MatthewMiller

He's talking about the header primary tubes. As I've explained about one million times, it is true that the diameter of the primary tubes can be too big for best power in a certain operating range. But as I've also explained several times, it's not because backpressure is good - it's because the getting the gas velocity correct in that resonant system ("system" being the valve, exhaust port, primary tube, and collector) will time the negative phase of each pulse to the period of the exhaust valve opening, so that backpressure is reduced. If you just stick an obstruction in there (like a muffler) it won't do anything except create backpressure, and that will harm performance just like it would anywhere else downstream.

And I've also told you several times that the post-collector exhaust system is not tunable for more power. You cannot change anything downstream of the collectors to tune the negative-pressure pulse at the exhaust valve. Probably the best source for understanding this is Vizard's writing on exhaust systems.

Yes.
Again, this is in reference to header primary tube diameter. It is NOT about the post-collector exhaust system. You haven't found ****. The fact that I've gone over this several times in writing, and yet you still continue to conflate primary tube diameter with exhaust system backpressure is pretty amazing. You need to do a lot less typing and lot more re-reading and learning.

ddahlgren

If I am following this correctly potential mufflers are none or round glass pack type or magnaflow X or stock. Large oval or box type are out. Hopefully someone that does have some useful thoughts before Kingtal0n's bat signal goes off and he needs to take a crap on this thread.

84 4+3

The eliminators on mine are mini glass packs basically. So yes I'd say that theory seems to hold true especially reading others posts. No mufflers at all seem to be hit or miss though as I seem to recall some having drone with eliminators that just are tips.

MatthewMiller

Corsa has round mufflers, too. They just aren't the ones they use for the C4 system. I guess the challenge is in being able to buy just their mufflers, without the whole cat-back system. It doesn't look like they sell their mufflers separately.

EDIT: This is something the OP said earlier. That said, it may be worth scouring used listings for people selling Corsa mufflers or a whole system used and using just the mufflers.

aklim

So does Borla and they are noisy things. Neither Borla or Corsa sells it individually. So if buy the system and sell the rest after taking the muffler out, the next guy will pay almost a system for the muffler. Tried to ask for it and got the answer from Corsa

ddahlgren

Other than stock or Corsa are there any real known works 99% of the time then. It seems like a straight thru glass pack would be no louder than the Borla as I can see from one end to the other with them. I am thinking some sound deadening material in the cargo area might help a lot too. I personally think the resonance is between the cargo floor and rear window all acting like an amphitheater.

MatthewMiller

I would think so, too. Keep in mind that there is a difference between attenuating sound throughout its frequency range (which is roughly what these things do), and attenuating a mode of excitation (drone) centered around one specific frequency. That is to say, the Borla or glass pack probably attenuates all sound somewhat (actually they probably attenuate high frequency harmonics more, because they don't have enough packing to do much to the long, low-frequency wavelengths). OTOH, the Corsa or any other quarter-wave resonator would attenuate specifically the drone, without doing much attenuation to other frequencies. So it depends on what you're after.

For sure, this. I think I posted a while back that just packing some luggage or a moving blanket in the back hatch area attenuates noise quite a bit. Even just taking the glass top off does, too. So there is no question that the rear floor and the ceiling amplify at least certain frequencies of sound.

84 4+3

Also from what I've been reading the sound deadening doesn't help too much. A resonating frequency is a resonating frequency. It sometimes dulls it but it probably won't kill it. And yes, the rear glass is mostly what causes drone I would believe. Easy way to test is pop the hatch and drive down the road.

I don't know of any others that work 100% with any setup. My system is basically stock other than the cat back so one could speculate my system is good for a stockish exhaust. The hooker areochamber in my cousins transam is about the nastiest sounding muffler I've heard on a stock l98. You also can't have a conversation at all inside the car and it drones like a sob. He comments all the time how throaty mine sounds driving past and how quiet it is inside especially with the top on. Sometimes things just end up working and you get lucky.

I have heard having turndowns on the muffler tips tames the drone quiet a bit. I know my stockers were damn quiet and they had turndowns. The cutouts in the rear bumper look awfully weird empty though. More than likely why they went straight out on 85s and up. There should be an official muffler test sticky and each popular system rated on a control vehicle. Then updated with owners systems with specific mods. Headers, cam, h/x pipe etc. Unless I missed it and there is one already. I'm sure there's a vendor that would lend out warranty systems or something. Misaligned tips or something like that that can be tested.

ddahlgren

Am I correct in thinking that cams or intakes have virtually nothing to do with the drone?

GREGGPENN

On the packing issue, I put (household) fiberglass batting behind my rear wheel wells about 10yrs ago. It's helps A LOT. Probably more for road noise than exhaust but I recommend it....especially since it's lasted that long and hasn't caused other problems (mold, fire?, etc..)

Both in my case (sidepipes) and yours (others), this area of packing doesn't block exhaust outlets -- meaning it's not going to stop drone. I did buy a cargo mat too -- which probably helps more. Not sure because I did the undercarpet sound-barrier thing at the same time. The combination of under carpet sound barrier PLUS rear wheel insulation changed my C4 to something way more refined.

Personally, I think glass packs (and chambered pipe) attenuate mid-bass more than low bass. I think the LONG LENGTH of pipe run forms/carries your low frequencies...even around bends. I think drone is midbass vs lowbass. And again, I think box mufflers can CREATE mid-bass when not correctly baffled/cancelled....like a Corsa.

If you want significant dampening of drone/sound, use the longest glasspack you can find/fit. I'd even consider belly-mounted and tail-mounted combos. IOW, series of them. I remember a few C4 owners being happy with chambered pipe...which is probably 24" or longer? That said, I'm not sure if/how chambered pipe differs in attenuation vs equivalent length glasspack (or cylindrical) mufflers? I'm guessing the later is better-priced. (I haven't tried chambered pipe myself...though I've used glasspacks and bullet resonators...which are very similar.

No idea whether tapered cylinders are better than the flat-end variety. You would think...but I don't know. Like MM, I got some of my exhaust knowledge from Vizzard's articles. Yet, he's no where near clear about the shape of a "good muffler".

I would add that part of my (our?) original goal would have been to determine what a "terminator box" is? More precisely if other components (like a muffler, x, or y) can create the termination wave necessary to "fool" the system into "seeing" a shorter after-collector pipe system. My GUESS is that an X/H goes a long way by doubling the amount of pipe from that point on? And....that's why an "H" placed directly behind the collectors has measurable HP improvement.

I remember another member (who had a website with tons of good builds/info) recommending the narrowing of pipe ... to keep velocity up as gases cooled. I haven't seen this mentioned in recent history but the talk of piping made me think of it. I'd be curious to hear your thoughts on whether this was misinformation?

I can't remember for sure but I'm thinking it was recommended that catbacks/axlebacks could be chosen one size smaller because temps were significantly lower by the rear axle.... Of course, it has also been suggested (over the years) to put an X/H at the location where paint would stop bubbling/burning off the pipe. It seems that might be too far back -- considering the prior post-collector recommendation.

Obviously, I'm an expert on this...just spouting a few things here/there. Heck, I was even thinking exhaust pipe COULD serve as an extension of the header collector. I know I've read that.

The particular tri-Y's I bought came in two designs. One had a much longer collector and was SUPPOSED to "tune" exhaust for DD rpm ranges. IOW...better low torque. I haven't reread Vizzard's stuff lately but I was thinking primary diameter and secondary LENGTH had the most effect on "tuning". From this standpoint I thought this was what ton-of-kings was getting at so awkwardly? Like intake runner tubes, the exhaust "tunes" as well. I guess my question is whether an "extension" of collector tubing (e.g., the pipe itself) LOWERS where the engine is most efficient in terms of wave tuning/power?

So...the prior question of WHERE to insert an X/H pipe...for wave termination would SEEM to link to a discussion of how much after-the-collector exhaust pipe could/should be added -- if you want better low-end response? I apologize for bringing it up because it invites comments from our two best buddies -- but it IS (after all) part of the discussions you find online about exhaust design.

Obviously, stock/small cammed builds don't matter nearly as much due to the lack of overlap.

In my own knowingly compromised, superlong, sidepiped setup, it SEEMS possible I don't have as much top-end run-out as when I used to have a muffler/converter in the belly. That's partly why I wonder what qualifies as a wave terminator box.

SurfnSun

TA

This is exactly what I did. The drone from my 40 series Flowmasters was really getting old, but as the rest of my system was pretty nice (2.5" SS duals with an X-pipe), I wasn't looking to replace the whole system. I knew Corsas would solve my issue, but I wasn't putting a $1200+ exhaust system on my $8000 car. I scoured the for sale section daily until finally a set of Corsa mufflers only showed up. I believe I paid $400 and I'll tell you it was the best $400 I've spent on this car. The car is now enjoyable to drive and the wife will even ride in it again (on occasion)

69427

As I mentioned above, just a canister with a sudden significant increase in square area, along with sufficient volume to contain the exhaust pulse without a significant pressure rise.
I made one for my '69 a few years back. I wanted a bit more oomph coming off the corners at 4000 RPM, and I did the calculations. Due to the small ground clearance in my car the box had to be flat instead of round. It seemed to work, given the rapid difference in exhaust sound/volume at the 4000 RPM range, but the flat surfaces made it feel l was sitting on top of a bass drum at WOT. I suspect a round termination box would be quieter, but like I say, it wasn't a packaging option, so I eventually took it out for a slightly quieter car at WOT.

GREGGPENN

I have to agree with you -- especially after what Matt has said. I was susceptible to the idea from the standpoint of negative waves, their affect on reversion, and....

I was even thinking exhaust pipe COULD serve as an extension of the header collector.

The extra pipe length exists in MOST of our systems. As for what it does is the reason I posted it BECAUSE I looked at it in the same way you just "quantified" it. IOW, you'd THINK we all have super long header collectors with the use of a terminator box (or an X/H pipe unless, as you propose, pressure doesn't drop enough).

So...Do most of us have 10' long header collectors!

I hadn't thought about this before...the calculations. Were you able to measure pressure inside the exhaust pipe and determine how many times bigger a cross-section would need to be -- to lower pressure to atmospheric?

Sounds like you didn't measure the results?

Curious how big (what the dimensions) of your box was?

69427

The box was (IIRC) 550 cubic inches. Due to ground clearance limitations of the car the approximate dimensions were 3"H x 18"W x 16" L (tapered length into single 4" tailpipe to the mufflers). I had a pressure gauge tap at the box, and at WOT I had 8" H20 (about .3 psi) measured in the box.
According to Vizard's formulas and my target RPM and exhaust timing, I needed a port-to-box length of 40 some inches. Fortunately this placed the box in the only available large space (behind the transmission). If I was drag racing, the higher RPM target point would have put the box further forward, and made installation a bigger headache.
As I mentioned earlier, the sheer noise under the car due to this box just made this experiment unpleasant, and I went back to 3" tubing wye'd into the four inch tubing (one big pipe is lighter than two smaller pipes). The 4" pipe then wye's back out under the differential into 3" pipes feeding the mufflers.

ddahlgren

I can say with decades of dyno work behind me that rpm range is the job of the length of primary pipes with the diameter set by the hp level so the amount to get out. Collectors are more for broadening the range the header works well and the paint stripe test deal does work fairly well and where the collector is generally cut off at. Then come the reality that has nothing to do with perfect and that is fitting the header in the chassis so lengths and diameters have to be compromised most times to get them on the car. Merge collectors work best and do allow for a somewhat smaller collector tube. I wish I saved that endless stack of dyno sheets but did not. I have never tested a complete exhaust system so don't have the slightest idea what it might do other than form a restriction.

MatthewMiller

I also agree that you are correct. It may be that a cam with more overlap has a little more drone just because of the throughput of both valves being open at the same time for longer, at a low rpm where it isn't "on the cam" yet (i.e., there is more still-burning gas getting past the exhaust valve, adding energy to the exhaust column). But not much, I think.

Gregg, yes the exhaust pipe connected to the header collector can and does serve as additional collector volume, up until the point that it hits the first termination. A termination can be a box (as 69427 mentioned), cat, a resonator, or a muffler. The most effective for minimal power loss is an empty triangle box that tapers toward the exit pipe. Also, Vizard wrote that to be most effective, a termination box needs to have 8x the volume of one cylinder (regardless of how many cylinders the engine has).

But here's the thing about collector function: they don't work on velocity like we talk about in the primaries, but rather on volume and/or length - wave resonance. It's a big difference. Keeping a collector smaller to maintain velocity doesn't result in a lower torque peak - that smaller collector volume will result in a torque peak at a higher rpm. The lower the peak you are seeking, the bigger the collector volume should be. This is exactly backward to the thinking about velocity in the primaries. But it's analogous to the thinking on primary length, where the longer the wave can travel correlates to the lower the rpm it's resonant tuning will peak. My guess is that most collectors are too long. I know mine is.

ddahlgren

If it is the floor rear window combination that amplifies the resonance how does that work out in a convertible?

MatthewMiller

Good question. With the top down, it should be relatively less noisy, just like my hardtop with the top removed. With the top up, I wonder if the fabric roof and flexy rear window is quieter than the hard top. I also wonder if the factory solid hard top is quieter than the "glass" top, since it has some headliner padding (doesn't it?).

GREGGPENN

The key word in ddalgren's question is amplify. Drone is likely louder in coupe than verts, though both would still hear it. It's commonly found that the location of outlets affects sound as much as anything -- but that's gonna be similar to the coupe vs vert issue.

To me, drone can exist in any car because it's like a poorly balanced stereo setup -- where the midbass overwhelms everything else. (I would add that box/oval mufflers exist in a cabinet size that I would associate with a small, POTENTIALLY boomy speaker. Unless it was tuned properly.

Actually, for sticks, it can be an issue of the exhaust note being tuned lower -- since ZF6's run on the hwy at lower rpms than autos. LTx cars are different than L98s because of the piping diameter difference. I also agree that cam duration can play a roll too...as well as how the exhaust cam events are timed.

Thanks for refreshing my memory (and bringing it into this thread) about the 1/8th CI rule. I had forgotten if that was in Vizzard's article. Assuming that's true, then anyone with bullet cats attached to their header collector SHOULD be "good". The body on most bullet cats is 4" and they are about 12" long. That would be 48 cu/in...or about 1/8 of my 383.

My particular brand of header has a Kooks "pickle" collector. I believe that's also supposed to help with the termination wave...if fed into a large enough component. By the math (above), it would also stand to reason that you could feed headers into a section of 4" pipe...say 12-18" long, then taper to 2.5 or 3" (depending on power output). If you tapered into an x-pipe, you'd be feeding right back into "dual outlets".