

General BMW Questions
Use this forum to ask general question about the BMW ownership and keep up on the latest BMW news. Direct all model specific questions to model specific forums below. 

Thread Tools  Display Modes 
#1




Horsepower vs Torque Relevance and Explanation Attempt!
Quote:
Last edited by Bandem; 12222013 at 07:59 PM. 
Advertisement 

#2




Is this posted elsewhere in Bimmerfest?

#3




This is wonderful. My favorite post so far. Excellent write sir.

#4




Thanks a lot for posting this. Very well written and explained.
__________________
The Carpatia Express, Romania ED 2015 228i, 3,095 miles  Germany, Czech Republic, Slovakia, Hungary, Romania & Austria ED 2012 328i Coupe, 1,013 miles  Germany, Czech Republic & Austria ED/PCD 2007 328i, gone  1,523 miles  Germany & France ED 2004 330i, gone  2,390 miles  Germany, Luxembourg & France ED 2002 325ci still going  5,914 miles to Portugal and back to Nice ED 2001 325ci, gone  6,163 miles  Germany, Austria, Liechtenstein, Italy, France, Monaco, Andorra & Spain 
#5




I didn't read the whole thing but it seems OK. There is a far easier explanation of the two terms, however. Torque is just rotational force. It takes torque to spin the wheels against load. As long as you have more torque than the resistance, the vehicle moves. An 18 wheeler needs a lot of torque to get the load moving. The diesel engines they use have many times more torque than they have hp.
To accelerate quickly we need to do work quickly. In physics, work is defined as force times distance. The rate at which work is done is power. Our electric companies measure power in watts and kilowatts (thousands of watts). Watts and hp both measure power. 1 horsepower is defined as 33,000 lbft/min and is equivilent to almost 746W. It makes a lot of sense to me that what makes us accelerate more quickly is not force but the rate at which work is done. That is why hp is more important than torque for acceleration. Jim 
#6




Quote:

#7




Great explanation. It's like a engineering degree in a single post. Except in plain English.
__________________
2006 330i  ZSP/ZPP/ZCW/iDrive/Steptronic Sold 2007 Z4 3.0si Coupe  ZSP/Xenons/Navigation/6MT Sold 2011 535i MSport  ZMP/ZPP/ZP2 
#8




I have a degree in mechanical engineering which may be part of the reason the shorter explanation works well for me.
With respect to gearing, what is really going on is you have to get into and stay into the rpm range where peak hp is developed to maximize acceleration. It is correct to think of the gearing as multiplying torque but what we really care about for acceleration is the hp. If you shift early, the hp available is less so you accelerate more slowly. If you shift around red line, you are typically spending most of your time near peak hp so you accelerate more quickly. A really high numerical advantage at the start gets you into peak power very quickly. Tight gear to gear spacing keeps the fall off less when you shift gears. A manual transmission is in some ways helpful to acceleration so a lot of the time the manufacturer will compensate with the gearing. A 128i manual is faster than a 128i automatic. But in a 135 they are essentially identical. At least in 2009, the 135i automatic has more gearing advantage at the start to make up for the lower efficiency of the torque converter automatic. With the DTC the losses of the torque converter are eliminated so the automatic may be quicker if it still has a gearing advantage. Shifts of an automatic are quicker but torque converter automatics are somewhat inefficient due to the need to transfer power through the torque converter. Jim Last edited by JimD1; 12072012 at 11:56 AM. 
#9




Quote:
Quote:
Quote:
Oh yeah, and I am also going to add a great reference to my example to give people a sense of reality in acceleration times. From what I found/derived. I see that ((Velocity^2/2))*(Mass/Power)=Time Is this right, just to confirm? I want to show people that if you had optimal gearing, it would take a lot less power in cars to accelerate to 60mph (for example). A CVT running at peak HP can cut 060times off by as much as half in some cases. A toyota Camry with 200hp can do a 060mph in well under 5s with a CVT. Goes to show how critical gearing is, which is why I emphasize it. 
#10




I am a Chartered Management Accountant with an MBA...not a ME...however, the detailed and simple explanations above explained it all...thanks!
__________________
One should treat others as one would like others to treat oneself. 
#11




I don't think there are any simple equations to turn hp or torque into acceleration. We know that distance=initial velocity times time plus one half times the acceleration times the time squared. We also know that force equals mass times acceleration. The problem is with the force part of the f=ma equation. We can rearrange it to a=f/m but where do we get force? At any instant, I think it is the torque of the motor times the gearing minus any losses. But it constantly changes as the rpm changes and the gears change. So you'd have to do a lot of work to try and calculate this.
Maybe that difficulty is why the online calculators are empherical. They use published (or test) information to develop a relationship. Jim 
#12




I realize rpm is changing, but ignore automotive for a minute.
Analyze this equation purely from a physics and theoretical standpoint: ((Velocity^2/2))*(Mass/Power)=Time ^Does that equation mathematically work out? 
#13




One of the simplest ways to check an equation is to plug in the units and see if you get the result you want.
If we square velocity in Ft/sec we get feet squared over seconds squared. I like pounds for mass. 1hp=33,000 ft lbs/minute which we can convert to ft lbs per second by dividing by 60. That gives us units we can deal with. But if we go through the math on the left side of your equation, we get ft/sec or velocity. We do not get time. Jim 
#14




I added a section I forgot about differentials. Some more updates with pictures to be added.

#15




Quote:

#16




Added a section on CVT!

#17




Enough, already. Torque vs. HP? Lots of these guys don't even know the difference between acceleration and velocity, and what's more they don't care.

#18




Well, if they don't care they won't read it. This is for the ones that do .

#19




Awesome thanks so much!

#20




It is quite easy to get a link between power, torque and acceleration if you are willing to ignore friction and air resistance (and don't change gear .
Take the kinetic energy of the car, which is E= 0.5*m*v^2 where m is the mass of the car and v is its speed. Actually, there should also be a contribution from the spinning wheels, and the spinning engine, but we can imagine those have been absorbed in the definition of the mass m, ie the relevant m is a little higher than the actual weight of the car. Now, the rate of change of energy with time equals the power P. So, by the magic of calculus, we obtain: m*v*a = P where `a' is the acceleration (ie the rate of change of the velocity). So, at a given speed v your possible acceleration is proportional to the available power P, as you would expect. Of course, as you accelerate, your speed v goes up, so depending on how P changes with vehicle speed you may accelerate at different rates. Now, we use the fact that power is torque times (angular) velocity of the propshaft and that the angular velocity is proportional to the speed of the car (if you don't change gear), so P is proportional to v times the torque T, say. That means the v cancels on both sides and we get that m*a is proportional to the torque T! So as you speed up you find your acceleration is proportional to the torque T offered by the engine. The vehicle speed and the engine rev speed cancel out. This means that with a flat torque curve your acceleration will be constant, which is why people rave about "linear" acceleration when the usable rev band has a large chunk of constant torque, as it does in the V8 M3, for example, which has some 4000 rpm of usable rev band with essentially flat torque.
__________________
Now: 2011 M3 sedan 6MT. Was: 2009 328i xdrive wagon 6MT, 2008 Audi S4 convertible. Last edited by Candide13; 04252013 at 12:21 PM. 
#21




Not as easy as you may think. I can't tell you how many people I know who have the mindset of:
"Power is just a calculated value and therefore unimportant, your torque is what's important because its what actually accelerates you" or "My muscle car has only 200hp, but a billion lbft of torque and therefore is as fast as car with more power" And most of all, what drives me nuts is the extremely common confusion when people think torque is the same thing as work since both involve force and distance...Torque has nothing to do with work or energy. A lot of people don't realize that and its probably in the roots of that misconception. 
#22




That is a pretty thorough explanation on the difference between torque and horsepower. Really explains why horsepower is more important than torque in acceleration. Nicely done.

#23




Great texts here:
Let's not forget though that the application also plays a role in this discussion. If one wants to scream the engine at max power, power is everything that maters. However, high torque will keep the driving experience "effortless" in normal everyday driving. If you can afford it, you want to have the most power capable, large engine (torque), and the most sophisticated large transmission (more gears) that money can buy. That's what many cars are labelled as a delight to drive in any conditions (eg, Aston Martin 12 cyl., except race track), and others recognized as quite laborious to drive (eg, Ferrari V8, anywhere but in the track). Not very difficult, just a matter of money and/or application. LeoRochesterMI 2010 30ixDrive Black/Black 
#24




Reminds me of that old adage ...
Understeer is when you hit the wall with the front of the car.
Oversteer is when you hit the wall with the rear of the car. Horsepower is how fast you hit the wall. Torque is how far you take the wall with you 
#25




loled at the last one.
__________________
2015 X1 sDrive28i  xLine  Alpine White  Oyster  Lighting Package  Ultimate Package

See More Related BMW Stories 

Forum Navigation  

Today's Posts Search 
Thread Tools  
Display Modes  


