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## Shift Points

 To: mgs@autox.team.net Shift Points Simon.MATTHEWS@st.com Thu, 12 Dec 96 12:56:24 +0100
 ``` I'm sorry, but the 'shift to optimum torque point' is just total BS. It may be an approximate rule of thumb, but to apply equations and comment on how difficult it is to apply is just beyond reality. So what is reality: well as any engineer will tell you, it is power that counts -- power at the road wheels. Not strictly BHP, since we have a dynamic situation and one has to consider not only the inertia of the car, but also the inertia of the engine (note that in a higher gear, less energy is used accellerating the engine than in a lower gear, when accellerating the car between the same 2 speeds). Also, losses through in the gearbox may be different in different gears (remember that BHP is usually quoted at the flywheel). There is a peak power point for the engine -- unless you have a CVT gearbox (continuously variable transmission) -- and when accellerating the car, you have to change the engine revs, so usually, you will want to start below the optimum power point and take it past the optimum power point -- spanning the peak. The Saab turbo example just shows that the advice not difficult to apply to engines with flat torque curves, it shows that it is WRONG! What does all this say about when to change up: that the point can only be determined by a detailed study of the engine's power curve, the gear ratios, losses in the gearbox, etc. There is no easy *accurate* solution, although an *approximation* may *perhaps* (I don't personally know) by changing up at the point that will give optimum torque in the higher gear. But since this is only *rule of thumb*, *don't* try to use it to obtain accurate, definitive numbers. Finally, one is rarely driving on a straight endless track, without other traffic, so the optimum shift point may be varied for drivability and control reasons. Regards, Simon ```
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