Should You Use Foam or Gauze?
Gauze filters offer superb airflow with a good dust-holding capacity. They were originally designed for race cars with the goal of maximum airflow and engine protection for the duration of the race. For a race car or for a high-performance vehicle driven occasionally, a gauze filter will allow the engine to develop the most power. The down side is they don't filter as well as other materials, so you have to trade between maximum power and maximum service life. The up side is that a properly designed foam filter will offer a good blend of filtering performance and power-making airflow.
Foam filters typically combine great airflow capability, a huge dust-holding capacity, and very high filtration efficiency for extremely small particles. They are considered by many dirt bike riders and buggy drivers as the filter of choice. High-performance foam filters are made up of tiny interlocking cells that trap and hold dirt particles throughout the entire volume of the foam and thus are often referred to as full depth filters. As GCA Industries marketing materials explain, "the cell strands of the foam stop the dirt while the oil holds it until the filter is cleaned."
GCA Industries tested the airflow capacity of a racing-type gauze filter, one of its foam filters, and a factory paper element on a Super Flow S.F. 600-flow bench. The company tested each in clean and dirty conditions using a flow range between 6 and 4 inches water resistance. According to GCA's tests, in both the clean and dirty tests the gauze and foam filters far outperformed the factory filter. And when clean, the gauze out-flowed the foam. However, under dirty conditions, the foam out-flowed the gauze. The superior flow of foam compared with gauze when loaded with silt is wellknown to off-road performance enthusiasts and it's why many use this type of filter extensively.
So What Did We Observe in Our Test?
Our test rig is a '99 GMC Sierra with a 4.8L engine backed by the four-speed automatic. It's totally stock with 60,000 miles on it. The factory says it makes 255 hp at 5,200 rpm and 285 lb-ft of torque at 4,000 rpm. We won't see those numbers on the dyno test. We expected power values at the rear wheels approximately 25 percent less than the factory flywheel stats due to rotational inertia and driveline frictional losses, and that's what we got.
The baseline test generated 192 hp at 5,100 rpm and 218 ft-lb of torque at 3,700 rpm. Those figures equate to about 75 percent of the advertised net power figures, so we're reasonably confident the GMC's powertrain and the Dynojet chassis dynamometer are functioning correctly.
With the True Flow intake system installed, we saw peak horsepower improve from 192 to 203, an increase of 11 ponies. Torque went from 218 lb-ft to 230 lb-ft, an increase of 12 lb-ft of torque. Furthermore, the True Flow intake improved the average power out put from 3,400 to 5,100 rpm by 8 hp and 12 lb-ft of torque. Keep in mind these values reflect the power losses mentioned above. An estimate of the flywheel horsepower assuming losses of 25 percent reflected in the measured values would give us an average increase of 10 hp and 15 lb-ft of torque with a peak increase of 14 hp and 15 lb-ft respectively.
The increase in the average output indicates a substantial real-world power increase from this intake package. In addition, we also measured an increase of a little more than one mpg in our fuel economy. The kit definitely made a positive improvement in the performance of this truck. Considering we didn't test the truck with the True Flow filter installed until it had several hundred miles of dusty driving through the Santa Ana winds in Southern California, the performance of the new True Flow filter is even more impressive.