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Engineering/Technical Ask technical questions about cars. Do you know how a car engine works? 
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03142001, 03:08 PM  #1  
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What does the "b" in bhp stand for? what is the difference between torque & horsepower? I have heard detailed descriptions of horsepower but not torque....


03142001, 03:11 PM  #2  
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base horse power .......does that answer ur question


03142001, 03:36 PM  #3  
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1 of em


03142001, 04:41 PM  #4  
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actually the "B" stands for "Brake" because dynomometers used brakes(of some type) to resist the motor torque which is used with RPM to calculate HP


03142001, 07:07 PM  #5  
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Yes, it's "brake".
Torque is a rotational force. Kind of a strange concept, but I usually think of it like this....if you twist off a bottle cap, you apply torque. If you lift something, or pull something (straight line force) you are using power (hp). Torque in a car can be found by this equation..... (hp * 5200)/RPM I think that's it...don't remember exactly.
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03182001, 07:59 PM  #6  
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enzo
You're almost right: its 5252. In layman's terms, horsepower is simply an application of torque. There are in general two ways to make big horsepower: large amounts of torque at moderate rpm, or moderate amounts of torque at high rpm.
A better measure of an engine's capabilities than horsepower is its torque, and in fact its torque curve. From that you can easily extrapolate the horsepower curve, as the horsepower curve is similar to a derivative (an old term from calculus  which by the way I passed by the skin of my teeth  but it follows the same principle). Hopefully this example will help explain the horsepower versus torque principle. Texan posted a good explanation of this on SHO. I've got it on my computer at work, so I'll repost it here later. 

03212001, 04:23 PM  #7  
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hey enzo, you got that off of Popular Hot Rodding didn't you? I watch the same thing every chance I get. Thats what they said a few weeks ago.


03212001, 05:33 PM  #8  
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Here's that repost; this is quoted directly from texan in an old SHO thread
damn, I swear I have typed this explanation out like four times but never have saved it . OK, so here's the speech again in layman's terms, hope it covers everything (and accurately)...
HP= a measure of forced over time, where 1 hp is equal to 550 lbs lifted one foot in one second. Expressed in terms of torque, 1 hp equals 550 ft/lbs per second. Noticed this measurement is time based. Torque= the measure of instantaneous rotational force an engine can produce. Notice this measurement is not time based. hp= torque x RPM / 5252 torque= hp x 5252/ RPM With these two basic equations, you can figure out how much hp or torque you have at a given RPM with either measurement. Notice that because of this equation, an engine can NEVER make as much hp before 5252 RPM as it can torque, and the opposite is true above 5252 RPM. Any dyno chart you ever see where the two plotted curves don't cross at 5252 RPM is a bogus dyno chart. What's important to understand as this pertains to vehicles is that hp is not a direct determinate to accelerative ability. Horsepower is more a way of easily summarizing the abilities of a car, but not an exact or necessary thing. Horsepower is basically a way of stating torque with gearing already taken into account, which makes the pain of figuring out torque multiplication factors less needed for generalized performance figures. Here's the three most important factors in determining how fast a car can accelerate... effective torque at the wheels overall vehicle weight available traction total drag (constituted mainly of aero drag at speeds above 50mph) Since we are just talking about differences in power and how it effects acceleration, we can basically toss out everything except torque since the others can be assumed to remain constant(please remember this is just a simple hypothetical). So now you need to understand what torque multiplication is, and here's an excerpt from an older post I made on the subject... "What this all means comes into focus when you understand the concept of torque multiplication. Torque is the only productive force that an engine creates, so when talking about acceleration relative to gearing this is all that matters. Torque multiplication takes the torque output of the engine at any specific RPM and multiplies it by the overall gear ratio (think of this as gear reduction) to show us how much torque is actually being transmitted to the wheels at any given speed and in any given gear. An example: Engine A is making 150 ft/lbs of torque at 3000 RPM, and is in our earlier hypothetical 1st gear which has a ratio of 3.42:1 and a final drive of 4.27:1. Effective torque at the wheels @ 3000RPM = 4.27 x 3.42 x 150... which equals 2,190 ft/lbs of torque to the wheels! Ever wonder why acceleration is always better in lower gears than higher gears? That's why. The true accelerative ability of a car is defined by these laws, and in the most pure sense torque at the wheels is all that matters (meaning HP is not directly important or a determinate of potential speed). " So knowing all this new stuff, you should be starting to see the big picture. Now you have the definitions of horsepower, torque and torque multiplication, along with a basic understanding of what they mean. All the concepts surrounding this haven't been covered here, so if anything isn't clear or I forgot something important please just say so, and either I or someone else can fill in the blanks. Hope this helps, peace 

04062001, 10:28 PM  #9  
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Hey JD, thanx for the repost of that, I learned alot and have since developed a formula to calculate 060 mph time that will be within 5% of the true numbers.
I noticed something about those gears. The torque seems to be kinda on the small side so taking the tire size of a tempo (215/70R/14) with a redline of 6000 rpm, that gear will only last to 29.6 mph, and that is being a little generous. And also to the texan, torque is directly related to acceleration but horsepower gets you the top speed. 

04242001, 05:57 PM  #10  
Oldie

This is some good info, bu tI knew it all already. It took awhile, though. Could you post that 060mph formula? Please?
Also, you can calculate horsepower from the quarter mile time. It can be off by a fair bit (usually less than the car has) but it is a rough approkamentation. hp=(mph\234)to the exponent 3, *vehicle weight. Examle, it goes 125 mph in the quarter. 135/234=.534 .534*.534*.534=.15 .15*3000=457 bhp Try it with other vehicles. If it is driven really well, it should be close. But some cars (like big SUV's) can be up to 20% off. Still, a general rule is better than none
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04252001, 03:59 AM  #11  
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Psman32@af Actually, I have never understood why people think hp is what determines top speed. Now I'm not saying it's not, but I would like you to answer this hypothetical on top speed....
A vehicle has an engine producing a perfectly constant torque output. For this example, we'll say 200 lbs/ft at all RPM, with a redline of 10,000 RPM. So it's now easy to calculate hp from this, but what about top speed based upon that hp output? Let's say this vehicle's top gear allows it to put down 640 lbs/ft of effective torque to the wheels (a 0.8:1 gear with a 4:1 final drive) throughout all of said top gear. Now we'll say this motor at redline, in this gear, can reach 200 mph. Now at 150 mph the total resistance to motion is 640 lbs/ft of pressure, while 200 mph is 700 lbs/ft of pressure. So since the horsepower curve is still climbing (to a total of 380hp @ 10,000 RPM), would the vehicle continue to accelerate or reach top speed at 150 mph?
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12092004, 08:09 AM  #12  
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Re: more stupid questions!
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All other things being equal hp DOES determine top speed. IE. If the cars are drag equivelant (rolling and aero) the car with the greatest HP can go the fastest. (assuming final gearing is correct) Both cars will accelerate until all possible work energy is used. Please note that drag is not linear. eg. To double your speed takes 4 times the energy. Jim SR Racing 

12092004, 11:16 AM  #13  
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Re: Re: more stupid questions!
Quote:
HP comes from torque, so in order to get more HP, you have to either have more torque, or make the torque peak higher in the RPMs so that it generates more HP. Without torque in a rotational engine, there is no such thing as HP.
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12092004, 11:51 AM  #14  
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Re: Re: Re: more stupid questions!
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BTW, a quiz. Ever seen an engine that produces maximum torque at zero RPM? Jim SR Racing 

12092004, 12:02 PM  #15  
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Re: more stupid questions!
Yeah a steam engine. Good question. I love those old steamies. My dad and I used to go the Grainger's Picnics when I was a kid. Growing up in Amish country, there were so many of those old steam tractors. *sniff* I can smell the coal burning now.
You're right... "inertia" was a bad word to use. I tried to use it as a metaphor for demonstration's sake, but it didn't come across.
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