FAQ: The Exhaust Thread

For performance, The stock manifolds and down pipe are the most restrictive areas. Doing cat-back work looks impressive, costs some money, and changes the sound, but it won't give much of a HP gain, imo.

Just changing to a 3" downpipe is a nice way to gain about 10 HP on a stock engine. Ported manifolds or headers will raise that a little. With other mods, these mods can multiply the HP you get with each one.

I'm wanting to upgrad to a coated plog and a downpipe. Just got to get the $ together!

Actually, I wanted to replace worn parts of my exhaust primarily. I figured that if getting some performance gain was a matter of using one or another component instead of an OEM replacement, I'd do that. So performance is important and so is quiet because I spend so much time driving highways. So I was thinking if I went with 2 1/4" pipes and scuh all the way back (instead of reducing to 1 7/8") it's be fine.

What are your further thoughts?

Albertj

Based on reading/rereading the exhaust articles Jason posted, I'm thinking a Riv "enthusiast" exhaust - let's call it a Wildcat-1 Exhaust, might consist of:

- stock manifold and downpipe
- high flow cat
- resonator 18" long with 3" inlet/outlet
...and for a dual exhaust:
- split to 2" at the rear axle Y, optional 2 1/4" for a little louder
- matching mufflers (2" or 2 1/4") with flow at least 300 CFM each
- turndowns or tips of owner's choice at muffler outlet extensions, ending under rear bumper.
...or for a single exhaust
- reduce to 2 1/2 after the resonator (this is to control noise and increase gas velocity a bit)
- bend to right (passenger side) into high flow (550 CFM or better) muffler
- turndown or tip of owner's choice at muffler outlet extensions, ending under rear bumper.

I'd like to keep the Buick Burble at idle, and I'd like the system to be QUIET at and under 2000-2500 RPM - for highway cruising. I do a LOT of highway driving and that's why I listed an 18" resonator. Above 2500 RPM it can make noise up to the legal limit.

WHAT TO DO:

Please comment on this setup - I'm specifying a new exhaust system for my car over the weekend, want to get the specs to the builder ("Never-Rust" of Avalon PA, see http://www.never-rust.com) early Monday and have him fab/install it on July 6.

For instance, should I stick with the stock pipe sizing all the way through, or maybe from engine through resonator, and just open up the tailpipe and mufflers to 2" - 2 1/2" ? Should I put a shorty glasspack muffler after the resonator/before the Y and use glasspacks as rear muffler(s) instead of tweeking the stock setup?

Please advise.

Oh, and if you have some time on your hands, you might spec a Wildcat-2 exhaust for modified Rivs, modifications to be listed. For instance, specs for a Riv with a smaller supercharger pulley, tuned headers, or any other mods that increase exhaust volume at the collector.

Thanks

Albertj

Quote :

I'd like to keep the Buick Burble at idle, and I'd like the system to be QUIET at and under 2000-2500 RPM - for highway cruising. I do a LOT of highway driving and that's why I listed an 18" resonator. Above 2500 RPM it can make noise up to the legal limit.

You've probably read this before: I'm against pipe, resonator, and muffler upgrades, for the most part. The reason being, Buick figured out a balance that works to keep the exhaust quiet. I believe it is very difficult to recreate that balance using random aftermarket parts. We can speculate at best, but it's very likely going to be louder than stock.

The most restrictive part of the exhaust is at the manifold and downpipe area, before the cat converter and resonator. This has been essentially proven with tests conducted on the W-Body by Thrasher Performance, and documented with dyno tests on a G-Body by INTENSE Racing. Thrasher proved that other than the U-bend (which we don't have), there is very little power gained from upgrading cat-back, and INTENSE was able to get 400 HP out of a Bonneville using stock exhaust after the cat.

Improving the downpipe to 3" and replacing manifolds (especially front side) with headers or power log is going to free up extra HP. The cat converter can be replaced or deleted, but I've read the OEM unit flows fairly well. Deleting and replacing with straight or taper pipe is ultimately the best flow solution. I know, this not street legal.

There is some debate on how well the exhaust system flows after the cat. I'm convinced it is more than adequate for 90% of us who mod Rivieras. And more importantly, it is very quiet! Why must we mess with a good thing? Maybe some of us like the noise of a loud 90º V6, or we "think" we're getting something when we really aren't. I know, that "T" pipe looks terrible, and those "soda straw" sized tail pipes seem narrow, but in my opinion they are not as bad as we like to believe. They're also made of stainless, and so will last a long time. Furthermore, I hear a nice gurgling idle at low RPM through the stock exhaust. Speed up and you don't hear a thing. No drone. The only noise I really notice is from the thin walls of the tube headers - a necessary evil I guess.

I've thought about getting rid of the stock exhaust many times, and so far I just can't do it. There is no good reason to do so unless it is to make more noise, change the appearance (tempting), or to build a car that makes 350 WHP+. If I didn't drive my car 60-75 miles per day, I would think about it. For what you lose, I don't think the very small performance gains are worth it.

albertj wrote:

- stock headers ...

I seem to see this phrase used more frequently lately for some reason. There is no such thing. The stock parts are manifolds, aftermarket pieces fabbed with welded steel tubing are headers. Don't know why this bugs me, but it does.


Excellent comments by AA.

from what i know about exhaust, bigger pipes flow better at high rpm,high speed, smaller pipes,stock, are for common sense driving, around town ,low end torque. bigger pipes kill low end torque. the question is, can your stock ecm support exhaust mods? can it deliver the required fuel at the top to match the exhaust flow? on my 88 acura legend i installed a KN air/fuel monitor . i watched the monitor before and after exhaust mods, 3" down pipe,3" straight pipe,cat delete,high flow muffler. and i did see a leaning out of the fuel at high speed ,whereas before exhaust mods, the fuel ratio was in limits. my acura didnt have a s/c but the bigger pipes really opened her up on the top,i reached 140 mph on a long straightaway in long island, but it killed the bottom end. very anemic low end. also with the biger pipes ,at the top, she got a little hotter temp wise than before my guess because she ran lean. the parameters of the stock ecm are defined. the ecm can give you a little more my guess .anything to get the exhaust out the engine faster is a good thing. are you going to have the ecm modified? or have you done that allready?

Albert,
Is your exhaust system in NEED of replacing?
Is that why your building a new system?
Just curious.
After installing my headers and 3" high flow cat (Magnaflow from ZZP) I measured what I could use and the available space for a resonator and mufflers in case i wanted to do a full exhaust and came up with the following;
Maximum space available for a resonator is 28" by 7" round. Mufflers should be no larger than 20" x 12" ( stock is 15 x 11 ).
Was considering a stainless Magnaflow 4" x 22" (overall lentgh = 28") round muffler for the resonator (2 1/2" dia.) P/N 10436
Then building remaining system with 2 1/2" stainless pipe, custom with Y to 2 Dynomax Super Turbo Mufflers (side inlet - center outlet ) 4 1/4" x 9 3/4" P/N 17747 or 17748. They flow 404 and 415 SCFM respectively.
Was going to buy the mufflers and have a local shop do the stainless pipe bending and welding for me.

robotennis61 wrote:

bigger pipes kill low end torque.

While this is true for NA engines, especially smaller overhead cam engines, our supercharged engines have two nice advantages: 1) we already have a huge amount of low end torque to begin with, and 2) any lost torque can be regained by dropping SC pulley to smaller dia, increasing boost (and torque). A long as you keep KR in check (add fuel), the engine will be making plenty of torque, and additionally flowing more air (more mid & top end HP).

The only objection I have against modifying the cat-back exhaust is that it's plenty good in stock form for making a decent amount of horsepower, and it's quiet.

Thanks for the reminder, Aaron.

And Rick - the context is - I'm replacing the exhaust because after 200K miles, the stock mufflers are rusted through at the rear edges and the pipes from the downpipe back, including the cat in/out, are a bit soft. Thanks for the measurements, they will help.

So since some of the better discussion on this editboard seemed to concern exhaust, I thought this thread might interest people.

Further Comments:

Regarding the soda-straw tail pipes after the T - well, they only have to flow ~250 CFM so, now that you guys mention it, making them bigger does not buy anything although it could change the sound - lower the note a little. Matter of fact you could argue they should be smaller to increase exhaust velocity and reduce condensation. I suspect that making the cat and resonator bigger without also retuning the ECM will just cause backfiring.

I don't intend to delete the cat. Period. My cat case is a bit soft, though, so that is why I am thinking about replacing it. I have 200,000+ on the car. Be nice if I could check the cat.

As for the T pipe, I think Rob (Rob McIntosh is "Never-rust") is going to make an elbow, cut a notch at the apex, weld the exhaust pipe to that, and call it a T. Well, maybe call it a Y. Maybe weld the notch cutout back into the elbow as a baffle. Rob is talking about straightening out some of the turns. I suspect the curvy tail pipes are set up as they are in part so that they will in fact make the burble sound. I agree with you, Aaron, the stock exhaust has a real nice burble at idle.

As for the exhaust job - just the OE replacement parts for the mufflers back total almost $400. I can have a replacement SS exhaust for less than the OE parts will cost (incl shipping) from GMPartsDirect. In part because for some reason GM wants over $400 for the cat... heck, I can get a Walker direct-fit for half that from NAPA, and NAPA is the high-priced spread so to speak. Bottom line is the difference in price between a custom T304 SS exhaust and buying a trunk full of OE parts and hanging it myself is not that hard to swallow. I discounted the aftermarket part option because they typically don't last as long as I plan to keep the car.

So anyway, I promised Rob I'd get back to him to say what I wanted but I could not make up my mind about much of it. Thanks everyone for your input!

Albertj

ever thought about running 2, 2" exhaust pipes ,from each cylinder head,bypassing the 3" down tube mod,and running them all the way to the back of the car and run them into a true x pipe out to the mufflers? it sounds a little radical but also sounds like a trick mod. think it would work?

There isn't the space under the car for 2 pipes to run the whole length without sacrificing a lot of ground clearance.
I'm sure it can be done but you'd be dragging the left pipe.

is there space to run them side by side?

I'm not sure, I would have to get it up in the air and take some more measurements.
Next time I get to use a buddy's lift I'll try and remember to take a closer look.

Robotennis: to your twin pipe mod: Yes and no. You'd have problems with engine control and clearance. As it is the riv has two oxy sensors. with your 2-pipe setup you'd have to reprogram the ECU to take data from one bank only - or - use a DSP to merge the signals from 4 o2 sensors (could probably be done with a BASIC Stamp in a module that would work in a car environment.). There might be room to weld in a couple shorty SS cats just below the collectors from headers, or just below the exhaust manifold on each side. The Chrysler 2.2L and 2.5L engines used cats that went right by the exhaust manifold. They were not much bigger around than an exhaust pipe. Maybe one could obtain 2 new ones of those and 4 shorty flex couplings and cut/weld it all in place. Would have to deal with the underhood heat though. On the one I used to own (1986 Dodge Lancer ES Turbo) they used muffler wrap to insulate near the cat. In the case of the Riv, I am also not sure how some of the fitting would be accomplished.

As for getting the pipes under the car, maybe mandrel-bent 2 1/2" pipes to fit and then flatten them side by side & strap or weld them together for the run under that center hump.

On the highway it would drone and buzz like a bad hangover because there'd be no resonators.

Let's just say it'd be a bit of a stretch.

Albertj

it makes sense. if im not mistaken the 95 riv only has 1 oxy sensor. maybe this mod is possible for me. ive looked into modifying a 97 .slp i think, header to fit my 95. i would have to use my stock header manifold plates and just bang em apart and rig them up . i think its doable take alot of work though..... as for the oxy sensor maybe it can be mounted down stream on top of the x pipe?

robotennis61 wrote:

as for the oxy sensor maybe it can be mounted down stream on top of the x pipe?

The front O2 sensor doesn't NEED to be mounted downstream to pick up all cylinders, you can mount it further up and just read from 3 cylinders (one bank).
If that would make your installation easier it wouldn't hurt anything. Then just use an emulator for the rear sensor.

I think the thing to remember here is what Aaron pointed out earlier - the exhaust as it is is well designed and is not the constraint for horsepower. Stated more directly, it ain't broke. Don't fix it. If you're modifying it, it's more likely that you want to get a rumbly sound, not because you want it to breathe better - it already breathes fine.

And Rick is right that the ECU will work with oxy readings from one bank of cylinders.

This thread summed things up for me. What I'll do is have a new SS system hung on that more or less meets factory specs, except I will probably use the best (quietest) resonator I can find. What I have not figured out is if I should use a glasspack as a resonator or if I should get one that essentially is only a Helmholtz trap (no fill just baffles). Any experience with these? Please comment.

Albertj

I'm sure you already know all of this but I've attached a link to some information on mufflers and resonators.
http://www.howstuffworks.com/muffler.htm
I'm actually looking for for some other data that I printed out some time ago that references not only the different types of construction for the most efficient noise cancellation but also has manufactures names for each specific type that I found very useful.
Unfortunately what I printed out does not have the web address on it. I will continue to look for it or type the whole page of info here if i can't find it.

The page of info I have is titled "Types of Mufflers"
I took the liberty to condense it some.
Mufflers can take care of the silencing chores by three major methods: Absorption, Restriction and Reflection. Mufflers can use one method, or all three, to attenuate sound that is not so pleasing to the ears of the Highway Patrol or you and me.
The Absorption method is the least effective at quelling engine roar. But the benefit is that "absorbers' are also best at letting exhaust gas through. Good examples of absorbers are the mufflers found in GReady BL-series exhausts, DynoMax UltraFlow and the good old fashioned Cherry Bomb glasspack.

Restriction
Doesn't that word just make your skin crawl? It's right up there in the same league with words like "mame" and "rape".
Obviously, a restrictive muffler doesn't require much engineering expertise, and is almost always the least expensive to manufacture. Thus, why we find restrictive mufflers on almost all OEM exhausts systems.

The one paragraph in particular that I think is useful is as follows:

Reflection

Probably the most sophisticated type of muffler is the reflector. They often utilize absorption principles in conjunction with reflection to make the ultimate performance silencer. remember any of your junior high school math?
Specifically, that like numbers cancel each other when on a criss-cross? That's the same principal used by the reflective muffler. sound is a wave. And when two like waves collide, they will "cancel" each other and leave nothing to a corpse but a spot of low-grade heat.
There are numerous engineering tricks used in the reflective muffler. Hedman Hedders makes a muffler that looks a lot like a glasspack. In fact, it is a glasspack with a catch. The outer casing is sized just-so, so that high-pitched engine sound (what we deem "noise") is reflected back into the core of the muffler...where those sound waves meet their maker as they slam right into a torrent of more sound waves of like wavelength coming straight from the engine. And, this muffler is packed with a lot of fiberglass to help absorb any straggling noise that might be lagging behind.

There is more, but I think the object here is to find a muffler / resonator that is of the 'Reflector type". It should provide the best combination of sound attenuation and performance / efficiency.

Please let me know what type/make/part numbers you decide to go with, Thanks

The fabricator (Rob) has two types of mufflers. One type has straight-thru pipes and a combination of fiberglass and t304 stainless wool batting to attenuate sound. He also has "baffled" mufflers. He's getting them from a fabricator in the midwest somewhere. I plan to go with his baffled (reflector type) mufflers, but somewhat oversized for the application so I don't have a flow problem. That is the system will be stock from the manifolds to the Y but then at the Y will be 2 1/4" instead of 1 7/8". Rob plans to straighten out the bends some and use offset "baffled" mufflers. I'm getting oval shaped resonator tips (he does not have). I was thinking of getting an "ultra quiet" resonator, but am puzzled that it's only 14" long. I was also thinking about replacing the cat.

I may or may not be able to get part numbers for you BUT here is the Never-Rust site with some of the numbers:

http://www.never-rust.com/products/mufflers.html
.

Albertj

albertj wrote:

I was thinking of getting an "ultra quiet" resonator, but am puzzled that it's only 14" long. Albertj

What vendor are you looking at for the "Ultra Quiet" 14" resonator?

Not sure - this is the Vibrant resonator:

Link

and this is one vendor source for it:

http://jdmultimate.com/Store/Product/ItmGroupDetail.aspx?GroupId=50550&Make=UNIVERSAL

selling it for about $80.- If you have other source please PM. However...

As it is, I think most likely I'll use one of the Never-Rust baffled 'torpedo mufflers' as a resonator. They are 4" diameter, something like 25" long. They are shown on the Never-Rust site I already posted.

What I have not done - the math to figure out what the resonators are damping. The length is significant, because as you know what happens is the sound in a muffler is dissipated by reflecting it back on itself. At various frequencies, the standing waves will cancel themselves. So the point is to have a resonator that cancels the primary sound and its harmonics when the car is running 1500 - 2100 RPM. 1500*6/60=150 Hz, 2100*6/60=210Hz. What has to happen is that the muffler and resonator have to cancel out a number of low frequency noises and many of their harmonics that are coming at your ears in very rapid succession.

I am thinking the 14" resonator gets rid of the drone and lets a lot of other higher and lower frequencies through - kind of like a midrange filter - and the longer (22 1/2") resonator gets rid of more harmonics but also probably emits low frequency, low amplitude sound. Think of these resonators like organ pipes. Longer tube = lower note. The other cool thing about it is that in general the lower the pitch, the less audible the sound will be close to the pipe. So if the pipe is sized and baffled right it will pass the 90 dB test (usually muffler laws limit vehicle sound to around 90 dB, a practical maximum) but in the field sound off nice rolling down the highway - without making nuisance noise (the drone) in the vehicle.

In other news - I learned (well, I should have guessed) that my cat is shot although it still has physical integrity. It was explained to me. if you want details, PM me.

Albertj

I have never heard of Vibrant but it sounds like they make a quality product.
During my quest for knowledge on building an exhaust system for the Riv. I've tried to figure out what would work for me. Ended up with paralysis from analysis.
The problem is you don't want spend a lot on a specific resonator or mufflers only to find out they are too loud or not to your liking (tone wise). I personally found it difficult to gather the necessary data on any individual resonator or muffler to make an educated decision. It's somewhat of a crap shoot unless your willing to try various parts and see what happens
So, I've decided to take AA's approach and just leave the cat back stock for now, temporarily at least. I do not need to replace any of the components due to corrosion but wanted to provide a free flowing system without droan or undue noise.
So I'm naturally interested to see how your system comes out and how you like or dislike certain components.

Quote :

I am thinking the 14" resonator gets rid of the drone and lets a lot else higher and lower frequencies through - kind of like a midrange filter - and the longer (22 1/2") resonator gets rid of more harmonics but also probably emits low frequency, low amplitude sound. Think of these resonators like organ pipes. Longer tube = lower note.

This also applies to the overall proportions of pipe diameter vs. length, and the overall volume of air inside the system. I don't know for sure what the numbers are for our vehicle, but as a whole, an exhaust system itself works as a Helmholtz resonator.

When I used to build subwoofer enclosures as a hobby, I remember that for a given enclosure volume, the ported duct used for venting the box could be "tuned" to a very specific frequency. This resonant frequency, or "Fb", was determined by the proportion of the vent. When tuned, it would cause the enclosure/woofer system to "sing" at that frequency, much the same as what happens when you blow across the opening of a glass jug.

As this applies to exhaust, it is correct that longer exhaust tubes will create a lower tuning frequency, but it is not true that larger diameter tubes will create lower frequencies. Since the relationship is proportional, when a tube gets longer, Fb gets lower. When the tube diameter gets larger Fb gets higher. It's complicated of course because the resonators and mufflers each are working as acoustic resonators themselves, but for a given length of exhaust tubing, changing nothing other than the tubing's length and diameter will determine to some extent at what frequency the system is easily excited. If the exhaust length and other components remain the same, increasing tube diameter to 2.5-3" or more will raise the resonant frequency, which as Albert mentioned will become more audible and unpleasant (our ears are more sensitive to higher frequency sounds). This is why a very large diameter exhaust tube (3-4") tends to sound raspy and often much louder than a smaller diameter one.

Conversely, decreasing tubing diameter will tend to create a lower, growling sound. The trick is, when you narrow the diameter enough, you can also decrease length because the proportional relationship is preserved. This is how you can have that deep sound coming from Harleys and other bikes. The tubes are narrower, but they're also shorter. A bonus is that exhaust flow is also improved by shortening the length of the system.