 at 12:01 AM.
02-25-2007, 12:06 AM
#18
i shift 200 rpm short of the limiter (200 rpm after the shift light) or as close to it as possible, why becasue thats where it makes the best tire chirping noises and sidesteps the most
02-25-2007, 12:35 AM
#19
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As far as I know, this dip is when going from Variocam stage 1 to stage 2. When my hall sensors failed and stage 2 never engaged, the dip/drop was very noticeable and it felt like driving in to a wall.
Ciao,
J.
Ciao,
J.
02-25-2007, 04:00 AM
#20
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Originally Posted by Derick Cooper
To measure the relative torque difference between gears, just divide by the RPM drop. In the example above, the 3rd gear overall torque multiplication would be 75% of second gear (6000/8000). In other words, the engine would have to produce 25% more torque at 6000 RPMs than at 8000 RPMs in order to justify using a higher gear before redline.
There is an interesting anomaly seen in many modern cars with forced induction. These cars often have Torque curves with a profile like Table Mountain. absolutely flat from say, 2,500 to 6,500 rpm - this effect is more noticable in Euro standard engines than US spec engines, incidentally. So one can change gear and experience no difference in Torque.
Most journalists write about this and think its terrific. In reality, of course, its Very Boring and makes driving less interesting.
When I first drove the Ruf with the AS Supercharger I was impressed in part because the torque curve was more like that of normally aspirated cars and the higher one revved the greater the rate of increasing power delivers, very satisfying.
As far as calculating optimal shift points is concerned, this is a nice problem for people coming to grips with linear programming, there are some interteresting conclusions to be drawn from exploration of the dual.
R+C
