Bottom Line
High Performance Diesels
Let Your Engine Breathe

Q: "I only need a lot of power for pulling the mountains but I don't want the engine to hurt itself. What can I do?"

A: Engines don't hurt themselves. It's the man with his foot on the throttle and hand on the shifter that hurts the engine. Years ago, an older Cummins engine technician told me an engine should always be in a gear that will allow it to accelerate while pulling a hill. This applies to all engines-diesel and gasoline. Let the engine breathe and run free. If your foot is flat on the floor and the rpm is dropping, get out of the throttle and shift into a gear that will allow the engine to pull the load using moderate turbo boost.

An NTC BigCam III engine develops approximately 17 hp per pound of boost. If you're pulling a mountain and using 30 lbs. of turbo boost you are developing 510 hp.

Assuming that you have installed our performance injectors, fuel pump, dual fuel line kit, mapwidth-enhanced turbo, and Fuel Preporator on your 400 Cummins, the turbo boost will produce 36 pounds of pressure. Take 36 times 17 and your engine is producing 612 hp. If the engine has stock pistons and timing, be very careful as to how much of this boost you use to get the job done. I personally would only use 30 pounds of boost to pull long hills or mountains. If you rebuild the engine and install the ceramic and TeflonT coated pistons and retard the timing without using the mechanical variable timing (MVT), you can safely use up to 35 pounds boost on long pulls. With the high lift camshaft and MVT, using 40 pounds of boost or 680 hp is relatively safe. Keep in mind that you don't have to use all of this power on every hill or mountain.

While traveling the interstate, I have noticed that many mountains have a break in the percentage of grade about half way up the hill. Most drivers keep their motors mashed and grab the next higher gear, only to drop that gear in another half mile. This driving technique never gives the engine a rest. My suggestion is to stay in the lower gear and ease up on the throttle (allowing the pyrometer and water temperature to decline), thus giving the engine a rest. You know that you are going to be back in that gear anyway for the rest of the mountain. Let the engine breathe and always use high rpm when working the engine. High rpm is 1800 to 2200. This rpm range gets the piston out of top dead center faster and less damage is done to the piston. At top dead center (TDC), using low rpm, the piston is being hammered because of internal engine pressure. Your pyrometer heat may be low, but if you're using high horsepower and low rpm, the internal pressure is high and that is what cracks pistons.

You can have a stock engine with high horsepower, but you'd better have the proper performance gauges and know what they are telling you. An NTC 300 has 130 pounds of fuel pressure. A 350 has 157 lbs., 400s have 176 lbs., 444s have 196 lbs., and a 475 has 200 lbs. By having the liquid-filled fuel pressure gauge in the dash you know how much power you're using. A stock engine has slightly more than two hp per pound of fuel. Assuming the fuel pressure has been increased in your BCIII 400 from 176 to 225 pounds, you no longer have a 400 hp engine but a 475 hp engine. As fuel pressure continues to rise, the rate of two hp per pound starts to diminish.

The true way of producing horsepower is by lowering the compression ratio, retarding the timing, and increasing the injector flow and turbo airflow. So by increasing fuel pressure and injector flow, the horsepower increases at an alarming rate. Think about how much power you're using to get the job done and the more you use, the less engine life you'll have in the long run. Fuel pressure, boost, pyrometer, and tachometer gauges are vital to long engine life. 

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If you would like our booklet on high performance Cummins diesels give Aimee a call at 724-274-4080. There is a $9.00 charge for shipping and handling. Just call us with your Master Card, Visa, AMEX or Discover card number and we will get one out to you. Our address is:

Diesel Injection of Pittsburgh, Inc.
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Cheswick, Pa. 15024

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The above column is the opinion of the writer and does not necessarily reflect the opinion or beliefs of Land Line Magazine or Cummins Engine Co.