Backpressure: The myth and why it's wrong.

Thank to Moe02 for finding this. I couldn't find it and its a very good explanation.

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 For example, "Honda's 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 efficiently. 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?

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 upgrade 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).


P.S. Could a MOD please sticky this. Its very good info

I'll stick it for a little while but I'd like to get more experts to chime in

From Wikipedia:  http://en.wikipedia.org/wiki/Back_pressure#Back_pressure_in_automotive_.28four-stroke_engine.29_exhaust

Back pressure in automotive (four-stroke engine) exhaust

Back pressure caused by the exhaust system (consisting of the exhaust manifold, catalytic converter, muffler and connecting pipes) of an automotive four-stroke engine has a negative effect on engine efficiency resulting in a decrease of power output that must be compensated by increasing fuel consumption.

More good info: From http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

We've seen too much misinformation regarding exhaust theory. What kind of misinformation? For starters, there are a lot of people in the "Bigger is Better" camp. We're talking about exhaust pipe diameters. Even the big magazine editors are boldly smattering statements like, "For a turbo car, you can't get an exhaust pipe that's too big." Also, terms like "back pressure" and the statement, "An engine needs back pressure to run properly!" really rub us the wrong way.

Let's start from the beginning. What is an exhaust system? Silly question? Not hardly. Exhaust systems carry out several functions. Among them are: (1) Getting hot, noxious exhaust gasses from your engine to a place away from the engine compartment; (2) Significantly attenuating noise output from the engine; and (3) In the case of modern cars, reduce exhaust emissions.
Hardware

In order to give you a really good idea of what makes up an exhaust system, let's start with what exhaust gas travels through to get out of your car, as well as some terms and definitions:

After your air/fuel mixture (or nitrous/fuel mixture) burns, you will obviously have some leftovers consisting of a few unburned hydrocarbons (fuel), carbon monoxide, carbon dioxide, nitrogen oxides, sulfur dioxide, phosphorus, and the occasional molecule of a heavy metal, such as lead or molybdenum. These are all in gaseous form, and will be under a lot of pressure as the piston rushes them out of the cylinder and into the exhaust manifold or header. They will also be hotter 'n Hades. (After all, this was the explosion of an air/fuel mixture, right?) An exhaust manifold is usually made of cast iron, and its' primary purpose is to funnel several exhaust ports into one, so you don't need four exhaust pipes sticking out the back of your Civic.

Exhaust manifolds are usually pretty restrictive to the flow of exhaust gas, and thus waste a lot of power because your pistons have to push on the exhaust gasses pretty hard to get them out. So why does virtually every new automobile sold have exhaust manifolds? Because they are cheap to produce, and easy to install. Real cheap. Real easy. Like me.

"Ok," you ask, "so now what?" Ah, good thing you asked. The performance alternative to the exhaust manifold is a header. What's the difference? Where a manifold usually has several holes converging into a common chamber to route all your gasses, a header has precisely formed tubes that curve gently to join your exhaust ports to your exhaust pipe. How does this help? First of all, as with any fluid, exhaust gasses must be treated gently for maximum horsepower production. You don't want to just slam-bang exhaust gas from your engine into the exhaust system. No way, Jo-se'! Just as the body of your '94 Eclipse is beautiful, swoopy, and aerodynamic, so must be the inside of your exhaust system.

Secondly, a header can be "tuned" to slightly alter your engines' characteristics. We'll go in-depth into header tuning a little later.

Nextly, exhaust gasses exit from your manifold or header, travel through a bit of pipe, then end up in the catalytic converter, or "cat". The cat's main job is to help clean up some of the harmful chemicals from your exhaust gas so they don't end up in your lungs. In most cars, they also do a great job of quieting things down and giving any exhaust system a deeper, mellow tone. You'll see a lot of Self-Proclaimed Master Technicians (SPMT's) telling people that removing a cat will get you tons of power. There's room for debate on this, but in our experience, removing a catalytic converter from a new car won't gain you much in the horsepower department. It can also get you a $1500 fine if the EPA finds out! If you drive an OBD-II equipped car, you'll also get that damn annoying CHECK ENGINE light burnin' up your dashboard. (And for all you racers concerned with OBD-II's fabled "limp mode", you can put your fears to rest.)

From the catalytic converter, the exhaust gasses go through a bit more pipe and then into a muffler, or system consisting of several mufflers and/or resonators.
Are you a muff?

Exhaust gases leave the engine under extremely high pressure. If we allowed exhaust gasses escape to the atmosphere directly from the exhaust port, you can well imagine how loud and cop-attracting the noise would be. For the same reason gunshots are loud, engine exhaust is loud. Sure, it might be cool to drive around on the street with that testosterone producing, chest-thumping, 150 decibel roar coming from your car? for about 5.3 seconds. (Not 5.2 or 5.4 seconds? 5.3.) Even the gentleman's gentleman has gotta use a muffler, or system of mufflers, on their exhaust.

Again, you may hear a few SPMT's tell you that "Borla mufflers make horsepower!" Or "An engine needs some backpressure to run properly!" Nonsense. A muffler can no more "make" horsepower than Wile E. Coyote can catch roadrunners. Any technician with any dyno experience will tell you that the best mufflers are no mufflers at all!
Types of Muff

Mufflers can take care of the silencing chores by three major methods: Absorption, Restriction, and Reflection. Mufflers can use one method, or all three, to attenuate sound that is not so pleasing to the ears of the Highway Patrol.

The absorption method is probably the least effective at quelling engine roar, but the benefit is that "absorbers" are also best at letting exhaust gas through. Good examples of absorbers are the mufflers found in GReddy BL-series exhausts, DynoMax UltraFlow, and the good old-fashioned Cherry Bomb glasspack.

Absorption mufflers are also the simplest. All of the above named mufflers utilize a simple construction consisting of a perforated tube that goes through a can filled with a packing material, such as fiberglass or steel wool. This is similar to simply punching holes in your exhaust pipe, then wrapping it up with insulation. Neat, huh?

Another trick absorption mufflers use to kill off noise is, well, tricky. For example, the Hooker Aero Chamber muffler is a straight-through design, with a catch. Instead of a simple, perforated tube, there is a chamber inside the muffler that is much larger than the rest of the exhaust pipe. This design abates sound more efficiently than your standard straight-through because when the exhaust gasses enter this large chamber they slow down dramatically. This gives them more time to dwell in the sound insulation, and thus absorb more noise. The large chamber gently tapers back into the smaller size of your exhaust pipe, and the exhaust gasses are sent on their merry way to the tailpipe.
Restriction

Doesn't that word just make your skin crawl? It's right up there in the same league with words like "maim" and "rape".

Obviously, a restrictive muffler doesn't require much engineering expertise, and is almost always the least expensive to manufacture. Thus, we find restrictive mufflers on almost all OEM exhaust systems. We won't waste much time on the restrictive muffler except to say that if you got 'em, you might not want to flaunt 'em.
Reflection

Probably the most sophisticated type of muffler is the reflector. They often utilize absorption principles in conjunction with reflection to make the ultimate high-performance silencer. Remember any of your junior high school math? Specifically, that like numbers cancel each other when on a criss-cross? That's the same principal used by the reflective muffler. Sound is a wave. And when two like waves collide, they will "cancel" each other and leave nothing to call a corpse but a spot of low-grade heat.

There are numerous engineering tricks used in the reflective muffler. Hedman Hedders makes a muffler that looks a lot like a glasspack. In fact, it is a glasspack with a catch. The outer casing is sized just-so, so that high-pitched engine sound (what we deem "noise") is reflected back into the core of the muffler? where those sound waves meet their maker as they slam right into a torrent of more sound waves of like wavelength coming straight from the engine. And, this muffler is packed with a lot of fiberglass to help absorb any straggling noise that might be lagging behind.
The Exhaust Pulse

To gain a more complete understanding of how mufflers and headers do their job, we must be familiar with the dynamics of the exhaust pulse itself. Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.

Keep in mind that for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving. A good Mr. Wizard experiment to illustrate this is a coffee can with the metal ends cut out and replaced with the plastic lids. Cut a hole in one of the lids, point it toward a lit candle and thump on the other plastic lid. What happens? The candle flame jumps, then blows out! The "jump" is caused by the high-pressure bow of the pulse we just created, and the candle goes out because the trailing portion of the pulse doesn't have enough oxygen-containing air to support combustion. Neat, huh?

Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.

Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively "sucking" it along. This is what's so cool about a header. The runners on a header are specifically tuned to allow our exhaust pulses to "line up" and "suck" each other along! Whoa, bet you didn't know that! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don't always exactly line up. Thus, the reason for the Try-Y header, a 4-into-1 header, etc. Most Honda headers are tuned to make the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM. A good 4-into-1 header, such as the ones sold by Gude, are optimal for that high winding horsepower you've always dreamed of. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you'll realize that since stock exhausts are so good at restricting that they'll actually ram the exhaust pulses together and actually make pretty darn good low-end torque! Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses. Also, a header does a pretty good job of additionally "sucking" more exhaust from your combustion chamber, so on the next intake stroke there's lots more fresh air to burn. Think of it this way: At 8,000 RPM, your Integra GS-R is making 280 pulses per second. There's a lot more to be gained by minimizing pumping losses as this busy time than optimizing torque production during the slow season.
General Rules of Thumb with Headers

You will undoubtedly see a variety of headers at your local speed shop. While you won't be able to determine the optimal power range of the headers by eyeballing them, you'll find that in general, the best high-revving horsepower can be had with headers utilizing larger diameter, shorter primary tubes. Headers with smaller, longer primaries will get you
slightly better fuel economy and better street driveability. With four cylinder engines, these are also usually of the Tri-Y design, such as the DC Sports and Lightspeed headers.
Do Mufflers "Make" Horsepower?

The answer, simply, is no. The most efficient mufflers can only employ the same scavenging effect as a header, to help slightly overcome the loss of efficiency introduced into the system as back pressure. But I have yet to see an engine that made more power with a muffler than an open header exhaust. "So," you ask, "what the hell is the best flowing muffler I can buy?"

According to the flowbench, two of the best flowing units you can buy are the Walker Dyno Max and the Cyclone Sonic. They even slightly out flow the straight through designs from HKS and GReddy BL series. Amongst the worst, are the Thrush Turbo and Flow Master mufflers. We'll flow some of the newer mufflers as they become available at our local Chief auto.
Resonators

On your typical cat-back exhaust system, you'll see a couple of bulges in the piping that are apparently mini-mufflers out to help the big muffler that hangs out back. These are called Helmholtz Resonators and are very similar to glasspacks. The main difference is that firstly, there is no sound-absorbing fiberglass or steel wool in a Resonator. And secondly, their main method of silencing is the reflective principle, not absorption. An easy way to tell the difference between a glasspack and a true Helmholtz Resonator is to "ping" one with your finger. A glasspack will make a dull thud, and a true Resonator will make a clear "ping!" sound.
Turbos

Another object that might be sitting in your exhaust flow is a turbine from a turbocharger. If that is the case, we envy you.

Not only that, but turbos introduce a bit of backpressure to your exhaust system, thus making it a bit quieter. All of the typical scavenging rules still apply, but with a twist. Mufflers work really well now! Remember, one of the silencing methods is restriction, and a turbine is just that, a restriction.

This is actually where the term "turbo muffler" is coined. Since a turbine does a pretty good job of silencing, OEM turbo mufflers can do a lot less restricting to quiet things down. Of course, aftermarket manufacturers took advantage of this performance image and branded a lot of their products with the "turbo" name in order to drum up more business from the high performance crowd. We're sad to say that the term "turbo" has been bastardized in this respect, and would like that to serve as a warning. A "turbo" muffler is not necessarily a high-performance muffler.
Pipe Sizing

We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha? NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.

Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 � inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!
Other Rules

A lot of the time, you'll hear someone talking about how much hotter the exhaust system on a turbo car gets than a naturally aspirated car. Well, if you are catching my drift so far, you'll know that this is a bunch of BS. The temperature of exhaust gas is controlled by air/fuel mixture, spark, and cam timing. Not the turbo hanging off the exhaust manifold.

When designing an exhaust system, turbocharged engines follow the same rules as naturally aspirated engines. About the only difference is that the turbo engine will require quite a bit less silencing.

Another thing to keep in mind is that, even though it would be really super cool to get a 4 inch, mandrel bent exhaust system installed under your car, keep in mind that all of that beautiful art work won't do you a bit of good if the piping is so big that it gets punctured as you drag it over a speed bump! A good example of this is the 3 inch, cat back system sold by Thermal Research and Development for the Talon/Laser/Eclipse cars. The piping is too big to follow the stock routing exactly, and instead of going up over the rear suspension control arms, it hangs down below the mechanicals, right there in reach of large rocks! So when designing your Ultimate Exhaust System, do be careful!

Well you can find anything on the internet. 

If it's a modern car/truck   

First off, this thread looks to be long since dead. Secondly, I'm am WAY not an expert on exhaust.....as evidenced by me finding this thread while looking for exhaust mods..... Been doing my reading on CAI's and consensus seems to be that the best "bolt on HP mod" for Avalanche is exhaust.... Having said all that, I read this by somebody supposedly "in the know" and I'm not buying it(as stated):

""""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.""""

Maybe this guy just picked a REALLY bad analogy and I shouldn't pick it apart..... But this is WRONG in my opinion.... If you stuff an opened ended hose in a bucket, it will fill the bucket full of water as fast OR FASTER than a hose with a nozzle on it. So to say "it's moving faster" is wrong. Moving faster at the point of exit maybe, but certainly not moving thru the hose(I.e. exhaust tubes)faster..... Your certainly creating "back pressure" in that hose, but faster? Umm, no. Anybody else see that quote and think the same?

I don't know enough to get in a big ol' exhaust knowledge pissing contest, but that quote from the article COMPLETELY contradicts the article IMO.... So I'm still lost.

Well, I'd say you are both right.

Giving the author you quoted some credit, the water that is exiting the hose is moving faster, at the exit.  But to your point, in that example there is less water flowing through the exit, and hence the flow rate in the hose itself is lower.

But how does the EPA feel?

moosc said:

But how does the EPA feel?

Click to expand...

You been hanging out on the Harley Forums again?


Great research and info provided.

Hanz Groober said:

First off, this thread looks to be long since dead. Secondly, I'm am WAY not an expert on exhaust.....as evidenced by me finding this thread while looking for exhaust mods..... Been doing my reading on CAI's and consensus seems to be that the best "bolt on HP mod" for Avalanche is exhaust.... Having said all that, I read this by somebody supposedly "in the know" and I'm not buying it(as stated):

""""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.""""

Maybe this guy just picked a REALLY bad analogy and I shouldn't pick it apart..... But this is WRONG in my opinion.... If you stuff an opened ended hose in a bucket, it will fill the bucket full of water as fast OR FASTER than a hose with a nozzle on it. So to say "it's moving faster" is wrong. Moving faster at the point of exit maybe, but certainly not moving thru the hose(I.e. exhaust tubes)faster..... Your certainly creating "back pressure" in that hose, but faster? Umm, no. Anybody else see that quote and think the same?

I don't know enough to get in a big ol' exhaust knowledge pissing contest, but that quote from the article COMPLETELY contradicts the article IMO.... So I'm still lost.

Click to expand...

  I agree with Hanz Groober.  I get what the guy in the article was saying about the garden hose, but he missed the mark.  An open hose has more flow capacity than one that is in any way restricted...once again the "back pressure" demise.  While the water actually exiting the hose is "faster", there is not as much of it, and it certainly isn't "vacuuming" the water building up behind it.

  The following is a FACT:  If you double the diameter of a pipe, you increase it's capacity four fold.

  No one has ever narrowed the diameter of a pipe so they could stuff more of it into the same space in an attempt to make it's contents flow faster.  Fluid, gas, electricity (wire diameter)...you name it.  A radiator has a series of long, thin tubes in an effort to slow the coolant down to allow more time for cooling in the face of the oncoming air.

  There IS  an exact science to exhaust sizing...and there are many variables to take into consideration.  I have posted this question several times on this board, and no one can answer.  I even gave the spec's of the intake size, the head spec's, cylinder spec's, cam spec's, etc.  No one, on this forum at least, knows the formula.  I have all the confidence in the world that if you know every aspect of the flow, minus one, that you can figure the final piece of the puzzle to make that piece be the most effective it can be.  I also am confident, that you could build an engine with the flow in mind and come up with what diameter intake and exhaust...and every thing in between...needs to be for optimum flow.  Optimum flow equals optimum efficiency equals optimum performance.

ms38w said:

  There IS  an exact science to exhaust sizing...and there are many variables to take into consideration.  I have posted this question several times on this board, and no one can answer.  I even gave the spec's of the intake size, the head spec's, cylinder spec's, cam spec's, etc.  No one, on this forum at least, knows the formula.  I have all the confidence in the world that if you know every aspect of the flow, minus one, that you can figure the final piece of the puzzle to make that piece be the most effective it can be.  I also am confident, that you could build an engine with the flow in mind and come up with what diameter intake and exhaust...and every thing in between...needs to be for optimum flow.  Optimum flow equals optimum efficiency equals optimum performance.

Click to expand...

The problem with applying the exact science is your first need to define exactly what you are shooting for.  It's about like using exact science to define the perfect cam.  There are tradeoffs made and the trades each of us would make would not be the same.

Somewhere around here if I can find it I've got a book with some exact science equations, charts, etc. on this topic that would make your head spin.  Besides optimum exhaust and cam you've got exact science for the intake, and they all play together.

The engineers at GM collectively know a lot more about these exact sciences than we collectively probably ever will, and the trades they make are "safe" ones where many of us like to a bit more sporting with the trades.

Exactly.  The GM experts use the "safe" formula, instead of the "optimum" formula...which actually makes sense for them.  For performance enthusiasts, "optimum" would be the choice...the trade off would differ based on the level of performance and the end goal.  If someone said, this is my goal and what I'm willing to sacrifice to get there...now build me a power/drive trane.  But if someone said, I have A,B and D...what should C be to give me the most performance...we should be able to find "C".  It's basic algebra...with a lot of engineering in the middle 

It's not as simple as you believe it to be.

Yes it is pretty basic to find the optimum given the variables A, B and D as you describe but keep in mind one of those will be RPM.  I don't think you're always going to running your engine the same RPM and because of that it becomes greatly complicated solving your "simple" equation for a wide range of RPM values.

Then you've temperature (air and coolant), humidity, barometric pressure which also aren't constants and for which you'll have to rerun the "simple algebra" equation to cover the variations in those....

I realize it's not "simple".  I'm just saying that there is a definite science that is equatable.  All the variables you listed still come into play.  At any given RPM, temperature, etc, there is still an optimal solution.  The PCM already takes into account barometric pressure, temperature, RPM, etc., making them, in effect, a constant...that is, a controlled variable or a compensatable variable.  That leaves the physical "hardware" that is constant. "A,B and D", if you will.  These would be the things such as the cam, the  crank, the bore and stroke, etc.  If you take the mathematical formula that conclusively determines all the hard, unchanging inputs, then allow the PCM to adjust for the ever changing, non-constants, then you could have the perfect set up based on your needs.  Sure...each of us have different need and goals in mind, but that's where we are able to change the hard constants and let the PCM do the rest with the non-constants. This is where a tuner can input the necessary bits of information into the PCM, and let the computing take it from there.  That is why modern cars are more efficient than those of 40 years ago.  The PCM compensates for the ever changing inputs of our world. 

Fathom this...

  Engine Q, bore of 4.02", stroke of 3.99", head chamber of x and piston dish of y, cam spec of z exhaust of r and intake of h...etc.
  PCM reads air temp at 84*, MAF at m, RPM of R and engine temp of E, etc.

The PCM adjusts fuel, air, spark and timing, taking into account all of the parameters.


Back to the exhaust size...

If you input all of the known variables (hardware, etc)< the exhaust would be one of the known constants> and mirror them into the equation with a given set of non-constants...pick the optimum of each...and use that as a base line. The PCM would adjust to optimize the adjustable parameters.  This would be as close to the "perfect" set up as you could get.  We already know that the PCM will make the adjustments for the non-constants.  We know what the "ideal" conditions are for a given set of performance features.  Just as we know one cam/engine combination is better suited to one goal, we also know one exhaust/engine combination is best suited to one goal.

Now my meds are wearing off and I am loosing lucidity, as you can see from my incoherent ramblings.  You have no idea how many times I had to re-type a word that was either constantly misspelled or just plain jibberish...   

If someone is really serious about building a motor and how the different components affect horsepower and torque, then you spend a few bucks and get one of the dyno simulators. Comp Cams has a few, one that I have is Desktop Dyno. You can impute cams, compression, intakes, exhaust (yes ms38w, even long tube vs shorties ). So you can build a motor to your needs..

About $50.00  http://www.summitracing.com/parts/cca-186011/overview/


Randy

OMG!  It took you long enough, Randy 

ms38w said:

The PCM already takes into account barometric pressure, temperature, RPM, etc., making them, in effect, a constant...that is, a controlled variable or a compensatable variable. 

Click to expand...

The PCM optimizes spark timing and fuel injector firing duration for the amount of airflow it calculates.  In the newer engines it can optimize airflow to a limited extent by deactivating cylinders and variable valve timing, but I don't think you've got that in your truck and I don't have it in either of mine.  In ours the only control the PCM has over airflow is idle airflow and for electronic throttle bodied engines when the rev limit is reached.

The PCM by itself cannot optimize airflow nor make the variables cited a constant.  For instance the PCM cannot make the exhaust pulses into the header a constant frequency over changing RPM.  The only way the PCM could make that a constant is to make the RPMs a constant.

You all are making my head hurt. Best line I heard in awhile,  can't out engineer a engineer.

Why is a flowmaster bad? Ive got a FM 40 series with the 3 inch inlet and dual 2.5 inch outlets that I have dumper before the rear axle but whats wrong with that?

09chevyavy said:

Why is a flowmaster bad? Ive got a FM 40 series with the 3 inch inlet and dual 2.5 inch outlets that I have dumper before the rear axle but whats wrong with that?

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I don't think it is - I've got a flowmaster 70 series on one truck and a 50 series on the other.

Loggie has noted a slight hp increase (measured on dyno) using exhaust cutouts.

Thats what I am thinking, its a lot less restrictive than the stock muffler, just to be sure I cut open my stock one when I took it out and compared it to the "ghosted" picture of the 40 series on flowmasters site and its a ton less restrictive

I had the 50 series on my 2500 dual 3" inlets and dual 2.5" outlets and it rusted out from the inside. A large crack developed (see pic) and it leaked exhaust and sounded crappy.. It was on about 5 years. I switched to Dual stainless Magnaflows..


Randy

NASCAR runs open headers but they only hope the engine lasts 500 miles.

It's an interesting topic: durability vs. performance

I have heard of that, they usually last about 5 years, which is good as I want to go louder at this point but I am thinking I will grow out of it but thats also a good thing.

I'm on my second flowmaster 70, but I'll have had my truck 10 years in March and 267K miles.

Muffler shop suggested a Magnaflow when I replaced a 7 or so year old flowmaster but with the wife happy with the sound I thought I'd do another 7 or so years with the flowmaster.