With fuel prices continually on the rise, it’s no wonder diesel enthusiasts are asking themselves, “How can I save money at the pump?” Many have already installed aftermarket upgrades to their trucks, such as intakes, exhausts and programmers to boost power, which in turn, often improve mileage. But for those wanting to go even further to maximize both power and mileage, there is another alternative: Compressed Natural Gas, commonly known as CNG.

So what is CNG and why should you consider installing a system on my truck? Compressed Natural Gas, or CNG, is a domestically produced, readily available alternative to diesel fuel that is made by compressing natural gas to less than 1% of its volume at the standard atmospheric pressure.

Consisting mostly of methane, CNG is clear, odorless, and non-corrosive. In fact, many municipal bus fleets, in an effort to “go green” have done 100% CNG conversions on their diesel powerplants. The end result is a clean burning vehicle with reduced emissions.

Make no mistake, CNG for the diesel enthusiast is considered a secondary fuel to the primary fuel, which is diesel. In fact, several companies offer similar setups utilizing propane. But since propane costs rival that of diesel fuel, CNG is the next most economical secondary fuel option. Compressed natural gas can average a 40% savings on fuel costs, according to PDI. As a matter of fact, second quarter prices for 2008 showed an average savings of $2.73 per gallon when purchasing natural gas over diesel.

Performance Diesel Inc, (PDI) is one of Utah’s leading diesel shops that have been boosting power and mileage for truck owner for years. When diesel fuel prices went well over the national average of $4.77 per gallon, PDI turned its focus to finding a way to get the maximum fuel efficiency for diesel engines. The result is a Compressed Natural Gas (CNG) System, which runs on a 70% natural gas and 30% clean diesel mixture, which increases performance while decreasing emission particulates.

The new PDI compressed natural gas system enables light-duty and medium-duty diesel engines to operate primarily on natural gas. The diesel engine is basically unchanged and retains its high performance and high efficiency diesel cycle. According to PDI, by efficiently burning compressed natural gas, customers benefit from lower fuel costs and lower emissions. If the CNG supply runs out, the system allows the truck to switch back to 100% diesel fuel, giving a complete diesel operation backup.

PDI uses a fumigation method, so natural gas is drawn directly into the intake system. The CNG is combined with air and directed into the combustion chamber. This secondary fuel allows for a more complete burn in the combustion chamber. According to PDI, most diesels burn 78%–80% of the fuel mixture introduced into the combustion chamber. By introducing CNG, the burn efficiency increases to roughly 95%, boosting both power and fuel mileage while reducing emissions. In fact, several states offer tax credits for CNG conversions.

Each PDI natural gas system is specifically designed for truck applications to operate during a predetermined boost pressure and can also be controlled by a toggle switch mounted in the cab to turn the system on and off during periods when the user wishes only to operate on diesel fuel.

We visited PDI’s shop in Saint George, Utah, where a compressed natural gas system was being installed on a 2008 Dodge Ram 3500. The big Dodge 6.7-liter Cummins was stone stock with no aftermarket upgrades, the perfect candidate to find out just how well a CNG system improves performance and mileage. And since the Cummins was also fitted with a DPF, it was a good chance to find out how the CNG system would affect the filter and associated sensors.

Make no mistake, installation of PDI’s CNG system is a full-day install. Since you’re dealing with natural gas, special care needs to be taken not only with where the tank is mounted, but each line connection as well. The photos in this story highlight a typical install, but more specifically, how the system was installed on this ’08 Dodge.

After installation, the Dodge Ram was placed on the dyno for the results. Truck owner, Steve Benson, was ecstatic to find that his truck pulled over 490 rwhp and more than 990 lb-ft of torque, with the stock intake and exhaust. That compared to a baseline of 335 rwhp and 623 lb-ft of torque.

According to Benson, “My Dodge was getting 13.5 mpg on the highway before PDI installed their CNG system, and now I am getting 24 mpg in town, and 31 mpg on the highway.”

Keep in mind that mileage is calculated by the amount of diesel fuel burned. Sine the truck is running mainly on CNG at a 70% ratio, big mileage gains are not uncommon. But since CNG is cheaper than diesel, and the truck is making nearly 500 hp without the addition of an air intake, programmer or exhaust system, CNG makes a lot of sense for diesel owners.

Since the fuel burn is more complete, what does this do for EGTs? According to PDI, EGTs actually drop 200° under light loads, which remaining the same under heavy tow conditions. And best of all, because of the cleaner burn, exhaust smoke (and soot) is reduced.

In fact, on the 2008 Dodge, PDI claims the cleaner burn reduced the number of regeneration cycles on the DPF. If you’ve already exhausted the aftermarket bolt-on approach for both power and mileage, and want to try something that’s “outside the box” for improve mileage, power, look into a CNG system for your diesel.

1. PDI’s Compressed Natural Gas system comes complete with a high-pressure, 12.6-gallon tank, mounting brackets, a high pressure CNG fill valve, regulators, pressure sensors, cab-mounted activation switch and all the hoses and fittings for an easy install.

2. The guinea pig for this install was an ’08 Dodge 3500 fitted with the new 6.7-liter Cummins.

3. The first order of business was to install the mounting brackets for the CNG cylinder in the bed of the truck.

4a,4b. PDI selected the fuel fill area for mounting the high-pressure fill valve. The valve mounts in the plastic just above the diesel fuel filler neck.