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 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===
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".

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.

Check out this pair of Big Block Chevy Headers

[[Image:unequal.jpg]]

the driver's side rear tube (in yellow) must be about 10"-12" shorter than the next tube (in red).

This would mean that 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? Thats why equal length is important: so you can tune your car. 

-edit see edit above. Its just not the way this writer would like you to believe.

This has been demonstrated on a '69 corvette with Headman side exhaust. The engine did not respond to idle screw adjustments at all. There was an 18" difference in primary tube lengths. Switching to an equal length header made adjusting the carb easy and idle vacuum went up 2 inches of vacuum. 

-edit Claiming something happened on a single car with no proof to back it up and nothing else to show except that we should just trust you leaves your character in question. Where are the dyno sheets? 

"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. 

edit- a flat torque is a very good thing to have. A flat broad torque curve makes better drivability, and a smoother power band than a peaky engine. Much more fun to drive to the majority of the driving public, including hot rodders.


I guess all we can do is create our headers properly or reconfigure existing units to work as they should and not give ourselves tuning headaches. When big name header manufacturers say equal length, they must be talking out their collectors.

Now we get to the collector - 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 most 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.

-edit There is no way ANYONE can prescribe an exhaust system size without knowing a lot more about the engine design being used. Will a 2 1/2" exhaust be the right choice for a 1.X liter Geo Metro engine? How about a 5.4 Lightning? A single 2 1/2" for a warmed - over 454? No way. 

While we are on the subject, I see the proliferation of 3" exhaust systems out there. If you have very healthy big block, are running a blower, some turbo, or have a ton of nitrous, you go boy! 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.

-edit Here we go again. A larger than needed exhaust will NEVER kill your low end torque as much as the writer claims. 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 engines 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: 

1. "shortie" or a "block hugger" - is ''not'' a header

-edit Shorties ARE headers. They still flow better than stock manifolds and help produce more power than stock. They are also easier to install since they will bolt up to the rest of the exhaust system.

2. Bigger diameter primary tubes are NOT better in most street applications

3. Longer tubes are better for torque

-edit Where? Low end? Top end? Another baseless claim, needs more proof.

4. 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.
5. A long and smooth collector is a good thing

6. Just like header tube size, be reasonable with the size of your exhaust system

[[Category:Engine]]
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