Header design
by: 1169884748, 427v8, 48 Dodge, Cobalt327, Crashfarmer, Crosley, JerryMopar, Jon, Powerrodsmike, SAATR, Schnitz
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Contents |
[edit] Introduction
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?
[edit] Collector size
[edit] 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, turbo, or 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 a lot 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] Argument for larger collectors
A larger-than-needed exhaust system won't necessarily "kill" your low end torque. Any engine that makes respectable low end power will continue to make low end power regardless of a sewer pipe sized 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 torque "lost" from the bottom end of the power band has actually been moved upwards in the powerband, into the midrange. This can help performance during highway driving and towing, and may provide better passing power when needed. A more powerful midrange tends to 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.
[edit] Summary of header design principles
- A "shortie" or a "block hugger" header flows better than a stock manifold in most cases, and helps to produce more power than a stock manifold. However, it is not technically considered a header. They are easier to install than long tube headers in many cases, especially in late model vehicles, which makes them attractive.
- Bigger diameter primary tubes are NOT better in most street applications.
- The torque/power boost available from any header will occur over a specific RPM range. This is accentuated when using equal length headers; they tend to have a higher peak at the cost of average power/power under the curve, or the width of the powerband. Equal primary tube length is important for maximum power output, but not to the point of obsessing over it. A regular Tri-Y or 4 into 1 header will work better than the stock manifolds. Unequal length headers are a good compromise for a street driven vehicle in many cases; after all, engine design is a compromise.
- A long and smooth collector with a moderate diameter will tend to favor the bottom end, conversely a short, large collector tends to favor the upper range.
- Just like header tube size, be reasonable with the size of your exhaust system. Just do not undersize the system. If you're going to err, err on the larger side.
[edit] 180 degree exhaust system
One thing that has not been discussed is the unequal firing order from one bank to the other. One college research paper on a Jaguar racing engine having a 105% volumetric efficiency, the torque peaks vs rpm looked like the Grand Tetons.
The 1963 Ford 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. This system is called a 180 degree exhaust. It has some advantages in racing situations, but for the most part it's a very difficult and costly system to manufacture, package and install/service in a production vehicle.
[edit] See also
Wikipedia article on exhaust manifolds
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