Intake Resonance

[Quadstar Tuning LLC]

A not so fun part of making more power with a 6.5 that we run into is the annoying intake resonance they can produce. The factory plenum, turbo, and air intake had intentional design features to get rid of most of it, but all that goes away in favor of moving more air through the engine.

Has anyone done any work in this area and found some root causes? Cam profile, head work, etc?

A few things we've noticed over the years is that the dual plane intake masks "some" of it and it's not related to the combustion event or injection timing since it does it at higher RPM, even in deceleration fuel cutoff (handy feature of the EFI pump). Surely, we aren't crazy and this bothers other owners too!

 

[94DieselSub]

Almost all oem gas/diesel induction systems nowadays implement resonance traps and desinged countermeasures.

These are the strange looking molded plastic appendages on the intake tube before (after) the filter box. Actually even the filter boxes are designed as part of that sys. Vehicle noise/vibration/harshness is at a high level of engineering design now. Everyone expects even the new econo box vehicles to ride like a caddy - so extensive design effort goes towards that now.

Since my 6.5 currently runs the stock turbo/filter I haven't yet experienced the excessive cab drone effect yet. Just the usual normal idi self destruct sounding clatter. My flowmaster equipped Vett will make your teeth fillings rattle from drone though!

So resonance in a cavity or pipe is really Helmholtz constructive standing waves. The cavity/pipes volume and shape determine what Fq that resonance sets up at, and the type of stimulus required to energize said resonance.

So reducing amplitude or eliminating resonance (modal, contructive wave interference), several approaches can be implemented.

(1.) Simple absorption/damping, or tuned absorption.
(2.) Designed countermeasures, such as secondary resonator chambers, cavities, and shapes that induce additional resonant frequencies that attenuate or mask the primary resonance via harmonics, or cancellation via nodal deconstructive wave interference.

BTW, that audiable drone/noise is a direct side effect of induction or exhaust resonance that many designs utilize to "tune" a system. Like N/A multi-port injection manifold designs that induce positive manifold pressure at the specific tuned rpm operating range. This is also how plused negative exhaust pressure waves are established (scavenging).

Are you testing the 6.5 market waters for solution demand, or just trying to wrap your head around the issue? As its a simple problem that requires complex fixes, (esp on the intake side).

 

[Husker6.5]

A not so fun part of making more power with a 6.5 that we run into is the annoying intake resonance they can produce. The factory plenum, turbo, and air intake had intentional design features to get rid of most of it, but all that goes away in favor of moving more air through the engine.

Has anyone done any work in this area and found some root causes? Cam profile, head work, etc?

A few things we've noticed over the years is that the dual plane intake masks "some" of it and it's not related to the combustion event or injection timing since it does it at higher RPM, even in deceleration fuel cutoff (handy feature of the EFI pump). Surely, we aren't crazy and this bothers other owners too!

Turn the radio up! Simple and very cheap solution!

 

[ak diesel driver]

Another factor you don't hear much about is temperature. I have exhaust resonance but only when it gets down to zero* and below

 

[FellowTraveler]

Another factor you don't hear much about is temperature. I have exhaust resonance but only when it gets down to zero* and below

This is what I used just after my down-pipe on the Cummins full 4" system no more drone or other resonance issues from system.

Flo Pro Twister F5 Exhaust Muffler - Race / Diesel / Resonator 4" In - 18" Long | eBay

Muffler Type: Race Muffler / Resonator. Muffler Series: F5 Twister. Notes 1: Use as a muffler or resonator. Works as a Resonator on Noisy Diesel Applications. Race Muffler That Fits In Tight Places. Dual flow path cancels noise and creates a vacuum effect that accelerates the exhaust through the...

www.ebay.com

 

[JayTheCPA]

As mentioned, an open versus closed filter box makes a noticeable difference in drone.

Two thoughts where both presume a closed filter box connected to the fender's air intake port:
- add a sound absorption blanket to the intake plumbing, including the filter box. Might also apply some damping material to the inside of the fender opposite of the intake port (think: dynomat or similar).

- see if there is something of a shorty muffler / resonator to allow the sound wave to chop-up and self destruct. If there is nothing available, find a friendly welder to make one and see what happens

Another path to noise in general is a sound deadening blanket covering the valve covers and oil sump. Saw commentary where this was done for the Cummins IDI with significant noise reduction.

 

[Quadstar Tuning LLC]

Are you testing the 6.5 market waters for solution demand, or just trying to wrap your head around the issue? As its a simple problem that requires complex fixes, (esp on the intake side).

This is purely for our two trucks right now. Just trying to figure the right path to go down so we can enjoy the trucks more. Anytime you want to go somewhere around here, you end up on the highway for extended periods and keeping it at 55-60 mph isn't an option. The engine setups we have are perfectly happy at any speed but our ear drums are not!

The exhaust side is good. We run JAMO fluted resonators up front and MBRP perforated tube mufflers back further. Still has a great tone since both devices are straight through designs. But when you take 4.10 gears to 75-80mph, the intake noise is unbearable. No radio is going to hide the noise these motors are capable of making.

As mentioned, an open versus closed filter box makes a noticeable difference in drone.

Two thoughts where both presume a closed filter box connected to the fender's air intake port:
- add a sound absorption blanket to the intake plumbing, including the filter box. Might also apply some damping material to the inside of the fender opposite of the intake port (think: dynomat or similar).

- see if there is something of a shorty muffler / resonator to allow the sound wave to chop-up and self destruct. If there is nothing available, find a friendly welder to make one and see what happens

Another path to noise in general is a sound deadening blanket covering the valve covers and oil sump. Saw commentary where this was done for the Cummins IDI with significant noise reduction.

We know the open intake is part of the issue, but just aren't experts in NVH to solve it. Thus coming here : ) Even with a wrapped enclosure around a front mounted filter, it still reverberates into your brain at times. No amount of turning up the radio will hide what we are talking about as it's only tolerable with the windows down at times.

We do have some 6.7 intake tubes we were going to play with since they are in the scrap pile but in looking at them, it seems the devices built into them are solely to eliminate turbo whistle and not resonance. But we plan to find out since it's only a time investment and can be rigged up temporarily.

 

[Twisted Steel Performance]

A lot of it is the very small plenum area, a larger plenum will give the resonances somewhere to go.

 

[Will L.]

Double edged sword of the improved air flow on intake mods. The better the air flows, the more sound waves can travel back.
Chris is on point. The larger reserve closest to the valves the better for sound and for performance.

Getting the intake away from sheet metal like the fenders helps, so do resonator tubes on the intake. If you have to keep air coming in there, consider Dynomat in the fender. Use same math that you would adding a resonator tube to the exhaust system. Get a frequency app on your phone, drive and pick the target issues. Often 2 resonators is used- Honda was the king of this back in the 80’s.

 

[94DieselSub]

So an out of the box solution that many wouldn't think about is active noise cancellation. It would require a aftermarket stereo installation with high output subwoofer(s) and lots of amplifier pwr ( which is cheap nowadays). A DSP unit with microphone input would process the noise cancellation. A side benefit would be you'd also have a bumping sound sys.

A major aspect of vehicle cabin resonance is "cabin gain", which larger volumes like in tahoes/suburbans exhibit in maximum amounts at low Fq's. You can achieve >12dB of cabin gain. 10dB of acoustic SPL is logarithmic, IE equivalent to amplifier pwr going from 1 to 10watts, 100 to 1000watts, ect.

 

[Quadstar Tuning LLC]

We've definitely been considering trying to add resonator tubes to the intake. We doubted that anyone actually tried it yet but figured it was worth asking.

A lot of it is the very small plenum area, a larger plenum will give the resonances somewhere to go.

That makes me think we could try a Banks plenum since we have a j-code intake on one truck. Just to see what the difference is and how it helps. Not that it would be ideal by any means.

 

[Will L.]

Oh, something else that helps is disturbing the wave via screen. It forces the longer (deep) sound wave to Be disturbed and muffle in the process.

I followed these intake gaskets a bit concerned for my turbo so close to the valves and if it comes apart I can loose valves, pistons, etc. As well as sound improvement. (You think 6.5 trucks get loud at high rpm, you need to see inside Hummmer/hmmwv) In reading about them, the techs that have installed a bunch said the can hear the difference and identify if the engine has been done or not by the sound alone. I studied about sound in school, this screening would have far less in sound control than hemholtz chambering but may be helpful in both fronts.

 

[MrMarty51]

Could T into the intake piping and experiment with PVC piping of various diameters and lengths, sort of like the intake on My 2000 year model.

 

[JayTheCPA]

So, 4.10's at 75 - 80 mph. With OE tires that is 2,500 - 2,700 RPM's. Little wonder it is loud

Are the 4.10's for towing? Or bragging rights for off-the-line starts?

If the latter, 3.42's and a NV4500 will net about the same jump off the line. Bonus is that this combination will make the beast a LOT more quiet at the target cruising speeds

 

[JayTheCPA]

One additional thought. Given that an open air box is under the hood, my vote is to first find a K47 box and see if that drops the sound enough. Make sure that the box's intake port has tubing to the fender, as without it IAT's above 200* F are likely when towing. (Don't ask how I know).

If the K47 works good enough, then you are past the brush fire and can decide whether any additional work is worth the effort.

 

[Will L.]

Mind you I just passed the over 40 hours no sleep mode (which bothers me more now than a year ago) but a small muffler installed pre turbo? Mufflers are obviously sound killers in exhaust. And the turbine is basically sucking in the air, and the airflow is already disturbed tremendously at the filter...
While resonators are to tune out specific frequencies, mufflers kill it all.

Ok, you guys make fun of me while I try for a solid 8 hours- haha.

 

[schiker]

I only know a little about sound. It is confusing because some of it is quantitative and some qualitative.

I think the root cause is hi compression and only 2 valves. The acceleration of air movement is higher once you start increasing mass of air flow and the noise increases. It's all cumulative. Air starts expanding and compressing in the intake runners near the valves as more mass of air "pumps/rushes" through the motor. I think of it as this makes something like an "accordion" movement I suppose and the springyness of the air in the runner is highly variable but it makes sound. Some sound is this movement of energy as air accelerates expanding and compressing. Add more air turbulence in the plenum with variable air springyness in runners and you have added sounds like when you blow across a bottle top.

Sound waves are line of sight waves like light. They bounce off of stuff and change direction at the speed of sound. All this sound energy bouncing back and forth and intermix to build standing waves (resonance). Some sound just bounces down a pipe to exit the end but some passes through the wall. The sound waves bounce all around the plenum and back out the intake. Some sounds come out the opening of the intake box but some pass through the walls of the intake. Just like the exhaust not all the sound comes out the end of the pipe. Some are made by the air "accordioning". Its cummulative and anywhere the air is compressing and expanding its building noises.

To quiet sound, I think you have to do something with the energy: dissipate it and or absorb it or help cancel it out with opposite wave energy. In our discussion here Resonance sound are sound waves that "stack up" I guess from the pulsing of the air and distances the air "accordions". Then there are frequencies that are irritating and others that are more pleasant to our ears and can cross over or be different than just amplitude or intensity of the sound wave.

A filter will quiet some sound a muffler will quiet some sound but not all.

I don't think putting a muffler before the air filter (ambient side) will eliminate resonance as good because the waves are bouncing on the clean side of the filter and I am not sure they will be the same on ambient side due to having to travel through filter media. I assume cancelling out would work better in the same chamber the air is "Accordianing" in. I guess why those mesh gasket work is they dampen the accordion action as air rushes into cylinder the sound energy it bounces back and forth down the intake runner.

 

[ak diesel driver]

Do you guys remember when those mesh gaskets were sold as a mpg improver? LoL

 

[MrMarty51]

Do you guys remember when those mesh gaskets were sold as a mpg improver? LoL

YUP, sure do.

 

[94DieselSub]

@ak diesel driver,

Yep, me too!

But ironically, unlike the million variation of the bent sheetmetal "tornado" inserts seen on ebay. The screened gaskets at least had a shred of science behind them.

On a carbureted engines, intake manifold fuel dropout (fuel separation) did occur. It was detrimental to emissions and fuel economy. The screened intake gaskets would (in theory) brake down the precipitated large fuel droplets into a finer mist.