the science behind back pressure

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09SMOKINHEMI

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so i've read a few discussions on the forum and many talk about back pressure, either too much, or too little, etc.... so what exactly is it and how do you correct it. are the 4th gens with the dual exhaust better than the ones like mine with only a single side exhaust?
 

SILVER_BULLET

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GOT THIS FROM ANOTHER FORUM

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.

- Adapted from Thomas V.
 
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09SMOKINHEMI

09SMOKINHEMI

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wow... some pretty complex stuff here

i thought that with a restrictive exhaust it would increase back pressure. there's a guy here with a saleen mustang that had some exhaust work done and he was complaining about back pressure after getting a new exhaust system installed, and come to find out that in addition to his new exhaust system, they needed to fabricate another spot for the exhaust to dump because of too much back pressure.....

so it just made me think if having a dual exhaust would've been better than just staying with a single exhaust. maybe my thinking is wrong but i wondered if a dual exhaust would flow better, decreasing back pressure as opposed to a single exhaust
 

SILVER_BULLET

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wow... Some pretty complex stuff here

i thought that with a restrictive exhaust it would increase back pressure. There's a guy here with a saleen mustang that had some exhaust work done and he was complaining about back pressure after getting a new exhaust system installed, and come to find out that in addition to his new exhaust system, they needed to fabricate another spot for the exhaust to dump because of too much back pressure.....

So it just made me think if having a dual exhaust would've been better than just staying with a single exhaust. Maybe my thinking is wrong but i wondered if a dual exhaust would flow better, decreasing back pressure as opposed to a single exhaust

im not sure exactly whats better for what vehicles since theyre all different but what ive always heard is you gotta have your air flowing as freely as possible but only to the point where your exhaust is too free flowing that it slows down the speed of the air flow
 
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09SMOKINHEMI

09SMOKINHEMI

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i see.... i'm sure these long tubes that i'll be installing would help in that regard.
 
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09SMOKINHEMI

09SMOKINHEMI

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yes and while having them installed they'll install my e-cutout too
 

SILVER_BULLET

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Thats my set up long tubes to the y pipe and the cutout right at the end of the y pipe
 

mattman_13

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From what I've read is it's more about the velocity of the airflow. The gasses come in waves with each combustion. Like water you want it to flow without the waves coming back and canceling each other out. But you also don't want it to flow so quickly that they just escape. Each wave sort of makes a vacuum and helps expel and pull the next. Very complex and over my head. I just put on a dodm80 and forgot about it...haha
 

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That's a good read. I'm thinking of going with true duals with a X pipe, two Cherry Bomb Extremes, then out the factory tips in the bumper. Do you guys think this would be a good set-up? What size pipe should I run with this? I was thinking 2.5 inch.
 

SILVER_BULLET

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that's a good read. I'm thinking of going with true duals with a x pipe, two cherry bomb extremes, then out the factory tips in the bumper. Do you guys think this would be a good set-up? What size pipe should i run with this? I was thinking 2.5 inch.

i think 2.5 inch will be fine i know people that have 2.5 inch pipes with cammed trucks as long as the exhaust flows enough for what you have theres no real reason to go with bigger pipes
 

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You are going to get 1000 answers to this question. I am one of the guys that has always preferred an open exhaust with very little backpressure...for example, longtubes with an open cutout behind the Y. If you want the same flow with less noise, then a 2.5" true dual setup is the way to go. I have seen alot of people talk about losing power with their cutouts open, but track any dyno numbers have said otherwise for as long as the new hemi has been out.

You also have to consider the powerband you have to work with. An open exhaust isnt going to benefit a lifted 4wd with 35" tires and a factory converter. But that same open exhaust on a truck with shorter tires and a higher stall converter is going to be perfect.

This is just a tough one to answer because there are so many correct answers.
 

SILVER_BULLET

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you are going to get 1000 answers to this question. I am one of the guys that has always preferred an open exhaust with very little backpressure...for example, longtubes with an open cutout behind the y. If you want the same flow with less noise, then a 2.5" true dual setup is the way to go. I have seen alot of people talk about losing power with their cutouts open, but track any dyno numbers have said otherwise for as long as the new hemi has been out.

You also have to consider the powerband you have to work with. An open exhaust isnt going to benefit a lifted 4wd with 35" tires and a factory converter. But that same open exhaust on a truck with shorter tires and a higher stall converter is going to be perfect.

This is just a tough one to answer because there are so many correct answers.

hmm ive never tried running mine with the cutout open i have mine right after the y pipe but i always close it right before i run since ive heard alot of people say it slows you down but ill give it a try next time i race
 

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The first time I ever raced my 03, I had a superchips, cold air intake, and a cutout. Cutouit closed I ran a 15.6 @ 88mph. Cutout open I ran 15.2 @ 92mph. I made 3 passes with each, and the results were the same on every pass. Now with your longtubes and no tuning, you might not see the same gains because you dont have any added fuel or timing in the tune. It is definately worth trying though.
 

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Here are two passes I made last Friday.....
The slip on the left was with the e-cutout closed and the slip is the right is with the air filter off the vararam and e-cutout open. You can see I actually gained on my 60' by two one hundredths and almost knocked half a tenth off my ET. So I would say having less back pressure isn't an issue as long as you have mods to back it up like headers, a TB,an air ram type intake, and a tune.

2012-12-07234533.jpg
 

SILVER_BULLET

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hmm ill give it a try just for the heck of it without the tune
 
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09SMOKINHEMI

09SMOKINHEMI

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Here are two passes I made last Friday.....
The slip on the left was with the e-cutout closed and the slip is the right is with the air filter off the vararam and e-cutout open. You can see I actually gained on my 60' by two one hundredths and almost knocked half a tenth off my ET. So I would say having less back pressure isn't an issue as long as you have mods to back it up like headers, a TB,an air ram type intake, and a tune.

2012-12-07234533.jpg

wow that's impressive..... do you have your cutout right after your Y?
 

TheBlackHemi

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wow that's impressive..... do you have your cutout right after your Y?


Yeah after the Y and before the muffler....it's a 3" raceland e-cutout.

I like it because I have a 22" magnaflow muffler (same as the catback kit) and it's pretty quiet until you get on it. It makes for a great set-up because I can take long road trips without any annoyng drone/tone of the exhaust that can give you a headache after listening to it for hours. Then on the streets I can open it up and go Nascar on that ass .:gr_grin:

22" Magnaflow with a Raceland E-Cutout sound clip - YouTube
 
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09SMOKINHEMI

09SMOKINHEMI

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Good stuff makes me wanna go to a 22 from my 14

sent from my Samsung Galaxy S3
 
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