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Discussion Starter #1 (Edited)
Well i added my 4-1 header to my modified stock exahsut of no mufflers or resonator
EDIT:I have a Cold Air inttake BTW
SO my exhaust setup is as follows with no tune yet

  1. 4-2-1 long tube header(with EGR still connected)
  2. stock flex
  3. glass pack


well one thing im for certain unless something happened to mess up after install:i cannot chirp the ATX anymore on takeoff it feels so slow now compared to when i had my cat hollowed[scratch]
but every so often when the car shifts into second the tires give a nice wheelspin or chirp

i feel like ive lost all low end power but def feel it in the higher rpm bands after install

Now i've been advised to look into backpressure
what should i do
Please explain Backpressure and also does my results have anything to do with this? should i add something to this setup to help me?
Thanks All For[idea] ideas,opinions,and advice[idea]
 

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I copied and pasted this as it was pretty good. basically the less backpressure you have, the lower torque you have down low, but u gain it up top. Torque is what spins the tires, and since you loose it off the line, thats why you no longer can chirp ur tires!!




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.
Up NextIn 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.
 
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Discussion Starter #3
rep added mlbbaseball
ive looked on a couple sites and none explained it better than this one

anyways i dont kno what i was expecting tho
but i definitely need to add a muffler at the end of the pipe cause the cherry bomb just doesnt quiet it enough ITS LOUD!!

any suggestions instead of a muffler?
 

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I've never really come across a definition of "back pressure" that I like. Like the one above, it implys that with the header (which by it's very nature is more free flowing than the stock manifold, larger primaries more of a radiused bend/smoother flow) more power should be gained. But you are seemingly seeing the opposite, some of which can be blamed on the runner length. For the most part though it doesn't explain your situation, nor anyones elses really. Close but no cigar.

I kind of look at it this way, between the exhaust stroke and the intake stroke there is a period of time called the overlap, or cam overlap (if you have adjustable cam gears you can vary this time along with the opening and closing of valves, goes hand in hand). Our car runs a batch fire fuel injection system (as most car do), that means that all the injectors fire at the same time. Now odds are that the overlap period will fall in partially with the injection time. Which means that some fuel will be lost in the overlap period. So what happens when the Oxygen Sensor gets hold of that? (One unanswered question I've had for some time)

What I am getting at is the with free flowing exhaust it may be possible for the ECU to lean the mixture because it's seeing unburnt fuel, but that fuel is just from the added flow during overlap.

So what can you do to test that or fix it? I really don't know, but I'd suppose adjusting cam timing may help some, but I really don't know.
 

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Let's try another way to look at it.


Basically, you are asking if reduced back pressure causes a reduction in torque or isn't performing properly.

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to power loss and 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 and THIS leads to a reduction in torque. 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 cars don't have to worry about the effects described above, because the ECU (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 ECU'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.

This is particularly important. When you read the ad about the HP and torque gains from purchasing ONLY an exhaust system, read the fine print.

You WILL absolutely be slower on a stock system with big exhaust unless you open up the front end, enable more air/fuel into the combustion chamber and program the ECU to make this happen while using an air/fuel managment system (larger fuel rail, injectors, etc.) that can adjust for the freer flow.
 

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Discussion Starter #6
good theory iminhell
so my next noob question is can i add adjustable cam gears to my stock cams?
to maybe tinker with this problem?

not for power gains but to maybe smooth out if i do have problems?
ive heard you can bash up your stock valve cams from adding adjustable cam gears on stock cams
 

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Discussion Starter #7
WeeAsp i read that one i didnt kno what to think cause the guy who wrote that kept talking about BMW ECU's adjust their parameters now would you kno if ours adjust the same?
 

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Regardless of make and model, an ECU is an ECU.

The point is that simply putting a larger exhaust system on the car isn't sufficient.

The lack of back pressure (depending on how large you go) will lead to a slight reduction of "power" - especially at the low end because the stock fuel system cannot compensate enough on the air/fuel mixture side of the equation to make the mod truly effective.

This is particularly true of vehicles that run a stock tune, stock ignition, stock manifold, and stock fuel delivery system.

Quite simply, a larger exhaust system - COULD exceed the capability of the remaining stock components and this leads to a power loss.

For example, I used to own a 96 Formula Firebird. In it's day, very quick. My issue was that I bought a muscle car that didn't sound like one. I put a Borla exhaust system on the car and ran it wide open.

The car was NOTICABLY slower (much like your situation). I called Borla for advice and they told me that I NEEDED to put a diffuser disk in the exhaust. Otherwise, I would experience an approximate 12% loss in torque.

I tightened up the exhaust and created more restriction and the car ran like stink.
 

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This is particularly important. When you read the ad about the HP and torque gains from purchasing ONLY an exhaust system, read the fine print.

You WILL absolutely be slower on a stock system with big exhaust unless you open up the front end, enable more air/fuel into the combustion chamber and program the ECU to make this happen while using an air/fuel managment system (larger fuel rail, injectors, etc.) that can adjust for the freer flow.
I don't get this part. So there is no point on getting an exhaust if you don't have an increased fuel delivery system??? I might really sound noob here, but i can't really phrase this proparly. [poke]
 

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Back pressure is resistance to flow in the exhaust system. When you do away with back pressure you loose bottom end torque, the reason you loose it is when there is back pressure in the system it is harder for the engine to turn over (keep on rotating) so in turn it HAS to produce more torque to continue running.

Just like everything else with these cars when you loose backpressure you loose bottom end power.... but you gain top end power due to the free'er flowing exhaust.

Now to the "burnt valve".. our ECU's actually do a great job of keeping the AFR where it needs to be and there should be no problem with the ECU compensating for the free'er exhaust. Usually you get a burnt valve from going extremely lean (like either a very bad tune or fuel system failure).

Also I'm pretty sure that all 4 of our injectors do not fire at the same time.... if they did the car simply would not run since you would be getting fuel at all 4 stages .... intake, compression, combustion and exhaust.
 
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Well basically just about every bolt on/ exhaust mod you do causes some low end loss... but its a trade off for top end power.
 

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Discussion Starter #13

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I don't get this part. So there is no point on getting an exhaust if you don't have an increased fuel delivery system??? I might really sound noob here, but i can't really phrase this proparly. [poke]
The real issue that a lot of exhaust companies will claim increased HP and torque (for example - 30HP and 10-25% torque).

That is pretty much a fallacy in my opinion. You may see marginal HP gains, but there will be a loss of torque.

HOWEVER, if you change the intake, alter the fuel delivery system (i.e. tune the ECU), then it is possible to see significant increases in power.
 

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Discussion Starter #16
so basically what we have figured so far is:

Throwing a header on your FoFo without a tune will result in a low end decrease
due to the ECU on a stock tune adjusting the Air Fuel Ratio without a tuner giving it the Information that a free flowing exhaust has been added
since there is less back pressure the engine does not work as hard to push exhaust gas from the chamber therefore making a noticeable loss in low end power

simply put: a header on a stock tune will produce an increase in high end power(HP) rather than noticeable gains on the low end power(TQ) until a tuner is added to the equation

anyone for correction so i dont seem stupid?
 

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so basically what we have figured so far is:

Throwing a header on your FoFo without a tune will result in a low end decrease
due to the ECU on a stock tune adjusting the Air Fuel Ratio without a tuner giving it the Information that a free flowing exhaust has been added
since there is less back pressure the engine does not work as hard to push exhaust gas from the chamber therefore making a noticeable loss in low end power

simply put: a header on a stock tune will produce an increase in high end power(HP) rather than noticeable gains on the low end power(TQ) until a tuner is added to the equation

anyone for correction so i dont seem stupid?
First, not stupid. It is a valid question.

It is a bit oversimplified, but yes. An aftermarket header with no additional changes to the ECU or the air/fuel management system will result in a "low-end" performance loss.

I crew for a Grand Am Cup team and we campaign a BMW 330ci. The car uses the factory header with a free flowing cat back system (required).

We put a header on the car from Bavarian Motorsport and were nearly a second slower on lap times. We reprogrammed the ECU and the car performed back at "stock" levels. When we put the factory header on the car, we were nearly 2 seconds quicker.
 
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