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Old 04-26-2002, 05:17 PM
dabenz dabenz is offline
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Join Date: Apr 2002
Location: eastern ND
Posts: 657
Engine rpm = long life. Torque = fuel mileage and acceleration

Thanks to this web site I’m going to tackle the rear end springs and rubber stuff this summer, then my trip meter, then my clock, then maybe rebuild the parts car engine for McDonalds fuel. In return, here’s some old school diesel driving tips that are still useful today, using daBenz (1970 220D) as an example.

daBenz doesn’t have a tachometer. The dots on the speedometer represent the maximum speed for a particular gear. So when do I shift and how do I make the engine last? I use the old 80/60 rule.

(Attach graph here.)

It looks a lot more complicated than it is. First, find the maximum speed for each gear, preferably from the owner’s or service manual. Place a dot for each gear then draw a line from the dot to 0 mph and 0 rpm. Find the rpm that corresponds to the maximum engine (SAE) horsepower. Multiply that rpm by 0.8 and draw the 80% HP line. If I was to overlay the SAE horsepower curve on my graph I’d see that I develop about 90% of available horsepower at the 80% HP rpm, which corresponds to a top speed of 65 mph in 4th gear.

Where I shift comes from the torque. Find the rpm that corresponds to the maximum engine torque and draw the Full Torque line. Multiply that rpm by 0.6 and draw the 60% Torque line. If I was to overlay the torque curve on my graph I’d see that I develop 95% of the available torque at the 60% Torque rpm, which is about where I’d like to shift.

On the flat, shift somewhere between the 60% Torque and the Full Torque and give it just enough throttle to accelerate to the next speed. Try the low gears below the 60% curve (progressive shifting). If the engine pulls without smoking or heating up then you’re not lugging the engine. Practice makes perfect. Drive up to the Full Torque rpm and shift quickly to get maximum acceleration.

There’s no need to accelerate beyond the Full Torque rpm unless the road is too steep and the engine lugs in the higher gear. Downshift, accelerate to the 80% HP line and shift up. If the engine still lugs, then downshift and enjoy the view. I’ve pulled 7% grades in the summer at 45 mph in 3rd gear and kept the temperature gauge from moving more than a needle’s width.

Old rule also works with new turbocharged truck engines that don't have an injector pump. HP is HP, torque is torque, diesel is diesel.

Attached Thumbnails
Engine rpm = long life.  Torque = fuel mileage and acceleration-torquespeed2.jpg  
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