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post #11 of 16 (permalink) Old 05-17-2007, 09:37 AM Thread Starter
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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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post #12 of 16 (permalink) Old 07-12-2007, 09:36 AM Thread Starter
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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_demart...s/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...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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post #13 of 16 (permalink) Old 08-15-2007, 10:30 PM
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I'll take 2 Monsters, a cup of coffee and a Redbull.

This is a lot of reading and comprehending!

On a Scale 1 to 5, My cars a 6
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post #14 of 16 (permalink) Old 04-24-2009, 05:33 PM
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QUOTE (Stretch @ Oct 14 2005, 01:26 PM)
Quote:
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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post #15 of 16 (permalink) Old 07-16-2010, 11:10 PM
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Crossbow: You da man

Yes CROSSBOW, as per your
Quote:
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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post #16 of 16 (permalink) Old 11-30-2010, 02:20 PM
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Talking

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.

06 M6 V6 | AWR 88 duro mounts | CAI | Oil Catch Can | MSDS Header | Magnaflow High flow cat | Custom 2.5" Exhaust with Dual Flow-Pro mufflers | Exedy stage 1 clutch | Exedy racing flywheel | 17" Toyo Proxes4 | 50 watt 6000k HID High Beam | 50 watt 5000k HID Low Beam | Silverstar turns | DICE aux ipod mod | All LED retro | De-badged front and rear | shaved | 35% tint gauged at 22% | Scangauge II |
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