Thoughts on Engine Sound at High RPM's (kinda long)

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?

Gunzero

You blew my mind away, but nice theory. Test it out

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homeryansta

you sound like you either had a strong emphasis in mechanical engineering or physics in your field of study.

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jtsmith1

Just finished my BSME, haha. Wanted to explain it a little more in depth to provoke thoughts of others on the board.

I would love to test it out, but just dont have the time to do it. However, I would be interested in hearing some feedback from others who may have used a light weight flywheel option (not sure if anyone is doing this with their 370's).

If it can be confirmed that a lighter flywheel causes the problem to get worse, than it stands to reason that using a heavier flywheel (one way to overbalance an engine) would reduce vibration and sound.

WhiskeyHotel

O.k. - the V-6 is always unbalanced, firing-wise correct (2 on one bank, 1 on the other or similar). This is a second order rocking motion - correct?. I guess that rotationally a V-6 can be balanced (first order -?). Are these both torque (rpm) sensitive? Would a weighted fly-wheel do anything for non-rotational ( other than first order) unbalanced?

I'm an EE so excuse my lack of knowledge.

Red__Zed

I don't think the roughness/harshness is an issue with resonance TBH.

I think it mostly has to do with the fact that the engine isn't balanced. You almost never see a v6 revving up this high, especially not one with this old of a design.
It's really not possible to have a smooth, high-revving V6, and our engine just helps demonstrate it.


EDIT:

Yes, a heavier flywheel would likely smooth things out, but that's not a performance knock I'd be willing to take...

Especially since it likely would only make a VERY small difference...

ChipsWithDips

Keep in mind the VVEL is changing lift and duration and only maxes them out in the higher RPMs and at high load. Changing the cam profile like this is going to affect the sound of the engine. At lower rpm and load, the engine acts like it has a mild cam, and at higher rpm and load it's more of a performance cam.

jtsmith1

Not an expert in this area either, but I do believe almost any second or higher order vibrations can probably be neglected due to significantly lower magnitudes, cannot say this for sure though.

As far as the firing order, I never really put much thought into it. It seems that it would also be a function of RPM due to the change in momentum of the masses. I guess a good set of mounts (essentially dampers) and a properly balanced crank would be key here.

Not sure what you mean about non rotational unbalanced? The flywheel can essentially act as a bandaid, covering up some imbalances in the engine and absorbing some of those vibrations. Obviously, from a performance standpoint, we would like to reduce the mass of any rotational parts, but at what cost? I think a heavier flywheel would "overbalance" the engine, reducing any first order vibrations and probably minimizing any higher order vibrations as well.

Again, not an expert in vibrations, just have a basic undestanding. I do have some software on my laptop that we used in school to analyze engine parameters during design, such as balancing. If I knew every detail about the engine, such as bore, stroke, all the masses (major components such as wrist pins, piston, conrod, and crank), I could do a little analysis and see what sized flywheel provided the smoothest torque output and minimized vibrations. Question is, how much heavier would this be and what would the performance trade off be?

Gunzero

The engine noise or vibration isn't even bad, just adds to the excitement.

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Kingbaby

I suggest you go to a local track/car meet/pm local members on here...

VQ37VHR or any VQ to me sounds even better at higher RPM! Some are expecting some magical tone when adding bolt-ons! Doesn't happen that way, you don't get power without lose comfort. Now that exhaust note at higher RPM's to some is bliss....I think playing this out in theory is pointless! I'm sure Nissan has put countless hours into this.

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wstar

The VQ37VHR out in the 6K+ RPM range is music to my ears, no idea what you're talking about I wish I had a camcorder that could capture the sound better on dyno runs.

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Davey

Yeah, I've said before, I read the reviews of the car being "harsh" and "noisy" and "unrefined" in the higher RPM ranges and I just don't see it to any degree I could care about. What's to complain about, with a V6 spinning at 7500 RPM?

jtsmith1

Either I am missing the sarcasm boat or there is a significant difference between members definition of refined. My Z is far from it at high rpm's, especially off throttle. Maybe some are just not familiar enough with the topic to know it when they hear it.

jtsmith1

Next time you guys are driving, rev up to redline and let your foot of the gas, allowing the engine to spin down by itself and see what you think.

Again, for some, this may not be an issue.

RCZ

haha, yeah the car is "rough" pass 4500RPM. You're not going crazy. The way it sounds and the way it feels are two totally different things. No one should appreciate an engine that feels like you're breaking it when you take it up in the rev range...which ours does.

I don't think its that we're hitting its frequency, it just feels rough. It does it for several thousand RPM and that should tell us its not just resonance, its just that the damn thing vibrates a lot. Be it from the vvel system or more solid engine mounts or what. I've had three types of flywheel on my car and they all made similar vibration at high rpm. You could say the stock fw feels more muffled because it doesnt seem to get transferred into the cabin and chassis as much, but its still there loud and clear.

I had solid transmission mounts installed a while back and the vibration was completely unbearable. At high RPM it felt like the world was ending. I think the best we're going to be able to do is isolate the vibrations as much as we can, maybe we can get some liquid filled mounts like porsche does... that gt3 feels like silk at 9k rpm.