Exhaust Q & A

[IRKillroy]

I'm pretty sick of seeing the same questions being asked a hundred different ways.

Back-pressure = good

All you need is one 90 degree bend in your exhaust (or enough bending to equal 90 degrees so two 45 degree bends that add up to 90) to produce adequate back pressure. What this does is prevent the vacuum in between each exhaust pulse from getting sucked back into the pipe and keeps the flow going out.

Headers....

You'll hear multiple theorys on what kind you should go with, there are three types. Shorty, Tuned, full-length. Everything else will fall within one of these categories.

The shorty are ideal if you don't believe in tuned headers, and need to have a bigger engine bay but can't... these were designed out of lazy mechanical engineers who didn't want to talk to another design team to make a better product.

The Full-lenth are ideal again if you don't believe in the tuned headers, and were inspired by the aftermarket and the theory that we just want to get the exhaust away from the engine... good theory, too bad the guy who invented EGRs didn't believe in this.

Tuned headers, these are awesome. Imagine each exhaust pulse as a pool ball rolling down your pipe, now look at the length of each pipe on your stock, shorty, and full-length headers... are they the same length? No. so those pulses will collide with each other and cause turbulance... do you like turbulance while flying in a plane? Yeah, turbulance in your exhaust is bad too. so tuned headers have equal length pipes so the pulses never hit each other and will even create a scavanging effect in your other cylinders which will get even more fuel and air mixture into your combustion chamber. Yeah!! more fire go boom!!!

Pipe sizes....

stay with-in 25% of your stock size or you'll have crappy performance.

Mufflers....

Doesn't matter, they are supposed to do what the name implies, muffle. Any aftermarket muffler will do a great job compared to a stock muffler... stock mufflers do their job by restricting flow. Aftermarket used science of sound waves (physics) to muffle which is less restrictive.

Cats and why you should keep them....

Catylitic converters have come a long way since the 70's and many of you think they suck and are restrictive... well some of you are right. Well the ones who think they are restrictive, and I'll tell you why you're right. Cause your stupid and have plugged them from running too rich or did something that caused too much carbon to build up inside of them. The rest of you should stop listening to that 17yo sitting in your auto shop or if you're old... forget what that 17yo told you back in the day.

A properly tuned engine will benifit from a catylitic converter in that it won't polute the environment and you won't have crazy nut jobs like Sarah Palin putting cross-hairs over the location of your house on google/maps for ELF to hunt you down and shoot you in the face (too soon?? she's off her resperator and doing fine). Besides, summit has a high flow cat that you can bolt up if you need to replace yours.




So, in conclusion... your 2.3l - 4.0l is not a 1970's 429 and you don't need a 3 inch exhaust with cherry bombs that dump right after the headers for good performance and sound. Most of you who want a better sound only need to buy ANY after market 2 or 3 chamber muffler for a decent rumble.

Now stop spamming the forum with your idiotic threads!!!!!

Stay Motivated.

 

[BlackBII]

Fixed.

I'm pretty sick of seeing the same questions being asked a hundred different ways.

Back-pressure = bad

All you need is the correctly sized pipe to keep the volumetric efficiency of the exhaust gas as high as possible

...............................

 

[IRKillroy]

why should I care what the volumetric efficiency of exhaust gas is?

VE = volumetric efficiency is a ratio (or percentage) of what quantity of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions.

you want just enough back pressure so you don't have gases going in both direction depending on the voids between exhaust pulses. Yes you can do this with pipe diameter, but to create the amount you need you're restricting the flow through the whole system, two 45 degree bends in a freer flowing pipe will give you the resistance you want and need.

 

[BlackBII]

Ok, we'll say exhaust gas velocity instead.

But I'm not even going to try anymore...

Maybe someone else will have the patience that I now lack.

 

[IRKillroy]

In one attempt you give up. I agree that exhuast gas velocity is important, but you feel that back pressure is bad and can't back it up. If you assumed you had no back pressure of any kind you wouldn't have proper flow as the gas would rather try to equalize in the pipe rather than find the exit and then equalize out there. You need to think of it like water in a hose. take a 3 inch waterhose and a 1 inch waterhose, both with the same pressure pushing the water... which one has more velocity?

ad a kink to the 3 inch hose and you get increased velocity but more free flowing capability than the 1 inch. It's just not as efficient as say a 2 inch hose with a slight bend.

there are many factors to this, simply saying it's bad makes you wrong.

 

[BlackBII]

Instead of trying to type it all out..I'll just steal it from elsewhere.

http://www.thumpertalk.com/forum/showthread.php?t=659727

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

 

[rickf92592]

While I agree with most of the OP's original rant, you are over-simplifying the 'back-pressure' issue.
The really difficult issue is to reflect the sonic waves from the exhaust back to the port at the optimum time to reduce 'over-scavenging' and increase cylinder fill and compression. Real easy to state, really hard to design within the parameters and restrictions of a production car.
Simply adding back-pressure may or may not do this, it is for people way smarter than me to engineer the solution.
Completely agree about cats, properly functioning there is little lose of performance and a world of good done to the emissions.
Flame away!!

BTW, for all of you who are shopping for a aftermarket intake (filter) some of the same principles apply.

 

[IRKillroy]

@rick, I didn't simplify it... 'cause most people think that back pressure is bad, even Black contradicted himself with his post. There are so many ways to create back pressure, and all of them increase your "velocity" and if you couple that with tuned headers you get a scavenger effect in your cylinders that helps get inert gases out of the combustion chamber during your cam overlap of say... the standard 10 degrees of rotation.

One train of thought is to simply get the exhaust away from the engine... so a straight pipe goes away... it does it's job, the power is in the engine and any loss of power is so minimal you won't notice.

another is tuned exhaust... which Black was trying to covey in a someone else's "article" which seems to be pulled from some class material. Your exhaust should cover your RPM range, not just your power band... the same principles apply to setting up your intake/carburetor. There are mathematical equations to keep you from screwing this up.

SO, back pressure = good.

'cause black, as it states in your post "The trick is to have a pipe that that is as narrow as possible while having as close to zero back pressure as possible at the RPM range you want your power band to be located at. " even though you post is extremely contradictory at times such as this "You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream.". Because those you can do a few things to have a happier median

larger pipes with BENDS in them create back pressure (friction) in the exhaust stream which will prevent your reversion of flow and increase the velocity of the gases assuming you don't have a tuned exhaust system. Which makes all that crap pointless as each pulse will fall in line behind another one and will be merrily pushed out the pipe at whatever speed you want them to go.



Disclaimer: I'm getting to the point that I'm repeating myself different ways... It's 2am here in Okinawa. I understand what you're saying black, but there are different thoughts in the science of exhausts. Just because you read it somewhere and it sounds smart and well written doesn't mean that it's the end all, be all of theories. Your statement did nothing more than to dispel a notion that "extreme" back pressure is thought to be good (for whatever reason).

My intent was to tell these guys that the 3 and 4 inch exhaust stacks with rain caps won't make the truck perform better, that they in fact need back pressure for the engine to run right... your article says the same thing, except that it's needs to be perfectly balanced... dude, we're talking about the same ****ing thing.

I have debated editing down this retarded post but I don't care and I'm putting it up anyways... Stay Motivated, and thanks for not quiting on me.

P.S. took me so long to write this I had to re-login.

 

[Yellowsplash]

Once again.....here is another one of "those" threads...

BlackBII is right....if you were to search the forums you will find that this has come up a million times and has been corrected a million times...

backpressure=bad
scavenging effect=good

End of story.... Your use of the term "backpressure" is misleading, and your theory is abit skewed. But since my postings (and many others) about this very subject are all over this forum, and with you being a member and the ability to search for them, I'll leave that up to you to find because like BlackBII I've lost the motivation to correct the myths by which we discuss and teach others on here because it never does any good anyways...

 

[IRKillroy]

look, you can't get rid of backpressure from your exhaust system even if you wanted to. Perhaps I'm misleading people with my statement but in fact I'm not wrong in my intent.

I wanted to explain to people that a 4in exhaust in the name of reducing back pressure will not net you the gains that you want. (my way explaining it sucks balls though, and after reading it again I went in the wrong direction with it... my bad)

There are people out there who honest to god don't realize that a muffler no matter the type creates back pressure, and that each exhaust system should be custom built for the application if you really want to max out your performance.

I will admit that velocity is important, but velocity means nothing without back pressure, Temperature, and pipe sizing as they all are factors in an exhaust system.

Even with all of this, I still believe that an engine should have back pressure, however the amount of back pressure should be kept to a minimum.

 

[Yellowsplash]

Backpressure is not what you want...you want velocity. In choosing the right size piping you are creating a scavenging effect that will increase the velocity and therefore the amount of flow that can be drawn out of the cylinder without the piston having to push it out alone. Its all about the previous exhaust pulse creating that vacuum or scavenging effect of pulling the exhaust out of the cylinder. This is the name of the game...less energy wasted on parasitic losses and more to the wheels, not to mention a cleaner incoming air charge as more exhaust gets spent out the exhaust causing an even better burn during the next revolution. It has nothing to do with needing backpressure. Your looking at it all wrong because by thinking you need a smaller pipe you are automatically creating more backpressure because of it being smaller...your assuming by association and it is simply wrong.

Ever heard of the term "Correlation does not imply causation"...

Is a 10" pipe going to incur less "backpressure" than a 4" pipe...sure, if you had a motor that produced enough exhaust gasses to even warrant that big of a pipe or even make a measurable change...

Also, by creating bends in a "larger" exhaust pipe, your only causing the gases to tumble around the bends and slowing the exhaust flow, and in certain cases causing backpressure. Due to already having too large of a pipe and the lack of vacuum properties or scavenging effect being created due to its size (lack of velocity), the piston is already having to push the exhaust out of the cylinder and the exhaust pulses to play bumper cars as they escape out the tailpipe.

We can talk about Ideal gas laws all day...I've done it numerous times. I dont care to anymore.

Take it as it is... (Im not directing this towards you or any other member, but rather the populus who read this)
Backpressure is bad.
Too large of a pipe is bad.
Finding the correct size to accomodate velocity and you are miles ahead of the rest.
Its your truck, do as you please, but dont spread myths...

 

[IRKillroy]

I understand... it's why it's a Theory. And if you read my previous posts you'd see we're on the same sheet of music, I just suck at expressing what's on my mind without having a back and forth with someone.

Besides all of that, If there are flaws in ones Theory the only way to really test it is to add pressure and fix the leaks.

You can't get away from backpressure, you can only mitigate it in your exhaust design. End of story.

 

[Bondo]

So a shorter exhaust pipe (like dumping before the rear tire) would be better for flow and scavenging as well? No?

 

[RangerFabWorks]

So a shorter exhaust pipe (like dumping before the rear tire) would be better for flow and scavenging as well? No?

not exactly. you want an exhaust that has the least amount of bends or restrictions in it as possible. and actually the longer and smoother the pipe is. the better the scavange effect.

but sizing is key. too big. means the exhuast can actually stop flowing and revert back to the engine during the engine firing pulse waves.

too small. the engine cant push the gases out fast enough. and this causes backpressure and your engine will choke itself.

the ideal exhaust setup would be to start big at the manifolds and progressively step down in size to the tailpipes. that would creat the ideal scavange effect but the results would be null. it would be enough to benefit the engine.


basically. your stock exhaust does a pretty good job in flowing decent. and being quiet. your not going to gain much other than sound. maybe 2-3 horsepower at most on the newer cars.

i have found you actually FEEL like your going faster with a stock quiet exhaust than you would with something like a flowmaster. you dont hear the engine as much so it SOUNDS like its working less. this is my own personal opinion and do not in anyway push these ideas among others.

 

[Bondo]

Ok, thanks BW67.
Hey, are you a LI Jets fan?