Next up is a performance exhaust. Whereas
induction kits and replacement performance filters are to
do with reducing pressure drops on the way into the engine,
performance exhausts are to do with minimising pressure restrictions
on the way out of the engine. Some performance systems have
an increased bore to reduce pressure losses but most retain
the standard bore and have more freely flowing exhaust boxes.
However, just opening up the
exhaust is not necessarily the answer. The extreme would be
to remove the exhaust system altogether but, for those of
you who have experienced it, if the exhaust breaks near the
manifold, all power is lost. For a normaly aspirated (non-turbo)
engine in particular it is important to retain a certain amount
of back pressure. It can also be important where the first
exhaust box is situated. The reasons for this are all to do
with exhaust scavenging which is to do with negative pressure
pulses reflected back up the exhaust manifold to remove the
last of the exhaust waste from the engine. All a bit technical
I know - a new page describing all this will be posted soon.
Having said all that, the situation
with turbocharged cars is different. The turbocharger is positioned
as close to the engine as possible in order to maximise its
efficiency because it needs to be as close to the hot source
as possible. And because the turbocharger is spinning at such
a high speed compared to the engine this means that any pressure
pulses from the engine are evened out and exhaust scavenging
is not possible to the same extent as it is in normally aspirated
engines. Therefore turbocharged cars can increase their power
much more easily by simply increasing the bore of the exhaust
system and removing exhaust boxes.
So the outcome of all this is
that typical turbocharged cars such as Imprezas and EVOs can
be producing more power as they get mega loud due to reduced
back-pressure (i.e larger bore systems and free flowing boxes)
but a normally aspirated car will actually reach the point
where it becomes less powerfull as it gets louder. This is
where the performance
exhaust manufacturers come in. They know what is best
for each kind of car.
The biggest brand names in the
UK are Magnex, Mongoose
and Scorpion who all produce
a large range of stainless steel performance exhausts for
the most popular cars for tuning in the UK market. Other popular
names are Piper, Sebring
and Supersprint with
Sebring and Supersprint manufacturing a huge range of part
numbers and Piper (of the cams fame) manufacturing an impressive
range of both mild and stainless systems and backboxes with
a vast range of tailpipe options - all with a 2-3 day delivery!
Other well known names include Janspeed
and Ashley Competition Exhausts
from the 70's who still produce a good range of performance
exhausts and manifolds.
Ah, manifolds.....that is another
subject unto itself. For normally aspirated cars in particular,
the design of the exhaust manifold is all important. The standard
manifold supplied on a car is a compromise between cost, power
and space. However, a well designed aftermarket manifold will
fit in the same space but be tuned to produce significantly
more power. This is achieved through carefully choosing the
correct length for each of the manifold branches, thus maximising
the cylinder purging through pressure waves. Again, more details
coming soon...! There are not actually that many manifolds
available in the market. Magnex
do some as do Supersprint
but there are more available from Janspeed
and Ashley Competition Exhausts
although they do major on older applications like the Mk2
Escort.
Prices for a stainless steel
performance exhausts start at around £350. Top of the
range exhausts can release around 8-10bhp.
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