Bottom Line
High Performance Diesels
Your Questions Answered

Bruce Mallinson

With this issue of Land Line we are going to change the format of this article to a question and answer column. Got questions? Let's hear ‘em!

Q: "I own a Big Cam II NTC 350 CPL 632, what should I do to obtain more power? My engine has 236,000 miles since the last in-chassis rebuild and I don't want to change the pistons."

– John, Charleston, WV

A: The first thing we need to know is how much horsepower and torque you want. We are going to answer this question based on a horsepower requirement of 400 to 425 and then 450 to 500 hp.

For 400 to 425 hp, the first item to be added to the engine is a dual fuel line kit. Next are the injectors. BCII and III 350s have very small injectors and the flow must be increased by two sizes in order to obtain power. To supply fuel to larger injectors the fuel pump must be re-calibrated to supply more volume. These three changes will deliver the additional 75 hp. We recommend that a turbo boost gauge, pyrometer, and liquid-filled fuel pressure gauge be installed to monitor the engine. For 450 to 500 horsepower, the head gaskets should be replaced with our .013-inch thicker gaskets to lower the compression ratio. The camshaft should be removed and the off-set key changed to retard the timing. With these two changes we would go to a larger cup on the injector and increase the flow by four sizes and, naturally, the flow on the pump would be increased. The fuel suction line should be increased to size number 12. As the horsepower goes up the volume of exhaust also increases, so a larger turbocharger should be installed. With more flow in the exhaust system, dual exhaust is preferred with straight KTA 600 mufflers made by Donaldson.

Q: "What is a dual fuel line kit?"–Sam, Hartford, CT

A: Several years ago while troubleshooting a power problem on a small cam 350 we made a discovery. The COE Kenworth would only run 26 mph and the injectors were one size larger. The flow of the fuel pump was increased by 50 percent. The truck wouldn't perform even though the fuel pressure gauge (which was recording fuel pressure at the shut down solenoid) was recording 250 PSI.

I removed the fan support to gain access to the plug in the front head of the fuel galley and installed a second fuel pressure gauge and went for another test ride. There was still 250 PSI at the fuel pump. However, there was zero fuel pressure at the front head. The fuel was not getting through the heads. I noticed the fuel line from the solenoid to the rear head was a number six Aeroquip rubber hose. After removing the hose, we cut it into small sections and found the hose was swelled shut from the reaction of diesel fuel to rubber. A Cummins copper-coated steel line was installed and the power problem was resolved. During the final test run we had 250 lbs. of fuel pressure at the pump but only 200 lbs. of pressure at the front head. From injector number six to injector number one there was a loss of 50 lbs. pressure.

Since some Cummins engines produce two horsepower per pound of fuel, there would be a 100 hp loss from the rear cylinder to the front cylinder. Could this be why the rear pistons usually are the first to fail, and could this be why the engine runs rough when pulling a hill?

Marvin Winship, from Buffalo, NY was our first client to try the dual fuel line kit on his NTC 475. On the Cummins dyno in Buffalo, his Peterbilt put an additional 21 hp to the ground with the dual fuel line kit. That is 26.25 hp at the flywheel without increasing the fuel pressure. LL

Fax, or e-mail Diesel Injection of Pittsburgh Inc., PO Box 82, Cheswick, PA, 15024. Fax # (724) 274-8003. Our e-mail addresses are:,

If you would like a copy of our booklet on high performance Cummins diesels, send us a check or money order for $9.00 or give Aimee a call at 724-274-4080 with your MasterCard, Visa, AMEX or Discover card number and we will get one out to you. Our address is Diesel Injection of Pittsburgh, Inc., 1403 Freeport Road, Cheswick PA, 15024 My booklet is now available for viewing or download from the World Wide Web. The address is

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.