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?

==A bigger header isn't necessarily a better header==
The two most important aspects of header design are tubing diameter and primary tube length. This is definitely one area where the "Bigger is Better" philosophy doesn't cut it. Most very mild small blocks out there would perform better with 1 1/2" primary tube headers on them. Ever try to find primary tubes that small?

===Tubing diameter===
Just like putting a 300 degree duration cam in a 350 inch small block with 8:1 compression will kill any drivability and torque (but the idle sounds neat - until you hear a high compression big cam motor), putting a set of 1 3/4" headers on a mild small block will kill torque and drivablility, not to mention fuel economy. 

What horsepower does your engine ''REALLY'' make? Most people overestimate horsepower, RPM range, etc. of the motor in their ride. Consider that the GM ZZ4 crate motor makes 355 hp and the Mopar Performance 5.7 Hemi crate motor makes 360 horsepower with great heads (as for the Hemi, excellent heads), roller cams and brand new everything. How much power is your 350 with 50,000 miles, stock iron heads, 268 degree cam and 8:1 compression going to make? The two engines mentioned above would be ideal candidates for headers with 1 5/8" primary tube headers at 36" long with a 2 1/2" collector and exhaust system.

===Primary tube length===
That brings us to primary length. First of all, those "shortie" headers are not headers, just tubing manifolds designed for clearance -- not horsepower or torque. Although they look like they would flow better than manifolds (and probably do in many instances), unless you are running a supercharger, you need more than flow out of a header. The bothersome part of the "shortie" (other than length) is that the collector is so short and causes a lot of turbulence right where the flow needs to be smoothed out.

Most street engines that are operated in the idle to 5500 range (yes, your 350 may rev 6500, but is it making any power up there?) work very well with 36"-38" primary tubes. This is the length necessary for the shock wave in the tube to reflect back to the exhaust valve and create a vacuum which will suck the burnt gases out of the combustion chamber. This is provided that the primary tube has the proper diameter to keep the velocity of the gases up. 

====Exhaust pulse====
Velocity is created in the exhaust system from an exhaust pulse traveling through the primary tube and as rear part of the pulse cools, will create a vacuum. This vacuum will help to pull the next exhaust pulse out of the cylinder. This leaves a cleaner cylinder with less spent exhaust fumes and more room for the incoming air/fuel mixture. More air fuel = more power and torque.

====Equal primary tube length====
=====Argument for equal primary tube lengths=====
If the length of the primary is part of the tuning equation, how well does an engine run with different primary tube lengths? Try and jet that carburetor without pulling your hair out! Most of the commercially available headers out there have a large variance in tube length. Check out a set for a big block mopar in a B or E body for an example. The variance between longest and shortest tubes on these units can be as much as 16".

[[Image:unequal.jpg|right|frame|Big block Chevy headers. Note how the driver's side rear tube (yellow) must be about 10"-12" shorter than the next tube (in red).]]

In the header photograph to the right, the short primary tube would scavenge at a higher RPM and the long primary tube would scavenge at a lower RPM for the respective cylinder. Therefore the cylinder with the short tube will be running lean at low RPM and the long tube cylinder will be running lean at the high RPM and would require different jetting and timing than the others. How do you do that with a standard kettering distributor and a simple carburetor? That's why equal length is important: so you can tune your car.

=====Argument against equal primary tube lengths=====
Equal length headers are good for a certain part of the RPM range of a typical engine. However, when buying an equal length header you are left with the length the manufacturer wanted to use, NOT the correct length for the engine you are building. How do you know if its the right length?

Different primary tube lengths are not nearly as hard to tune. This type of header shown has proven itself for decades to be a well designed, good flowing header that will free up a good amount of horsepower compared to stock manifolds.

"Equal length" is usually defined as the longest and shortest tubes being within 2 inches of each other (about as close as you can measure with a tape measure at the swap meet). 

There have been claims by some manufacturers that unequal lengths broaden the torque curve due to different cylinders performing better at different RPM. It is left to the reader to decide if flattening the torque curve is a good thing to be doing with headers. However, a flat broad torque curve makes better drivability, and a smoother power band than a peaky engine.

==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's sometimes not technically considered to be a header. However, they are also typically easier to install since they bolt up to the rest of the exhaust system.
#Bigger diameter primary tubes are NOT better in most street applications.
#Longer tubes are better for torque ''(this claim needs to be confirmed and expanded upon. Top end torque? Low end?)''
#Equal primary tube length is very important for tuning and power output, but not to the point of obsesing 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.

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

[[Category:Engine]]
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