Back Pressure, is it needed? - Hyundai Genesis Forum
Hyundai Genesis Coupe Performance Genesis 2.0l turbo and V6 Coupe (Ex: Intakes, Exhausts, Programmers)

 
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Old 05-29-2009, 10:06 PM
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Back Pressure, is it needed?

This is a pretty good description of back pressure from Vagabond off of Zuwharrie.com:

During the exhaust cycle, burnt gases depleted of oxygen are "exhausted" from the combustion chamber. The more difficult it is for the gases to escape, the more "back pressure" there is. The greater the back pressure, the more oxygen depleted gas there is remaining in the chamber during the intake cycle, resulting is less available space for fresh air containing oxygen to support combustion during the combustion cycle. However, if the fuel provided is already fully burnt, there is no significant increase in power if the back pressure is reduced. This is why some people complain that their back pressure reducing new header or large bore exhaust system has not provided an increase in power. In order to optimize an increase in power, the engine needs to be re-tuned to provide additional fuel to take advantage of the additional available oxygen.

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Old 05-29-2009, 10:08 PM
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Backpressure: The myth and why it's wrong.

Here is another good read off of thumepertalk.com motor cycle forum that has some more insight.

Backpressure: The myth and why it's wrong.

I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?
I often wonder how the myth "Engines need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door.

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Old 05-29-2009, 10:09 PM
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Destroying a myth

Another good read on back pressure:

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.

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Old 05-29-2009, 10:10 PM
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More detailed explanation on back pressure

B]More detailed explanation on back pressure:[/B]

Exhaust backpressure can cause a variety of problems. A plugged catalytic converter can strangle engine breathing and cause a big drop in engine performance and fuel economy. And if the converter plugs up completely, it can make the engine stall. The same thing can happen if a muffler, resonator or double walled exhaust pipe collapses internally. Anything that restricts exhaust flow will create excessive backpressure in the exhaust system.

EXHAUST BACKPRESSURE SYMPTOMS

The classic symptoms of too much backpressure include things like a lack of high speed power, poor fuel economy and even overheating. Anything that backs up exhaust pressure into the engine will also back up heat. About a third of the heat produced by combustion goes out the tailpipe as waste heat, so if the heat can't escape it can overload the cooling system and make the engine run hotter than normal, especially at highway speeds.

If there is a complete blockage in the exhaust, the engine may start and idle fine for a minute or two, then die as backpressure builds up and strangles the engine. In some instances, backpressure may buildup to such a degree that it blow out a pipe connector or the converter shell. That makes diagnosis a lot easier, but in most cases you may not be sure if there is an exhaust restriction or not. So in these instances, you need to measure backpressure.

HOW TO MEASURE EXHAUST BACKPRESSURE

To measure exhaust backpressure, you need a pressure gauge with a scale that reads zero to 15 psi, or zero to 100 kPa or higher (note: 1 psi equals 6.89 kPa, and 1 kPa equals 0.145 psi). If you don't have a low pressure gauge, you can buy a basic exhaust backpressure test kit for around $60. If you want to be really accurate you can use a digital manometer or pressure gauge that displays pressure readings in a variety of different units of measurement (psi, kPa, inches Hg, inches H2O, bar, etc.). A tool that reads from 0 to 15 psi will typically cost around $170.

Measuring exhaust backpressure is not as easy as it sounds because there is no quick and easy way to tap into the exhaust system. If the engine has an air pump, you can tap into the exhaust system at the air pump check valve. Disconnect the check valve and install a pressure gauge. For accurate test results, however, the check valve must connect to the exhaust system ahead of the converter. Note: if the air pump plumbing hooks up at the converter, this technique won't give you reliable results.

You can also check backpressure by removing an oxygen sensor from the exhaust manifold, and connecting a hose fitting to your pressure gauge. If the O2 sensor is fairly easy to reach and the vehicle is fairly new, it should come out without too much effort. But on an older vehicle, O2 sensors can be difficult to remove. And there is always the risk of damaging the sensor.

A third option is to drill a small hole into the exhaust pipe just ahead of the converter and attach a fitting for your pressure gauge or manometer. This may be easier than trying to remove a 10 year old O2 sensor, but it also means you'll have to plug the hole afterwards with a self-tapping screw or a small spot weld.

On some diesel engines (Ford diesel trucks, for example), there is a backpressure sensor in the exhaust system that measures exhaust backpressure directly. You can see the actual value by using a scan tool and looking at the exhaust backpressure sensor PID.

BACKPRESSURE READINGS

Backpressure readings at idle on most engines should generally be less than 1.5 psi (10 kPa). This will vary somewhat from one vehicle to another depending on the design of the exhaust system, the size of the pipes, how restrictive the converter, muffler and/or resonator is, and whether it is single or dual exhausts. We've seen some idle readings as high as 2.75 psi on a few vehicles, but for most 1.5 psi or less at idle is normal.

A partially restricted converter, muffler or pipe may flow enough exhaust at idle not to cause a problem, but chokes breathing at higher engine speeds. So to test this possibility, you need to rev and hold the engine at 2000 rpm. A "good" reading on most engines at 2000 rpm should be 3 psi (20 to 21 kPa) or less. Again, there may be some vehicles that will read a little higher that don't have a problem, but the reading should not be significantly higher.

Pay close attention to what the backpressure reading does while you are holding it at 2000 rpm. If it remains steady, chances are there is no restriction. But if the reading gradually increases, it means backpressure is building up and there may be a blockage.

If you want to rev the engine higher, say to 4000 rpm and hold it, the backpressure numbers will shoot up. Most stock exhaust systems will show backpressure readings from 4 to 8 psi (27 to 55 kPa), or even higher. As before, if the backpressure reading is unusually high or it continues to climb at a steady rpm, it usually means there is an abnormal restriction causing an unhealthy increase in backpressure.

INSPECTING THE EXHAUST SYSTEM

Higher than normal backpressure readings mean something is restricting the flow of exhaust out the tailpipe. Though the converter is usually the trouble spot, restrictions can also occur inside mufflers and resonators if a baffle collapses or the fiberglass sound-absorbing roving clogs up an internal passageway. Double-wall exhaust pipes can also collapse internally causing a blockage.

If the system has a blockage, inspect the exhaust system end to end for any obvious signs of damage like a crushed pipe, severe corrosion, etc. You can thunk the converter to see if it rattles inside (indicating the catalyst substrate is broken).

The next step would be to disconnect the exhaust pipe just behind the converter to see if that makes a difference in the backpressure readings. The readings will go down a bit when the exhaust system aft of the converter is disconnected, but if you don't see any drop it means the converter is probably plugged. The other possibility is that the head pipe between the exhaust manifold and converter has collapsed internally.

If you see a big drop in the backpressure readings when the exhaust system aft of the converter is disconnected, it means the converter is flowing okay and the blockage is somewhere in the rest of the system (bad muffler, resonator or tailpipe).

VACUUM TEST ANOTHER WAY TO CHECK FOR BACKPRESSURE PROBLEMS

Another way to check for a backpressure problem is to check intake vacuum at the engine. It's much easier to hook up a vacuum gauge to a vacuum hose or port than it is to remove an O2 sensor. Vacuum gauges typically display readings in inches of Hg (Mercury). One inch Hg equals a pressure reading of 0.49 psi or 3.38 kPa.

Normal atmospheric pressure is around 14.7 lbs. per square inch at seal level. This will vary a bit with temperature and humidity, and it goes down at higher elevations. Vacuum is created inside the engine's intake manifold by the intake stoke of the pistons trying to overcome the restriction created by the throttle butterfly. On most engines will develop 16 to 22 inches Hg of vacuum at idle (except for most diesels which have no intake vacuum because they have no throttle plate). The vacuum reading at idle will depend on engine wear, throttle opening, camshaft overlap, exhaust backpressure, air temperature and density.

To check vacuum at the intake manifold, start with the engine off, and disable the EGR valve by removing or disconnecting its hose or one of its solenoids. Connect a vacuum gauge to a ported vacuum source on the intake manifold or throttle body. Start the engine and note the vacuum reading at idle with the transmission in neutral.

If the idle vacuum reading is lower than normal, or it continues to drop while the engine idles, an exhaust restriction is causing exhaust pressure to build up and backup into the engine.

If you increase engine speed, the vacuum reading will drop slightly, then stabilize and rise back up to within 2 to 3 inches of the vacuum reading you noted at idle. Any sudden drop of over 10 inches Hg of vacuum may indicate a blockage problem. Erratic swings of the vacuum indicator may indicate periodic blockages caused by loose components temporarily blocking the exhaust system.

Vacuum readings can also be affected by other factors such as weak or broken valve springs, overadvanced or retarded valve timing and/or ignition timing. So if the needle on the gauge is bouncing around, it could indicate a mechanical problem in the engine.

REDUCING BACKPRESSURE

Reducing exhaust backpressure can improve fuel economy and performance. Reducing restrictions in the exhaust system allows the exhaust to flow more easily so the engine can breathe more efficiently. The most common modification is to replace the stock muffler with a low restriction aftermarket performance muffler, or to replace the entire stock exhaust system from the catalytic converter back with a free-flowing aftermarket performance exhaust system

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Old 05-29-2009, 10:11 PM
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A Backpressure Study

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.

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Old 05-29-2009, 10:50 PM
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Nice reads thanks for posting them... I think what many people need to realize is that an engine has MANY components that work together... intake/exhaust timing, ignition, fuel delivery, exhaust flow, intake flow, etc... on top of the fact that the manufacture has to engineer and design a total system that not only performs but also is emissions compliant. Oh yeah they have to try and please as many people as possible with how the car sounds... and to think we haven't even discussed whether this car could use an X or H pipe! lol
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Old 03-22-2011, 04:08 AM
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great informative post. i still feel back pressure is not myth.
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Old 04-25-2011, 05:47 PM
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Quote:
Originally Posted by Sabbasaun View Post
Nice reads thanks for posting them... I think what many people need to realize is that an engine has MANY components that work together... intake/exhaust timing, ignition, fuel delivery, exhaust flow, intake flow, etc... on top of the fact that the manufacture has to engineer and design a total system that not only performs but also is emissions compliant. Oh yeah they have to try and please as many people as possible with how the car sounds... and to think we haven't even discussed whether this car could use an X or H pipe! lol
Depending on cylinder pressures and ve (volumetric efficiency) the back pressure needed for optimal performance accross the rev range will differ. For boosted setups the higher the boost pressure or larger the turbo increases cylinder pressure and therefore needs a less restrictive ( Larger diameter) exhaust to reach a good ve.
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Old 04-27-2011, 09:54 PM
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On my SHO I could tune the MAF for exhaust changes. The more open the exhaust the higher rpm the power would move. Don't think any actual HP was made, if so it was negligible. But it was obvious where the peak torque happened at.
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Old 04-28-2011, 12:04 AM
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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.
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