Chevrolet Impala SS, Handbook

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SSHandbook
Modifications: Exhaust
applications, where mileage and performance when the nitrous is not in use is the most important, pipe size
should not be changed to suit the nitrous.
Headers -- Primary Pipe Lengths
Misconceptions concerning exhaust pipe lengths are widespread. Take for instance the much-overworked
phrase "equal-length headers." More than the odd engine builder/racer, or two, have made a big deal about
headers with the primary pipes uniform within 0.5 inch. The first point this raises is whether or not what was
needed was known within 0.5 inch! If not, the system could have all the pipes equally wrong within 0.5 inch!
Trying to build a race header for a two-planed crank V-S with lengths to such precision is close to a waste of
valuable time. Under ideal conditions it is entirely practical for an exhaust system to scavenge at or near
maximum intensity over a 4,000 rpm bandwidth. Most race engines use an rpm bandwidth of 3,000 or less
rpm. If the primary pipe scavenging effect overlaps by 3,000 rpm then it matters little that one pipe tunes as
much as 1,000 rpm different to another. Since this is the case, then all other things being equal, pipe lengths
varying by as much as 9 inches have little effect on performance. A positive power-increasing attribute of
differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe
routing to the collector in often confined engine bays.
Apart from the reasons just mentioned, there is also another sound reason why we should not unduly
concern ourselves about equal primary lengths. In practice, the two-plane cranks that typically equip V-S race
engines render the exhaust insensitive to quite substantial primary length changes. Experience indicates inline
four-cylinder engines are more sensitive to primary pipe length, but a two-plane cranked V-S is not two inline
fours lumped together. It is two V-4s and, as such, does not have even exhaust pulses along each bank.
With a conventional, as opposed to a 180-degree header, exhaust pulses are spaced 90, 180, 270, 180, 90
and so on. The two cylinders discharging only 90 degrees apart are seen, by the collector, as one larger
cylinder and accounts for the typical rumble a V-8 is known for. This means the primaries act like they do on a
four-cylinder engine, but the collector acts as ifit were on a 3-cylinder engine having different sized cylinders
turning at less revs. (Doesn't life get complicated?) This, plus the varied spacing between the pulses appears to
be the cause of the system's reduced sensitivity to primary length. These uneven firing pulses on each bank
seem to work in our favor. Evidence to date suggests that single-plane cranked V-8s, which have the same
exhaust discharge pattern as an in -line four-cylinder engine, make less horsepower and are more length
sensitive. Dyno tests with headers having primary lengths adjustable in three-inch increments show that
lengths between 24 and 36 inches have only a minor effect on the power curve of V-8s that you and I can
typically afford, although the longer pipes do marginally favor the low end.
Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the
collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary
diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-
into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively
easily and it is often best optimized at the track rather than the dyno.
As for the secondary length-that is from about the middle of the collector to the end of the secondary (or
the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the
primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice,
collector length and diameter can have more effect on the power curve than the primary length. A basic rule
on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end.