Exhaust systems and how they work

[Rev ATARI]

Exhaust systems and how they work

Exhaust systems and how they work

"THE MYTH OF BACKPRESSURE"

Backpressure is one of the most common terms used win talking about exhaust systems it is also highly misunderstood.

So what is Backpressure?

Backpressure is the resistance of air flow

So if Backpressure is resistance than why does a loss of Backpressure because a loss in HP, well I will tell you know that is does not.

"THE TRUTH OF BACKPRESSURE"

Backpressure is used on engines to

(1) help with fuel economy
(2) help with meeting emissions laws

On a tuned high performance engine Backpressure is not needed in fact Backpressure will hinder HP gains. There was a test done in SCC car magazine where they did a header comparison on a B18C in a Acura Integra and guess what just for fun they did a dyno run with no header at all. The run with out a header made more gains threw out the full RPM range than any header they tested. Now keep in mind no header no Backpressure.

Now with all that said and done lets move on to what Backpressure and the lack of Backpressure do to the exhaust system.


Reversion this is the start of all exhaust systems.
So at the stat of the intake stroke you have cam over lap and the gasses in you header are under high pressure causing the gasses to be pushed back into the cylinder causing a bad fuel/air mix.

Exhaust with Backpressure

Scavenging

This also starts at the intake stroke win you have cam overlap. The momentum of the exiting exhaust gasses creates a brief vacuum in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new fuel/air charge to be at its best

Exhaust without Backpressure


How to rid Backpressure

Backpressure is resistance in the exhaust so to rid it we have to have bigger piping but at the same time not to big. Why not to big because the exhaust gas moves slower threw larger piping. But also considering that the gasses flow at different rates at different spots in the RPM band you also have to trade off some high or low RPM power with all that said


The magic size for most N/A Honda engines is about 21/4 to 21/2 inch

[OdiedogCX]

Someone on this site .. and i have no idea who .. said that a decrease in backpressure=less torque.. so thats untrue?

[Rev ATARI]

From what I have read yes its untrue

[PseudoRealityX]

Originally posted by OdiedogCX
Someone on this site .. and i have no idea who .. said that a decrease in backpressure=less torque.. so thats untrue?

a lack of backpressure will not reduce torque, however, if the exhaust system was largened to reduce the backpressure, then the overly large exhaust system and the corresponding drop in exhaust gas velocity WILL hurt torque.

[pssst!]

What is 21/4 & 21/2?

[ricerocket2002]

i think he means 2.25-2.5 inch exhaust piping diameter. hey ich_folge, what are your sources?

[Jumpan23]

I heard that the piping size matters becuase the exhuast moves in puleses. With the perfect size piping the exhuast pulses will move in a single line. If you go too big...the pulses will move around and bounce off of the walls of the pipe, making it move slower. If you go to small...c'mon you know.

I forgot where i read it but off of the net somewhere....I guess this does agree with the above. That's why when the exhuast is removed completely (head pipe only) you don't lose HP...you don't have to worry about exhuast bouncing around.

[Rev ATARI]

Originally posted by ricerocket2002
i think he means 2.25-2.5 inch exhaust piping diameter. hey ich_folge, what are your sources?

My sources are simple research followed by the physics of air flow

[Targa250R]

Originally posted by ATARI
The magic size for most N/A Honda engines is about 21/4 to 21/2 inch

Yep, for mild to moderate NA applications, I usually recommend 60mm piping. For high-power NA applications, 2.5in or even a hair bigger is fine. All mandrel-bent piping, of course.

I have some more good exhaust info here. This deals with the myth of backpressure:

* * *

Originally posted by Prelussion at AutomotiveForums.com

A lot of people have different thoughts on backpressure, and often confuse it with Velocity and Delta Pressure...
I will now post a colaboration of posts from P u r e h o n d a . c o m

"THE MYTH OF BACKPRESSURE"

…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace.

Here's a reply to the above post:

"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.

There is one thing I'd like to add to texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that texan describes.

Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.

Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.

Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.

Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.

Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.

There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "

Credit given to Texan and Fritz for their info on this topic.

* * *

Cliff notes: Backpressure is your enemy. High flow velocity and positive delta pressure are your friends.

[Targa250R]

I quoted this from one of Primax's posts - Primax really knows his stuff when it comes to exhaust and backpressure. : I believe the original source of this article was at h o n d a l i f e . c o m.

* * *

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 "Hondas 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 "Hondas 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(s).

VI. Conclusion.

SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

* * *

Some additional links dealing with turbo exhaust needs:

http://vishnutuning.com/exhaust101.htm
http://vishnutuning.com/news_pvnrt.htm

Credit goes to ironmonkee for posting these links. :

[Primax]

I also had another thread explaining bp but we pretty much got it whooped here. I had some little ms paint drawings too to show the pulses and ill try to find it and put it up for easier understanding.




yea, i know its 3rd grade but u get the point.

[eghybrid]

Keep in mind the only reason we have exhaust is too quiet our cars down, exhausts rob power in an effort to silence the engine. For example in an NA car all the exhaust scavenging needed to remove the exhaust gas from the ports properly so as not to damage the motor is accomplished in the header primaries, and or collector. Beyond this point everything is there just to quiet the car down. No backpressure is best but, when speaking of exhausts you are creating backpressure, and this is where too much or too little comes into play.

And no power(very minimal if any) will be lost anywhere in the powerband running an open exhaust. Why? Because backpressure is a combination of things such as exhaust stream velocity, temperature, and resonance(turbulance or reversion). Running an open header or downpipe gives you the best of eveything, high velocity, higher temp., and no resonance. This give you an optimum exit of exhaust gas without allowing it to cool, slow down, or revert on itself.

Too big of an exhaust cause these problems for example...let's say you blow up a balloon you put it to your mouth and blow...easy right?? Now put a 1-foot long piece of tubing 3" in diameter between your mouth and the balloon and try..harder right? Because, the "cavity" the pipe creates requires you fill this tube up with pressure prior to the balloon blowing up, this large tube also allows your breath to cool, slow down, and the flow becomes turbulent.

I made the above post a LOOOONG time ago, and thought it may have some relevance here.

[quikflip27]

so why do the high powered cars...jgtc, topfuel, f1...keep some sort of small exhaust? why arnt they juss running no headers if they restrict some hp?