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Discussion Starter · #1 ·
I am wondering how to get that classic, perfect race car sounding exhaust where the sound is smooth and even through any rpm range with no droning, brapping, popping etc. we have a 67 chevelle with a GM crate 350 HO engine, Headman mid length headers, dual exhaust with flow master mufflers and I think 2 1/4 or 2 1/2 inch pipes. It’s plenty loud but at certain rpm “spots” the sound is not ideal. The pipes turn out behind the rear tires. I know some of what I’m hearing might be just due to being inside the car but I don’t think that’s all of it.
I currently do not have a crossover (H) pipe.
We had full length headers on it first but had to change them out due to ground clearance problems. The sound did not change much when we put the mid lengths on.

Is pipe routing, muffler position, H pipe or anything like that responsible for differences in sound?
 

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I think lots of people have different views of what the perfect sound is. Mine has changed several times during the last 10 years.
To me, its a quietish idle where you can hear the cam lope popping up near the collector and a nice low burble after the muffler. All bets are off when the secondaries come open because you dont want to choke it out with a quiet muffler.

I liked the pop with Flowmaster 50 series, and they were still fairly quiet. But the guys on here spoke highly of Dynomax, and their straight through design so I tried some and absolutely love them!

As far as the classic race car sound. A healthy motor with some converter and gear, is going to sound distinctly different than a stock drivetrain setup with the exact same exhaust system. If you know what that sounds like, it cant be duplicated by any system out there.

But it sounds like you have a nice car, so we want you sounding good!!!
 

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Since you have a system already, I would fab a crossover. I went from 3" collectors to 21/2 pipe with a 13/4 balance tube near the bell housing/transmission parting line. I choose this style because of my pipes must pass through frame diagonals, which keeps the pipes wide in the frame until after the end of the transmission. Other cars where the pipes can hug the transmission, may work better with an Xpipe.
I was told, that if you have an annoying rpm that has a crap sound. You should go to a welding store, and get a heat sensitive Crayon, and draw a line along the length of pipe leading to the muffler on each side. Then run the engine at the annoying
Automotive tire Motor vehicle Hood Trunk Bumper
RPM and watch to see where the crayon line turns color first(or burns off), That is where the xpipe would do the most good.

I used the chamber baffled 9X14 muffer approx. 48" from collector, which place it behind the front seat, then over the rear axle and through a 2X12 resonators then bumper exit pipes. All stainless. It has a pleasing idle grumble, while maintaining a very nice sound through the RPM range.
 

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Discussion Starter · #5 ·
I think lots of people have different views of what the perfect sound is. Mine has changed several times during the last 10 years.
To me, its a quietish idle where you can hear the cam lope popping up near the collector and a nice low burble after the muffler. All bets are off when the secondaries come open because you dont want to choke it out with a quiet muffler.

I liked the pop with Flowmaster 50 series, and they were still fairly quiet. But the guys on here spoke highly of Dynomax, and their straight through design so I tried some and absolutely love them!

As far as the classic race car sound. A healthy motor with some converter and gear, is going to sound distinctly different than a stock drivetrain setup with the exact same exhaust system. If you know what that sounds like, it cant be duplicated by any system out there.

But it sounds like you have a nice car, so we want you sounding good!!!
As for gears I have a 4L65e transmission and the original rear at 3.08. Converter is 2400-2800
 

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Discussion Starter · #6 ·
Since you have a system already, I would fab a crossover. I went from 3" collectors to 21/2 pipe with a 13/4 balance tube near the bell housing/transmission parting line. I choose this style because of my pipes must pass through frame diagonals, which keeps the pipes wide in the frame until after the end of the transmission. Other cars where the pipes can hug the transmission, may work better with an Xpipe.
I was told, that if you have an annoying rpm that has a crap sound. You should go to a welding store, and get a heat sensitive Crayon, and draw a line along the length of pipe leading to the muffler on each side. Then run the engine at the annoying View attachment 619712 RPM and watch to see where the crayon line turns color first(or burns off), That is where the xpipe would do the most good.

I used the chamber baffled 9X14 muffer approx. 48" from collector, which place it behind the front seat, then over the rear axle and through a 2X12 resonators then bumper exit pipes. All stainless. It has a pleasing idle grumble, while maintaining a very nice sound through the RPM range.
What do the resonators do for you?
 

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The Brappp sound that I find most objectionable is caused by pulses that tend to stack up in length of pipe. It interrupts that frequency harmonic, so that a pressure wave travels forward in between pulses. sound cancelling without restriction.
Same for the H.. X pipes, they change the frequency dynamics.
My system has no length of pipe over 48", this places the crossover roughly the same distance from the muffler and the muffler the same distance to the resonator, and the resonator the same distance to the bumper. just worked that way.
 

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Discussion Starter · #8 ·
The Brappp sound that I find most objectionable is caused by pulses that tend to stack up in length of pipe. It interrupts that frequency harmonic, so that a pressure wave travels forward in between pulses. sound cancelling without restriction.
Same for the H.. X pipes, they change the frequency dynamics.
My system has no length of pipe over 48", this places the crossover roughly the same distance from the muffler and the muffler the same distance to the resonator, and the resonator the same distance to the bumper. just worked that way.
So there is such a thing as “standing wave” pattern that develops in transmission lines (electronics) and also in sound traveling inside pipes. I guess that is somewhat the operating principle behind horned musical instruments. I have dealt a little bit with tuned audio pipes. I assume the same principle applies to exhaust systems. At certain frequencies a pipe of a given length will develop a resonance. And at harmonic frequencies there will also be some resonance.
But in the case of exhaust systems not only does the frequency change (with rpm) but the pressure also changes as more gas is applied. Also factor in that there are restrictions in the line such as changing pipe diameter and the mufflers and you now have a mathematical mess.
Thats why I was asking about muffler position. But also H-pipe position. And I’m still not sure what resonators do.
The fact that my mid length headers do not have same length pipes also probably complicates things.
I was hoping someone else had already figured out an optimal spacing of components. It sounds like your model might be one to follow since it’s working for you. However, you do have full length headers and probably a bigger engine? But maybe it’ll be close enough.

what are the resonators for?
 

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In the 70's a resonator was often placed behind the rear axle crossways, with an inlet and outlet on each side. The can was common to both sides of the exhaust. This effectively became a second crossover point, and had cancelling effect.

only a 302 sbf with shorty headers, I placed an extended 3" sweep on the collector to bring the flange down to the rear facing orientation.
 

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Discussion Starter · #10 ·
2 1/2 pipe is minimum.
Needs h-pipe or x-pipe to tame the thump of 14 feet of pipe.
I know that pipe size should scale up with engine size but I have know idea how to calculate the right size. I have heard that too much pipe diameter can work against you. But then the length of pipe plays a role too as you mentioned.
5000 rpm is max rating on this engine but I won’t be pushing it that hard most of the time as I’m not a racer (not yet anyway).
Im interested in knowing how to determine this because I’m building a 283/292 and I understand they rev higher. Is there a formula based on engine flow and muffler restriction and pipe length? Or is bigger just always better? I guess only street legal folks have to worry about such things.
 

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Attached is a video of my exhaust, it’s a 350 Chevy ( basically, it’s a 94 lt1). Has hotcam, 1.6 roller rockers and tune, idle set for 750, exhaust is 2.5 inch corvette rams horns and 2.25 pipe, h pipe, cherry bomb turbo muffler placed near rear and tail pipes exiting in stock position. I love the sound, mild at idle, no drone at all! And if you step on the loud pedal it roars!!
 

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Discussion Starter · #14 ·
I just looked at the firing order of the Chevy crate 350 ho that I have in my chevelle. It’s no wonder a true dual exhaust can have strange sound issues. Treated as two separate 4 cylinders each with a single exhaust pipe running with this combined firing order the “pulses” are not evenly spaced on either side. The firing order is 1-8-4-3-6-5-7-2 where all even numbers are on one side and odds on the other. Without over analyzing this I can see how it might sound a bit wacky in some spots. Given an H-pipe or an X-pipe ties the two sides together it probably does help fill in the gaps in both pipes and smooth out the sound. Probably helps to have the two exhaust pipes coming out nearer to each other too like both out the back instead of separated more with one on each side of the car like stock pipes (like mine). Mine is “brapping” a little bit centered at around 2000 rpm. Starts at maybe 1800 and stops at maybe 2200. On the even numbered side there is fire, fire, pause, fire, pause, pause, fire, pause then it starts over. That’s one helluva mess to try to apply any kind of frequency analysis to because instead of one frequency you have elements of at least three different frequencies all in the same pipe.
Again I can see why a crossover helps. I think I’ll order an H-pipe kit.
 
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An H pipe will mellow the overall tone. The position can be pretty much anywhere from just under the bell housing upsweep to the tranny case to just behind the trans cross member.

If you’re running long tube headers right where the collector joins the hard pipes is good if space otherwise permits.

I shy away from side dumps anywhere along the side of the vehicle. If you haven’t put the thing in a wind tunnel there is no way to know if the air flows low and to the side is hindering or helping flow out of the pipe. The only place that without aero testing that is guaranteed to not hinder the exit flow is behind and under what ever passes for your rear valance whether that is a bumper or tailgate. When in motion the area immediately behind the vehicle is a low pressure area where the air flow under, over and around the vehicle is detaching. So essentially you’re exhausting into a partial vacuum. Plus the exhaust sound is aimed behind you, the doppler effect lowers the tone of the note you hear that same as what you hear of an approaching train horn versus the tone after it passes you.

As for mufflers my personal preference is glass packs. Longer and larger diameter cases are quieter and more low frequency rumbly like the good old days sound. Shorter lengths and smaller cases get louder and sharper in tone.

Bogie
 

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You have to think of an exhaust like a trumpet or Tuba. Take for instance a Trumpet in Bb and a Tuba in Bb. The trumpet's tubing is about 4' long while the Tuba's is 16' long and plays about two octaves lower than the trumpet. The way it does this is by taking the vibes from your lips and makes a series of resonances inside the tubing. If you've ever played one, the strange thing is that you vibe your lips, but the instrument itself decides the frequency that your lips vibrate based on the feedback it gets from the resonance in the instrument. Without pressing any keys you can only play notes within the overtone series. So you can play Bb-1, F-1, Bb-2, D-2, F-2, and Bb-3. Pressing keys changes the length of the tube by opening up a loop.

Now imagine putting a cotton ball inside the tubing of the instrument about halfway in its length. The resonant waves hit the cotton and diffuse, so it can't make the bouncy resonance and the instrument will either not play at all, or it will be out of tune or a different key altogether.

That's what a muffler does. That's also why a muffler is usually not right in the middle of the pipe, otherwise it wouldn't be as effective. If you have two equal lengths of pipe before and after the muffler, you are splitting it up into two identical frequencies that are one octave higher. If you put it at the 2/3 point or 3/4 point, you are making the resonance frequencies different and they can't amplify each other.

Adding a resonator is kind of like adding another free-flowing muffler. It's job is to not restrict, but give another point where resonant waves have to fight with a soft diffusing material.

That's the long answer. The short answer is "it's really hard to guess what yours will do." About all you can do is google videos about exhaust sounds and listen. I had a Dynomax on my LT1 with resonators. I thought it sounded wonderful for not much money. It didn't last very well, but not bad. They start pretty quiet, but as the packing breaks in and gets matted it settles in to its sound very nicely. Don't be shocked if it starts too quiet for you. Give it a couple months for the final sound. Mine ended up rusting just behind the mufflers (in front of the axle) so I cut the tailpipes/resonators off. That was very aggressive sound. I loved it. The inspection station didn't because in PA the tubing has to exit from under the car somewhere.

I also really like MagnaFlow's Performance line (not the XL 3-chamber) but you'll pay a bit more for them. Borla has so many different lines that it's hard to tell. Some of them are super quiet, others are loud and blatty. Flowmaster also makes a buttload of different flavors. They all pretty much sound the same, just louder or quieter.

Other generalities: Larger diameter usually means louder. Cam and port design have a huge impact on how it sounds. Overall length usually has a large impact on sound and resonance (shorter exhausts are louder). Downturn tips can make drone way worse as the sounds bounce between the ground and the floorpan. Not quite as much in a truck since the bed doesn't share the same floorpan as the cabin.
 

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Discussion Starter · #17 ·
You have to think of an exhaust like a trumpet or Tuba. Take for instance a Trumpet in Bb and a Tuba in Bb. The trumpet's tubing is about 4' long while the Tuba's is 16' long and plays about two octaves lower than the trumpet. The way it does this is by taking the vibes from your lips and makes a series of resonances inside the tubing. If you've ever played one, the strange thing is that you vibe your lips, but the instrument itself decides the frequency that your lips vibrate based on the feedback it gets from the resonance in the instrument. Without pressing any keys you can only play notes within the overtone series. So you can play Bb-1, F-1, Bb-2, D-2, F-2, and Bb-3. Pressing keys changes the length of the tube by opening up a loop.

Now imagine putting a cotton ball inside the tubing of the instrument about halfway in its length. The resonant waves hit the cotton and diffuse, so it can't make the bouncy resonance and the instrument will either not play at all, or it will be out of tune or a different key altogether.

That's what a muffler does. That's also why a muffler is usually not right in the middle of the pipe, otherwise it wouldn't be as effective. If you have two equal lengths of pipe before and after the muffler, you are splitting it up into two identical frequencies that are one octave higher. If you put it at the 2/3 point or 3/4 point, you are making the resonance frequencies different and they can't amplify each other.

Adding a resonator is kind of like adding another free-flowing muffler. It's job is to not restrict, but give another point where resonant waves have to fight with a soft diffusing material.

That's the long answer. The short answer is "it's really hard to guess what yours will do." About all you can do is google videos about exhaust sounds and listen. I had a Dynomax on my LT1 with resonators. I thought it sounded wonderful for not much money. It didn't last very well, but not bad. They start pretty quiet, but as the packing breaks in and gets matted it settles in to its sound very nicely. Don't be shocked if it starts too quiet for you. Give it a couple months for the final sound. Mine ended up rusting just behind the mufflers (in front of the axle) so I cut the tailpipes/resonators off. That was very aggressive sound. I loved it. The inspection station didn't because in PA the tubing has to exit from under the car somewhere.

I also really like MagnaFlow's Performance line (not the XL 3-chamber) but you'll pay a bit more for them. Borla has so many different lines that it's hard to tell. Some of them are super quiet, others are loud and blatty. Flowmaster also makes a buttload of different flavors. They all pretty much sound the same, just louder or quieter.

Other generalities: Larger diameter usually means louder. Cam and port design have a huge impact on how it sounds. Overall length usually has a large impact on sound and resonance (shorter exhausts are louder). Downturn tips can make drone way worse as the sounds bounce between the ground and the floorpan. Not quite as much in a truck since the bed doesn't share the same floorpan as the cabin.
That’s an interesting comparison. And my pipes do turn out and down so maybe that’s part of the problem. This resto is relatively new with only a few hundred miles on it. It doesn’t sound bad but I guess I’m just picky. Maybe mine will change over time too but I’ll still try the h pipe at some point just to hear the difference for myself. It’s hard to get anything from a youtube video. I’ve listened to a few before and after videos and I can hardly tell the difference and that is in the rare occasion that they record them both in the same environment.
 

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That’s an interesting comparison. And my pipes do turn out and down so maybe that’s part of the problem. This resto is relatively new with only a few hundred miles on it. It doesn’t sound bad but I guess I’m just picky. Maybe mine will change over time too but I’ll still try the h pipe at some point just to hear the difference for myself. It’s hard to get anything from a youtube video. I’ve listened to a few before and after videos and I can hardly tell the difference and that is in the rare occasion that they record them both in the same environment.
The problem with most youtube videos is that they put their cell phone on the ground 3 feet away from the tip. It just distorts and blows out the noise. I watched one where the phone was so close to the tip, it blew the phone over when they started the car. Useless.
 

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Discussion Starter · #19 ·
The problem with most youtube videos is that they put their cell phone on the ground 3 feet away from the tip. It just distorts and blows out the noise. I watched one where the phone was so close to the tip, it blew the phone over when they started the car. Useless.
I like it when they record the before video inside the garage and the after video outside. 🤪
 
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