jtsmith1

So most people have spoke of how the sound of the VQ37VHR becomes less desirable at higher RPM's. After spending some time in my vehicle, I am curious as to what is actually causing this.

It seems to me that some of it may be related to the balancing of the engine. I feel a bit more vibrations in the higher RPM's, which I think could be a potential source of this noise.

Some quick vibrations 101:

From a purely fundamental vibrations standpoint, everything has a resonance frequency, including the overall "system" under the hood, or the engine. One main design parameter to consider when designing an engine is the overall RPM range it is intented to operate within (which, at any given RPM, corresponds to a certain frequency). Once you understand the operating range, the next thing to consider is how to balance the engine to ensure you will not run into a condition that may result in resonance.

Resonance is a term used to describe a state at which a "system" has been excited by a frequency that is equivalent to that "systems" natural frequency. When this happens, things get nasty in a hurry (serious deflections and movement to the point of catastrophic failure). Think of suspension bridges swaying like crazy during earthquakes.

So in order to keep this from happening with an engine, you add weight here and there to change the "systems" natural frequency, balancing the engine. This is also used to help reduce the overall torques felt within the components of the engine, so they dont end up translating into the drivers seat. Also, a flywheel is used as a damper, essentially absorping some of the vibrations, so you don't feel them when you are sitting in your car.

So here is what I am curious to know: did Nissan miss the mark with this? Did they sacrifice a smoother operating engine for a lighter one?

If so, I think some of the noise we may be hearing is due to the vibrations of the materials at higher rpms. This is good and bad. The bad part is, that means there was more room for improvement on the design of the overall engine. The good news is, I think this could be improved on significantly with the addition of a heavier flywheel.

Yep, thats right, a heavier flywheel. While I have no real world experience with this, I believe moving to a light weight flywheel would actually cause this situation to get worse, while increasing the weight of the flywheel would reduce the vibrations of the engine at higher RPM's and potentially provide a smoother feeling and better souding engine.

Any thoughts?