Driving With That Classic Cummins

In the March issue, I implied that even back in the pre-1985 “Dark Ages” (before sophisticated engine and drivetrain designs), a well-trained, professional driver could achieve 10 mpg (Imp.) in fuel efficiency. Well, that brought a flood of calls asking what that driver was using. I was thinking specifically of the 400-horsepower Cam-IV Cummins NTC of the mid-to-late Eighties-tough puppies. Let’s look at what I learned about how to squeeze maximum miles per gallon out of that block.

(A word of caution: I’m speaking specifically about that engine. Don’t try the following technique with even a Cummins 444, because it ain’t gonna work. The 400 had a crankshaft design life of about 400 years, it seems, and that’s one of the keys to success here.)

The beauty of the Cummins PT injection system was that it had only an idle regulator and a high-rpm no-load cut-off at 2238 rpm, with nothing in between except your foot, your buns, and your head.

Let’s start with your foot. The key to driving this engine is throttle control, and that means foot control. I used to wear a tensor bandage on my right ankle to eliminate any flutter on the throttle that might transmit unwanted fuel demand to the engine.

Your foot, at cruise, must be at a right angle to your ankle (forget about wearing those big-heeled cowboy boots). Sit in the cab with engine running and bring the rpm to about 200 above loaded cruise. Have a buddy outside hold a carpenter’s square along the throttle pedal, and from your heel to the back of your knee. Adjust the throttle rod until the angle is exact by the square.

Now you can drive with a relaxed muscle structure in your leg and can hold the throttle absolutely steady for long periods of time. That’s important for fuel efficiency with any engine, but really vital for this one.

The PT injection system measured fuel input to the engine in pounds of fuel per hour, and that was totally determined by the position of your foot on the throttle. Let’s say you’re cruising at 60 pounds per hour of input, or about 8 mpg. You hit an “invisible” grade of 0.5% (a six-inch rise across 100 feet of road), but the power draw to hold road speed instantly demands another 50 hp, (i.e., input of another 17 pounds per hour of fuel). Because your foot is your computer, you hold steady and let your unit slow by three or four kilometres an hour. You’ve just put the equivalent of $4 an hour into your pocket.

Going down that same grade, slack off a little as speed begins to rise. Let kinetic force take over, and the only fuel the engine needs is for the idle governor (just about nil).

Acceleration costs big, so regulate that requirement. Your 60-pounds-per-hour cruise requirement goes stratospheric during acceleration, and even worse on a hard push. You’re spending well over 200 pounds just about every time you squeeze down on the accelerator. That tensor bandage should let you feel when you have too much toe down.

Accelerating into a grade costs you 250 to 300 pounds per hour in extra fuel burn, so don’t do it! You have only about 1000 pound feet of torque available with this engine (compared to maybe 1500 for the guys with the new-age electronic wizards under the hood), so don’t try to compete. Let your revs fall back on the grade until they’re still dropping at about 1400. That’s your shift point (peak torque is actually 1300 rpm, but you need shift time above that).

Now, never shift without the clutch, and a loaded downshift requires you to break torque by touching the clutch while you are still on the throttle, to keep flame on the turbo. Feather the throttle just as the clutch breaks torque, slide out of gear, and slide in just as the rpm rises to match (no clutch-in required), and you’ve now grabbed your new gear without losing the torque you already have. You should still have about 800 pound feet on the wheels.

Lastly: road speed. Optimum loaded average road speed for any tractor is 89 km/h. Below that, your time is worth more than the added fuel savings; above that, you’re paying more than your time should be worth. Increasing cruise speed beyond 90 km/h will cost you about $100 per hour at the end of the day, when you compare time saved versus increased fuel cost. This applies to your ’89 KW or a ’99 full-dressed anymake/anyengine beauty.

Electronic control units are marvelous things, but like they say: where there’s a will, there’s a way.even if you’re still cruising with that classic Cummins!

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