ISSN: 1391 - 0531
Sunday, November 12, 2006
Vol. 41 - No 24

Ok, now back to headers

So last month we talked about building your own exhaust system so this month we go to the root of the system which is called a header.

On a side note a few friends of mine were bringing up this issue of the good old long arm of the law stopping cars with 'beat' barrels and fining them for noise pollution. Noise pollution?! Come on give us a break for crying out loud! That's the most ridiculous thing I have heard! If they really want to do something about noise pollution I can first suggest they pull over all the diesel fume belching AIR polluting buses and then fine them for their NOISE polluting horns. Those horns are in excess of 100 decibels; trust me I know I live by the main road next to a bus stand.

I cannot for the life of me figure out why they have such loud horns. I mean it's a bus, so like its huge, how can anyone miss the blasted thing rumbling down the road! Those horns are just plain obnoxious and on top of polluting the air they pollute our ears, big time. I guess it's just another example of our misguided and badly managed systems showing us how shiningly retentive, and retarded they can be.

Ok, now back to headers.
The reasons why headers work are where opinions really become confused with facts, an area full of old hot rodders wives tales and myths. As we said before, headers can produce substantial amounts of power on a motor with very few negative compromises. Headers work so well in producing extra power with no negative side effects such as mileage losses common with other mods, that they are a rare, win-win modification with hardly any negative trade-offs. This makes them a mod that is almost essential for any serious engine build-up.

If headers are so good, why don't cars have them from the factory? Well a few cars do, hidden under the factory heat shields, however for space, cost and catalytic converter light-off reasons, most cars come with a crude, cast iron, log-type manifold stock from the factory. A log manifold is simply a tube with stubby legs connecting the exhaust ports to the main tube. This is good for conserving heat to quickly light off a catalytic converter during cold starts, and it is compact, preserving valuable under hood space within today's crowded engine compartment. However a log manifold is detrimental for power production.

A header is an exhaust manifold fabricated from tubular sections of pipe. Full radius mandrel bends are preferred so the pipe's tight radiuses will not be crushed down. Each individual exhaust port is treated to it's own separate primary runner instead of merely dumping into the shared main pipe of a log manifold. The equal length, or close to equal length primary pipes converge at a single, larger diameter point or collector. The collector then leads to the main exhaust pipe.

An old hot rodders tale is that headers produce more power by reducing backpressure and by the long individual runners preventing the exhaust blast from one cylinder from blowing into the next cylinder, contaminating the charge on overlap. While this is partially true it is not the primary reason why headers produce more power than a stock manifold.

Headers make more power by primarily using resonance tuning to create a low-pressure reflected wave rarefaction pulse during the overlap period. The overlap period is in between the end of the exhaust stroke and the beginning of the intake stroke (remember our 4 stroke cycle from part one?) where both the intake and exhaust valves are open at the same time for a few degrees of crankshaft rotation. Engine designers use overlap to help the engine breathe better.

The way a header is tuned is much like how an organ pipe is tuned. The optimal length used is the one needed for the primary pipe to have a fundamental note corresponding to the time when the exhaust valve opens. When the exhaust valve opens, a high-pressure pulse of hot expanding exhaust gas travels down the exhaust port at approximately 300 feet per second. This wave of hot, moving, high-pressure gas has mass and inertia of its own which pulls suction or a low-pressure rarefaction behind the pulse.

Well that's it for this episode I will be adding more to this in the next installment so make sure you tune in! Feel free to e-mail me with your questions on and I will do my very best to answer your queries.

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Copyright 2006 Wijeya Newspapers Ltd.Colombo. Sri Lanka.