Power & Torque

[Kifakw]

hello fellow petrolheads.

 

just want to clarify some doubts and gain some knowledge from the old birds.

 

Firstly, am i right to say that in general, torque is something that accelerates your car?

 

and power is something that determines the top speed of your car?

 

 

let say for a figure of ::

Civic Hybrid Engine Specs

Power: 82 kW , 110 HP SAE @ 6,000 rpm; 123 ft lb , 167 Nm @ 2,500 rpm

 

this engine produces the most torque when you keep your rev @ 2,500? and produces the most power when it's @ 6,000?

 

let say you wish to accelerate at its maximum best performance.

 

how will you go about doing it?

 

when you rev to 2500 - 3000, you change to a higher gear? since it generates the most torque at 2500.

 

thank you have a nice day!

[Thug]

Ehh...useful torque created by a internal-combustion engine only within certain rpm.

but peak of torque is always lower than the peak of power

 

Power is the product of rpm and torque....the questions u asked i'm not sure man...

Edited March 20, 2008 by Thug

[Thug]

Maybe u might wan to read this...

 

 

http://www.epi-eng.com/ET-PwrTrq.htm

[Kifakw]

okay read up the thread alr.

 

so for a real race breed engine, the most optimised power band will be peak torque at low rpm and whereas peak bhp at high rpm.

 

such that you don't have to always change gear and maximum acceleration at its best?

 

hmms.

 

but for a normal everyday car, how do you get the maximum acceleration? based on the figures above. anyone?

[Kyojin]

Well it really depends on what you mean "best performance" if you are talking 0-100 sprints, you should just rev past 6000 and upshift quickly. As you change gears, the revs will drop and you want the next gear to engage above the max torque and near the maximum power.

 

it's best visualized if you can get a torque/power curve graphed out.

 

But if you are going for economy and driveability, then you want to just go past 2500 and upshift there. it's no use reving all the way till 6000rpm, unless you are racing. that's not why u got a hybrid car to begin with.

[Imuya]

The example you have given is an exception.

 

The HCH has the IMA (motor assist) which brings the peak torque down to 2,500rpm.

 

While the max power output is at 6,000rpm when running WOT and on the gasoline engine only.

[Normal_aspirated]

wat you described in yer post is very practical and correct.

 

general motorists will not understand jargons and techie stuffs.

 

from the specs u mentioned, change gears between 2k to 3krpm, and WOT not above 6,000rpm will help not to waste more fuel.

Edited March 21, 2008 by Normal_aspirated

[Kyojin]

yeah i forgot about that. the IMA will shut off if the throttle is opened. The ICE alone will not give you that same kinda of peak torque.

 

which brings me back to my point that it's not meant to be driven like a Type R.

[Freestylers09]

just curious..heard from workshop people about this..

 

if highest bhp produced let say 5k rpm....to have the effect,we actually have to rev to 6-7k instead?as 5krpm is the moment the power being produce,not yet for the output to feel the effect...just bring across my mind since on this topic..

[Kelpie]

wat you described in yer post is very practical and correct.

 

general motorists will not understand jargons and techie stuffs.

 

from the specs u mentioned, change gears between 2k to 3krpm, and WOT not above 6,000rpm will help not to waste more fuel.

 

Some have mentioned that high bhp is not important on city roads but torque is more important. But I tend to think that for petrol cars, usually the higher the bhp, the better the torque and the faster 0-100Km/h result. So, when car shopping, I look at bhp 1st then Torque and follow by 0-100km/h. For the bhp versus rpm, there should be a balance, too early or gasty a torque may not be a cup of tea for normal drivers too.

 

Regards,

[Kyojin]

for city traffic, like in Singapore, i would go for something with low-end peak torque. considering that our highways are 90km/h max and even doing 120km/h on NSH, a turbo diesel makes a lot of sense.

 

but say if i am going for a small car with smaller petrol engine (1.5 or less), i would still look for something with lower end torque and tall 1-2-3 gearing, since it'll be used mainly for transport purposes.

 

that's of course for a pure transport point of view. of course if you are talking about driving pleasures, track time, or the autobahn in Germany, then something with high BHP, high up in the revband would be ideal.

 

anyway, horsepower is what car manufacturers would always like you to see first. it's a marketing gimmick. because it's easy for them to squeeze horsepower. this of course is referring to mass market models and not high-end sports/super cars. there's no doubt that a higher displacement engine will produce both higher BHP and torque. there are many variables and manufacturers can vary these specifications (bore, stroke, compression, cams, valves, gearing) to manipulate the power and torque curves. so it's not always the case that, ceteris paribus, a higher BHP engine means more torque.

[Kifakw]

Cause I was lazy to find for other specs thus I used a hybrid specs.

Okay, actually what I'm really interestd in is that. With the numerous specs figures given by the car maker, how does one actually make use of the figure in your everyday driving?

 

In detailed, at which RPM do you get the most acceleration out of it for overtaking and at which RPM the ride it's the most torquiest etc.

For example a Euro CTR specs::

Power bhp (kW) at RPM: 198(148) / 7800

Redline at RPM: 8000

Torque lb-ft (Nm) at RPM: 142(193) / 6500

 

Am i right to say that, this kind of engine is specifically made such that whenever the driver upshift to a higher gear, the RPM still stay within the maximum torque&bhp power band? In this way, the driver does not waste a few seconds for waiting to reach the desired rpm?

 

Oh wells, power and torque are really a myth and like women, hard to understand.

[Kifakw]

if it's really easy to squeeze horsepower.

why does car manufactures does not squeeze the most out of its model? Such that it can put itself as a cut above the other direct competitors? Like maybe for a 1.6 litre model, squeezing as much as 200+ horsepower just like the type R variant.

Or is it cause if their low end model is already available at 200+ horsepower, its very hard to market the type R model with the same engine specs?

 

sorry, if i'm asing too much haha.

[Exowraith]

Well to my understanding high bhp usually means a higher top speed whereas higher torque means better acceleration.

 

take ie Bugatti Veyron got 1001bhp only 270hp is needed to reach 155mph/250km/h the remaining 730hp to bring the car up to 253mph/407km/h

 

diesel engines have high torque but their peak hp is near the red line(5000rpm) around 4500rpm to 5000rpm.

 

the power band is usually around the peak torque and hp curves if you can take a look at dynocharts.

[Kyojin]

you are right. there are many variables, including model line positioning/marketing etc.

 

but technically, when they try to "squeeze" more HP by tuning (cams, valves, etc), the low-end will suffer. which means they have to strike a balance between max power and daily driveability.

 

taking an extreme example: F1 cars - they need super high revs to get maximum HP, but they cannot be driven at low revs. (of course gearing here comes into play too)

[Kifakw]

i see. yups it sounds logical for the balance between the low end performance and high end performance at the same time.

 

thanks alot for the invaluable knowledge. hee hee

 

yea the 700+ bhp is to overcome all the other external factors right, such as wind resistance and etc.

[Imuya]

wat you described in yer post is very practical and correct.

 

general motorists will not understand jargons and techie stuffs.

 

from the specs u mentioned, change gears between 2k to 3krpm, and WOT not above 6,000rpm will help not to waste more fuel.

 

Some have mentioned that high bhp is not important on city roads but torque is more important. But I tend to think that for petrol cars, usually the higher the bhp, the better the torque and the faster 0-100Km/h result. So, when car shopping, I look at bhp 1st then Torque and follow by 0-100km/h. For the bhp versus rpm, there should be a balance, too early or gasty a torque may not be a cup of tea for normal drivers too.

 

Regards,

 

The best thing to see is the torque curve of a car. But too bad sales people don't give out dyno charts of the car they are selling.

 

Edit: after reading the thread again, just wanna add that:

 

IT'S ALL ABOUT TORQUE!

 

Specifically torque over your RPM range.

High torque, low rpm range (vans, lorry) = low horsepower

Low torque, high rpm range (VTEC!!!) = relatively high bhp from a small engine

HIGH TORQUE WITH 10,000RPM RANGE = WTFOMG OWNINGZZZZZ

 

Just had a 12hr workday on a Sat, forgive my rambling...

Edited March 22, 2008 by Imuya

[Eyke]

what you want are your optimal shift points, but for that you'll need to need your gear ratios (which also determines your top speed) and horsepower curve:

http://www.bgsoflex.com/shifter.html