Optimal Shifting RPMs (Long post)
06-03-2001, 05:19 PM
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Optimal Shifting RPMs (Long post)
Not too long ago, there was a question regarding the best rpm to shift.
I put together a spreadsheet for the 180, using the tranny gear ratios (allowing for the different shaft for 5th gear). It shows that a shift from 1st to 2nd allows the rpms to drop to 51% of the 1st gear rpms (assuming same speed) and subsequent shifts drop the needed rpms to 64%,72%, & 79%, respectively with each shift (5 speed).
Assuming you shift at about 5600 rpm, where the HP maxes, and where the torque curve starts to fall, This will drop the rpms from 5600 to 3100, 3600, 4000, & 4400 as each shift occurs.
Each of these rpms is well within the range of max torque. One could hold the rpms to 6400 to keep the rpms higher on the HP curve, but the torque curve is rapidly falling by 6000 rpm.
Does anyone know which is most important, the HP or the torque at the time of engaging the next higher gear. I would guess torque, but it IS only a guess.
I can e-mail the Excel file to anyone who wants to examine the figures more closely.
But, my question is: Is this just mathematical hocus-pocus? Does this mean anything in the real world? Are my figures correct?
Comments?
Phil
I put together a spreadsheet for the 180, using the tranny gear ratios (allowing for the different shaft for 5th gear). It shows that a shift from 1st to 2nd allows the rpms to drop to 51% of the 1st gear rpms (assuming same speed) and subsequent shifts drop the needed rpms to 64%,72%, & 79%, respectively with each shift (5 speed).
Assuming you shift at about 5600 rpm, where the HP maxes, and where the torque curve starts to fall, This will drop the rpms from 5600 to 3100, 3600, 4000, & 4400 as each shift occurs.
Each of these rpms is well within the range of max torque. One could hold the rpms to 6400 to keep the rpms higher on the HP curve, but the torque curve is rapidly falling by 6000 rpm.
Does anyone know which is most important, the HP or the torque at the time of engaging the next higher gear. I would guess torque, but it IS only a guess.
I can e-mail the Excel file to anyone who wants to examine the figures more closely.
But, my question is: Is this just mathematical hocus-pocus? Does this mean anything in the real world? Are my figures correct?
Comments?
Phil
06-03-2001, 05:30 PM
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Torque. You should also account for inertia and drag...
so that acceleration (in g's) in next gear = or exceeds that in current gear. You are trying to keep the car accelerating at the max.
Assume that acceleration curve in any gear very closely mirrors the torque curve. As engine torque x overall gear ratio drops off in 2nd it will be at some point equal to rising (or steady) torque in third. That's the theoretical shift point.
Performance Trends has programs which do that well.<ul><li><a href="http://www.performancetrends.com/">http://www.performancetrends.com/</a</li></ul>
Assume that acceleration curve in any gear very closely mirrors the torque curve. As engine torque x overall gear ratio drops off in 2nd it will be at some point equal to rising (or steady) torque in third. That's the theoretical shift point.
Performance Trends has programs which do that well.<ul><li><a href="http://www.performancetrends.com/">http://www.performancetrends.com/</a</li></ul>
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