Vibrations can be challenging to troubleshoot. Without sophisticated equipment, the diagnostic procedure is to change parts until the vibrations cease. This is how most shops attempt to fix vibrations in trucks. This is how we have tried to fix them in the past. We have since upgraded to using a vibration analyzer. This powerful tool is how we can determine the source and the intensity, and determine the fix, of a vibration.
There are two components to vibrations: displacement and frequency. The displacement of a vibration can be thought of as the intensity of a vibration and is measured in milli g-force (mg). While some vibration analyzers measure intensity in acceleration or velocity, in this article we will stick with mg. The second component of vibration is frequency, which is measured in cycles per second or Hz.
To find a vibration, we first ask a series of questions to the driver about which gear, speed, and engine speed the vibration occurs. Then we attempt to replicate that scenario on the dynamometer. We can then determine from the analyzer which component is harshly vibrating by supplying the truck with variables such as engine speed, tire size, axle ratio, and transmission gear ratios. Anything above 65 mg is considered too much vibration. However, the threshold will vary across the frequency range. Once the data is collected, we can use orders, or multitudes of frequency, to determine the failed component.
To think about this simply, let us imagine a wheel and tire with a red dot at one point on the tire. With every tire rotation, this red dot will touch the ground once. As the wheel spins faster, the frequency, or the number of times the dot touches the ground, increases in the number of times it touches the ground per minute or per second. This frequency is the tire revolutions per minute. Mathematically, if we divide the engine rpm by the product of the transmission and axle ratios, we will get the tire RPM. Example: 1600 RPM / (1.0 * 3.55) = 450.7 RPM.
Now, let us replace the red dot on the tire with a flat spot from skidding the tire. This flat spot will cause a vibration. As the tire rotates, the ground will encounter the flat spot at the same frequency as the tire’s RPM. This is what is known as a first-order vibration. A first-order vibration occurs at the same frequency as the rotating object. For wheels, this could be an out-of- balance tire, a brake drum, wheel run out, etc. An example of a first-order vibration from the engine is the torsional damper, crankshaft, torque converter, and clutch assembly. When we see a vibration that exceeds the failure threshold, we can use formulas like the one previously mentioned to calculate which component is rotating at the frequency of the vibrations. In the next article, we will dive deeper into half, second, third, and further orders of vibration.
Most of the calls we get about poor fuel mileage result from a truck that is not set up correctly and requires 18 to 22 pounds of turbo boost to pull a load on level terrain. There are many causes of high turbo boost when on level ground. Driving too fast for the conditions, the turbocharger is too small, the turbine housing is too small, the rear gears are too high or too low, the exhaust system is restrictive, an uneven load, the alignment is out of adjustment, the trailer is not lined up with the tractor, the engine is underpowered, using the cruise control when the ground is not totally level, the driver is not paying attention to the turbo boost gauge (or worse yet the truck is not equipped with a boost gauge or pyrometer), the air filter is too small or restrictive, the tires have too high of a rolling resistance, the transmission is a 10-speed, the wheel bearings are set wrong, and/or poor truck, trailer, and load aerodynamics.
As you read through that list, not much is mentioned about the engine other than being underpowered. Other engine related issues could be the road speed governor is set too low, which does not allow the truck to be driven using momentum to go over the rolling hills, or the overhead is set wrong. If the engine is equipped with a VGT (Variable Geometry Turbo) and the vanes of the turbine housing are set too tight, a high boost situation will exist when on the level ground. Whenever the engineering department at Pittsburgh Power does a tune on an electronic engine, they adjust the VG vanes to lower the turbo boost on level terrain. When power is needed to pull a hill, the ECM adjusts the vanes for more boost.
2002 and older engines are not equipped with VG turbochargers, so we change the turbine housing size to lower the turbo boost. Now, don’t get confused by thinking if we reduce the boost on the level, the engine will not produce enough turbo boost to keep the exhaust gas temperature at an acceptable temperature. Just the opposite happens – when the turbine housing is matched correctly to the cubic inch of the engine and the amount of horsepower it produces, the EGT will stay cooler because the backpressure in the engine can vacate the combustion chamber, travel through the turbine housing, and out the exhaust pipe easier.
By the way, stock mufflers on 2007 and older engines create an exhaust dam and will not allow the engine to breathe correctly, so don’t use them. Our ported and polished exhaust manifold will give your engine 20% more exhaust flow, and our Max Mileage Fuel Borne Catalyst will also help to keep the engine and exhaust system clean of soot and carbon, and that will give your engine a slight increase in torque and fuel mileage, as well. If you have questions, stop by our shop in Saxonburg, PA or call us at (724) 360-4080. Thanks once again to Leroy Pershing for helping me with this month’s Performance Zone article!
