If anyone is interested, I found this info that might be helpful to our
Alpine street racers.
Jan
The following info is typed straight out of a Headers By Ed
catalog on the topic of Header tube length:
"Header Lenght. The length of a header (the length of the header tubes
AND collector) has two basic effects on an engine:
1) The lenght of the header determines at what point in the RPM range
that the header "tunes" or maximizes its effect on hp output. It does
this by using the sound waves reflecting back and forth inside of the
tubes and the collector to further aid in extracing exhaust gases from
the engine's cylinder. The effect on the engine is that as the
header's lenght DECREASES, the part of the RPM range where
the "tuning" effect adds power, INCREASES (and vice versa). However,
it should be pointed out that if lenghts are WAY OFF, the header can
literally be "out of tune"- actually PUSHING the exhaust gases back-
wards into the cylinder!!!
The range of "usable" lenghts can vary considerably- relative to, not
only the RPM RANGE of the motor but, the intended use of the vehicle
concerned as well. For example: While "Pro Stock" drag cars may use
headers with tube lengths as short as 24" and collector lenghts
basically no longer than the collector taper itself, the power to
weight ratio of these cars along with the rpm range these engines run
in, basically dictate header designs that only need work from 9k rpms
and up! In contrast, most "street" vehicles as well as many
competition vehicles have engines that are barely revved over 6k rpms
and consequently need headers MUCH LONGER so that the header "tunes"
in the engine's actual rpm range! 98% of the time, "tuned" lenghts
should never be shorter than 30". On the street, where increased
bottom and mid-range power improves initial as well as overall accel-
eration (and also improves throttle response and gas mileage), there
should be NO tubes LESS than 34" in lenght (yet mass-production
headers are quite often MUCH shorter than that!)
2) The lenght of a header actually represents a specific volume. This
volume contains a MASS (or weight) of exhaust gases. While the util-
ization of the SPEED of these contained exhaust gases creates "exhaust
scavenging" and, when optimized, can increase horsepower considerably,
over the years we have also seen that LONGER headers (because they
contain a larger MASS of exhaust gases) are quite beneficial when used
on steet cars as they further improve exhaust scavenging! Yet, in the
interest of reducing cost (short headers are much cheaper to build),
mass-production header manufacturers often make short headers that are
not only, NOT optimized to increase bottom and/or mid-range power, but
are often SO SHORT that they actually REDUCE bottom end power when
compared to stock manifolds."
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