marshall13
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the first step is to admit your ignorance
Posts: 301
Fort Lauderdale FL
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SV og LS wrote on 05/27/09 at 11:51:47:marshall13 wrote on 05/27/09 at 10:58:41:SV og LS wrote on 05/27/09 at 01:17:19:All dynamometers measure torque from which power is derived. Inertia dynos such as DJ 250 and its clones can print out a lot of information depending which sensors are connected and included on a printout. I part timed on a major dyno shop from 2000 to 2002.
inertia dynos cant measure torque.... they calculate HP by how quickly the wheel can accelerate a large mass... if you track rpm during the run, you can interpolate torque fairly accurately, but you cant "measure" it... the dyno sheet Jim posted previously in this thread is an inertia dyno run.... road speed and HP only, no engine speed or torque figure.... a strain-guage dyno works like a prony brake... as the dyno spins, a load is applied to the roller(a brake is applied)... the restraining arm for that brake has a strain guage on it.. that supplies the torque figure(accurately measured, not estimated by formula), that is then interpolated to HP by keeping track of engine rpm... all engine dynos are strain guage type, but chassis dynos can be either.... needless to say, any mis-calculation or measurement of the rollers mass will effect the accuracy of the inertia dyno... strain guage type can be tested for accuracy with a torque wrench.... Even with inertia dynos I always think it as torque which is inertia times acceleration and horsepower comes only when torque is multiplied with engine rpm. Is the strain gauge similar to old water brake dynos, measuring at a constant rpm? Around here (Denmark and northern Germany) I think most if not all dynos are inertia dynos. a water-brake dyno is pretty much a low tech strain guage type, as is the prony... though they dont run them "constant speed" anymore.. the controller applies load at a particular rpm until the engine slows, the distortion of the restraining arm at that point give the actual torque... a run would tend to start at redline, then be loaded to the torque peak rpm(engine stall point, or the threshold of it), then from low revs up to the peak torque point (or vice versa, one can do the low to peak first, and high to peak second, results will be the same)... a prony brake does the same by by using a scale at the end of a lever attached to a brake that slows the flywheel of the engine... the lever is the restraining arm, the scale the strain-guage... by running a constant engine speed, and applying the brake, one gets the "weight" measurement of the scale at the point of the engine slowing... you then repeat for each ordinate of your rpm scale... the weight registered multiplied by the arms length gives the torque....
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