The Great Muffler Debate

[theexbrit]

Sooooo, I've been reading up on mufflers & I'm interested to get input from you guys. Taking into consideration back pressure (for low end torque), what would you guys say is a good pipe size (I read it's only supposed to be 25-30 % over stock or you'll lose too much low end power) & muffler combo for the VG33?

[CNAM]

I love my Flowmaster.. Dual in/out setup(VQ35DE). They have a wide variety depending on preferred application. They are west coast easy to talk to if you have questions... Then again I have heard lots of Magnaflow for this VG33 you speak of..?

[ferrariowner123]

THANK YOU SO MUCH ANDY! I've been wanting to get input on this for a while!!

 

If you have an exhaust on a VG33 i want to hear exterior and interior audio, i want to do my exhaust soon!

 

-Kyle

Edited May 6, 2014 by ferrariowner123

[theexbrit]

I'm not too concerned about noise, I'm more interested in hearing about engine performance. Higher top\end with Flowmaster/Magnaflow, loss of low end torque, etc...

[ahardb0dy]

I like Borla, 2.25 pipe on the VG30, should be the same for the VG33.

[theexbrit]

I ripped the stupid tubular resonator thingy off the last 2 feet of the tailpipe so it's just straight pipe from the muffler box to the end of the tail pipe. It gives it a little more of a throaty sound, not much but a little. It doesn't seem to have affected performance any, I think it was just added by Nissan to remove any highway "drone" noise for the mall cruizer crowd.

[adamzan]

I put a dynomax on my 94, sounds ok I guess. Not much louder than stock and I didn't notice any difference in power. I used a 2.25 version. Next muffler will be a magnaflow or borla.

[Precise1]

It seems to me that all you would want/need to replace is the choke point of the muffler where you have 2 in and one out. I don't have any recommendations as our R50 is stock, but I would suggest making sure what you get has an outlet volume (area really) equal or slightly less than the sum of the two inlets. Agreed, nothing else is needed, just have it piped back from that point.

 

For all the people with WD21 set up input, that not might be valid because the R50 has twin cats/pipes into the muffler and 1 outlet; a much different animal.

 

B

[ferrariowner123]

It seems to me that all you would want/need to replace is the choke point of the muffler where you have 2 in and one out. I don't have any recommendations as our R50 is stock, but I would suggest making sure what you get has an outlet volume (area really) equal or slightly less than the sum of the two inlets. Agreed, nothing else is needed, just have it piped back from that point.

 

For all the people with WD21 set up input, that not might be valid because the R50 has twin cats/pipes into the muffler and 1 outlet; a much different animal.

 

B

So get a muffler that is roughly the same size at stock? We can still increase the piping size, correct? I ask to confirm my thinking as this was the route I was planning on going.

 

-Kyle

[CNAM]

http://s835.photobucket.com/user/CNAMDAFENOM/media/IMG_1701_zpsdf5e3849.mp4.html

 

[ahardb0dy]

Borla does make a dual in dual out

[CNAM]

Pick up the May issue of 4Wheel parts "Off-Road Adventures" magazine. There is an article/profile and history of the Flowmaster company, products and applications.

[Precise1]

So get a muffler that is roughly the same size at stock? We can still increase the piping size, correct? I ask to confirm my thinking as this was the route I was planning on going.

 

-Kyle

Technically you can increase the size of the piping from the cats, to the muffler, and back from there, but I think that would be overdone with a stock motor. My guess is to leave the factor piping up to the muffer, replace the muffler with a free flowing aftermarket design, and to make sure the outlet of said muffler has the cross section area of both the inlets combined so it is not a choke point.

 

B

[theexbrit]

Pick up the May issue of 4Wheel parts "Off-Road Adventures" magazine. There is an article/profile and history of the Flowmaster company, products and applications.

 

Just read that article, pretty interesting.

 

Here's a great article about the Backpressure Myth, it's a long read but worth it.........

 

"Destroying a myth.

 

Some say that "an engine needs backpressure to work correctly." Is this true?

 

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

 

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

 

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

 

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque. Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

 

 

 

Exhaust Backpressure Study

 

Replacing the stock production exhaust system with a low-restriction, free-flow one is usually one of the first modifications made to any vehicle in the name of performance. We all know they're louder, but how much performance do they really add? We've all seen supposed dyno tests, usually run by the exhaust manufacturer's themselves on their own dyno, indicating vast power gains, and psychologically, we always equate a healthy exhaust rumble with increased power in the seat of the pants, but how much power are we really gaining? To find out, we're running a simple backpressure study, and our results will be posted here as they come. Admittedly this study is not totally scientific as there are many uncontrolled variables, but it should be sufficient to provide a rough estimate.

 

It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury - inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.

 

1 inch Hg backpressure = 1 HP lost

 

For reference, we have the following conversions factors:

 

1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar

 

Our test vehicle is a '97 Buick Regal GS with 3800SC engine transversely mounted. It's exhaust system consists of a cast iron exhaust manifold on the left side of the engine which connects into a tuned tubular header on the right side, both banks connected to a single downpipe into a catalyst. The output of the catalyst runs into a resonator and then into a single muffler; all pipes are 2.25 inch. The exhaust system is very similar in the Pontiac GTP, the differences being that the GTP splits into 2 mufflers after the resonator. Our sources indicate that the GTP system results in approximately 3 in Hg less backpressure than the Regal, hence there is 3 less horsepower loss.

 

To measure system backpressure, a sample tube was mounted before the catalyst into the downtube

 

A flexible hose is run from the sample tube and attached to a pressure gauge inside the car for monitoring. We first ran the test with the complete full factory exhaust, and next dropped the entire system from the catalyst back. The final test was run with the muffler removed. Only the catalyst, resonator, and the majority of 2.25 tubing up to the muffler remained.

 

Next we took a look at the restrictive U bend that houses the post O2 sensor. It's function is to protect the O2 sensor from damage by positioning it straight up.

 

This restrictive U bend is completely removed, and replaced with a straight 3" piece. (Note this 3" piece is not the actual replacement pipe - it's just a scrap piece for the photo.)

 

This U bent tube is replaced by this straight 3" pipe. The O2 sensor is mounted to the side - less protected, but it'll be OK unless you go off-roading as we recently did!

 

Results & Conclusions

We ran three tests, observing three runs with each configuration and averaging the three. Peak backpressure occurred near the engine RPM redline of 5700-6000 rpm, at a max boost of approximately 7-8 psi. We took all our readings at WOT immediately before the 1-2 shift. Although we performed our tests on a Buick Regal GS, we predict a GTP will have similar results, taking into account the 3 in Hg difference. We realize our tests are not totally scientific, and they were not meant to be. Our goal is to obtain a ballpark estimate which, as the saying goes, is "good enough for government work." [before you government employees start flaming us - one of our associates worked for the US Army Corps of Engineers for several years, so we know how it is. 8^) ]

 

1. Full factory exhaust system of catalyst, resonator & muffler: 28-30 in Hg = 28-30HP lost [system is whisper quiet]

2. Only catalyst in place, no resonator or muffler after the cat: 13-14 in Hg = 13-14HP lost, thus approximately 14-17 HP gained over stock full exhaust [system is unbearably loud and shakes the entire car, conversation is impossible]

3. No muffler, just resonator, catalyst & tubing 20 in Hg = 20HP lost, thus approximately 10HP gained over stock full exhaust [system is bearable, but has some bad resonances and drones at particular RPMs.]

4. "U" pipe replaced with straight 3" pipe Testing to be determined.

 

The catalyst was never removed as we were only interested in achieving an optimal cat-back system. We can see from our results that the muffler is costing approximately 10HP loss while the resonator accounts for a 6HP drop, with everything from catalyst to the engine costing 14-15HP.

 

Therefore, it's evident that at best, a free-flow system will gain perhaps 10HP - and that's for a noisy system, while one which controls irritating resonances and drones better would probably gain less than that. Therefore, a 5-7HP gain from a cat-back exhaust system is probably in the ballpark for achievable gains.

 

Will removing the catalyst help? Definitely, but that's illegal for street use and probably more importantly to some folks out there, it sets an OBDII Malf code. Replacing the factory catalyst with a high-flow unit will not result in a significant increase either, as those "high-flow" units outflow a production unit by a couple of inches of Hg at best. In fact, our sources indicate that the catalyst on these cars are actually one of the least restrictive available. With a FWD platform, we're stuck with uneven header lengths due to the transverse mounted engine, limiting one's ability to truly optimize the header design. Therefore, it is probably more fruitful and definitely more cost effective to examine the situation after the catalyst.

 

The difficulty in designing an effective exhaust system is in minimizing backpressure while achieving a desirable exhaust tone with minimal resonances and drones. It has been suggested that replacing both the resonator and muffler with a single large staight-thru muffler (with dual outlets for the GTP) may be the best solution.

 

Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders. When you look at the four-stroke cycle in How Car Engines Work, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.

 

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.

 

In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder or eight-cylinder engine, there are four cylinders using the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the *muffler. It turns out that the manifold can be an important source of back pressure because exhaust gases from one cylinder build up pressure in the manifold that affects the next cylinder that uses the manifold.

 

The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.

 

Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

 

When you look at the four-stroke cycle in How Car Engines Work, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine. During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.

 

You can increase backpressure easily by sticking a potato into your exhaust pipe.

 

Potatoes are not known to be a power booster in most cars."

 

[Precise1]

Funny that they talk about 10hp from a cat back system, but discount the 2 they state could be had from replacing the cat (20% increase gain), and they do not test for high flow cat and cat back system...

 

Replacing the factory catalyst with a high-flow unit will not result in a significant increase either, as those "high-flow" units outflow a production unit by a couple of inches of Hg at best.

 

Nor did they talk about scavenging, since they have a car with boost, so makes this scenario specific.

 

 

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.

 

Split the difference, and it is right where I recommend. Keep it stock until the muffler, make sure the muffler isn't restrictive and pipe it without restriction from there. Some scavenging, less pressure at rpm, minimal component replacement, no CEL or smog issues. Best of all worlds.

 

Of course, my R50 is stock and my WD21 has headers, high flow cat, 2.25" piping into a Flowmaster Delta 60 (ditched the Magnaflow) so I can't talk from experience.

 

B

[bushnut]

Had a talk with one of the Custom muffler guys here in town (both 4x and street rod stuff). Since mine is swiss cheese and I have to replace everything from the cats back , I asked him about performance options. He said surprisingly that the VG33 stock exhaust system has pretty good flow, even with all the weird twists and bends, add a decent sounding muffler and things should be good.

[theexbrit]

I'm leaning more towards precise's scenario, on a stock motor you don't want to go too big. I'm thinking stock 2" up to the muffler then 2 in & 2 out (why have the different exhaust pulses hitting) then 2.5" to the tail pipes. The only other thing that might help would be newer cats that have better flow, but they're around $300 a pop so until I get a cel & codes (420 & 430 I believe) I'll leave them alone. The old girl is running pretty good, acceleration is good, etc, so I don't want to change things too much.

[silverton]

I came here to say that backpressure is a myth, but someone beat me to it.

 

2.5" is fine for our motors and even the 3.3L. 3"+ and you're getting in to turbocharged exhaust volume.

[Precise1]

I'm leaning more towards precise's scenario, on a stock motor you don't want to go too big. I'm thinking stock 2" up to the muffler then 2 in & 2 out (why have the different exhaust pulses hitting) then 2.5" to the tail pipes. The only other thing that might help would be newer cats that have better flow, but they're around $300 a pop so until I get a cel & codes (420 & 430 I believe) I'll leave them alone. The old girl is running pretty good, acceleration is good, etc, so I don't want to change things too much.

2 in/2 out scenario sounds like a good idea as well, it would probably increase scavenging VS RPM. I'd just make sure your piping size is the actual inlet/outlet size. You might want to consider going with 2.25" from the muffler back, especially if you are running dual pipes. 2" increased to 2.25" is a 26.5% increase in area, 2" to 2.5" is a 56.2%. Since you would be running dual exhaust, each only one bank out of a 3.3 liter motor (300-330ish total CFM @ redline), how much increase do you really need? Just enough to lessen restrictions...

You might run into some room constraints as well, IIRC the R50 is asymmetrical underneath towards the rear and you may need to have duals run side by side. Might want to take a quick look...

BTW, Flowmaster is made here in town.

 

After taking a look, I think dual out from the muffler back is overkill, but find a local custom exhaust shop and run a few scenarios past them as well.

it looks like this is the closest to your application, so you mught want to look at a different company as well. http://www.flowmastermufflers.com/?page_id=451&partno=524553

Or http://www.flowmastermufflers.com/?page_id=451&partno=524554

 

B

[theexbrit]

2" increased to 2.25" is a 26.5% increase in area, 2" to 2.5" is a 56.2%. Since you would be running dual exhaust, each only one bank out of a 3.3 liter motor (300-330ish total CFM @ redline), how much increase do you really need? Just enough to lessen restrictions...

B

 

Thank you Einstein, you lost me at "26.5%" :laugh: :laugh:

 

Seriously though, I think you're correct with the 2.25" if you go with dual out. I was thinking of dual out so that the pulses didn't have to merge but I think I'll follow your advice & talk to my local muffler shop.

[Precise1]

Easy there 'dry Brit humor', that is Meisinger to you...

I'm used to many decimal places so adding one significant digit is like you tossing a tarp over radio gear on a dry desert night...

 

They should know better than I, obviously, but the math/physics don't lie either.

You can search here to see plenty of threads of people who think a 2.5"+ pipe with no cat on a 3.0L motor is the key to power as well. Mileage may vary.

 

B