Rebuilding Older Trucks

2018, Air Filter, Big Cam Cummins, Peterbilt -

Rebuilding Older Trucks

Many of us who have been in the trucking industry all our lives, and even some younger people, have a fascination for antique trucks.  There’s something special about the rebuilding of “Old Iron,” a Class 8 truck from the mid 1970’s and older. This article will be about a commonly overlooked  problem when installing a newer diesel engine into an older truck. Please keep in mind that the older low horsepower and smaller cubic inch engines do not require a lot of fresh air measured in CFM (cubic feet of air per minute).  The small cam Cummins engines that were 335 horsepower and lower only required about 850 CFM. The older J series Cummins, 237 Mack, and the 671/8V71 Detroits all required less than 800 CFM. We all know the duel Vortox polished stainless-steel air filters equipped on the newer Kenworth, Peterbilt, Western Star, and Freightliner Classics are beautiful, but they are also functional. The 15-inch diameter filters are 1700 CFM each giving the engine 3400 CFM of air.  A 600 horsepower 14, 15, or 16 liter diesel engine requires 1600 CFM. Many of you now have 750 horsepower and that will require a whopping 1987 CFM. One air filter under the hood of the aerodynamic trucks of today will not supply that amount of air. Fortunately, those of you who have that kind of horsepower are wise enough to not pull mountains with your foot flat on the floor.

Back to the older trucks, we currently have in our shop a beautiful 1968 Peterbilt that has been re-powered with a Big Cam 3, 350 horsepower, 14 liter Cummins engine.  This truck is based in Texas and is owned by a crop duster who works out of town most of the time. His wife drives the motor home in which they live, and he drives the ‘68 Pete which pulls a stacker trailer with their vehicles and rock crawler Jeeps.  He was not able to find any Big Cam Cummins mechanic in Texas who could help him with his problem, which was the engine always fell flat regardless of how much fuel they gave it. There is more to performance than just adding fuel. After several conversations on the phone I had him bring the ‘68 Pete to our shop In Saxonburg, Pa.  After a few hours of working on the engine we uncovered several problems. The engine was supposed to be built to 400 horsepower, but has 350 horsepower pistons, which we can work with. Next, the timing is set for a 400, which is good, and it had 400 injectors, which is also good, but the fuel pump, wow. It was rebuilt twice before an older Cummins guy made changes to the fuel pump without a fuel pump calibration stand to test what he did.  Sometimes you can get away with this if everything else is tuned for the same performance level. The next problem was the turbocharger, it was too small for the horsepower requirements of the owner. We also found worn main and rod bearings, several oil leaks, and the rear engine mounts fell out in our hands.

As I was standing there looking at this ‘68 Pete I thought of a good friend of mine that is retired from trucking and lives in the Weirton, West Virginia area by the name of Paul Lambert.  Paul was a great mechanic and owner operator. He owned a skinny windshield 1971 Peterbilt which was powered by an NTC 335 Cummins. Before I met Paul, he installed a Big Cam 400 into his Pete.  There was no turbo boost gauge or pyrometer, (exhaust gas temperature gauge) in the instrument panel. If you’ve read my articles before, you know these gauges are essential on any turbocharged vehicle. After Paul installed the gauges, we realized his problem was a lack of air.  The exhaust manifold was flaking from too much heat. Driving without a turbo boost gauge and pyrometer is like driving blind. You do NOT know what is going on in your engine. I was working with Paul at his home and standing next to the ‘71 Pete’s single air cleaner housing it appeared to me that the air cleaner housing was for a 335-horsepower engine which requires 300 CFM less air than the 400-horsepower engine.  I took the measurements of the inlet, outlet and the part number of the air filter and on Monday I called Donaldson, and they reaffirmed my thoughts, the air filter housing was too small. I ordered Paul the largest air filter housing Donaldson had which was 1600 CFM. He installed it and called me the next day. His statement was” My truck finally runs free, I can go down the highway without having to push hard on the throttle, the exhaust gas temperature is much lower.”  Problem solved, no more flaking of the exhaust manifold and now we were able to give the engine some more horsepower to increase the fuel mileage and driving pleasure.

Back to the ‘68 Pete in the shop, I was looking at the outlet of the single air filter housing and it was 5.5 inches, removed the top of the air filter housing and the inlet bore was also 5.5 inches.  I knew these engines need at least a 7” inlet and outlet to supply enough air to the engine. Again, no part numbers on the air filter housing, so I called Donaldson and gave them the specs. I was correct, the air filter housing had 760 CFM at 6 inches of restriction, 880 CFM at 8 inches of restriction and 980 CFM at 10 inches of restriction.  This is another reason this 1968 Pete would not cruise effortlessly along the level highways. We are installing a 1700 CFM Vortox air filter housing, along with all the other work we are doing it will be finished in about 2 more days. Then it will go on the dyno and test driven and I will have the end results for you in the next article.

Written by;

Bruce Mallinson

Pittsburgh Power Inc.

3600 S. Noah Dr., Saxonburg, PA 16056