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Discussion Starter #1
Starting off...
http://www.rx8club.com/showthread.php?t=73332&page=1
http://www.uucmotorwerks.com/html_product/...etorquemyth.htm
http://forum.mazda6tech.com/about3061.html...ht=backpressure

Recently posted by flamearrow!

http://www.magnaflow.com/05news/magazine/05sportc.asp

http://www.isuzuperformance.com/isupage/tech/exhaust.html
http://www.cobbtuning.com/info/?ID=3222
http://www.isuzuperformance.com/isupage/tech/intake.html

http://www.nissanperformancemag.com/june02...ofthenerds1.php
http://www.nissanperformancemag.com/july02...ofthenerds2.php
http://www.nissanperformancemag.com/august02/revenge_nerds/
http://www.nissanperformancemag.com/november02/nerds/
http://www.nissanperformancemag.com/december02/nerds/



Terms to google search...

Backpressure
Exhaust Sizing
helmholtz resonance
Sound Waves
Exhaust Pulse/Overlap

Quotes and posts.

Two things. Backpressure is always bad. ALWAYS.

People always confuse having too little backpressure as being a negative..but thats not the negative. The negative is having too little exhaust velocity.

If you increase pipe diameter too much, you lose exhaust velocity, which results in less of a scavenging effect.

In a RWD vehicle, the exhaust pulses have a spot where they overlap. If you don't place a H or X junction in this area, you'll lose velocity as the pulses aren't given the ability to overlap with each other (think waves of energy). This is pretty difficult to do with a FWD vehicle, as the engine is transverse mounted, and getting equal length piping is almost impossible. Thus you usually end up with two seperate pulse junctions instead of just one.

In terms of efficency, most of the contour owners find that a single 2.5 mandrel system setup, or a dual 2.0 mandrel system (Internal Diameter), work best with the 3.0 duratec system.[/b]
here's a simplified explanation of whats going on.

Pistons are moving up in down in a 4 stroke engine. On the exhaust stroke, the results of combustion are pushed out of the cylinder, into the cylinder head, and out the exhaust valve and exhaust port.

At this point the exhaust is incredibly hot...sometimes in excess of 1000F...its also got alot of velocity. The hot gas wants to escape, and it wants to escape NOW. The velocity of the gas is determined by the amount of force initally expelled, and the amount of volume that gas has to fill.

A small restrictive pipe would immediately fill with exhaust, and move at a considerable velocity, but excess gas would be left behind, which in the worst case scenario would actually remain in the combustion chamber and mix with the incoming fresh air, reducing power.

A large pipe would immediately absorb all the exhaust volume, but drop velocity to the extent that scavenging doesn't occur. Exhaust would still flow out, but would not be as efficent as it could be.

If the piping size is just right, the velocity is high enough so that it creates a suction effect at the cylinder head, and literarly sucks the exhaust out of the cylinder. This makes it easier for the piston to move up, and recovers a good amount of previously wasted energy.

Because there are more then one piston, you also get a pulse overlap, as each exhaust stroke adds its velocity to the previous exhaust stroke. Getting both banks to overlap can greatly increase velocity and aid in scavenging effects.

Maintaining velocity is a factor of keeping pipe length short (removing unneeded bends), keeping pipe diameter consistent (mandrel bends), and keeping the exhaust from bleeding away heat (ceramic coating).[/b]
Sizing an exhaust is all about tuning for a specific rpm. Too large an exhaust will result in too little exhaust velocity in lower rpms, which prevents effective scavenging, and will result in a loss of low end torque. Too small an exhaust restricts the engines ability to vent exhaust gases at higher rpms, and results in a loss of power, and possible exhaust gas left in the cylinders (after the exhaust stroke ie. mixing with intake air). [/b]
Definition of Backpressure
Restrictions in the exhaust system that slow the exit of exhaust gases from the combustion chamber.[/b]
(Edit: Updated Links: No Dead links)
 

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I just learned a few useful things about exhausts.

Awesome finds. Kudos crossbow.
 

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Wasn't trying to "duel", Cross--I was serious. You should work for a race team or write a book.

But here's an analysis of what the Magnaflow article said:
"Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters [small relative to WHAT? What calculations are involved?]. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all."

Sounds like all I have to do is take the TWO mufflers off my 6 and I'll have more Hp/Tq because the exhaust flow "velocity" would be faster and less restricted. Now all I have to do is to wait for someone else here to say "You idiot! You are actually LOSING hp/tq by removing those mufflers!" Talk about busting myths...

I'm just curious what kinds of sensors/tools/calculations are employed in figuring out what is the perfect pipe for a certain engine. And I wonder if some place like Magnaflow actually does those measurements and calculations, or if they just bend some bigger pipes and louder mufflers for sales and then simply claim that you get more HP/TQ. Seems the best approach is to do the math first, but in this hyped up aftermarket world you don't often see a lot of data. It seems like everyone is saying "The only way to find out is to buy it, install it, then dyno it." Doesn't give me a whole lot of confidence.
 

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In a RWD vehicle, the exhaust pulses have a spot where they overlap. If you don't place a H or X junction in this area, you'll lose velocity as the pulses aren't given the ability to overlap with each other (think waves of energy). This is pretty difficult to do with a FWD vehicle, as the engine is transverse mounted, and getting equal length piping is almost impossible. Thus you usually end up with two seperate pulse junctions instead of just one.
that's not entirely true!

just because a vehicle is RWD doesn't mean it has split banks and true duals to deal with :D

some RWD vehicles are 4-cyl, V-6 or inline-6 which have similar exhausts to our own
 

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wickedfast,

Pipe sizing is not an exact science for a local tuner. Especially since cost of parts and off the shelf parts availability are limited. So basically you have to work with what you got. So your typical pipe diameters for catbacks are 2", 2 1/4", 2 1/2" etc... You can see Sport Compact Car (originator of the Build Your Own Exhaust articel) gives a rough guideline on catback sizing dependent on engine displacement.

http://www.magnaflow.com/05news/magazine/05sportcpg03.asp
1,500cc-2,000cc engines : 2-inch
2,100cc-2,500cc engines : 2.25-inch
2,600cc-3,000cc engines : 2.5-inch

But what is failed to be mentioned is that a lot of the exhaust velocity is lost up front in the exhaust manifolds, and catbacks complement headers output and not the other way around.
 

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Yay for more links of knowledge!

The bottom line is that equal length headers rock, and the RPM they're tuned for depend on the diameter and length. Speaker (subwoofer) modeling software works great for understanding pressure waves.
 

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Discussion Starter #8
Sorry wickedfast. I blame lack of coffee! (Look at that I'm apologizing...maybe someone secretly replaced my mazda 6 with an evo IX....)

You have a 6s. Your exhaust looks like this.


A few blinks and you'll see the problem. The japanese engineers figured they'd have a good laugh at the US's expense. There are more bends and twists in the first few feet of our exhaust then turns in the nuerburgring! (Ok thats exaggerating slightly! Regardless most people get sick just looking at that).

In this type of situation, modifying the catback or mufflers will provide little or no gain, because all of the restriction is further upstream. This is verified by dyno's, as no one besides the various exhaust manufacturers have shown any gain whatsoever with any of the available catbacks on the market. (on owner dynos).

If anything, some of the catbacks are actually reducing overall power, because they're slowing down what little exhaust velocity there is, by the time it gets done going through the twists and honeycombs of doom.

Now if you were to replace the exhaust manifolds and the restrictive (and dangerous) pre-cats with the wagner or lovely CP-E manifolds, then you could start working on the exhaust velocity of the rest of the system. This is assuming that the catback is even a restriction to begin with. Its a single pipe, with two mufflers handling the exhaust streams. Not exactly restrictive. Wagner saw zero gain on the dyno on the 6i headers/ypipe and with/without the stock catback or wagner catback. I'd say if anything the catback section of the car is actually fairly well designed in terms of bends and restrictions.

Picking an exhaust size for a particular engine...tough. Two ways to do it. Wait till a bunch of other people dyno their car with tons of different exhaust sizes, write down the info, and then purchase the best setup for your application. (http://www.contour.org)

Or... Calculate the maximum volume displacement at the particular rpm you are making maximum power at. Match the exhaust size up to a similar volumetric displacement. The higher the rpm, the less velocity you'll have at lower rpms, which will result in a loss of low end power. Conversely you can setup the exhaust for max flow at your torque peak, and get a completely different effect.

Toadster, the comment on the RWD vehicles was referring to V style configurations. Aka V6, 8, 12, 24, 32, 128. Etc :)
 

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Originally posted by da_fatman@Oct 14 2005, 01:13 PM
Anybody offering the Cliff's notes? God I'm lazy.  :nana:
[snapback]533837[/snapback]​

:wstupid:
 

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Originally posted by crossbow@Oct 14 2005, 10:15 PM
If anything, some of the catbacks are actually reducing overall power, because they're slowing down what little exhaust velocity there is, by the time it gets done going through the twists and honeycombs of doom.
[snapback]534052[/snapback]​
Oh well, a small price to pay to make the car look better.

From this:


To this:


:)
 

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Discussion Starter #11
This is an old thread, which discusses the technology and importance of proper exhaust design. I'm bumping it because of some conversations which have come up recently, for those who might want to learn more information.

Please look at the first few posts (up to the exhaust diagrams).

This information applies to all cars, whether they use forced induction, naturally aspirated, or lawn mower engines.
 

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Discussion Starter #12
Quotes from 84fordman.

A hell of alot math and science. I asked a similar question awhile back and someone told me to use the Helmholtz resonance theory. Here is a javascript Helmholtz calculator:

http://home.new.rr.com/trumpetb/audio/fboxjs.html

but you will need to know the volume of the pipe (Requires Excel):

http://pages.prodigy.net/rich_demartile/files/pipe-vol.xls

and you may want to switch it to metric and use this gallons to liters conversion calculator, then switch back to English measurements.

http://www.nomoreodor.com/gallons_to_liters_conversion.htm

I used 9 feet or 108 inches of pipe as a reference and at 2" pipe it says 4.2. Don't exactly know what that means, but I did find this tidbit as well.

"Back to exhaust. Think about how the 4 stroke engine works for a
moment. The exhaust valve is opened somewhere before BDC on the power
stroke and stays open until sometime after TDC on the exhaust stroke.
It stays open past TDC because the exhaust gas has inertia and so that
the exhaust system can continue to scavenge after TDC. Meanwhile the
intake opens a little before TDC on the exhaust stroke and stays open
awhile after BDC on the intake stroke. The period during which both
valves are open at the same time is called the overlap period. It opens
early to allow any vacuum created by the exhaust system to start the
intake flow early and it stays open to allow the ram effect of the
intake system to push additional charge in after BDC. Let's look at
some real numbers. These numbers are for the cam I use in my turbo
engines:

Intake opens 10 deg BTDC
closes 46 deg ABDC
Exhaust opens 45 deg ATDC
closes 1 deg ATDC
overlap 11 deg

The only time "overscavenging" could occur would be during the 11
degrees of overlap where there is a direct open path between the intake
and exhaust - 11 degrees in this instance. But overscavenging could
occur only if the headers were operating - returning scavenging pulses
at the proper instant - and that only happens in the speed range the
headers are designed for. At that speed, the inertia of the gas streams
prevents it. At any other speeds, overscavenging isn't possible because
the headers are not working. What DOES happen in engines with high
overlap at low speed is the bacpressure in the exhaust system pushes
exhaust back through the exhaust into the chamber and ultimately back
out the intake. This dilution effect is one of the things that makes a
high overlap cammed engine idle roughly and load up. The other is the
aforementioned reverse flow in the intake.

Now consider the ideal situation. In a perfect world the headers would
return a perfect vacuum pulse just as the exhaust valve closes. This
would leave nothing in the cylinder to dilute the incoming intake charge.
Of course, we can't achieve the ideal but properly designed headers can
return a substantial vacuum pulse to the cylinder."

He also recommends resonators of about 9" long immediately after the collector ends to terminate sound waves that produce that "boominess" or "ringing" common with headers.

Found this from Sport Compact Car:


"When the exhaust valve opens, a high-pressure pulse of hot, expanding exhaust gas travels down the exhaust port at approximately 300 feet per second. This wave of hot, moving, high pressure gas has mass and inertia of its own which pulls a suction or a low pressure rarefaction behind the pulse.

Depending on the engine, the pulse can have a positive pressure of anywhere from 5 to 15 psi with the low pressure rarefaction behind the pulse being anywhere from 1 to 5 psi of negative pressure. As this low-pressure rarefaction is several milliseconds behind the initial high pressure pulse, it can be exploited to help suck residual exhaust gases out of the cylinder toward the end of the exhaust stroke as the piston approaches TDC. The build up of this negative pressure and its timing in the exhaust stroke is closely associated with the primary pipe's length and diameter, just like an organ or other musical instrument.

As the exhaust valve starts to close and the intake valve starts to open, the engine enters the overlap period. During the overlap period the piston is starting to slow down as it approaches TDC and gets ready to reverse directions. To maintain good scavenging, a negative pressure must be maintained near the exhaust valve to help continue to suck stale exhaust gas out of the cylinder to make room for fresh fuel and air. As the main column of high pressure gas is almost out of the end of the header's primary tube, the pressure near the exhaust valve starts to rise again. All is not lost, however.

As the pulse of high-pressure, high-energy gas leaves the end of the primary tube and is diffused in the larger diameter header collector, a reflected pulse of sound energy just like a musical note is generated, much like that of a organ. This reflected sonic pulse travels down the exhaust pipe at the speed of sound, which is usually around 1100 to 1900 feet per second in thin, hot exhaust gas, causing a slight rise in pressure at the valve. The wave is then reflected back toward the open end of the primary pipe. Just like the initial exhaust pulse, the reflected sound pulse has an area of rarefaction, or low pressure, behind it. If the pipe is of proper length and diameter, this reflected wave can be exploited to lengthen the amount of time the condition of low pressure exists around the exhaust valve.

These phenomena are harnessed by the smart header designer to tune the pipe to help get the maximum amount of burnt gas out and to help pull the most fresh fuel in. Of course, because a header is tuned like a musical instrument, a header can only be optimized to produce the greatest scavenge-improving vacuum in a band of several hundred rpm."

laying around with my theory on "tuning" the exhaust pipes for sound using the helmholtz resonance theory. The calculator I posted above does not convert existing figures into metric or imperial properly, so you have to do your calculations in one or the other only. Hope my math is right, but here is what I found from pipe diameters ranging from 1.75" to 3.0" and using a test length of 13' and 10':

396.239 cm length of pipe (estimated 13 feet)
4.44 cm diameter pipe (1.75")
helmholtz resonance 13.7

396.239 cm length of pipe (estimated 13 feet)
5.08 cm diameter pipe (2.0")
helmholtz resonance 13.7

396.239 cm length of pipe (estimated 13 feet)
5.715 cm diameter pipe (2.25")
helmholtz resonance 13.6

396.239 cm length of pipe (estimated 13 feet)
6.35 cm diameter pipe (2.50")
helmholtz resonance 13.7

396.239 cm length of pipe (estimated 13 feet)
6.985 cm diameter pipe (2.75")
helmholtz resonance 13.6

396.239 cm length of pipe (estimated 13 feet)
7.699 cm diameter pipe (3.0")
helmholtz resonance 13.6

304.8 cm length of pipe (estimated 10 feet)
4.44 cm diameter pipe (1.75")
helmholtz resonance 18.1

304.8 cm length of pipe (estimated 10 feet)
5.08 cm diameter pipe (2.0")
helmholtz resonance 17.9

304.8 cm length of pipe (estimated 10 feet)
5.715 cm diameter pipe (2.25")
helmholtz resonance 17.5

304.8 cm length of pipe (estimated 10 feet)
6.35 cm diameter pipe (2.50")
helmholtz resonance 17.5

304.8 cm length of pipe (estimated 10 feet)
6.985 cm diameter pipe (2.75")
helmholtz resonance 17.6

304.8 cm length of pipe (estimated 10 feet)
7.699 cm diameter pipe (3.0")
helmholtz resonance 17.8

To parallel what the resonance frequency means, the closest representation I could find was a MIDI keyboard, but the lowest frequency they have is 27.5.

http://www.phys.unsw.edu.au/~jw/notes.html

I find it intriguing that sometimes going to the larger pipe produces a higher pitched note than a smaller pipe.
 

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QUOTE (Stretch @ Oct 14 2005, 01:26 PM) index.php?act=findpost&pid=533813
Yay for more links of knowledge!

The bottom line is that equal length headers rock, and the RPM they're tuned for depend on the diameter and length. Speaker (subwoofer) modeling software works great for understanding pressure waves.[/b]
so wut do u think makes the better headers for the 4cyl 6 MSDS or F2USA
 

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Crossbow: You da man

Yes CROSSBOW, as per your
In terms of efficency, most of the contour owners find that a single 2.5 mandrel system setup, or a dual 2.0 mandrel system (Internal Diameter), work best with the 3.0 duratec system.
.
I found exhaust flattening and 2 wicked close bends from the Y pipe to the catalytic converter on my 2006 Mazda 6 V6 3.0 litre.
I replaced and rebent the stock 2 1/4" exhaust to 2 1/2". I also replaced the rear cat to 2 1/2" along with 2 1/2" Mazdaspeed exhaust------huge hp gains (everything is working together--why do cat back when restriction is cat front?). currently @ 201 HP @ wheels with an automatic!!! (stock is around 165). I still have stock exhaust manifolds with pre-cats and no computer modification!!!!! :)
 

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If you dont want to understand why this all works by reading all those threads, I just got this back from Marty at MSDS:

Hi Scott, 2.5" od will be fine on the 3.0 vvt. The npn-vvt guys use this size also and report good hp/tq. You will proll'y lose perf with 2.25" size.
Marty
tech/msdsinc.
 
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