if your serious about maintaining good peak hp numbers use the calculators in the linked threads to actually calculate the ideal matched header dimensions, this is not a guessing game its strait physics and easily calculated to maximize exhaust scavenging and max volumetric efficiency, resulting in max power , at any given rpm and displacement, compression , cam timing etc this stuff can be calculated, but when dealing with an average and without knowing all the specifics you go by averages,and assume that peak hp takes a higher value than off idle torque
ok as a rule of thumb,when calculating and engines expected exhaust flow rates you can roughly assume 2.257 cfm per horsepower produced
if you have a 500hp engine it will in theory produce 500 x 2.257=1128.5 cfm of exhaust flow
or 141 cfm exhaust flow per cylinder, but your exhaust valve is only open about 220-290 degrees and the time between power strokes gets shorter as the rpms increase.
Of the 720 degrees in a complete engine cycle, theres lets say 250 degrees , or 250/720=1/3rd of the time, but it doesn,t all exit instantly so you can figure on about 1/2 the time the header primarys under significant pressure and pressure and pulse frequency varies with the rpms so the exhaust dump rates not at a constant flow or pressure, the header must handle 141 x 2=282cfm /115 =2.45 sq inches for exhaust flow per cubic inch , you look at the chart and find thats about a 1.7/8 diam. subtracting the pipe wall thickness






you will occasionally see dyno test results similar to this, that show about a 20 hp spread in the power curve, that seem to indicate that the difference in header configuration is not overly critical,but keep in mind these results are almost always done on 350-383 displacement engines with fairly mild compression , below 9.5:1 and fairly low duration cams ( example ,below 230 deg @.050 lift )and stock or mildly restricted exhaust systems, an engine with a fairly tight LSA and high compression and a low restriction or open exhaust can take full advantage of the headers scavenging the cylinders in a tuned rpm range significantly boosting the power produced, the tuned exhaust header has a greater effect on a higher compression ratio engine of larger displacement with a tighter LSA cam
you might want to read these also
viewtopic.php?f=56&t=185
http://victorylibrary.com/mopar/header-tech-c.htm
PRIMARY PIPE DIAMETER FACTORS
The primary header pipe diameter is determined using basic engine mechanical specifications, such as: Bore Stroke Compression Ratio Valve diameter Cam specifications (lift and duration) Target rpm range
http://www.pontiacracing.net/js_header_length1.htm
http://victorylibrary.com/mopar/header-tech-c.htm
http://maxracesoftware.com/pipemax36xp2.htm
http://www.wallaceracing.com/header_length.php
viewtopic.php?f=56&t=1730&p=4310#p4310
http://www.boyleworks.com/ta400/psp/exhaust3.html
http://www.autolounge.net/calculators/e ... izing.html
http://www.bgsoflex.com/auto.html
http://www.custom-car.us/exhaust/header.aspx
http://www.mk5cortinaestate.co.uk/calculator5.php
http://www.custom-car.us/exhaust/default.aspx
http://www.superchevy.com/technical/eng ... index.html
viewtopic.php?f=56&t=352
http://www.superchevy.com/technical/eng ... index.html
viewtopic.php?f=56&t=361
viewtopic.php?f=56&t=260&p=315#p315
viewtopic.php?f=52&t=796&p=1156&hilit=volumetric#p1156
http://www.carcraft.com/techarticles/03 ... index.html
http://autolounge.net/calculators/exhau ... izing.html
most exhaust pipe is 16 gauge, or .065 wall thickness
http://www.engineersedge.com/gauge.htm
How do you judge the i.d of a pipe?
3" o.d = 2.87" diam. inside
2.75 o.d = 2.62"diam. inside
2.5 o.d = 2.37" diam. inside
2.25 o.d = 2.12" diam. inside
theres THREE distinct areas of pressure/velocity
(1)
gases in the cylinders and header primary tubes as the exhaust exits the cylinders
(2)
gases in the header primary tubes and header primary tubes as the exhaust exits the header COLLECTORS
(3) the exhaust system past the collectors
(1) and (2) must be calculated to match the intended displacement ,cam timing and intended rpm range
(3) (traditional back pressure ) should be MINIMIZED

keep in mind installing an (X) almost increases the effective cross sectional area, of the dual exhaust ,or collector cross sectional area to double what it had been behind a single header collector, by doubling the area that the exhaust flow sees, dropping the restriction to flow almost in half
looking thru an (X) pipe



http://www.wallaceracing.com/header_length.php
http://www.bgsoflex.com/auto.html
viewtopic.php?f=56&t=185
PRIMARY PIPE LENGTH
"The overall length of the primary header pipe is governed almost exclusively by the target engine's rpm range, and displacement which is dependent upon wave tuning. Typically, a lower engine rpm range likes a longer primary pipe, while a high rpm engine prefers a shorter primary."
SECONDARY PIPE DIAMETER (collector length in a 4 into one design)
While typical off-the-shelf street 4-into-1 headers do not have true secondary pipes, Burns' research has proven repeatedly that his Tri-Y designs make more overall power over a broader rpm range. While traditional lines of thought have street enthusiasts knowing Tri-Y pipes make more bottom-end torque, further research by Burns into the design have resulted in headers making more power all across the rpm range. With more components as part of the Tri-Y design, more tuning possibilities exist, and therefore more potential lives within.
"The secondary pipe diameter is determined by considering both pressure waves and reflective waves throughout the system. Since the pipes are paired according to the firing order, these waves can work together or against each other. Naturally, our designs work with the waves to increase the efficiency of the header, using the wave pulses to help pull gases from the engine.
"There are two basic kinds of waves we're dealing with. First, there are pressure waves. The pressure wave travels the length of the primary pipe in a 4-into-1 header, then is reflected from the collector where the area changes from the small-diameter primary into the larger-area collector. A reflection of negative pressure goes back up the primary pipe.
"In a Tri-Y design, the pressure of additional area changes (where the primary pipes become secondary pipes) produces additional reflections, so the Tri-Y must be designed in a different manner with respect to wave control. Given this, the area of the Tri-Y header between the first and second collectors becomes critical, and tuneable. The entire header is affected by this crucial length of pipe, and can be fine-tuned accordingly through proper sizing for optimal broad-range performance.
"The 4-into-1 pipe is also affected by altering pipe lengths, of course. But, without these secondary pipes it is impossible to tune with the same level of precision as with the Tri-Y headers. It's for this reason we prefer the Tri-Y design in most applications. The tuneability is so much more accurate, we're able to find more power over a broader rpm range. This is especially critical in engines expected to work well over a wide rpm range, like street machines."
Another huge reason for the move to Tri-Y headers is weight savings. Burn's claims most of their Tri-Y headers weigh in at about _ the weight of comparable 4-into-1 pipes for the same application, due to the smaller pipe diameters used throughout similar applications. Also, with less internal volume than comparable 4-into-1 headers, the Tri-Y equipped engine is typically more responsive. Tri-Y designs require physically smaller collectors as well, contributing further to space and fit concerns, and adding further to crisp engine responsiveness.
COLLECTORS
"There is much power to be found in researching collector design and size. The optimal collector is determined by several variables, and it's engineering interacts with the entire exhaust system. The internal volume, the outlet size diameter, and the angles at which the pipes come together within the collector are all factors that must be maximized for the header to perform to its full potential."
1 - Primary Pipe Entry Size
"Our computer model design program determines many of these hard dimensions based on data gathered over many years, including the length and diameter of the primary or secondary pipe entering the collector."
2- PRIMARY PIPE ENTRY ANGLE
"The pipe entry angle is typically between 10-20 degrees, with most pipes being right at 15 degrees. The cone (or goilet) formed between the pipes as they transition from primary to collector is formed as a consequence of these angles, nothing more. The mass of gases moving through the pipe does not want to change direction, so keeping these "pyramid" cones true to the pipe entry angle helps smooth the transition from the relatively small volume of the feed pipe to the larger volume of the collector."
3 - COLLECTOR OUTLET DIAMETER
"The collector outlet diameter is the most critical dimension in the header. It's what makes the merged collector work the way it does. Each collector we sell is custom-sized to each customer's engine, and there's no real 'formula' to get a broad-based general determination for street machines. As a rule, the overwhelming majority of aftermarket headers designed for the street market have way too big of a collector outlet diameter. Most street guys are losing power because of badly designed, manufactured, or engineered street headers. There is much room for improvement here."
4 - OVERALL COLLECTOR LENGTH
"Overall collector length is not critical. Once the other variables in the header design have been determined, the collector ends up being as long as it needs to be. We've found no benefit in lengthening or minimizing this dimension. It's more important to properly engineer what's going on inside the merged collector, and let the length determine itself once all the other important factors are optimized."
AFT OF THE COLLECTOR
One of the growing areas of research at Burns is the critical area just aft of the all-important collector outlet. Burns' dyno research led him to begin experimenting with interchangeable venturis, which slip into receivers just aft of the collector. While these prototype dyno parts were initially crafted to assist Jack in determining the critical overall collector diameter size, he soon realized they could be a marketable product. His initial "DynoSYS" product for dyno research evolved into a line of interchangeable sleeves called the Burns Tuneable Exhaust Collector, or BTEC for short.
The BTEC system has shown capability to alter the entire power curve of the engine. By changing only the insert, racers can change the entire tune on their engines to fine-tune for track conditions, weather, or driver preferences. Mostly used by drag racers, many in Pro Stock, the BTEC system offers enthusiasts a glimpse into the future of header design. While the drag racers have already embraced the benefits of BTEC, a number of road racers are beginning to experiment with the system as well.
X-PIPES
One area street machine enthusiasts are aware of is the evolution of the X-pipe. Early on, connecting the left and right halves of a true-dual exhaust system with an H-pipe resulted in measurable benefits. This theory evolved into the X-pipe, which allowed both left and right portions of the exhaust system to share some common flow area and resulted in even greater gains in power with a notable reduction in exhaust noise.

placing two x pipes in sequence seems to work well at both mellowing the exhaust note, and increasing the exhaust scavenging of the cylinders as it blends and smooths out the exhaust flow by allowing the individual cylinder pulse strength to dissipate rapidly, the first (x) reduces flow restriction, the second allows the exhaust pulse to run into itself further disrupting the individual pulse strength
http://www.bgsoflex.com/auto.html
http://www.engr.colostate.edu/~allan/fl ... /pipe.html
http://www.engr.colostate.edu/~allan/fl ... age7f.html

http://www.headerdesign.com/extras/engine.asp


http://www.carcraft.com/techarticles/he ... index.html
http://victorylibrary.com/mopar/header-tech-c.htm
http://www.headerdesign.com/
http://www.wallaceracing.com/header_length.php
http://www.bgsoflex.com/bestheader.html
http://www.porcupinepress.com/_bending/segmentbends.htm
http://www.pontiacracing.net/js_header_length1.htm
http://www.slowgt.com/Calc2.htm#Header
http://racingarticles.com/blog/2008/02/ ... aders.html