Mufflers or Cats ?

[vms4evr]

I have a small dilemna here. I have about $500 I can spend on the exhaust. So my plan for the Corsa is out.
So I've looked at 1 of 2 options.
1. Magnaflow mufflers. The ones specifically for the 92-96. Polished, look real sharp. I hear mixed opinions on how loud and how much resonance. Ragtop owners are happy but some of the hardtop owners complain. But no all.
2. Random Tech (or other) high flow cats. Should breath better and still use factory resonator and mufflers. I think this would make the exhaust system sound a little more throaty?

Which do you think would yield more HP/TQ and make a better choice overall?

Graham

 

[Ken]

The mufflers, no matter how high of a flow they can handle, will not do you one bit of good if they are restricted by the stock catalytic converter.

_ken

 

[BigRed]

Are you in a "controlled Pollution area"? If not ditch the cats all together and put a cheater plug on the O2 sensors (Avail at TPiS) then just Opt out for the Mufflers.

If you are in a regulated pollution area Do the Cats then get to the mufflers later. (Kens Right )

 

[vms4evr]

Guys,
Yes I'm in a Pollution area and the Emissions Test is actually the biggest part of the test. Last test my car was very low.
I found out that the factory LT1 mufflers flow about 250cfm each, so 500cfm combined.
So how much do the factory cats flow? This is why I asked the question. A lot of folks say " oh your cats are very restrictive", but no one has given me numbers to back that up.
I figure whichever flows the least I would replace.
So Ken, BigRed are you telling me the cats flow less than the factory mufflers? Got numbers to quantify that?
Graham

 

[BigRed]

High Flow Cats - There are several brands of catalytic converters. However the best is made by Random Technology. Random Technology catalytic converters actually have a higher flow capacity than most mufflers. As an example, a 3" diameter Super High Flow converter flows 501cfm. most 3" diameter high performance mufflers flow less then 475cfm at the same test pressure. Random Technology direct fit Super High Flow catalyic converters are now available for most domestic and imported vehicles. Custom catalytic converters with inlets and outlets up to 4" in diameter are available. What this means in laymans terms, a Random Tech Cat works as well as if not better then most mufflers available on the market today. One other brand is Catco.

Hope that helps you

 

[Redbob]

I'm going to take the contrarian view here, and recommend some good low-restriction mufflers. Restriction, or backpressure, is additive, and those mufflers on your '92 are pretty restrictive.
The reason "flow" numbers don't mean much on the exhaust side is that the pressures involved are so much higher than on the intake side. It is more common for exhaust manufacturers to work to "backpressure" objectives, under a specified condition of RPM and load (usually WOT, of course). Taking measurements on an actual vehicle (ie, the only meaningful measurements) can be pretty tricky if you don't have access to a closed course.
A technician in my employ once bashed up a ZR-1 when the brakes faded after several backpressure runs, and sent him off the end of our local airport runway at about 105 MPH.
Grass has SUCH a poor coefficient of friction!

But back to your question: go low restriction mufflers; 409 stainless will last longer if it's welded to than will the shiny stuff like 304. Get a pipe like the Walker "rear Y-pipe" made up, because it WILL be boomy in your coupe. Your coupe unfortunately has an acoustic standing wave frequency right about the same as engine idle firing frequency, and the boom can be defeaning. The "rear Y-pipe" (which I developed, BTW, not Walker) will reduce the input a lot, making the car much quieter inside.

 

[vms4evr]

Hey Redbob,
Haven't heard from you for a while. Given your background I'm going to take your recommendation and go with the mufflers. I looked at the Dynomax direct fit w/LT1 tips. Tried to order them but no luck. No one has them in stock. Some problem at Walker.
So I looked at the Magnaflow SS for the 92-96. They're polished and look good. Deep sound but there is some resonance in the hardtops. I also found someone who will sell me the Flowmaster cat-back system pretty cheap. That package is aluminized and doesn't look very good. Flowmaster brags some serious performance numbers with it that I'm skeptical about.
Now the resonance part. We discussed this some months ago. On the LT1 it doesn't use a Y-pipe. The resonator is 2 in 2 out so individual pipes run to the mufflers. How about an H-pipe or X-pipe after the resonator just before the pipes split off towards the mufflers? The guy who will do the exhaust says he can weld a balance tube between the pipes if that will help with resonance. Kind of a custom H-pipe.
Graham

 

[Redbob]

Originally posted by vms4evr
Hey Redbob,
Haven't heard from you for a while.

Yeah, this spring has been great for motorcycle riding in North Texas...

Now the resonance part. ...On the LT1 it doesn't use a Y-pipe. The resonator is 2 in 2 out so individual pipes run to the mufflers.

That's the the system layout that the 'rear Y-pipe' was originally developed for: we were trying to quiet a ZR-1 for Export, to meet European exterior noise law.

How about an H-pipe or X-pipe after the resonator just before the pipes split off towards the mufflers? The guy who will do the exhaust says he can weld a balance tube between the pipes if that will help with resonance. Kind of a custom H-pipe. - Graham

An "'H'-pipe" is exactly what it was originally called, but location is important: it needs to be at the rear, just about where the pipes separate and turn towards the mufflers, just about at the rear axle. A balance tube should work fine, and can be a bit smaller than the rest of the system piping if that makes it easier to fabricate and install, 2" diameter say, and it will still work fine.

And I'd forgotten about the resonator: it also is relatively restrictive. You might try replacing it with a couple lengths of pipe, see if the interior noise is bearable.
If not, if you have to use the resonator, here's something you might check on: see what the resonator outlet bushing size is compared to the system pipe downstream of the resonator. One big source of backpressure in these systems is the sudden changes in pipe size, both up and down, going into and out of the resonator and mufflers.
For example, and I may have mentioned this before: the early ZR-1's had (still have) 2-3/4" system piping, but the muffler and resonator connector "bushings", where the tubes connect to the endplates, are only 2-1/8". This abrupt change, entering the mufflers, and both entering and exiting the resonator, was the source of much of the ZR-1's backpressure. We got a 20% reduction in system backpressure by simply reducing the pipe size from 2-3/4" to 2-3/8".
There was less disturbance of the flow into and out of the resonator, and into the mufflers. Like I said, this was developed on a ZR-1, so I'm sure it's a valid approach at least to 400 real live horsepower.
A fringe benefit was a significant reduction in the weight.
So if you can't find resonators with larger internal tubes, you might consider reducing the pipe size to more closely match the size of the actual resonator internal tubes. And take a look at those mufflers you're considering, see what size inlet bushing and what size internal tubes it has. That's one indicator of the kind of backpressure you'll see. The ideal situation would be system piping that has the same internal diameter as the muffler and resonator inlet (and outlet) tubes.
A lot of this, of course, depends on just how sympathetic your muffler shop guy is...

Best Regards,
Robert
PS: None of this will have any effect on emissions.

 

[Vettelt193]

On my 93, and now on my 96, I cut out the rear mufflers... this makes a great sound, adds HP, and is cheap... just over $100 to get the work done.... If you do this, you may be able to do the mufflers and the cats

 

[vms4evr]

Vettelt193,
I've toyed with the idea of just going with muffler eliminators. Sure would lighten the rear end up and look very clean. Some folks said they had to add a lowering kit to the rear end just to get it back to the factory height. Apparently those LT1 cans are heavy...
Just seems like it would be quite loud and resonate a bunch with a coupe. I'm not looking to send women and children into a frenzy when I drive through the neighborhood. I'd just like better flow and a nice rumble. Nothing obnoxious...
Graham

 

[Vettelt193]

The loudness depends on a couple of factors... First of all the miles on the car. The more miles, the more the center resonator will have blown out, and the louder it will be. Secondly, and this is a widely overlooked item, but the tips play a big factor in the sound. On my '93 I had quad 3 inch chrome tips. The car also had over 100k miles on it... it was loud on the outside for sure... the good news is on the inside it wasn't bad at all... the only down side was resonance during low RPM accelleration... (6th gear at 45 MPH). On my 96 LT4, I kept the factory tips (they already had chrome tips over the factory ones so it looks good) and used smaller pipe... instead of expanding the pipe all the way to 3.5 inches (which is what the 93 had before the split to the twin 3" pipes) I actually went slightly smaller than factory, all the way to the rectangle tips... rectangles are not good at transmitting sound, so the car is much quieter, but still has a growl, especially at higher RPM.... The resonance is almost gone too... and from what I understand, LT4's are supposed to be a bit louder than LT1's.

 

[Redbob]

Don't know about the resonator blowing out with age; there's nothing inside it to blow out. No fiberglass or other absorber, I mean.
I suppose it could rust out; did you do a lot of short trips?

 

[Topless]

How loud would the LT-1 be with an X-pipe in place of the resonator, and no mufflers?

 

[Redbob]

How loud would the LT-1 be with an X-pipe in place of the resonator, and no mufflers?

Eh? What'd you SAY? Eh?

"Deafening" is the best description I can think of. More so at low RPM actually.
The 'X'-pipe reduces resonances but wouldn't really silence it any appreciable amount.

I did some testing to determine just how low we could the backpressure on a ZR-1, and we ran straight 3" pipes all the back, no mufflers, no resonator.
It was loud like a NASCAR stocker at idle and low RPM, but believe it or not, when you get the revs up it just seems to sing!
Of course, sleeping babies and Barney Fifes might not like that particular aria...
At light throttle, cruise condition at say 70 - 90 MPH, it just wasn't all that bad. If you lived in Arizona, New Mexico, or West B.G. Texas, you'd probably like it - if you never wanted to listen to the radio.

- R

 

[vms4evr]

I saw some threads about removing the resonator on another forum. This was done on Vettes, Z28s, and TransAms.

The end result was that the car gained no performance improvement, tested on a dyno. Sound increased but as Redbob noted the most sound was actually resonance. One guy sad it was litteraly deafening.

Now I'm thinking that a balance tube on the LT1 exhaust right where the 2 pipes form a Y on their way to the mufflers has some real merit. I keep on hearing people with ragtops complain a lot less about resonance than coupe owners. So what's the difference here? Well, on the coupe that rear compartment makes a nice echo chamber :L You can get some great bass out of your speakers back there because it echoes. Nice big chunk of glass to bounce those sound waves off. Well the bass from the stereo is a nice low frequency much like exhaust resonance. I have heard of using Dynomat and other sound-proofing material under that carpet in the rear to help baffle it.

I can't afford the Corsa system right now but if you look at their technology they used sound cancellation. Their exhaust system is supposedly a straight through design. They just beat the annoying frequencies around in chambers until they go away. So a balance tube in the right place would have a big impact on harmonics. Now exactly where to put it is the question

My muffler guy is sympathetic to my dilemna Redbob. He has no problem fabricating and welding in a balance tube for me. So if I go with the mufflers I'm really close to buying then we'll weld a 1.5-2.0" pipe in the Y section. He's actually very interested in doing this because he has dealt with resonance issues a fair bit so I would make a nice guinea pig for him...

Graham

 

[Redbob]

The difference between the coupe and roadster in their response to exhaust resonance has to do with internal dimensions of the cabin. In the coupe, you get an acoustic "standing wave" whose frequency is equal to the engine firing frequency, so exhaust noise "drives" the standing wave; that is, small increases in exhaust noise AT THAT FREQUENCY will make large increases in the amount of interior noise.
The particular standing wave that causes this problem is the one associated with the length of the coupe. This standing wave frequency unfortunately matches up with an acoustic mode in the exhaust, so there's a peak in exhaust sound at the same frequency.
There are of course standing waves related to the width (a sound wave bouncing back and forth between the side window glasses) and the height, but since these distances are much shorter, the frequencies are much higher. Sufficiently high that there are no exhaust acoustic modes that "drive" them.

For an analogy of what I mean by "driving", think of a kid on a swing: that kid swings back and forth at the natural frequency of that "system" with a very small amouunt of damping due to friction in the pivots, air resistance, feet dragging on the ground, etc. Let's say it takes the kid 3 seconds to swing up and back again; then he's got a "frequency" of 1/3 cycle per second (or Hz = Hertz). So if you were to push him every time he swings by, you'd be "driving" him at 1/3 Hz, and he'd swing higher and higher; you'd be "driving" him in "resonance."
If your "inputs" were twice as fast - 2/3 Hz - you'd still be pushing him on every swing, but every other time you pushed, he wouldn't be there. If you were to push half as fast - 1/6 Hz - you'd only be pushing him on every second swing, so he wouldn't swing quite as high, but he'd still be swinging pretty high.
If your "inputs" were at any other frequency, 1/2 Hz (every two seconds) for example, you'd miss the kid entirely, and eventually he'd come to a complete stop.
Is this clear, or has everyone gone to sleep?

Anyway, my point is this: the coupe is long enough to have unfortunately a cabin dimension whose standing wave corresponds to an exhaust problem; the roadster cabin is not as long, so it doesn't have that same problem.
Regards,
- R

PS: FWIW, the formula for standing wave frequency, is f= 1128/2/L where L is the length in feet, L is the speed of sound at sealevel.
e.g., for L=7', standing wave f=80 Hz, major boom territory.

 

[vms4evr]

Thanks Redbob. Once again you back up the problem with engineering proof. I love it. I don't follow you completely. But I get the gist. I need to stop pushing my kid on the swing and my exhaust will not resonate

Ok. So I can't change the engine pulses per say. Because the resonance will come and go at certain rpms. And i sure can't change the length of the cabin. So my alternative is to try and tune the exhaust to beat that frequency. I talked to my buddy about putting soundproofing in my cabin in the hatchback area. he said no problem we can do it easy. He does stereo installs so i get the material at cost and he installs it. What a pal.

Do you think I would need to insulate the doors and the floorboards or would the hatchback area cover it enough?

Graham

 

[Vettelt193]

If you are getting dynamat, they make a sound plus heat shield. I would put that under the carpet around the transmission, and any areas where heat comes through (On my car it seems like my right leg always gets hot because the carpet is getting hot). I put cheap heat/sound shield in from home depot on my 93, all in the back and around the tranny, and there was a noticable difference in sound, but a major difference in heat.(I bought a roll of shiny silver stuff)... hope this helps

 

[Redbob]

Graham:
Looking back, I can see I got maybe too caught up in the description of the standing wave phenomenon, and didn't really say what it all means. What it does is this: the cabin is so sensitive to small noise inputs, it's so sensitive to being "driven" at the standing wave frequency, that very small amounts of exhaust noise will result in large amounts of interior noise.

So I'm not sure that you can put enough sound absorbing material in the cabin to do any good. That's why I've always focused on reducing the exhaust noise, with good mufflers and a properly located crossover pipe ('H'-pipe).

Here's what a good interior sound package would consist of, if you wish to give it a try:
first, you want a barrier material under the carpets and on at least the rear wall of the luggage area. One such material is from Scosche, called AMT250. They have a web site.
Another good material is the heavy vinyl sound barrier from Silent Source (see Silentsource.com)
Barrier materials reflect the sound, and their principal property is mass; you want something that is at least 2 lbs per square foot surface density. The specs I've seen on Dynamat indicate it is a vibration absorber, not a sound barrier, and it doesn't have enough weight to do the job - unless it was layered; 3 layers of it would be required.

Second, you'd want a sound absorber, to dissipate the interior noise. Open cell foams are good for this; they don't necessarily have to have the "eggcrate" appearance you sometimes see on sound absorbers, but that slightly improves performace. Thicker is better: I don't think you would see (hear, rather) any effect at all unless the material is at least an inch thick, and for the 'boom' problem we're talking about, there might be very little benefit if it's less than 2" thick. Dynamat's "Dynaliner" is too thin to do the job you want done here.
For the absorber, you want as many square feet of it as you can fit in, on all surfaces. Even on the seat backs, if you can!
Keep in mind that there's no guarantee this package will do anything whatsoever for the boom problem, since theory says an absorber should be over 3 feet thick to absorb noise at this low a frequency; it'll probably reduce many other frequencies in the sound spectrum, though, particularly the higher frequencies that just sound "trashy".

Let me know if you do this and it works; if it doesn't though...

Best Regards,
- R

 

[vms4evr]

Thanks Redbob. I'll go look into those websites and the cost for the materials to see if it is a realistic project.

Now another question on the H-pipe and placement. I think we covered this a little before. Say after I add these mufflers resonance becomes annoying at a certain rpm. So I put the car on a lift and let it run up to that rpm. What inexpensive tool can be used to measure this harmonic problem and know where the best place is to put an H-pipe?

Graham