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Old 03-25-2014, 12:58 AM   #1
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Exhaust Fallacy

So, I guess this is just a gripe of mine with working on physics and cars for a while. Engines DO NOT NEED BACK PRESSURE. AT ALL. PERIOD. It is exhaust flow velocity that matters, balancing the pipe out to produce through the range of rpms you want. The smaller the pipe, the more torque at the lower end because the exhaust will be moving at a greater velocity, but when you step on it, your going to lose some from the back pressure because it doesn't have enough room to let it flow easily. The larger the pipes, they will allow for better torque and power at different throttle positions as they get bigger. Get to big and guess what, it will kill your power through all power bands.

But please, for the love of God. Can we fix this issue on the internet, back pressure is not necessary on any car in existence. The exhaust produces three frequencies as it empties from the valves and travels through the pipes. A high, Medium, and Low frequency. The low frequency, traveling in the very back produces a vacuum to help scavenge more of the exhaust and help pull it along. This system is why you do not want the largest pipes always. Even a .5 inch increase can be massively over what is necessary for the car. If you don't have extreme power differences, stay within .25 of the factory. (Start getting +50hp and yes, larger pipes will assist you more. But a CAI does not justify an exhaust size increase fyi.)

Now, Forced Induction is similar. The math does change and larger size pipes are necessary automatically in that case, but again they can still be to big. I've heard exhaust guys say you can't get to big with forced induction. The response to that, plug the numbers in to the proper physics equations and yes, yes you can.

Overall it is a balance, do you want low end, mid range, or WOT power and torque. If you want the WOT, then go a bit bigger, if you want the low end keep it small. I'm working on an equation to actually calculate what size to get based on where in the power band you want optimal performance. Please help kill this myth for good though. It is a misconception of a combustion engine that has been passed on for way to long.
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Old 03-25-2014, 02:26 AM   #2
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Dr. Pinkschism, you should design and market the next latest greatest exhaust we'll all want then.

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Old 03-25-2014, 03:22 AM   #3
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An engine may not need backpressure, I don't know. But I do know this, if you take off a muffler on a motorcycle, it will run like crap unless you put bigger jets in the carbs, so on an older car, it would make sense that you would have to get more fuel if you took off the cats and mufflers to make it run right. I guess the newer cars, the computer may take car of that. I am no scientist but backpressure is needed unless you change other things. On a motorcycle anyway.
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Old 03-25-2014, 08:21 AM   #4
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My favourite part about this is you invoke the name of some mystical deity to support your scientific claim:

"...please, for the love of God. Can we fix this issue on the internet, back pressure is not necessary on any car in existence."

There isn't anything really wrong with what you say but your explanation is overly simplistic too. Even explaining how a header works is a good deal more complex than you make out here. You can't treat the parts in isolation from the whole. The engine configuration, compression ratio, intake, cam timing, firing order, etc. all play a role in what exhaust suits a particular car, from a power production standpoint.

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Old 03-25-2014, 09:21 AM   #5
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Old 03-25-2014, 08:40 PM   #6
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So I will answer this in the order that the trolling began and be done with it.

1st.)Doctor is for someone who has a Ph.D. in a specific field. I'm still about three years off but thank you for your concern with the progress in my degree. It won't be to long and that will be completely appropriate.

As for designing exhaust, I do. I make almost every part that gets put on to my motorcycle, mustang, or drag truck. I make exhaust for other people as well. It's a slow process because I do it based on the car itself and the specifics behind that car. Me, and two friends of mine. (One who is a mechanical Engineer and the other is just an artist when it comes to working with metal.)

~thanks for the compliment compstall. but that wasn't even an argument to what i was addressing.

2.)Green 5.0 You are not included in this trolling you were just making a statement.
Back pressure refers to pressure opposed to the desired flow of a fluid in a confined place such as a pipe So if the point of exhaust is to get it away from the engine... why would it make since to push against the force of pressure moving out from your pipes >>>>>> and have a force pushing against that flow <<<<<<<. Motorcycles are not inherently different, they are more sensitive to changes in exhaust and the like because of the engine size. Does it need back pressure to work right? not at all. Period. It needs a proper exhaust flow velocity. Which is changed dramatically quicker due to the shorter pipes and the fact that the engine is pushing with less force than that of a car. (Your 5.0 compared to your motorcycle two different worlds in the velocity of exhaust.)

If you ever want to test this, simple. Chop the muffler off your motorcycle again, and take it for a ride. The lower end will probably feel like crap because now your exhaust is leaving much faster and you have changed the flow of the exhaust gases based on distance (not size). So driving around the neighborhood will be a pain. But at the higher end, it will flow more freely and you will get a better top end powerband. (My gsxr runs a very wide exhaust with no muffler. This kills its low end and stop and go traffic is a nightmare, but when i put it on the track and hit it, at WOT it runs like a champ.) Then weld the muffler right back on. You will see the change, will it be noticeable. Depends how dramatic that change is from the flow, but put it on diagnostics, and you will see it has nothing to do with backpressure, it all has to do with the velocity of the exhaust flow.

3.) 5LHO-so yours is littered with some ridiculous crap but ill handle it as i see it. First, I did not call upon some mystical diety, I myself am an atheist. I referred to God in a pantheistic manner. If you don't know what that is, then look it up. This is also notably used by several other scientists, one of the most well known being Albert Einstein. Complete atheist, but used God in a pantheistic manner to be ambiguous to knowledge. In no way is that a reference to a mystical super natural being.

2nd, Overly simplistic? If i wanted to write a dissertation about the internal combustion engine I would have. I was simply referring to a common problem with referring to something that has been referred to incorrectly for way to long.

You are right about one thing, the complexity of an engine is understated here. For a reason. The next time i write a 110 page descriptive analysis of the internal combustion engine, I'll be glad to send you a link to it to see if you read past page 2. I made it simple so it could be understood exactly what the problem was. Nothing more, nothing less.

4.)CatCobra... sorry you didn't even make the list of trolling that even made sense to glorify with some kind of thought out response.

Finally, what the hell happened to the internet. This forum is supposed to be for enthusiasts looking to gain knowledge and help each other with learning the systems of your specific car. I'm trying to correct one of those misconceptions. This isn't from a high horse stand point because it was only about four 1/2 years ago i understood the difference myself. I referred to backpressure all the time not knowing what it actually was I was referring to. The next time you go in to a performance shop though, you want to sound like you know a little bit more than just the dude dropping money on his car with no idea what it does. So when they say your exhaust is to restrictive, you can ask him how much he thinks you should change the size to increase the exhaust flow velocity and what ranges would be reasonable.

If your going to make an argument, make one. But don't sit here and troll on a community that is made to help each other out to learn and understand more about what we are doing. Side note: its not a bad idea to pay attention to what is being talked about to, if i wanted to talk about compression ratios, cam timing, engine configurations, or spark temperature I would have. (None of which are arguments against what was said, I might add.) Those are all things i learn more about on a daily basis as i tinker more and more. If what you say has no real quality to it though, don't talk.
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Old 03-25-2014, 08:53 PM   #7
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With all due respect you're 6 posts deep and acting like an industry elite because you worked on a motorcycle. Your 2 page post above that I didn't get past 2 paragraphs has earned a:

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Old 03-25-2014, 09:30 PM   #8
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I can't quote the Master's thesis above because, well, once is enough for anybody reading this.

Welcome to the forum, btw. Quite enjoying the first impression you're giving here that isn't getting better in the Katyusha barrage above.

I dunno, I spend my life trying to explain complex things to people that really don't know a lot. I teach auto shop at an alternate high school. This isn't how I do it but, to each their own. The problem is, the stuff you're saying that's right is, no-****-Sherlock to anyone with some knowledge but, to anyone who IS trying to learn, you just sound pompous and superior. Meh, I'm probably talking to myself here.
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Old 03-25-2014, 11:04 PM   #9
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If I fit a Tornado in my exhaust pipes will I gain power?

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Old 03-25-2014, 11:34 PM   #10
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I get what you're saying and you are correct in that statement. I think what rubbed people the wrong way and caused them to start trolling was your attitude to begin with. The way I took it and the way, I think, most of is did was that you already have large ego and are better than the rest of us because you are 3 years out from being a doctor. A doctor of what and who cares? 3 years out.... So you have a bachelors degree. Congrats. Just tone it down a bit alright and have some modesty about yourself. There are a lot of people on here that are willing to help if you ask. I mean you're already asking to change the internet! My guess is though and from the sound of it you don't need to ask is anything. You seem to already know it all.
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Old 03-26-2014, 08:23 AM   #11
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Ok then, are weep holes necessary, and if so how large and where to place on muffler. Are you on any other forums that I could enjoy your thoughts, you must be a member in the Society of Mensa.
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Old 03-26-2014, 12:51 PM   #12
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Well, I mostly agree. The term "back pressure" is a misnomer / gross oversimplification. I think what most people call back pressure is really exhaust tuning. There's a good deal of very complex acoustical stuff going on in an exhaust system. A well designed system can take advantage of power pulses (and the lulls) by designing in shapes, volumes and distances so, for example, a negative pulse produced by one cylinder can arrive at another cylinder just as it begins it's exhaust cycle to better evacuate the burned gasses. The negative pulses are caused when exhaust gas rapidly flowing from a cylinder's exhaust cycle are suddenly "chopped" by the exhaust valve closing. The gasses have mass and velocity so produce a negative or low pressure area in their wake.


Similarly, in an engine with a lot of valve overlap, a positive pulse reflection produced by one cylinder can be timed so that it arrives at the exhaust port of another cylinder at the very end of an exhaust cycle to prevent or reduce the amount of fresh charge that might escape through the closing exhaust valve. This results in a bigger charge in the combustion chamber as well as reduced HC emissions.


If an engine has fixed valve timing, this perfect timing of negative and positive acoustic pulses is usually only optimal over a small RPM range, hence the old term of "coming onto the cam" where a distinct power increase is felt or measured. Some manufacturers of performance engines have broadened the RPMs where exhaust tuning is optimal with movable flapper valves and such in the exhaust system that change the length and or volume and effect the timing of these pulses.


Today's engines with their variable valve timing make much broader use of these pulses by changing when they occur in the first place, eliminating/reducing the need for variable exhaust systems.
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Old 03-26-2014, 01:16 PM   #13
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Bullseye....now that was a nice way to express this well-known but often misunderstood set of concepts. Nicely put, sir.
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Old 03-26-2014, 02:23 PM   #14
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Im lost on this...

I am not correcting anyone, I want to understand.. maybe I am too old school or just because I traveled with Auto engineers on the road who liked to debate everything. Why would you not need some back pressure to maintain a degree of gain?
Example 1: Water running at a less defined exit ( focal point) loses pressure and then would then back up water flowing behind will cause some turbulence..
Example 2: Long tube header help produce more horse power than short tube headers.

Or does this examples of your point the exhaust is more for a controlled focal point and thus sound is a secondary attribute?

<--- in case I am stupid..
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Old 03-26-2014, 06:30 PM   #15
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Quote:
Originally Posted by Alerotaz View Post
I am not correcting anyone, I want to understand.. maybe I am too old school or just because I traveled with Auto engineers on the road who liked to debate everything. Why would you not need some back pressure to maintain a degree of gain?
Example 1: Water running at a less defined exit ( focal point) loses pressure and then would then back up water flowing behind will cause some turbulence..
Example 2: Long tube header help produce more horse power than short tube headers.

Or does this examples of your point the exhaust is more for a controlled focal point and thus sound is a secondary attribute?

<--- in case I am stupid..

If you had an engine that only needed to make useable power at a very narrow or specific RPM band, and you don't care how efficient the engine is with fuel, you could run open pipes with almost no back pressure. Example, look at top fuel dragster or funny car. The pipes are extremely short and individual, and you know they are blowing raw fuel right through the high valve overlap combustion chambers as evidenced by the flames exiting the pipes.


An engine like this is extremely peaky, almost unable to run at idle and certainly not suitable for a road car or even a road racing car where a broader spread of useable power is desired.


It's really all about the priorities the engine builder / tuner has for his application.
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Old 03-26-2014, 09:24 PM   #16
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The reason street gas cars need exhaust is because they operate in very wide operational range and need to be effective in all those different situations.

If they ran fixed rpms for long periods like aircraft engines, you could get away with a stub stack exhaust, as long as you tuned it to be efficient at that rpm.

A good example of what WJ was talking about up further here is the factory tri-Y header this car comes with. This header pairs holes as close to 180* apart as possible and this helps to scavenge gases from cylinders on the exhaust stroke. This creates torque quite nicely but, isn't ideal for high rpm applications. This is why longtubes, which isolate each cylinder longer before collecting the gases, favour high rpm application.
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Old 03-29-2014, 11:43 AM   #17
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Old 03-29-2014, 01:31 PM   #18
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Old 03-29-2014, 01:45 PM   #19
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