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This is my understanding of how a torque-rpm graph is produced. Start the engine and go to wide open throttle, there is no load and this is basically the right end point of torque-rpm graph with the highest rpm. Start applying brake to increase load torque, the rpm slows down. If you keep doing this, shouldn't the torque continuously increase as rpm goes down because you need to press harder and harder on the brake?

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  • $\begingroup$ Torque will vary with many factors - cam timing and lobe shape, engine rpm, ignition timing, fuel/air mix - so many... $\endgroup$
    – Solar Mike
    Jan 22, 2023 at 18:30
  • $\begingroup$ But shouldn’t torque equal to load torque in this case and load torque is progressively increasing? $\endgroup$ Jan 22, 2023 at 18:57
  • $\begingroup$ If you go uphill, crest the top and then go down hill, then load is not constant... $\endgroup$
    – Solar Mike
    Jan 22, 2023 at 18:59
  • $\begingroup$ Are you asking how to use a brake-style dyno, or how to make an engine have less low-end torque? Most dynos today use the acceleration of a cylinder to infer torque. $\endgroup$
    – Tiger Guy
    Jan 23, 2023 at 16:55

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Start applying brake to increase load torque, the rpm slows down. If you keep doing this, shouldn't the torque continuously increase as rpm goes down because you need to press harder and harder on the brake?

What happens is that the engine will suddenly stall once the rpms drop to the peak torque rpm. You aren't used to starting at WOT at max RPM, so it seems counterintuitive. The primary reason the torque peak is not at idle rpm is because of heat loss in the combustion chamber which lowers the MEP.

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  • $\begingroup$ If the engine stalls at peak torque rpm, how do you get the engine to go below that in terms of rpm? Because if I decrease load torque, rpm will go up again. $\endgroup$ Jan 22, 2023 at 19:38
  • $\begingroup$ yup, you have to operate at part throttle at reduced torque, then when you pass the peak torque, you govern the engine to increase throttle. It's a bit hit and miss without an electronic governor. $\endgroup$
    – Phil Sweet
    Jan 22, 2023 at 19:53

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