torque v power

[MarkMgibson]

Force per unit mass is the only meaningful measure of acceleration.

Power is the product of torque and rotational speed.

When I was once shopping for a car, I noticed that all of the cars in the "class" that I was looking at had roughly 2 liter engines, roughly equal torque, and the rated power was roughly proportional to the rpm where power was specified. I reasoned that I will never push the engine to its max rpm, so the power ratings were meaningless.

Yeah, peak power doesn't mean much for normal driving. As you say, not many people rev their motors that high. Torque is certainly more important, especially in smaller cars (like the 2L engines you mention). I've driven cars that go really well high in the rev range, but you're forever changing gears because the motors just die at low revs. Those sort of cars are very tiring to drive on a daily basis. You slow down to a near stop, and have to slam it back into 1st - that's a pain in the ass.

 

[fdeck]

Yeah, peak power doesn't mean much for normal driving. As you say, not many people rev their motors that high. Torque is certainly more important, especially in smaller cars (like the 2L engines you mention). I've driven cars that go really well high in the rev range, but you're forever changing gears because the motors just die at low revs. Those sort of cars are very tiring to drive on a daily basis. You slow down to a near stop, and have to slam it back into 1st - that's a pain in the ass.

I'd rather have the car slam itself back into 1st for me.

 

[Bob Lee (QSC)]

Force per unit mass is the only meaningful measure of acceleration.

Power is the product of torque and rotational speed.

+1

Gearing can change torque upward or downward, with an inversely corresponding change in rotational speed, so the power in and power out remain constant, minus parasitic losses.

When I was once shopping for a car, I noticed that all of the cars in the "class" that I was looking at had roughly 2 liter engines, roughly equal torque, and the rated power was roughly proportional to the rpm where power was specified. I reasoned that I will never push the engine to its max rpm, so the power ratings were meaningless.

Hah! I noticed that too. It was about that time I also realized I'd probably never encounter a car salesman who can answer my geeky questions.

 

[MarkMgibson]

You only need to drive a diesel to understand the importance of torque. Some of the modern diesel cars are damn nice to drive. My mate has a VW Golf TDi, and it's a great car to drive, especially long distances. Very quiet too, and much less vibration than a petrol driven car. Not to mention the fuel economy. Wouldn't mind one myself actually.

 

[Gopherbassist]

The driving experience is different, as you have already observed. Which is better is just a question of what you like. Remember that even if two engines produce the same maximum HP and torque, they can feel very different depending on where in the RPM range the maximum occurs and how steeply it rises and falls before and after the peak.

Also remember that 1 horsepower = 745.7 watts = 250 tube watts.

So, are you saying horsepower = volume? If I buy a better car my guitarist won't be able to drown me out at gigs? Why did no one share this with me sooner!? WE HAVE TO TELL THE WORLD!

 

[Lee H]

In REAL WORLD driving, most cars rarely see their peak HP numbers. 20+ years ago, I sat down a figured out numbers in a 1/4 mile car. At the time I was driving a 69 GTO Judge

I would launch with a transmission gear ratio of 2.48/ 1. I would climb to around 5000 RPMs, and shift. Second gear ration was 1.48/ 1. My RPMs would drop to about 3,800. I would pull back to 5000, and shift again. Third gear was 1/1, so I would only drop to about 4,100 RPMs.
in a 12 second car (bias play "slip and slide" tires) I would spend about 2.8 seconds in my peak horsepower range, and about 7.5 seconds in initial torque acceleration or shift recovery.
it did not take a rocket surgeon to figure out that I could get more building for where the car was spending the most time in

Oh and BTW...I was working for Jamora Racing when I figured this out. I was applying what I learned to the track, not just talking hypotheticals


as for the harley comparison... There will always be those who talk smack about harleys...but then again, there are chest beaters in every group. The harley engine is designed for much more than it puts out. But harley a has marketing department that does it's homework, and targets a demographic that spends money...lots of money.
I love catching the big talkers at bike events with dyno competitions. it is fun to see them kicking rocks and making their excuses when it is done

 

[Mike N]

In REAL WORLD driving, most cars rarely see their peak HP numbers. 20+ years ago, I sat down a figured out numbers in a 1/4 mile car. At the time I was driving a 69 GTO Judge

I would launch with a transmission gear ratio of 2.48/ 1. I would climb to around 5000 RPMs, and shift. Second gear ration was 1.48/ 1. My RPMs would drop to about 3,800. I would pull back to 5000, and shift again. Third gear was 1/1, so I would only drop to about 4,100 RPMs.
in a 12 second car (bias play "slip and slide" tires) I would spend about 2.8 seconds in my peak horsepower range, and about 7.5 seconds in initial torque acceleration or shift recovery.
it did not take a rocket surgeon to figure out that I could get more building for where the car was spending the most time in

Did you ever try shifting at 6000 rpm to see if you went any quicker?

 

[Lee H]

Did you ever try shifting at 6000 rpm to see if you went any quicker?

the problem was the old Kelly bias ply tires. They would not even stick to gum, if you ran over a glob. At the time, they were the only company making the replacement red wall tires that came stock on the GTO

I also had a 68 olds 442, that was a slower car, but consistently put up quicker ETs. The old 350R engine did not smoke tires. I could stuff the pedal on the floor and hold it there with no real fear of the back being loose.

 

[MarkMgibson]

In REAL WORLD driving, most cars rarely see their peak HP numbers. 20+ years ago, I sat down a figured out numbers in a 1/4 mile car. At the time I was driving a 69 GTO Judge

I would launch with a transmission gear ratio of 2.48/ 1. I would climb to around 5000 RPMs, and shift. Second gear ration was 1.48/ 1. My RPMs would drop to about 3,800. I would pull back to 5000, and shift again. Third gear was 1/1, so I would only drop to about 4,100 RPMs.
in a 12 second car (bias play "slip and slide" tires) I would spend about 2.8 seconds in my peak horsepower range, and about 7.5 seconds in initial torque acceleration or shift recovery.
it did not take a rocket surgeon to figure out that I could get more building for where the car was spending the most time in

Oh and BTW...I was working for Jamora Racing when I figured this out. I was applying what I learned to the track, not just talking hypotheticals

I remember driving big V8's with those horrible ply tyres, but I reckon those solid rear axles didn't help much in gaining traction either. I remember seeing a lot of big V8's get smoked by smaller, lighter 6cyd cars over a 1/4 mile.

It's even more true in normal driving, most people aren't revving their cars anywhere near the peek HP figures, so for accelerating out of corners or overtaking, good torque is more valuable for the average driver, even those who like to drive fast. Most people don't really understand this, so they tend to look at HP figures, and ignore torque and power to ratio figures which are much more important for daily driving. Most people don't even understand what "torque" means (in theory or in a practical sense), so I guess it's not surprising.

Mind you, I'd love to own a GTO!