Now that we’ve identified potential restriction, we’ll want to use what we know about differential pressure to determine just what effects these potentially detrimental features actually have on the intake system.
Anyone could perform the testing for differential pressure as it is relatively easy to do. Since we’ve already located what we believe to be potential points of restriction we know exactly where we should tap into the intake system to gather pressure data. One could accomplish this extremely cheaply by making their own manometers out of water and tube, and it has been done before. However, we did not want to rig up two of these (as they are usually large and hard to read) and spill them all over the place while doing first through third gear pulls. Instead we acquired a digital differential pressure manometer, specifically an “Extech HD750.” This was chosen for it’s low range (5psi) which lends to it’s accuracy, as well as the fact that it can datalog. Being able to datalog was ideal because we can show a chart of what is happening as we row through the gears, which is infinitely more interesting than if we were just to post peak numbers. This will also show if the pressure drop effects are due to speed of the vehicle, or if they are rpm dependent. Unfortunately, the sample rate is only 1 sample per second, which means that a longer pull is necessary to get a better data set, but this can be alleviated by performing more pulls. The units used for measurement during testing are in “inches of water.” 1 in H2O is equal to 0.036psi, and 27.67 in H2O is equal to 1psi. This is a relatively small unit of measure, so it is useful for showing small differences in pressure.
The manometer has two inputs for pressure, and will display the difference between the two pressure inputs. With how we hooked up the pressure signals a positive number indicates a pressure drop, and a negative number indicates a pressure gain. The manometer was to be placed in the cabin, so substantial lengths of hose were needed. Since we’re testing for pressure, the length of the hose was negligible. However, two 10ft lengths of .125in norprene hose were used, and were rated not to collapse under the expected vacuum.
The stock intake was tapped in various places, and fittings were added that would connect to the hose for the manometer. A fitting was placed at the inlet of the snorkel, at the top of the front of the airbox before the filter, at the top of the airbox after the filter and before the MAF housing, at the inlet of the rubber elbow after the MAF housing and before the bend, and finally right before the throttle body. Each fitting was sealed to prevent leaks, and caps were added to all fittings.