Philip,
My understanding: A header is a manifold that has pipe lengths and
geometry tuned so that backpressure is at a minimum while assuring that the
pulse of exhaust from one cylinder does no interfere with the exhaust flow
of from another cylinder. In addition to making the engine work harder
against a backpressure, the backpressure can force air out an intake valve
of a cylinder, which is later drawn into the intake, reducing the amount of
dense fresh air being drawn in. The tube lengths and geometry of a header
are chosen so that there is what is sometimes called an ejector effect (AKA
shock wave tuning or inertial extraction) where the pulse from one cylinder
is timed so that the momentum of the slug of exhaust leaves an area of low
pressure behind it that will cause an increase in intake charge during the
overlap period (TDC) by "sucking" in more on intake (no Monica Lewinsky
jokes, please). The header is sized to maximize these effects at various
RPM, and you choose the header based on what point (RPM) in the power curve
you want the most help.
----------
> From: Philip Hubbard <phubbard@carroll.com>
> To: spridgets@autox.team.net
> Subject: Exhaust Manifold vs. Header
> Date: Tuesday, April 28, 1998 11:47 PM
>
> After my description of my erstwhile manifold, the concensus is that I
have
> a header and not a manifold. Looking at pictures and hearing the
> explanations, I'm inclined to agree. Well, one of my goals for owning
this
> car is already coming true. I'm learning about autos. Now I know the
> difference in looks between a manifold and a header.
>
> So here's another question. There is obviously a reason for switching to
a
> header. What is it? More power? Switching back to a manifold, will
that
> cost me the 60 psi oil pressure I had before the crack up?
>
> A neophyte begs education. :)
>
> Philip
> 1974 MG Midget (Burgundy)
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