mt_goat

Well it is a hard concept to visualize, and I admit I had a hard time with it too at first, even though on my way to getting my degree in mechanical engineering I'd had 3 semesters of physics, 4 semesters of chemistry, and a semester each in fluid dynamics and gas power, it was still very difficult for me to see.

I like the way you're standing your ground and saying prove it to me, and there my not be anything I could say or do to convince you as I'm far from totally understanding all the dynamics of what's going on myself, (this kind of reminds me of the question "which came first the chicken or the egg?") lol.

With any big debate there is one main point that is confusing or misunderstood. Like back when people thought the earth was flat and some said no, it's round. In that case, the main point of misunderstanding was the gravitational force of a planet, lol.

Here IMHO the main point that's hardest to visualize is the fact that air has mass, and as such is subject to Newton's laws of motion. What often helps me grab a concept is to take the subject to the limit, so here goes an attempt at that. Think of the exhaust gases in the pipe as a coal train, ok, now you can picture some big mass. And the train has 3 locomotives or it could have 4 too if you want, or 6 or 8, but if it has 6 or 8 you have to split it into two trains at first and merge them together down the tracks into one train which is harder. And the locomotives instead of giving constant power are only allowed to give pulses of power, one at a time, with a brief pause in between each pulse where there is no power to move the train and the only thing keeping it moving is inertia. Now we have an exhaust train, and like the coal train it can be light (like with no coal loaded, just empty cars) or it can be heavy (like loaded down with coal). Now our exhaust train it is heaver when it's cold and light when it's hot, (or that's my theory anyway)

So when we step up to a bigger pipe we not only have more mass to push down the tracks (ie bigger railcars now) but we slow the stream down and allow it to cool more and get heavier. I'm pretty tired now, think I'll go home. More later maybe.

InternetRoadkill

This is complicated, so I'm going to grossly oversimplify. A larger pipe can cause higher back pressure... sort of. The reason is that a smaller pipe produces higher velocities and hence higher momentum of the outflowing exhaust gas. When the exhaust valve closes, the momentum prevents the outflowing gas from stopping suddenly. The result is that a momentary vacuum forms near the exhaust port. Headers are designed to promote this effect.

The trick in sizing exhaust is to choose a size small enough to promote a high velocity flow, but not so small that it begins to obstruct the flow.

jriebe

Personally, I ascribe to the "Exhaust Gnome Theory."

Think about it...the bigger the pipe, the more exhaust gnomes you can fit into the area. As with any other environment, too many hands generally become ineffeicient,too many gnomes = lack of personal touch = loss of efficacy thus proving that bigger pipes equal loss in effectiveness due to the ability of the gnomes to slack off...

Smaller pipe = more industrious gnome who knows he is gonna get his ass replaced if he does't work hard enough. he is efficient because he has to be.

You see, all the physics and holistic mumbo jumbo aside, it all really boils down to motivation. one highly motivated gnome is much more effective than many un-motivated lazy gnomes.

I believe this clears up the entire problem...

On a serious note, the only time I have ever heard of more flow damaging an engine is running open headers for any period of time. I am afraid i don't have a 10lb brain so I can't give you specifics, but i figue that the guys who designed the exhaust system had something going for them. If you are looking to spend eight million dollars on an exhaust for a 22r...do what you gotta do. But as far as general everyday use goes i think this argument is a little bit far out there.

I have really liked this thread as it seems everyone has put a lot of time and research into their posts, except me of course. Thanks for all the good info, whatever side you are on!!!!!


Jon

DaveInDenver

It's called Bernoulli's Principle. Velocity and static pressure are inversely related, so as velocity goes up, static pressure goes down.

InternetRoadkill

Actually, Bernoulli's Principle is not what is at work here. Bernoulli's applies to venturis.

DaveInDenver

A venturi is one demonstration of the Bernoulli Effect, but so is an aircraft wing, the nozzle on a rocket engine, a curve ball. Bernoulli simply described the phenomenon that as fluid velocity increases, the pressure goes down.

http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

Another graphic of Bernoulli Effect, plus you can see how velocity, pressure, area relate.
http://home.earthlink.net/~mmc1919/venturi.html

mt_goat

Nice links Dave!

InternetRoadkill

Actually, I'm familiar with the Bernoulli effect. But as I mentioned, it applies to venturis, not constant-area exhaust pipes.

The Bernoulli effect does apply to aircraft wings (think of them as half of a venturi), but it doesn't apply to rocket nozzles (Bernoulli only works with incompressible flows). A curve ball is an indirect result of Bernoullis. The curve is produced by the ball's rotation influencing the boundary layer of the flow across its surface.

Incidently, pressure doesn't drop just because the air is moving. You can have a high-velocity flow with the same (or even higher) pressure as a slow moving flow. Bernoulli's describes the change in pressure/velocity as the ratio of potential/kinetic energy changes. But this requires that the total energy be constant. This is not the case for an exhaust flow which is receiving the residual combustion blast and waste heat from the cylinder.

mochester

i'm going to go with the one that i can test for myself on a scale model. it should be very clear that when blowing through first a drinking straw and then a section of garden hose, which one has more backpressure

edit: that part about blowing between the two candles still makes me laugh
its totally irrelevant. he doesn't even attempt to explain how it relates to an exhaust pipe. that's the main reason i don't buy anything this guy says. he can't even give an example, and his explanations don't make sense. maybe i'm just not smart enough, or maybe he's full of ☺☺☺☺. air doesn't act the same way trains on tracks do. that's a horrible example too. air doesn't move on tracks, connected to other air in strings. air gets compressed and expands and bounces all over the place. not one example that works as well as mine, which disproves the bigger exhaust=more pressure theory in 2 seconds. what's a better measurement of backpressure than you're own lungs? think about it. they're like a big air pump, and you can be the computer, analyze what you feel, and make a decision about what has more backpressure - a smaller or larger diameter tube. if you have any common sense, you won't even need to test this, and you will conclude what i have concluded

unless you can think of an example for your theory that actually involves air moving through a pipe

DaveInDenver

Bernoulli works fine with compressible fluids, it works fine in an vessel. It's nothing more than an explanation that allows you to conserve energy and still explain the phenomenon. The standard presentation of Bernoulli does assume incompressible fluid, but the difference is that you just have to account for changes in density and stored energy, the principle still holds.

http://www.fluidmech.net/tutorials/b...-bernoulli.htm

Look, I'm not going claim I know much about the thermodynamics of an auto exhaust (or heck, thermo in general, I only took two classes in it anyway), I was just pointing out the physical reason. The sizing of an auto exhaust is something I'll trust the experience that people have found before. I'm more of a Maxwell, Frick, Moore and Faraday kinda person anyway. But I do happen to agree that there is an optimal balance of velocity, pressure and volume that tunes the exhaust to the engine. Two stroke guys have known that for a really long time.

Stevo3

Wedge a potato in your exhaust and see if thats too much back pressure .

I am totally lost on this post haha

dcg9381

Turbines work better than potatoes.

dijlop

word. some deep stuff here. ive been sonfused on this issue as well. this kinda helped. i think the only real solution is to turbo everything. lol.

crbocaj125

Dude, you are thick. It states here in this thread about 5 times that the ideal pipe size would be the small as you can go without restricting flow. Obviously the drinking straw is too small for your lungs, or else it would be a PITA to drink out of. Your lungs are not a good comparison, because your lungs will NEVER put out enough air to displace all of the air in anything such as a garden hose.

The candle example is a PERFECT example of how air traveling faster loses density. Which relates to how easily it can be expelled from the exhaust. You want to keep the air moving fast, becuase when it slows down it thickens. Too big of an exhaust also allows the air to cool, which is also makes it thicker.

I could explain much more about this, but it would be repetetive considering it has all already been said here once or twice in this thread.