Installing and Testing Prothane Motor Mounts on a 5.7L Hemi
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During the installation of TTI headers on our ’06 5.7L Hemi Police Pursuit Charger (“Go With the Flow,” July ’17), we discovered that the motor (engine) mounts were torn beyond the point of reuse. After a trip to the Dodge dealership, we had a new pair of mounts, allowing us to complete the exhaust testing. Following the installation of the mounts, we started to wonder how often these mounts fail? Will these mounts cause us more work in the future? An internet search pointed out that the LX/LD platforms all experience relatively short motor mount life spans. The stick shift–equipped vehicles tend to have less mount failures, but they too experience failures of the factory mounts. Our Charger’s 81-page repair work history showed the original mounts and a second pair of mounts required replacing during the Charger’s seven-year service (June 2006 until December 2013). The first replacement occurred in 2008 when the original mounts had accumulated just over 26K miles, and another replacement occurred after 27K miles in 2010. The pair of mounts replaced during the TTI story had accumulated over 34K-miles.
With a concern about the factory mounts failing in a 30K-mile window, we contacted Prothane Motion Control about testing a pair of their billet aluminum and urethane motor mounts (PN 4-512). Prothane started manufacturing quality urethane/polyurethane OEM replacement parts in 1991 offering about 100 products. They’ve added to their product total now providing over 5,000 products for imports and domestics spanning from the muscle car era to late-model vehicles.
Urethane components such as the Prothane motor mounts meet the needs for spirited performance street/strip applications, but many individuals have concerns about the harshness of the urethane due to its minimal compliance. To evaluate this concern, we’d assess the noise, vibration, and harshness (NVH) of the stock motor mounts and the Prothane motor mounts. The noise would be a subjective test based upon our ears. Are there differences in the sounds of the engine or in the occupant compartment? We would assess the vibrations with an Electronic Vibration Analyzer (EVA) and an MTS 4000 vibration analyzer. The harshness would be another subjective estimate. We’d use the “butt-o-meter” to determine the changes in harshness. Would there be a change felt in the seat, floor, or the steering wheel of the Charger? If so, how objectionable would it be, and would it be acceptable and merely considered a tradeoff for the added durability.
Before we installed the Prothane mounts on the Charger, we set up the EVA and the MTS 4000 to acquire the data of the factory motor mounts. We evaluated the vibrations at idle, 40 mph, and 60 mph. With the Hemi at idle (650 rp,), the testers noted a few subtle vibration frequencies (measured in hertz). The frequencies showed a first order engine speed-related vibration and a first order water pump pulley vibration. The vibrations were both less than .06 g’s (amplitude). Any reading exceeding .06 g’s is a vibration that the occupants of the vehicle can feel. We could feel a minor vibration in the driver seat, but it wasn’t objectionable. A first order, second order, third order, or fourth order vibration refers to the amount of pulsations of a component in a single rotation. A first order would experience a single pulsation per rotation, a second order would have two pulses per rotation, and so on. At 40 mph (1,400 rpm), we had a second order driveshaft vibration and a 1st order tensioner pulley vibration, both below .73 g’s, and at 60 mph (1,800 rpm), we measured a first order tensioner pulley vibration at 1.08 g’s. At 40 and 60 mph, we noticed some vibrations in the vehicle. In both cases the Hemi was in the multi-displacement system (MDS) mode, which is the four-cylinder mode. The factory motor mounts damped the vibrations to an acceptable level. We had no complaints about the quality of the ride. It was free of undue noise and harshness.
While we could install the mounts with the Charger on jackstands, we elected to drop by Pennsylvania College of Technology to perform the installation of the Prothane mounts while using a vehicle hoist. The removal of the factory mounts was straightforward. We placed a screw jack under the oil pan to support (and raise) the engine when we loosened the mounts. The removal of two fasteners at the base of each mount and backing off two bolts at the engine block freed the mounts. The passenger side did require the loosening and removal of an alternator bracket. To remove the passenger-side mount, we had to squeeze the mount between the oil pan, K-frame, and the rack-and-pinion. To gain enough clearance, we removed a bolt that held the power steering lines to the K-frame. The driver-side mount snaked out between the K-frame and the oil pan without any difficulty. The installation of the smaller diameter Prothane motor mounts was much easier than the removal of the factory mounts. The Prothane mounts easily cleared the K-frame and oil pan on both sides of the engine.
With the Prothane motor mount bolts torqued, we fired up the Hemi and noticed a lively aura pulsing through the driver seat. We didn’t consider it unpleasant, but it was noticeable. Since we had just replaced the mounts, we thought maybe we were feeling more “sensitive” to any subtle changes. We set up our EVA and the MTS 4000 in the same fashion we had for the factory mount tests. The frequencies at idle were a first order engine speed-related vibration, a fourth order engine speed-related vibration, a first order alternator pulley vibration, and a first order tensioner pulley vibration, which were all less than .25 g’s. The amplitude of the vibrations increased by approximately four times over the factory motor mounts, but they weren’t intolerable.
At 40 mph, we experienced a second order driveshaft vibration, a first order idler pulley vibration, a first order tire vibration, and a first order tensioner pulley vibration. When the Hemi was in MDS mode, the highest vibration reading was 1.51 g’s, which was twice the factory mounts. The vibrations were very noticeable, and when the engine lugged down, the vibrations increased. However, when the Hemi went back to eight-cylinder mode (MDS mode off), the analyzers had readings that were less than the MDS mode with the factory mounts. At 40 mph, with the engine in eight-cylinder mode, the ride was very pleasant, but the same speed in MDS mode was mildly disagreeable. At 60 mph, the MDS mode again was distasteful with the highest vibration of 2.91 g’s, which was almost three times the factory mounts. There was a first order tensioner pulley vibration, a first order water pump pulley vibration, and a first and second order tire vibration. When the Hemi went in eight-cylinder mode, the vibration frequency dropped to 1.00 g’s, which was less than the MDS mode with the factory mounts.
The Prothane motor mounts performed very similarly to the factory mounts whenever the engine was in eight-cylinder mode, but the MDS mode resulted in noticeable vibrations, noise, and harshness throughout the Charger. We used a DiabloSport tuner to disable the MDS, which would leave the Hemi in eight-cylinder mode permanently. That did the trick. We had a very compliant ride that was quiet (with exception of the exhaust), had a low vibration concern, and was void of any harshness. The only concern we now had was how would the fuel economy change with the engine tune permanently set to eight-cylinder mode.
The Prothane motor mounts are performance oriented, and even though they have a slant toward performance, with the MDS mode disabled, they operate very similarly to the factory mounts. If you’ve been pushing the envelope with your LX/LD, then you have probably already disabled the MDS mode, so a concern about any fuel mileage changes is minimal. If you’re looking for a motor mount that’ll last more than 30K miles and provide a compliant and lively ride, Prothane may have just what you need.