Car Exhaust - Hemholtz (1/4 wave) Resonator

[SoonerAviator]

So I did a thing with my wife's car (Cadillac CTS VSport) and put on the CTS-V rear diffuser and quad exhaust tips. Looks fantastic, BUT, I also got rid of the giant suitcase muffler in lieu of two more aggressive mufflers. The overall sound is great, not obnoxious, but louder than stock and a bit exotic sounding. Anyhoo, the only negative is that there is a good bit of exhaust drone/resonance around 2100-2350 rpms measured around 110-117Hz. I've read that j-pipes added before the mufflers can target a particular frequency wave by using a 1/4 wave to show up 180-degrees out of phase.

On my vehicle this calculates to roughly a 29" j-pipe per the math. My question is, with this car being dual exhaust all the way back from the turbo downpipes, would I need two j-pipes of that length, or 2 at half that length? My gut says they'd both need to be 29" but I'm not sure.

Any thoughts?

 

[Let'sgoflying!]

“This thread is useless without audio”

 

[Ted]

It all depends on the total exhaust setup. I'm not sure what the engine is in your wife's car or setup. But basically, you go upstream from where you put the J-pipe and determine the resonance frequency accordingly, and then size your pipe from that. So for example, a V6 with a true dual exhaust (no crossover) would be as two 3-cylinder engines. You determine the J-pipe length from that, and put one in each pipe.

I've read where some people have done this and found it to be "too effective", i.e. too quiet. So it might take some playing around with to get it really where you want it volume wise.

 

[SoonerAviator]

Engine is a 3.6L twin turbo V6. I believe there is a crossover pipe up near the mid-car point, but the two exhaust pipes run the length of the car. The factory muffler was a dual in-dual out design. So I was trying to figure out what length of j-pipe was going to be nessary given that they would have to be installed on each pipe.

 

[SoonerAviator]

“This thread is useless without audio”

Lol, want to get this sorted out before the video is recorded. A 500HP TT v6 isn't exactly going to rumble enough to pound your chest, but it's got a pleasant growl. Trying to avoid it sounding to "ricey". Still has to be civil since it's the wife's daily driver.

 

[Ted]

Someone who's smarter about acoustics than me would have to chime in, but my inclination would be that if there is a crossover pipe, the frequencies will end up being for a 6 cylinder, and you would then need a same length of pipe for each half of the exhaust after the crossover. If you did the pipes ahead of the crossover, you'd treat them as 3 cylinders.

Another thing to consider (although this is probably not doable) is changing the length of the exhaust to move the drone to a different RPM. One of my vehicles has a drone at an RPM that equates to about 72 MPH, which is an annoying location. Because of how it's set up, I can shorten the exhaust and I think that would move the drone up to about 85 MPH, which would be better.

 

[SoonerAviator]

Someone who's smarter about acoustics than me would have to chime in, but my inclination would be that if there is a crossover pipe, the frequencies will end up being for a 6 cylinder, and you would then need a same length of pipe for each half of the exhaust after the crossover. If you did the pipes ahead of the crossover, you'd treat them as 3 cylinders.

Another thing to consider (although this is probably not doable) is changing the length of the exhaust to move the drone to a different RPM. One of my vehicles has a drone at an RPM that equates to about 72 MPH, which is an annoying location. Because of how it's set up, I can shorten the exhaust and I think that would move the drone up to about 85 MPH, which would be better.

Yeah, I'm not familiar enough with the science to know of the exhaust pulses are being dampened so that a given length of pipe "effectively extends" the exhaust which targets the drone, or if it's harmonic to a certain resonance and acts like a pipe organ which would need the full 29" added to each pipe. I have a feeling it's going to need to 28-29" pipe on each pipe. This is an example from some other models of the same type.

 

[Ted]

Yes, that sounds like what I'd be looking for.

 

[Let'sgoflying!]

very civil:

 

[SoonerAviator]

very civil:

I'd love the v8 option if for the sound alone, but the CTS-V with the supercharged LS engine (640hp) is almost double what my VSport costed on the used market. The wife has enough problems with the lead foot, she doesn't need another 140HP helping her make bad decisions lol

 

[Ted]

I'd love the v8 option if for the sound alone, but the CTS-V with the supercharged LS engine (640hp) is almost double what my VSport costed on the used market. The wife has enough problems with the lead foot, she doesn't need another 140HP helping her make bad decisions lol

I realize that it won't happen, but I'd actually like to see some smaller V8s come out that can make the power the V6s do, but with the better visceral qualities and more in the affordable price point range. I haven't driven many of the newer ones so it may have improved, but I've not found an American V6 that I really enjoy the qualities of. The V6 in the Alfa Romeo (which you can argue the origins of, but is an Italian design) is fantastic, and I find the Italians make a great V6. Really they make pretty much a great anything engine wise as far as visceral qualities go (we'll ignore reliability).

The horsepower wars we're seeing now are great for us horsepower junkies, but let's face it - largely unusable on the street for more than a few seconds, and the required "driver aids" to make them not have the self-destructive insurance rates of early Vipers tend to interfere and take away from the enjoyment. I4s and V6s generally are less satisfying engines (although the odd-fire I4s are interesting). I3, I6, V8, those I like. But with the various other requirements of weight, emissions, and cost, I think we're not going to see those in new cars.

My wife's Alfa has 505 HP, and it is a really enjoyable daily driver on the whole. But that said, it is a very tame 505 HP compared to my 3000GT VR4, also an all wheel drive V6 powered vehicle with no driver aids (we'll ignore the early ABS system), and it was something around 400-450 HP.

 

[SoonerAviator]

I realize that it won't happen, but I'd actually like to see some smaller V8s come out that can make the power the V6s do, but with the better visceral qualities and more in the affordable price point range. I haven't driven many of the newer ones so it may have improved, but I've not found an American V6 that I really enjoy the qualities of. The V6 in the Alfa Romeo (which you can argue the origins of, but is an Italian design) is fantastic, and I find the Italians make a great V6. Really they make pretty much a great anything engine wise as far as visceral qualities go (we'll ignore reliability).

The horsepower wars we're seeing now are great for us horsepower junkies, but let's face it - largely unusable on the street for more than a few seconds, and the required "driver aids" to make them not have the self-destructive insurance rates of early Vipers tend to interfere and take away from the enjoyment. I4s and V6s generally are less satisfying engines (although the odd-fire I4s are interesting). I3, I6, V8, those I like. But with the various other requirements of weight, emissions, and cost, I think we're not going to see those in new cars.

My wife's Alfa has 505 HP, and it is a really enjoyable daily driver on the whole. But that said, it is a very tame 505 HP compared to my 3000GT VR4, also an all wheel drive V6 powered vehicle with no driver aids (we'll ignore the early ABS system), and it was something around 400-450 HP.

Yeah the Caddy LF3/LF4 TT V6 has some okay aural qualities as far as most V6s go, but I certainly don't consider it to be soul-stirring like an I6 or v8. The 500HP is definitely refined and well-controlled by the various stability control systems, but you can deactivate most of it if you really want to test your meddle, lol. The CTS is just a very balanced car and gives great feedback when you're approaching the limits of traction. It's based off of the Alpha chassis that the current Gen Camaros are, so it has performance roots. The ATS-V is essentially the same vehicle in a smaller footprint (think BMW M3 vs M5).

I got some Kilmat sound deadening material to line the trunk floor with, and some heat wrap insulation to put around the new mufflers just to minimize vibration. Then I can have the j-pipes installed. I might see if I can have a trombone-style design made at first, so that I could adjust the total length to target the best range of frequencies and then weld it in.