Editing Header design

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To some, a header is just a bunch of tubes that connect the exhaust port to the rest of the exhaust system. To the more mechanically curious, it is a system of tuned length and diameter tubes connecting to a device which amplifies and optimizes the wavelength of the exhaust, effectively sucking burnt gases out of the engine's cylinders.

In the street rod world, absolute mechanical efficiency often takes a back seat to appearance, clearance issues, and ease of installation. However, most of us overlook the benefits of a properly designed and built header and how it can improve drivability, power output and fuel economy. If you are building headers or modifying existing headers, why not try to keep the physical operation of a header in mind while working on it?

==Collector==
===Argument for smaller collectors===
Ever see a car header with 1 5/8" primaries that had a 2 1/2" collector? Hooker makes them for trucks, but you don't see them for cars. But that is the optimum diameter for many street headers. Run that right into a 2 1/2" exhaust and you have a sweet system with lots of torque where you need it, and better fuel economy to boot. The collector should have a smooth, gentle shape from the four tube area down to the final diameter to keep things moving smoothly.

If you have very healthy big block, are running a blower, some turbo, or have a ton of nitrous, a 3" header may be appropriate. However, about the only street application of the monster 3" diameter pipe is when you count on having an engine with large gobs of top-end power. Otherwise, velocity is king in exhaust and 3" is probably too big to keep the speed of the gases up in the exhaust, and there goes that bottom end torque again! Many recommend either 2 1/4" or 2 1/2" diameter pipe for street V-8's.

===Argument for larger collectors===
A larger than needed exhaust won't necessarily "kill" your low end torque. Any respectable engine that makes low end power will continue to make low end power regardless of the sewer pipe exhaust. SOME power may be lost, yes. Usually not enough to make a difference on MOST V8 engines that are even close to being tuned right. Even a stock big block can make complete use of a 3" single exhaust system. Some even come with that size STOCK.

The majority of "lost" torque is now in the midrange of the engine's powerband. This will help with highway driving and towing, and provide better passing power when needed. This more powerful midrange will always make the low end seem less powerful, simply because the engine pulls better in a different part of the RPM range. Most people that "think" they lost low end power are simply noticing that the engine pulls better in a different way, even if NO low end power was ever lost.

==Summary of header design principles==
#A "shortie" or a "block hugger" flows better than a stock manifold, and helps to produce more power than stock. However, it is not technically considered a header. They are easier to install in a stock style exhaust system, which makes them attractive.
#Bigger diameter primary tubes are NOT better in most street applications.
#The torque/power boost available from a header will occur at a specific RPM that can be easily determined.  The price paid for this boost is likely lower power at other RPMs.  All engine design is compromise.
#Equal primary tube length is very important for tuning and power output, but not to the point of obsessing over it. A regular Tri Y or 4-1 header will work wonders over the stock manifolds.
#A long and smooth collector is a good thing.
#Just like header tube size, be reasonable with the size of your exhaust system.

One thing that has not been discussed is the unequal firing order from one bank to the other.  I did a research paper  on Exhaust tuning when I was in college, A Jaguar racing engine had 105% volumetric efficiency,,, the torque peaks vs rpm  looked like the grand teton mountains   Start with a sheet of graph paper and draw the engine in plan view then draw a line 8 blocks long from cylinder no 1 . a line 7 blocks from cylinder firing next.... 6 blocks from the cylinder that fired third   etc  and so on following the firing order. .   You will see the lines are not uniformly distributed ,  the ford 63 indy engine had the "bundle of snakes" exhaust system on the top with pipes crossing over to have 4 equally spaced exhaust charges in each exhaust header,  some race car headers had pipes crossing under the trans to get equal charging in each header

==See also==
[http://en.wikipedia.org/wiki/Manifold_%28automotive_engineering%29 Wikipedia article on exhaust manifolds]

[[Category:Engine]]
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