Torque, rpm, kw. Does it make sense?

[spetz]

Hi guys,

Can someone explain to me

1. A car that makes 200nm at 4,000rpm is making 112hp.
Now, at 8,000rpm if it made 200nm it would be making 224hp.
My question: Is the rate of acceleration faster at 8,000rpm than 4,000rpm or is it the same?

2. Seeing as the torque can start dropping off but hp still rising, does the cars rate of acceleration decrease after the torque drop point or hp drop point?

[curtis265]

1. the rate of acceleration is higher when you have more torque - the actual acceleration you feel throughout the powerband is in the same 'shape' as the torque curve

2. yes.

Power = torque x angular velocity

when someone says ur honda has no torque, they mean you have no low-rpm torque

here's some brain food
http://craig.backfire.ca/pages/autos/horsepower

[spetz]

So, by the answer to the first question, I understand that considering you can never get more than 100nm per liter of capacity then all the modifying we do to an engine is doing nothing more than shifting the torque curve to higher rpm? And our rate of acceleration does not change, it just shifts?
Something sounds wrong...

[curtis265]

not quite, you can increase the torque output in certain areas. - for example, increasing to a well sized catback exhaust will generally increase torque throughout the RPM range.

if u get an exhasut slightly too big though, it can have the effect of losing some low end torque and shfiting it to the top as you are thinking.

who told u 100 Nm/L? There is a limit but i doubt you'll ever achieve it.

[mocchi]

isnt it in the gearing? torque from engine get multiplied with gear ratios x fd.

that's why 1st gear will have shitloads of torque to move car from standstill, as it has shorter gear (less teeth).
wont be able to launch car from standstill on 5th gear.

more torque on first gear so acceleration will be quicker on first gear rather than 5th. provided you have enough traction.
more torque will give more acceleration so car will accelerate quicker on 1st, less on 2nd, even less on 3rd. vehicle speed is other thing.

gearing is the 'replacement' for displacement.
also the reason why ppl prefer b16a gearbox rathen than c2 gearbox, it has shorter gearing, more oompf when you floor it even though engine stays the same.

[spetz]

Well to be honest I always assumed that because hp is in higher rpms, what it really means is the fact that at 8,000rpm a car can travel the same speed as another at 5,000rpm because of gearing, and even though torque may be less the gearing effect makes up for it.

I thought in an NA car 100nm/L is more or less accurate?
The only thing I was looking at the Mugen FD2R and it had 237nm however not sure if it is a 2.0 or 2.2L

[curtis265]

nah i'm not disagreeing with you, i just never heard of it

gearing is related to the actual torque output to the road, yes. so really, the faster car is the one with more power, not torque - you need a mix of a high redline and a flat torque curve

[aaronng]

Power is the rate at which work is done. So if you are trying to cover a particular distance (say 400m), you will reach the end quicker if you have more power.

Torque on the other hand is a measure of force that is applied to rotate an object around an axis, in this case the crankshaft.

So in the end, Power is related to how quick you reach the destination (assuming you use full throttle to get max power) and torque is an indication of how quick it accelerates.

The gearbox just multiplies torque that is applied on the road by sacrificing engine revolutions (RPM) through gearing.