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Ok to start out I feel it necessary to let you know that this is actually for my friends 2013 police auctioned Charger. He likes buying these cars at auction. Sometimes you win other times not so much. His most recent was a police charger that is actually not the police model but an RT. On his way across country with this new purchase he decided to let her rip. He reaches 120mph when all of the sudden it lost a few hp. It still drives normal just seems weak. No engine codes are showing up however he revels it for me and it sounded like a rc car. It no longer sounded like a reved hemi but rather my wife’s old Honda Civic. Someone told me there’s some sort of valve in the plastic manifold that may have gotten stuck causing the whine of a 4cyl and low hp but I’m not so convinced. Any help would be fantastic.
 

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There's no valves in the intake manifold ... it's just a molded composite shell (5.7L R/T). The 6.4L SRT intake manifold has a dual shaft Short Runner Valve (SRV) bolted to the rear of the intake manifold. The Dodge RAM trucks with the 5.7L also have the SRV but the SRV was not standard for the Charger R/T. It's possible your friend's R/T was modified and had a SRT intake manifold installed (a non-factory modification done by some owners). If a SRT intake manifold is installed it could be failing and not posting any codes because the 5.7L onboard diagnostics are not checking it (i.e. not standard equipment or an option for 5.7L R/T).
If you inspect the rear of the intake manifold, a stock 5.7L R/T manifold will have a Manifold Absolute Pressure (MAP) Sensor. A intake manifold with the SRV will have the MAP sensor and the SRV.
The 5.7L R/T also has Multiple Displacement System (MDS) ... which selectively deactivates cylinders 1, 4, 6 and 7 during steady speed (i.e. goes from 8-cyl mode to 4-cyl mode). The MDS solenoids are under the intake manifold.
The 5.7L R/T (starting with 2009 models) also has Variable Cam Timing (VCT). The cam phaser is oil flow controlled by a solenoid operated control valve. The valve rotates the phaser and alters the angular position (or phasing) of the camshaft relative to the timing chain and crankshaft rotation. The VCT control valve is also under the intake manifold.

Any of the above components could cause 4-cyl like symptoms but the car's onboard diagnostics will typically post error codes when something is not functioning correctly (except for the SRV). With the lack of any diagnostic trouble codes, and no SRV, I would check the intake manifold bolts to see if they are loose and causing an air leak (vacuum leak). The bolts should be torqued to 9 ft. lbs. Tighten in sequence from the middle bolts towards the outside in a crisscross pattern.
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2018 Charger Scat Pack
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...The 6.4L SRT intake manifold has a dual shaft Short Runner Valve (SRV) bolted to the rear of the intake...
...An intake manifold with the SRV will have the MAP sensor and the SRV...
...Any of the above components could cause 4-cyl like symptoms but the car's onboard diagnostics will typically post error codes when something is not functioning correctly (except for the SRV)...
Although a little old, this is a very interesting post for me. I have a 2018 Charger Scat Pack (6.4l obviously). Please tell me more about how the SRV works. Could it be the culprit of a strange noise I've been hearing for quite some time now? The noise is a whining sound that is 'sort of' like a supercharger whine; or if you've ever had an old car that had a speedometer cable that was going bad, it sort of sounds like that; it's a spinning, rotational sound. The sound seems to be coming from right where the SRV is located.

The sound started after I ran the car particularly hard one day, a five or six minute run of 130+ mph. The sound generally happens when I punch it and drop down two gears, such as in a passing maneuver and it lasts while the RPMs are high, but then goes away when the RPMs drop.

Oil pressure and temperatures all look good and normal and overall the car runs great. Notably, I have no error codes, which you stated the SRV does not produce. However, the car does seem to be running slightly rich at times, particularly at lower RPMs. Could the SRV be causing this sound I'm hearing and causing the rich fuel mixture?
 
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