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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / March 2005How do I calculate Torque?
What is the calculation for torque? I know the Horsepower is: HP = Torque(lb-ft) * RPM/5252 But how do I get Torque? Let's see if I have had enough coffee.... If HP = T * (rpm/5252) then you could divide both sides by (rpm/5252) and maybe end up with T = HP/(rpm*5252) Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's > What is the calculation for torque? > I know the Horsepower is: > > HP = Torque(lb-ft) * RPM/5252 > > But how do I get Torque? Crap that should be T = HP/(rpm/5252) Need more coffee... Mike > Let's see if I have had enough coffee.... > [quoted text clipped - 13 lines] > > > > But how do I get Torque? That's a great algrebra lesson but I am assuming that I have neither HP nor Torque to calculate so that doesn't help me. > Let's see if I have had enough coffee.... > [quoted text clipped - 13 lines] > > > > But how do I get Torque? You have to have one or the other to come close or go to a speed shop that tests them on a dyno. Math and the real world seldom match up. Mike > That's a great algrebra lesson but I am assuming that I have neither HP nor > Torque to calculate so that doesn't help me. [quoted text clipped - 16 lines] > > > > > > But how do I get Torque? > That's a great algrebra lesson but I am assuming that I have neither HP nor > Torque to calculate so that doesn't help me. Ah, then your only choice is either a dyno session or one of these accelerometers. With the accelerometer you can compute torque at rear axle but it also takes a lot of algebra. Knowing the rear axle ratio you can convert rear axle torque to transmission torque, and ultimately to engine torque, though that is a bit iffy with an automatic (fine to do with manual box, however). I've seen formulas to calculate - or estimate - hp from acceleration data. If I had nothing and wanted to do better than a guess, I would probably get one of those G-force accelerometer gadgets. >> That's a great algrebra lesson but I am assuming that I have neither HP >> nor [quoted text clipped - 6 lines] > engine torque, though that is a bit iffy with an automatic (fine to do > with manual box, however). > I've seen formulas to calculate - or estimate - hp from acceleration data. > If I had nothing and wanted to do better than a guess, I would probably get > one of those G-force accelerometer gadgets. If you carefully plot acceleration versus speed, yeah, you can do it. If you use an average, like a 0-60 time, you will get an AVERAGE torque, not a peak one, or a torque vs rpm. Further, 0-60 is too high, because at speeds above about 35-50 mph, air resistance is using a considerable torque. It would not be just a function of mass of car. So you'd have to compute acceleration vs speed up to 30, at several data points. This is getting a bit hard to measure accurately by a stopwatch and speedometer, which is why actual accelerometer is good. I have seen some for a little over a hundred bucks. Using vehicle acceleration to measure hp is useless. We've done it with airplanes on takeoff, and we get a horsepower figure of about one-fifth of what we know the engine is giving us. Any movement thru air represents drag, any rolling wheels represent drag, and the losses through the drivetrain are significant. Further, unless the engine is operating at the desired RPMs (redline or max torque) it's all irrelevant. The most accurate measurements use an engine brake and torquemeters. Torque x RPM x 6.28 divided by 33,000 equals hp. Dan In late model cars you maybe able to capture the needed data from the ODII port. The rolling and air drag losses can be calculated. Afar as the drive train losses you ignore this and what you have is very close to what you would get from a chassis dyno. Stan > Using vehicle acceleration to measure hp is useless. We've done it > with airplanes on takeoff, and we get a horsepower figure of about [quoted text clipped - 7 lines] > > Dan > Using vehicle acceleration to measure hp is useless. We've done it > with airplanes on takeoff, and we get a horsepower figure of about > one-fifth of what we know the engine is giving us. I know several people who have taken their car to the strip, weighed it, and used trap speed to compute horsepower, and THEN gone to a chassis dyno and checked the power that way. They've all said that the numbers are amazingly close- usually within 10-15%, and nothing like the 1/5 you're talking about. I would guess that with an airplane trimmed for takeoff with the gear and flaps hanging way out, the high aerodynamic drag just before liftoff invalidates the method. > > Using vehicle acceleration to measure hp is useless. We've done it > > with airplanes on takeoff, and we get a horsepower figure of about [quoted text clipped - 5 lines] > are amazingly close- usually within 10-15%, and nothing like the 1/5 > you're talking about. 10-15% ?? Struth. With my vehicle performance simulation software (details on the website) I can get within a couple of percent of true bhp if the vehicle performance and weights, gearing etc are known accurately. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) > Using vehicle acceleration to measure hp is useless. We've done it > with airplanes on takeoff, and we get a horsepower figure of about [quoted text clipped - 7 lines] > > Dan True, but the original poster didn't have access to a dyno. Your reasons are why I pointed out that all data must be taken at low speed, and would be a hell of a job. One would need to take data about every mph, and need some sort of automatic data acquisition system. I should have also mentioned measuring DECELERATION also. This gives data on drag sources. One can see the slope of the deceleration curve change as aero drag drops off but tire and chassis drag, much of which are independent of velocity, stay there. One should account for these forces when computing hp or torque. However, rolling friction is typically relatively low, so acceleration data at low speed will give a REASONABLE value on a car with manual transmission. BTW, data I have seen on chassis friction indicated less than 5% of power lost there. Tires were about the same. > That's a great algrebra lesson but I am assuming that I have neither HP nor > Torque to calculate so that doesn't help me. OK then... what DO you have? You have to know something to start the calculation. And any calculation that depends on knowing cylinder pressure will have to be converted to BMEP, and you'll have to know the engine's geometry as well (bore and stroke). Calculations like that aren't very precise unless you have a full-blown computer model akin to "Desktop Dyno." If you have a known HP and RMP simply insert them into the formula you gave and solve for torque. Otherwise, torque is something that must be measured on the dyno. It would take a rather complex formula involving many variables for you to calculate it. > What is the calculation for torque? > I know the Horsepower is: > > HP = Torque(lb-ft) * RPM/5252 > > But how do I get Torque? Which is precisely why I am asking...thnx. > If you have a known HP and RMP simply insert them into the formula you gave > and solve for torque. Otherwise, torque is something that must be measured [quoted text clipped - 7 lines] > > > > But how do I get Torque? > What is the calculation for torque? > I know the Horsepower is: > > HP = Torque(lb-ft) * RPM/5252 > > But how do I get Torque? Devide the horsepower at any rpm by that rpm value. The value of peak torque may NOT be at the same rpm as peak HP, so you cannot compute peak torque by dividing peak hp by rpm for peak hp. However, you can get the torque at THAT rpm by dividing hp by rpm at the rpm for peak hp. It just may not be the PEAK torque (which usually appears at a lower rpm). In other words, you really need a curve of hp vs rpm to derive torque. In fact, usually the dyno outputs torque vs rpm and you derive horsepower by multiplying torque and each rpm. At least, that is the way it used to be. Modern computerized dynos output curves of both, doing the multiplication for you. Torque = moment of intertia x angular acceleration It would be explained in a college physics text. That's where I'm looking at it. You can calculate the monent of interia of a flywheel and calculate the torque needed to get it accelerating at a given rate but trying to calculate the moment of interia of the whole driveline including transmission would, I imagine, be quite difficult. As mentioned it would better be done by measuring resistance on a tread mill. > What is the calculation for torque? > I know the Horsepower is: > > HP = Torque(lb-ft) * RPM/5252 > > But how do I get Torque? -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-FreeNet email must have "notspam" in subject or it's returned Get a mechanical engineering degree with emphasis on thermodynamics. You will learn this in your 4th year thermodynamics classes. Erich > What is the calculation for torque? > I know the Horsepower is: > > HP = Torque(lb-ft) * RPM/5252 > > But how do I get Torque? Actually, its first-year "Physics for Engineering Students," regardless of engineering discipline (even us EEs had to take basic physics and mechanics. > Get a mechanical engineering degree with emphasis on thermodynamics. > You will learn this in your 4th year thermodynamics classes. [quoted text clipped - 7 lines] >> >>But how do I get Torque? > What is the calculation for torque? > I know the Horsepower is: > > HP = Torque(lb-ft) * RPM/5252 > > But how do I get Torque? Not sure what you're asking here. Typically, torque is MEASURED, not calculated. Horsepower is computed from measured torque. If you have a horsepower curve and you want to back-derive torque, just solve for Torque in your equation above: (HP*5252)/RPM |
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