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Monday, July 2, 2012

Progressive Shifting - Save Time and Fuel!

I have found over the years that progressive shifting is the way to go, once you get used to it. By catching the next gear early, you not only speed up your shifts (you don’t have to wait as long for the engine RPM to drop) but you will tend to shift much more smoothly. It’s been a proven fact that you will accelerate more quickly than if you wind out the motor in each gear. Steve Sturgis has a great explanation of a 13-speed below below. It works just as well with any transmission. You can listen to a short uTube video by clicking the link above.

Great explanation thanks to Steve. Link to their site follows.
Steve Sturgess
Senior Editor  Heavy Duty Trucking Magazine

Most heavy trucks — those 18 wheelers you share the highway with — use bigbore diesel engines with displacements in the 14-15 liter range. They are relatively slow revving, usually topping out around 2,000 where they produce their maximum horsepower. The peak torque is usually down around 1,100 rpm. And since torque is what you want to get a truck under way, the driving technique for maximum acceleration is to keep the engine down close to this torque peak. It happens this is also the most economical way to drive a big diesel.
  Fortunately, these trucks also have multi-speed transmissions. Ten speeds are very common, but drivers who enjoy "gearjammin'" and who understand the need to keep the engine in as narrow an operating range as possible, specify transmissions with 13 or 18 speeds. (I did once see a truck with a 13-speed and a 4-speed auxiliary, giving it 52 forward gears, but I regard this as a little excessive.)
  Generally, when you shift a 13-speed, you'll roll off about 200-400 rpms between shifts, depending on gears and engine speed at the shift point. So ideally you should never need to accelerate to more than 1,500 or 1,600 rpm on the tachometer, then grab a gear and the engine comes back onto the torque peak at 1,100 rpm.
  However with the deep reduction in the lower ratios, you don't need to push it so hard, and the fewer revs you use, the less fuel you use. So we encourage "progressive shifting" where you use minimum revs just to get the truck rolling, progressively opening it up as you go through the gears.
  For example, when I drive, I'll select 1st gear then engage the clutch at engine idle (around 650 rpm). I'll tickle the throttle to add maybe 50-100 rpm then ease it into 2nd. With the engine speed so low and the closeness of the ratios in the low gears the rpm drops between the gears are very small. I'll carry on the same way through 3rd and 4th stretching out a little so that by the time I make the range shift to the high side of the transmission, I am maybe pushing it out to around 1,200 rpms. The means that as I complete the shift the engine is a little below the torque peak, but it is still plenty torquey enough to pull 5th. If this is a 13-speed, the next shift requires a decision: do I take a whole gear to 6th, or do I split the gears to 5th overdrive. Usually I like to split — because again, you need fewer revs to drop back on the torque peak. From there I go a 1/2 gear at a time through 6th, 6th over, 7th, 7th over, 8th and then into the big hole — 8th overdrive — which is the fastest ratio in the transmission. For these last couple of gears I'll usually let it out to 1,500 or 1,600 if I'm trying to get up to speed quickly, say on a busy freeway onramp.
  Progressive shifting is easier on the truck and, because you are keeping the engine near peak torque, it is actually the fastest (and most economical) way to get up to cruising speed.

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