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> From: Rader <rlr@rtp-bosch.com>
> To: dakota-truck@buffnet.net
> Subject: DML: Shorty vs. Long-tube headers
> Date: Thursday, July 16, 1998 9:33 AM
[snip]
>
> > I'll put on some good, long headers (not the junky shorties that
> > people are signing on to buy now).
>
> I'm curious about this comment. Why do you feel this way? Objective
> evidence is, of course, most convincing.
>
> Ron
I figured I'd hear back about this. Maybe "junky" was too strong a word.
Anyway, shorty style headers are a definite improvement over cast iron
manifolds. But, I figure regular long style headers are going to cost me
just as much, and they DO provide more power.
You see, the exhaust gas comes out of the cylinder head and rushes through
the header and into the connector in a kind of "slug" of exhaust. Its not a
constant flow through the header. When this slug hits the collector, a
slight vacuum is produced in the other three header pipes due to the
momentum of the exhaust slug in the collector. If one of the exhaust valves
in the three pipes opens now, the partial vacuum helps to "suck" the
exhaust out of that cylinder. You'll often here about improved exhaust
scavenging. This is what the people are talking about.
So, because you want the exhaust slug to pass through the collector at
exactly the moment that the next exhaust valve opens, there is an optimal
length for the header pipes for the best scavenging. Of course, the optimal
length depends on the engine RPM and the timing of the camshaft, chiefly.
On an airplane or boat where the engine has a certain speed its designed to
run at, you can pretty much nail the header length exactly. For cars,
though, we run the engine up through the RPMs as we go tooling down the
strip. The best we can do is get headers that are some compromise. In the
situation of a 300-400 cu in V8 running under 7000 RPM, that length is
usually such that the collector ends up next to the transmission. If the
engine is meant to run at much lower speeds, like 1000-2000RPM, a shorter
header will be about right. Between lowering production costs and the
low-rpm normal usage intended by the auto maker, the short manifold is
justified. But for racing, a longer header is better.
BTW, I'm a mechanical engineer that specialized in compressible fluid
mechanics, so this is my specialty. However, I'm no expert so I'm sure that
someone at hooker headers or similar might be able to give us more info if
we wanted it.
Aside from the logical explanation above, I could further prove that longer
headers are better through example. Go to the dragstrip and see how long
the headers are on the faster cars. If they use longer headers, it must be
better, right? But obviously there's some optimal header length, otherwise
I'm sure someone would build a header with the collector at the rear
bumper.
Hope that helps.
-Mike Miller
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