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
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 firstname.lastname@example.org
and I will do my very best to answer your queries.