Back Pressure, too much or not enough?

I did a search which anwsered the majority of questions, but I still cannot determine if an engine (lt1) runs optimally with lots of backpressure or will acheive more power with a free flowing exhaust. For example with my current setup I beleive the flowmasters are restrictive, however I want to go to a straight through muffler (ex. bullets) I have been told by some that If i do this my car will be a dog off the line because of no back pressure, but others tell me it will flow really good and I should see significant gains. This all seems really here nor there, I would like to know the logic in the backpressure debate.

Thanks in advance

Your bottom end loss will be minimal if you could even feel it at all. All engines will benifit from less back pressure. You will see the benifits in the higher rpm range. And when you think of it, what race have you ever been in where you've stayed in the lowest 1/4 of your rpm range for the whole race?

Ken R.

Only two strokes are going to significantly benefit from backpressure

Here is a thread I started a while ago.

It should cover everything you want to know. I highly recommend the book as well.

http://web.camaross.com/forums/showthread.php?t=215779&highlight=Myth

The benefits of small amounts of backpressure are only seen in engines with very large cams with alot of overlap. The danger is overscavenging. If you have a cam with alot of overlap, well-tuned headers, and a well-matched intake, you can actually draw your intake charge out of the exhaust. A little backpressure can help with this situation, but the ideal solution is to modify cam timing.

My point is that with a street LT1 cam and intake, too little backpressure isn't going to hurt you.

I don't mean to step on anyone's post, but I want to get a clarification:

If backpressure is HELPING an engine, there is an error with valve timing,
and/or intake & header dimension.

IOW, a proper system will have zero back pressure but proper length intake/exhaust
dimensions with IVO, IVC, EVO, EVC timed perfectly.

(of course this is limited to a specific RPM window)

Backpressure will exist at some moment in time and must be figured into the
picture.

Tuning the backpressure out of the window of RPM the motor spends most
of the time is the main objective.

i have yet to see a 4stroke engine ever benifit from backpressure. ever.

so long as the conservation of mass laws apply; Mass in = Mass out.
Last I checked, the end goal of all exhuast/intake tuning (heads, cam, rockers, exhaust setup) is to get as much air into the engine. air in = air out (or damn close to, lets assume steady state here)

Although extrodinarily dynamic, its still pretty simple and this rule ALWAYS applies: Its a pump. The goal is to get as much air in and out of it in a given time frame as possible.

Please do not confuse "exhuast tuning" and "backpressure."

but hey, i've been wrong before. maybe i'm missing something.

No you aren't wrong.How many Pro stocks have you seen running full exhaust.

Only reason the "backpressure" helps has nothing to do with the backpressure you changed the secondary length of the exhaust system and the motor didn't like it.

Time to go buy the new Pop Hot Rodding and read the article by Vizard in there on exhaust systems if you still think freaking back pressure makes power anywhere.

Bret

Only reason the "backpressure" helps has nothing to do with the backpressure you changed the secondary length of the exhaust system and the motor didn't like it.

Time to go buy the new Pop Hot Rodding and read the article by Vizard in there on exhaust systems if you still think freaking back pressure makes power anywhere.

Bret
Vizarrd? :irk:

ehh maybe.

I've never seen one individual test so many exhaust systems in my life.... The guy is in his element putting together a bad ass street car exhaust.

I'm not the only guy on here who will reffer you to him, Mindgame, OldSStroker etc...

I wonder if NateH read that article and would like to comment since he is the resident exhaust guy around here.

Bret

Thanks guys, its crystal clear now, zero to 69, that post is what I came up with when i searched before i posted this thread, I am going to install my bullets which is going to give me a straight through exhaust aside from the xpipe. Thanks again for quality posts, (really notice the difference between the advanced tech and general tech) ;)

I would say back pressure hurts power , what you want is velocity. Too big a pipe and you create back pressure , have the right size pipe and you have good ehxaust velocity.

To big a pipe does not create backpressure, it relieves it.

Vizzard rocks. He has a very scientific approach and the guy must live in the dyno cell! As far as back pressure goes, I agree with what's been posted about it being a bad thing, relatively speaking. But you have to keep in mind that the funtion of an exhaust system on a street car is different from a race car, in a variety of ways. Noise suppression and emissions are big considerations. Still, less back pressure = better, but the system will be compromised by having to serve multiple functions besides exhaust scavenging.

If I were doing a high-po NA 4th gen, the exhaust system would get a lot of attention. And eventually, I will get around to doing something else as it is (with boost and N2O). The stock configuration, with a single convoluted pipe running the length of the car, is so far from optimal that it's pitiful. Problem is that there is so little freakin' room to put a better system. So far, I haven't seen one I really like and a custom job would be needed. Big $$$ unless you can skillfully bend and weld tubes, which I cannot.

Rich

With regard to "Large Pipe Diameter releaving backpressure"

I disagree based on the text that I have read, and some real world examples
used to explain exhaust tuning.

There is a nice explanation of this in, "Scientific Design of Exhaust and Intake Systems",
as well as some less complex articles on Magnaflow's website.

Here is a quoted paragraph from their site:

http://www.magnaflow.com/07techtips/faq/question10.asp

It is a fine line to reduce backpressure while maintaining good exhaust velocity. It is not about getting the biggest pipe, it is about getting a more efficient pipe diameter while maintaining exhaust velocity.

http://www.magnaflow.com

With regard to "Large Pipe Diameter releaving backpressure"

I disagree based on the text that I have read, and some real world examples
used to explain exhaust tuning.

There is a nice explanation of this in, "Scientific Design of Exhaust and Intake Systems",
as well as some less complex articles on Magnaflow's website.

Here is a quoted paragraph from their site:

http://www.magnaflow.com/07techtips/faq/question10.asp

It is a fine line to reduce backpressure while maintaining good exhaust velocity. It is not about getting the biggest pipe, it is about getting a more efficient pipe diameter while maintaining exhaust velocity.

http://www.magnaflow.com

I think your post is valid, but sort of off-point. No one is saying the "the best exhaust system is always the one with the least back pressure." More like "whenever possible, back pressure should be eliminated." That's provided it doesn't interfere with some other desirable characteristic of the system. On a race car, the main goal is exhaust scavenging. To do that effectively, some back pressure is inevitable. If the only design criteria was back pressure, race cars wouldn't even have headers. The least back pressure for a given cylinder would be just dumping the exhaust port into the atmosphere.

Rich

from a little differant perspective. i have a 230-236-112 xe esct... long tubes and true duals out the back with bullits. it was so loud the interior was fallling apart sounded bad so i went on a search for something better. i found a flowmaster 430408 i think is the part number. its a single chamber 40 series. has better throttle responce revs smoother and quicker, and is noticably quicker.

i know its not scientific, but its what i experianced. and if the size is an issue this muffler is only 4" tall and 8" wide, i think 10" long case.

"I think your post is valid, but sort of off-point. No one is saying the "the best exhaust system is always the one with the least back pressure." More like "whenever possible, back pressure should be eliminated."

Agree

"That's provided it doesn't interfere with some other desirable characteristic of the system. On a race car, the main goal is exhaust scavenging."

Agree

"To do that effectively, some back pressure is inevitable."

Disagree. Backpressure is inevitable because the pipe dimensions
are static and can only be resonant at one specific RPM, and efficiency
falls off to some degree above and below that specific point.

IOW: The tuned pipe should target the engines peak HP RPM as the
resonant point with dimensions to get best power over a window of
about 2500 RPM.

The most waste removed from the chamber occurs when backpressue is lowest in the exhaust system at that point in time

"If the only design criteria was back pressure, race cars wouldn't even have headers. The least back pressure for a given cylinder would be just dumping the exhaust port into the atmosphere."

Disagree. There must be some sort of length to the port to tune the exhaust
pusles. I would also equate this to a 'pitot tube' for intake tuning which improves
the flow into the runner.

The exhaust gasses exiting the port would also benefit from a tuned length of pipe.

Of course, I'm speaking from retained information, not experience...but I have
to start somewhere!

Looking forward to a friendly technical debate!

To big a pipe does not create backpressure, it relieves it.
To big a pipe and you create backpressure. This is a fact . Example : If you had a 6 cylinder stock and you put in a 3 1/2 dual exhaust system you would lose alot of power due to backpressure created by running that too BIG a pipe. Good exhaust velocity would not be there and the flow would reverse. Part throttle would feel sluggish!

An effective exhaust system should have "negative back pressure" (ie vacuum) during the relevant portion of the combustion cycle (at the end of the exhaust stroke/beginning of the intake stroke). That's why a setup with a (good) exhaust makes more power than no exhaust system. So, back pressure IS good as long as it's negative pressure!

The less positive the pressure seen by the exhaust port during the period of an open exhaust valve the better. To accomplish the most negative/least positive pressure you need an exhaust system. Any exhaust sytem will create back pressure during most of the combustion cycle, that's why I said that some back pressure is inevitable with an exhaust system. You just want the system tuned so that it does no harm (by occuring when the exhuast valve is closed for that particular pipe).

Rich

What some people here are trying to get across is , that you never want "more" back pressure introduced into an exhaust system. Some comments here are completely wrong. Another example of a person not understanding any part of exhaust systems would be if they made a comment "hey , going to a 3 inch dual exhaust setup would be to much because there wouldn't be enough back pressure" (I heard this at exhaust shop) . Now if this person had substituted "exhaust velocity" in place of "back pressure" then he would have been correct.

An engine that has a cam cut with large lobe sepperation would need back pressure to hold the gasses in the comb. chamber, though. Right? How much is just right? Headers w/ cats and H/F "cat-back" ? I have heard an old racer say that he draws a line down the side of his collecter on the headders (open headers, 502) with a croyon or something like it. He takes it for a run down the track and where the line melts is where he cuts the collector...Is that accurate?

As far as the line with the crayon , it is not reliable.

the only time I want extra backpressure in my exhaust is if it caused by an exhaust driven supercharger ;)

To big a pipe and you create backpressure. This is a fact . Example : If you had a 6 cylinder stock and you put in a 3 1/2 dual exhaust system you would lose alot of power due to backpressure created by running that too BIG a pipe. Good exhaust velocity would not be there and the flow would reverse. Part throttle would feel sluggish!

Am I the only one who hasn't entered the twilight zone here?

Backpressure is positive pressure created in the exhaust system. A small pipe is very restrictive, thus causing pressure buildup in the header making it more difficult to pump the exhaust out the engine.

Build a 722 cid big block and install a single 2" exhaust and the pressure buildup in the header would be huge. Thus, too small a pipe creates backpressure. Install a big pipe, and the backpressure goes away.

Mike

Am I the only one who hasn't entered the twilight zone here?

Backpressure is positive pressure created in the exhaust system. A small pipe is very restrictive, thus causing pressure buildup in the header making it more difficult to pump the exhaust out the engine.

Build a 722 cid big block and install a single 2" exhaust and the pressure buildup in the header would be huge. Thus, too small a pipe creates backpressure. Install a big pipe, and the backpressure goes away.

Mike
I never said a to small exhaust pipe size could not choke a high hp engine. Once again what I am saying is that too big a pipe can and will hurt performance. The key words are "TO BIG". If you get the optimum pipe size it will have good exhaust velocity .

Am I the only one who hasn't entered the twilight zone here?

Mike
I think the problem is that the discussion is using a generic term like "exhaust system", without looking at the function of each discrete part of the total system... primary, collector, separation and tailpipe. Each has its own function and design parameters. Once you get past the collector, its all about minimizing pressure losses in the system. Maintaining velocity is no longer an issue. I think Rich put it best.

Let's get everyone on the same page. It seems like we're all trying to get
this across in our own analogy and nobody is understanding.

This is a summary of what I have taken from the text, "Scientific Design of
Exhaust and Intake Systems"

Instant in time A:

Exhaust stroke, 5000 RPM. Exhaust valve is fully open. Intake valve is fully
closed. Piston is about midway up the exhaust stroke.

Exhaust gas is being forced into the header primary at a certain velocity.

Exhaust gas is forward moving via positive piston/cylinder pressure.
A pressure wave is created equal in length to the period of time the exhaust
valve is open (duration @ 5000 RPM)

The pressure at the exhaust port is - 5 PSI.

The pressure behind the intake valve (closed) is 2 PSI

The pressure in the cylinder is + 50 PSI


Instant in time B

Overlap period, 5000 RPM. Exhaust valve is partially open. Intake valve is partially open. Piston is at TDC and virtually stopped (for a period of time
@ 5000 RPM)

Exhaust gas continues to move forward under it's own kinetic energy, but
has slowed down due to less help from the piston.

The cylinder pressure has decrease to possibly - 2 PSI from low pressure
area created by the exiting exhaust gas.

Scavenging is now happening

The intake charge begins to move into the cylinder (from high pressure area
to low pressure area)

The pressure at the exhaust port is - 3 PSI.

The pressure at the intake port is 4 PSI

Instant in time C

Intake stroke, 5000 RPM. Exhaust valve is fully closed. Intake valve is fully
open. Piston is mid way down the bore.

Exhaust gas continues to move forward under it's own kinetic energy, but
has slowed becasue of a closed exhaust valve (possibly creating a vacuum effect)

The cylinder pressure has decrease to possibly - 40 PSI from low pressure
area created by piston moving toward BDC.

The intake charge begins to move into the cylinder (from high pressure area
to low pressure area) at an increased rate. The intake port pressure may be
-10 PSI

A pressure wave is now being created in the intake port from the valve open
duration @ 5000 RPM

Repeat the above holding the same RPM
If the intake runners and exhaust primaries are at a tuned length, the reflected
pressure waves will be in phase with the next cycle's pressure wave to increase
cylinder filling.

If the intake runners and exhaust primaries are not at a tuned length for
5000 RPM, OR the RPM changes, reflected pressure waves will be
out of phase to a certain degree which will reduce cylinder filling.

Correct me where I'm wrong.

zero', post 29,
define 'tuned lenght'.

My understanding is, the primary pipes of any
'normal type' header is much too short,
30 inches, not even close.

30 inches, might be OK for a 1 cid model aircraft motor.

"you can NOT have too little amount of backpressure

the less the better
low backpressure will create low end torque as well as high rpm power

if you have something that will prove me wrong I would please like to see so. again just think of the meaning of word. from just the way the word is alone it is stating that you are putting pressure against the flow of the exhaust. how is putting pressure that your motor has to overcome to give you more low end torque
ideal would be a vac inside your exhaust ( that would have no pressure at all ) and with that it would help reduce pumping loss from the motor trying to force all your exhaust gas out. and also would help suck out a lot more of your exhaust which would create a high volumetric efficiency.
when you induce backpressure into the exhaust you are doing nothing more then making everything fight.
problems that come from backpressure is lower VE, pumping loss, high exhaust temps, high motor temps, more stress on your motor (small as it might be) and I am sure there are other things
just remember backpressure does not create a high velocity in the exhaust, and there is a difference between backpressure and velocity.
though by putting huge pipes on your car that is not really going to help you out either, it is not due to lack of backpressure though. what will go on if you put a large pipe on the car without the flow to back it up then the exhaust gas doesn't really have the walls of the pipe holding it into a controlled flow pattern instead the gas will flow around and have eddies in the system and try to turn back itself. if you get the right size pipes for your car you will reduce as much backpressure as low as you can but also keep the exhaust in check by not letting it just wander around but keep it all flowing in the right direction
and if backpressure was something that helped performance wouldn't ppl try to put something to create backpressure inside there intake also? instead ppl try to open up the intake, port it out, polish it make everything as smooth as they can to reduce the force the motor has to give to pull the air in. same thing on the exhaust side." QUOTE

This quote makes alot of sense. :)

*gulp*

You're asking a novice to define tuned length amongst a group of skilled engine
tuners and engineers? :eek:

I'll give it my best shot using supported links and examples:

When valves open and close they create a pressure pulse of a certain length
based on the frequency of the valve opening/closing over time.

As RPM increases, frequencies of the valve opening/closing events increase as well.

The wavelength of the pulses can be calculated using complex formulas.
Temperature, atmospheric pressure, speed of sound within a tube, etc.
would all become a factor to determine the wavelength.

I have no idea how this is formulated, so I simply use a standard formula
which assumes perfect conditions.

Here's a simple calculator:
http://www.sengpielaudio.com/calculator-wavelength.htm

Frequency__Wavelength___ 1/4 Wavelength
200 Hz______ 5.65 ft________ 1.41 ft
100 Hz_____ 11.30 ft________ 2.83 ft
_80 Hz_____ 14.13 ft________ 3.53 ft
_60 Hz_____ 18.83 ft________ 4.81 ft

It is my understanding that engines exceeding 5000 RPM can produce frequencies
of 200Hz, or higher which allow exhaust pipe lengths of full, half and 1/4
wave lengths to be used.

If the pressure/sound wave is extremely long, and not practical to build
into an intake/exhaust system, you can use a 1/2 wavelength, or 1/4 wave
length tuned runner to achieve similar results.

If the reflected waves mix with the new incoming waves and are in phase,
or mix such that there is a gain, the resulting pressure wave will be stronger.

Use this link to see how phase relationships with two pulses can mix:
http://www.ling.udel.edu/idsardi/253/sinewave/

I am basing most of my theory on speaker port tuning and audio which I know
more about than exhaust tuning. I'm banking on the fact that audio tuning
is damn near close to exhaust/intake tuning because we're still dealing with
air pressure and sound.

http://www.hpt-sport.com/tunedpip.htm

http://headerdesign.com/extras/design.asp

(search for paragraph titled "The Exhaust Pulse"
http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

What some people here are trying to get across is , that you never want "more" back pressure introduced into an exhaust system. Some comments here are completely wrong. Another example of a person not understanding any part of exhaust systems would be if they made a comment "hey , going to a 3 inch dual exhaust setup would be to much because there wouldn't be enough back pressure" (I heard this at exhaust shop) . Now if this person had substituted "exhaust velocity" in place of "back pressure" then he would have been correct.
flow is driven due to a pressure differential.
The idea in a nutshell is that the previous exhuast pulse pulls away fast and creates a negative pressure zone behind it.
The faster the exhuast pulse pulls away, the HIGHER your back pressure. Pressure = (V^2/2g)(specific weight)

with too big of a pipe, you will not have a negative pressure, and it will be close to atmoshere.
again, Pressure differential = V^2/2g (specific weight)

Its the same reason why a huge intake runner on a small engine will fail until very very high rpm, and even then it can be not as effective.

Also, keep in mind that all 2/3/4/5/6/8/10/12 (however many cylinders) pulses from the engine have to work together. This usually means that they have to meet in the collecter in a manner to support each other. Broken down again, this means not colliding with each other but one exhaust pulse pulling another one.
To do this, the obvious answer might be "equal length!" but no, not at all. Equal radii are faaar more important. Check out some basic head losses in a fluids text book. the losses due to length are mooy's number* length/diamter- (check out moody's diagram is usually VERY small number) in a system around 20'' with multiple bends and intersection when compared to geometric losses (bends, twists, change of size, intersections etc).Air, even more so than most liquids, HATES bending. it goes all disort fairly quickly.


getting a bit more complex, as far as i know, back pressure pressure doesn't equal to a restiction, which i should have put in my original post on page one.

For example- wrapping your headers will create more backpressure, but no more restiction bringing us full scale to what mr. bernuilli said.
Backpressure/SW = V^2/2g + Change in elevation + head losses (effectively, all restrictions)

Flowrate (v) = V*cross sectional area. as you can tell, you want the highest backpressure without adding restrictions. High velocity is where its all at.

- thats as far as I can see, please feel free to correct.

I'm not in a position to correct anyone's post, but I have a challenge to the
following:

"The faster the exhuast pulse pulls away, the HIGHER your back pressure. Pressure = (V^2/2g)(specific weight)"

Bernoulli also said a fast moving fluid (gas/fluid in motion) in a tube, will also
create a low pressure in the tube.

From this theory, I can infer that a fast moving pulse will create low pressure
resulting in a lower "backpressure".

Keeing in mind, the pressure throughout the pipe will be different because
we are dealing with a pulse, and not a steady flow.

For instance at the end of the pipe the pressure may be +10 PSI, the middle
of the pipe may be +5 PSI and the entry of the pipe may be +1 PSI along the
same pressure wave.

"Flowrate (v) = V*cross sectional area. as you can tell, you want the highest backpressure without adding restrictions. High velocity is where its all at."

I feel this statement is contradicting itself. With backpressure, you impede
velocity.

It is our job to time the pulses to arrive at their respective valves at open
to ensure a low pressure area for exhaust, and a high pressure ramming
effect at the intake valve.

Using a specific length of pipe at a steady RPM, the valve timing can be
matched to open the valve to receive the reflected pulse.

For those that don't believe that subtle changes in exhaust affect performance,
ask all of those people that have posted here why they lose torque when
the pipes are dropped from the collectors and back.

pressure differentials are what DRIVE flow. more pressure differential = more velocity/flow (since Area is constant)

restrictions will drop the velocity are "turn it into" pressure. think sticking your hand out the window at 70mph. Stagnation pressure I beleive is the word.

pressure differentials are what DRIVE flow. more pressure differential = more velocity/flow (since Area is constant)

I have no problem understanding that.

IE:

Intake runner pressure = 5 PSI

Cyilnder chamber pressure = - 10 PSI

Flow moves from intake runner to chamber



Intake runner pressure = 5 PSI

Cyilnder chamber pressure = - 20 PSI

Flow moves from intake runner to chamber with an increase in cylinder filling



Intake runner pressure = 5 PSI

Cyilnder chamber pressure = 30 PSI

Flow moves from chamber to intake runner (reversion)



It`s the back pressure statement in your previous post that I`m questioning.

backpressure, as far as i know, is stagnation pressure.

Stagnation according to one source:

"The stagnation or total pressure, p_0, is the pressure measured at the point where the fluid comes to rest. It is the highest pressure found anywhere in the flowfield, and it occurs at the stagnation point. It is the sum of the static pressure (p_0), and the dynamic pressure measured far upstream. It is called the dynamic pressure because it arises from the motion of the fluid."


If backpressure is due to stagnate flow of the gasses exiting the exhaust
system, I can understand how this term applies.

The dynamic changes in pressure within the exhaust tube are meeting
atmospheric pressure causing the exhaust gas to slow down upon exiting
creating backpressure.

By another definitoin, stagnate pressure occurs once a fluid hits a surface
and comes to rest.

Stagnate fluid/gas according to one source:

"cease to flow; stand without moving; "Stagnating waters"; "blood stagnates in the capillaries"


"The fluid along the dividing, or "stagnation streamline'' slows down and eventually comes to rest without deflection at the stagnation point."

The above definitions to the best of my knowledge cannot occur in an open
ended dynamic environemt (exhaust tube). This would imply that there is a constant blockage of
flow due to a surface obstruction. The above definiton also implies that
there is no reflection, compression, or rarefaction of the gas pulses.

With a muffler in place, I can see where the fluid along the dividing, or "stagnation streamline'' slows down and eventually comes to rest without deflection at the stagnation point where the gas flow meets a baffle in a
muffler.

Even then, there is a reflection and continued flow around the baffle.

In either case, how can either scenario improve gas velocity in a tube?
That is my question and challenge.

I may be misunderstanding stagnate to mean a fixed static pressure, as opposed
to a dynamic impedence to flow?

Hopefully somebody with supporting information can help us out.

zero_to_69, I have LE2 heads, and le1 cam jet hot long tubes and a 3" x-pipe with borla xr1 straight through mufflers. What do you think about it?

Oh my gawd! Somebody check hell, it must have frozen over; people are asking me for opinions! :D

I have no clue about the specs on the LEx components, but I've read good
things about them. Seems like you have a nice free flowing exhaust to support
the cam and heads.

Ok, so after all the info provided we all agree that backpressure is not something you want in an exhaust , right?

There goes another quote that explains it a bit more
"Scavenging has to do with exhaust gas velocity. Momentum of moving exhaust pulses help to pull the gasses out of the engine. Since momentum is mass x velocity, high velocity is good. This is where the confusion comes in. High velocity comes from smaller exhaust tubing. So, assuming smaller tubes could support the required flow, they are good. However, small tubes can't flow the same volume as easily as larger tubes (frictional losses, etc). So, we need to be careful not to confuse velocity with backpressure. Yes, small tubes create both. Velocity is good, but backpressure is bad. Actually, I'd say backpressure is worse than velocity is good, if I have to pick sides. Ideally, you'd want the smallest tubing that will support the flow that you need. Not oversized (reduced velocity/momentum) and not undersized (can't flow enough"

I believe that quote to be refering to header primaries, not from the collector back.

Oh my gawd! Somebody check hell, it must have frozen over; people are asking me for opinions! :D

I have no clue about the specs on the LEx components, but I've read good
things about them. Seems like you have a nice free flowing exhaust to support
the cam and heads.


since we are getting technical, its my opinion (not that anyone asked) that hooker/jethot headers suck. nowhere near equal length (not that its important), all kinds of different radii bends and different number of them. its all crazy!

even for trucks where packaging isn't a real issue, that problem persists:(

I believe that quote to be refering to header primaries, not from the collector back.
Believe me , when he is talking about tube size and maintaining velicity he is talking about the collector back.

That's a bunch of crap then. There's such a hodge podge of pulses from the collector back, that you could never organize them such that the decelerating gasses from one pulse causes low pressure to help draw out another. Think about the firing order: 1-8-4-3-6-5-7-2. That gives you L-R-R-L-R-L-L-R, so the pulses in one collector are not at all evenly spaced. It would be impossible to size an exhaust system to organize these such that they would tune properly to increase scavenging.

Now. . . the header primary is a whole different story. Since the pulses in the tube are very well organized (originating from one cylinder only), it is easy to tune the length and diameter to increase scavenging effect.

Besides all that, he says in the quote "small tubes", which lead me to believe he's referring to primary header tubes, not exhaust sytem piping.

Mike

since we are getting technical, its my opinion (not that anyone asked) that hooker/jethot headers suck. nowhere near equal length (not that its important), all kinds of different radii bends and different number of them. its all crazy!

Aren't headers supposed to "suck"?

Probably not what you meant. :)

That's a bunch of crap then. There's such a hodge podge of pulses from the collector back, that you could never organize them such that the decelerating gasses from one pulse causes low pressure to help draw out another. Think about the firing order: 1-8-4-3-6-5-7-2. That gives you L-R-R-L-R-L-L-R, so the pulses in one collector are not at all evenly spaced. It would be impossible to size an exhaust system to organize these such that they would tune properly to increase scavenging.

Now. . . the header primary is a whole different story. Since the pulses in the tube are very well organized (originating from one cylinder only), it is easy to tune the length and diameter to increase scavenging effect.

Besides all that, he says in the quote "small tubes", which lead me to believe he's referring to primary header tubes, not exhaust sytem piping.

Mike
Believe what you want but that is the way an exhaust works! If you have a good explaination(believable atleast) please offer it !

Bringing some more info so we all can learn! ;)
QUOTE-----"Erase your brain and start over. RX7Speed is correct. Read his posts and learn.

There are a few fundamental ideas you need to learn.

1. How does air flow? Air flows when there is a pressure differential. It flows from a high pressure region to a low pressure region. Temperature changes play a role here but I don't want to get into that.

2. What is a restriction? A restriction is anything that impedes the flow of air. This includes anything that changes the direction of air flow such as pipe bends, muffler baffles, o2 sensors in the flow path, pipe size changes, unions, joints, etc.

3. What is “backpressure?” It is a concept and a concept ONLY! It is conceptually any form of pressure that impedes the flow of air from the engine cylinder to the tail pipe. Personally, I hate the term backpressure. In all my years of studies in engineering, including 7 courses in fluid dynamics, I never once heard the term backpressure.

Imagine for a moment you’re in the engine cylinder. The piston moves up and pushes the air out of the exhaust valve. This creates a high pressure region at the exhaust valve. Air will flow to a low pressure region. This happens to be the tail pipe. Any restrictions in the exhaust system will impede the flow of air from the exhaust valve to the tail pipe, effectively creating “backpressure” in the exhaust system. This backpressure works against the air flow. ANYTHING YOU DO TO INCREASE BACKPRESSURE WILL DECREASE FLOW RATE and decreased flow rate will reduce your power output. Additionally, the interaction of the air streams from each header tube as they merge into the exhaust pipes has an effect on the air flow.

The purpose of an exhaust system is to purge air from the cylinders. The idea is to maximize air flow from the engine to the tail pipe. Different engines move different quantities of air at different engine speeds. The quantity of air you need to move determines the capacity of the exhaust system (pipe diameter). The time you have available to purge air from the cylinder determines the design of the header tubes. Keep in mind that the time available to purge air from the cylinders is dependent upon the camshaft profile and the engine speed. The smaller the cam the less time the valve is open. The higher the engine speed the less time the valve is open.

A street engine doesn’t move much air. In order to keep the air flow rate high enough to purge the cylinder in time, a small diameter header tube is employed (1-5/8). The rest of the exhaust system is sized appropriately for the volume rate of air that the engine flows. Again, anything you do to increase backpressure will decrease flow rate and decreasing flow rate will reduce your power output. Putting 3” duals on an ordinary engine is complete overkill because the engine doesn’t need that must flow capacity. In fact, due to the dynamics of air flow it’s actually hurting performance. But that’s too complex to describe here. A race engine moves a lot of air and it does it at high rpm. The shear volume rate of air needed to be moved increases the size requirement for the exhaust system. The header tubes will have to be increased in size but it’s still important to keep them small enough to efficiently purge the engine cylinder AT THE RPM THAT THE ENGINE WILL BE USED. Everything has to be sized appropriately. Bigger is not better. Small is not better. Getting it right is better" END OF QUOTE

I think the point you're missing here is the importance of scavenging. The objective is not simply to "purge air from the cylinders" (or do you mean exhaust gasses?), its to remove the exhaust gasses and help draw in the fresh air charge. The smaller tube primary increases pressure losses, but its a tradeoff because of the affects of pulse flow and the ability to tune the reflective pulse for the RPM range you want to maximize.

This is why you need to break the exhaust "system" down into at least four separate sub-systems. The design criteria for each subsystem.... primary, collector, separation and tail pipe... are different. Pulse tuning in the primary and collector is a proven benefit. High velocity has a purpose. But once you reach the optimum (for your particular) length of the collector, you've done all you can do with pulse tuning. That's why you want a physical separation at the end of the collector. To establish the effective length of the collector. After the separation, the "tail pipe" part is simply a matter of moving the exhaust gasses from one point to another... less pressure loss is better. Getting rid of that part completely is even better.

Again, to repeat what I posted a couple pages ago, people are speaking too genericaly of a system as a whole, rather than looking at the specific function of each part of the system.

I agree scavenging plays a critical part in an "exhaust system".

If you have a good explaination(believable atleast) please offer it !

I thought I just did. . . :irk:

If you want another explanation, just read Injuneer's post. The primary tube and collector work to tune the pulses, where "velocity [momentum] has a purpose", causing "scavenging."

But, "the tail pipe part is simply a matter of moving the exhaust gasses from one point to another... less pressure loss is better. Getting rid of that part completely is even better."

I just stated that scavenging is critical(important) so what's left to argue(discuss) :confused: ?

That the scavenging is caused by / set up by the header primary and, to a lesser extent, the collector, and not the collector-back exhaust system.

That the scavenging is caused by / set up by the header primary and, to a lesser extent, the collector, and not the collector-back exhaust system.
:shame: Oh, ok I get it now. I gave the impression that the "collector back exhaust system" had a critical part in scavening. Went back and read my replies and now I understand where I led you to believe that I believed that :D . and yes injuneer gave a good explaination.

One question so I know that we are all kind of on the same page. We all agree that you don't want to introduce backpressure in the collector back exhaust system right? Needing backpressure is a MYTH. :death:

I agree with that, except in certain rare cases.

For instance, a local engine builder built a Hemi that had a cam with alot of overlap. Due to a very efficient header and head design, it would over-scavenge, that is, draw air and fuel through the chamber and out the exhaust. Backpressure would probably help this engine. However, getting a camshaft with less overlap is the "right" solution.

Also, certain supercharged cars with too much overlap can use a little backpressure to prevent blowing air and fuel out the exhaust during the overlap period.

Mike

Ok, so after all the info provided we all agree that backpressure is not something you want in an exhaust , right?


you dont want RESTRICTIONS.

this thread is great