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This picture highlights the most significant difference between the stock header and the Gp A Rally
header. The primaries on the Gp A header are quite a bit larger. 50 mm vs. only 42 mm for the stock unit.
Thus the Gp A header provides a 42% greater cross-sectional area to the hot exhaust gases coming out
of the cylinder head.
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The secondaries on the Gp A Rally header are also larger than those on the stock unit,
though not by as great a percentage. The Gp A header secondaries measure 50 mm in diameter while the stock header
secondaries are 48 mm in diameter. This is still an 8% increase in cross-sectional area. Some exhaust experts
propose that the exhaust pipe diameter should constantly increase from the cylinder head to the tail pipes. The idea
being that as the exhaust moves towards the rear of the car it cools and loses energy and thus
has less ability to overcome flow
restrictions, Therefore the pipes should get larger so as not to impede the low energy exhaust. This may or may not
be true. In any case, BMW Motorsport obviously found a good overall torque curve on the Gp A Rally header
by using equal diameter primaries and secondaries.
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This photo shows how the O2 sensor is mounted in a slightly different place on the Gp A Rally header.
Turner Motorsport actually welds on the fitting for the O2 sensor, it does not come that way from
the factory. Note that the stock header effectively has two "H-pipes" whereas the Gp A unit
has only one, where the O2 sensor is located. It is not clear whether these H-pipes are located
such that the low pressure wave following an exhaust pulse can help scavenge the exhaust
from the accompanying cylinder. The "X-pipe" definitely DOES serve this function, as is borne
out by the decrease in torque when it is removed. The X-pipe essentially completes the familiar
"Tri-Y" layout, which is known to provide a good overall balance of power and torque.
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