main job of the exhaust is to expel all burnt gas from the cylinders.
must also help to draw fresh mixture into the combustion chamber.
must also drive the turbine on turbo cars.
The main job of the exhaust is
to rid the cylinders of exhaust gas and encourage inflow of air/fuel to fill
the cylinders. Any exhaust gas left behind takes up cylinder volume which
prevents max amount of fresh air/fuel in to produce max power. This also means
that heat is transferred to the inlet charge and charge density is reduced.
During the combustion phase, stale exhaust gas can block flame propagation
causing further power loss as well as increasing exhaust emissions of unburnt
Exhaust Turbo Performance.
maximum exhaust energy should be transferred to the turbine.
In the case of turbo engines,
there is the additional concern of spinning the turbine. The free flow of
exhaust gas is essential to initially spin the turbine and to keep it spinning
at top boost. Energy expended overcoming flow restrictions means less energy
to spin the turbine. Because of this every effort must be made to transfer the
max exhaust energy to the turbo, over a wide band of engine operating
Any efficiency decrease here will increase turbo lag. Any
improvements will profoundly influence the shape of the power curve and the
feel of the engine. When dealing with factory turbo car its in this area that
the most gains can initially be made. You stand to gain more horsepower per
euro than in any other area of the car, without compromising engine
The Exhaust Manifold.
- Maximise flow by ensuring internal surfaces are smooth, incl gaskets. - Machine away slag and extrude hone hard to reach areas.
Care must be taken not to
weaken the stock manifold, but bearing this in mind there is still a lot that
can be done to improve flow.
First make sure the internal
surfaces are nice and smooth. This encourages flow and discourages carbon
buildup. Casting slag should be removed as should any sharp edges or joins.
The manifold should match the exhaust ports and the manifold gasget should be
trimmed back as much as possible. The manifold to turbo joint should also be
Areas that cannot be reached
with a grinder should be extrude honed. But be careful not to overly increase
the internal size of the runners, because this will result in lower velocity
exhaust gas hitting the turbine.
The outside of the manifold
should also be smoothed out and any jagged parts left behind due to the
casting process should be removed and all sharp points should be smoothed
over, such as joins etc. this ensures that the constant heating and cooling of
the manifold does not cause cracking due to local temperature variations.
If each runner is joined due to
the way the manifold was cast, then they should be cut with a hacksaw so that
they are not in contact. You might think that they are joined to increase
strength but they are only joined like this because its easier to cast them
this way at the factory. The joining of the runners actually puts more stress
on the manifold due to expansion and contraction and cutting them apart
drastically reduces this stress and it also reduces the chances of a leaking
or blown manifold gasket.
Gasket Selection. - Use
good sealing insulator or conductor gaskets where appropriate.
The gasket is used to seal the
joints to prevent exhaust gas leakage. However, it does have a secondary role
which is very important and often overlooked. The gasket acts a heat insulator
or as a heat conductor depending on its material and thickness.
For some joins the gasket
should act as a conductor and so copper can be used as the gasket material.
Areas such as the exhaust to turbo inlet operate at similar temperatures. The
join between the cylinder head and the exhaust manifold is a different matter.
We don’t want the exhaust to lose heat energy by heating the cylinder head and
we don’t want the cylinder head to be heated by the exhaust making cooling
more difficult. Therefore, this gasket should be a good insulator while still
giving a good seal against exhaust gas. A metal reinforced gasket with fibre
composite an both sides will insulate well but won’t last indefinitely. A
fiber thermal barrier sandwiched between two light plates made of steel or
copper may last longer but doesn’t insulate as well.
A similar gasket should be used
on the turbo exit to minimise heat loss from the turbo to the dump pipe and to
keep the dump pipe cool. The external wastegate should also use an insulating
gasket on the entry side to insulate the diaphragm from heat. Use a conducting
gasket on the exit side of the external wastegate to allow heat to be drawn
away from the wastegate.
Tube Manifolds. - Tuned length pipes can be used to maximise exhaust gas scavenging. - Steam piping is strong but heavy. - Stainless steel is strong and light, but turbos will need additional
For the ultimate in exhaust
manifolds we have to ditch the cast manifold and fabricate a multi branch
manifold using tuned length pipes.
When turbos are located close
to the head, then steam piping can be used to go from the head to the turbo.
It is very strong, bends are readily available, it is will support the weight
of a turbo in the same way that a cast item will and it has good heat
retention qualities because of the thickness of the walls. The downside is
that it is heavy.
Alternatively, stainless steel
can be used and when the turbo is located further away from the head, then
steam piping is not really an option, so stainless steel or something more
exotic must be used.
Stainless steel is light and
strong and still retains good heat resistance because it is a poor conductor.
Also, a ceramic costing can be applied to the inside surface to enhance its
heat resistance. All bends should be formed in a mandrel bender. Welding bends
together should be avoided because it promotes cracking during its service
life and slag invariable falls to the inside of the pipe causing flow
The plates at the cylinder head
and turbo mounting faces should be at least 10mm thick.
For heavy turbos, a support
system must be employed to carry the turbo. Supporting it by the manifold
alone wont suffice. Hanging the turbo from a spherical bearing in the top of a
tripod, which is solidly fixed to the engine is the usual method used…. The
turbo cannot be rigidly fixed to the engine… it must be able to move as the
exhaust expands and contracts.
Manifold Dimensions. - Choose the minimum pipe size that will
give the flow needed.
Anything that adds weight,
complexity should be avoided.
If you are free to choose any
size turbo and any boost level you like, then a pipe length of 20in should be
used. The diameter of the piping will be dictated by the size of the engine
and whether you want top end power or low and mid range power. Its always best
to choose a diameter as small as possible to keep exhaust velocities high and
therefore, keep spool-up times low and responsiveness high. Only when all out
top end power is important should bigger diameter pipes be used.
The following diameters can be
used as a guide:
4cyl single turbo:
1300cc – 1.24 to 1.5in
1600cc – 1.25 to 1.625in
2000cc – 1.375 to 1.75in
2200cc – 1.375 to 1.75in
2400cc – 1.5 to 1.875in
6cyl single turbo
2000cc – 1.125 to 1.5in
2300cc – 1.25 to 1.5in
2500cc – 1.25 to 1.625in
6cyl twin turbo
2600cc – 1.25 to 1.625in
3000cc – 1.375 to 1.75in
3500cc – 1.5 to 1.75in
4000cc – 1.625 to 1.75in
8cyl twin turbo
4000cc – 1.375 to 1.62in
5000cc – 1.5 to 1.625in
5700cc – 1.5 to 1.75in
7000cc – 1.75 to 2.125in
8200cc – 1.875 to 2.375in
In 4cyl engines it is normal
practice to join all four pipes at the turbo inlet. But if space doesn’t
easily permit this then a 4 to 2 to 1 optin can be employed. If so then its
better to keep the primary pipes 12 to 16in long. Two primaries from cylinders
one and four should be joined( same for cylinders two and three). Then both
secondaries can be joined into the turbo inlet.
Connecting an External
Wastegate. - It
should tee off at a gradual angle and maximise flow.
It is important to tee off the
wastegate so that it doesn’t overly affect the smooth exhaust flow into the
turbo. At the same time the design should maximise flow through the wastegate
when it is opened.
The best place to locate the
wastegate is after all the exhaust tubes have come together at the turbo, or
even on the turbine inlet itself. It should not tee off at an acute angle,
otherwise flow will be turbulent.
External Wastegate Dump
Pipe. - If
emissions and noiseregulations allow, keep the dump pipe separate.
The wastegate pipe should
remain separate and not rejoin the main exhaust. Where the pipe must rejoin
the main exhaust it should do so as far away from the turbo as possible
because there is always turbulence at the join site. Sometimes the pipe must
be rejoined before the cat to satisfy emissions laws. If this is the case then
it should join just before the cat and if permitted the cat should be moved
further back along the exhaust system (even on
internal wastegate designs, it is a good idea to move the cat back as far as
Internal Wastegate Dump
Pipe. Internal wastegates are used to
minimise cost/weight and because they are more compact and are often built
into the turbo main casing. However, it is much more difficult to achieve
non-turbulent flow. Sometimes there is nothing that can be done to minimise
this turbulent flow. But if the exit of the wastegate port runs parallel with
the extruder bore then some simple dump pipe designs will give good minimal
Exhaust Pipe Size. - Bigger diameter bores are not always better.
There is no easy way to figure
out what the ideal exhaust size should be. The only foolproof way is to fit
several exhaust sizes and test them on the dyno. But some good guidelines can
narrow down the choices.
Sometimes good results can be
got from using pipes no larger than 15% of the extruder bore. Because of the
big weights of big diameter exhausts and big diameter silencers, bore sizes
should be kept as small as possible while still allowing maximum flow.
A good guide, based on the
experience of tuners is as follows:
1300-1600cc – 2.25 to 2.5in
2000-2300cc – 2.75 to 3.0in
2500-3000cc – 3.5 or 2x2.5in
3500-4000cc – 4.0 or 2x2.75in
5000-5700cc – 2x3.0in
These figures are based on
engines developing 120-150bhp per litre.
Rally engines developing
200-250bhp per liter need pipes that are 0.5 to 1.0in bigger.
The tailpipe can be reduced by
1/4in without suffering penalty.
But testing is the only way to
go. On group A two litre turbo engines, a 3in pipe will give excellent power
but a pipe 3.5in will cause a power drop and at 4in the power comes back. So
testing is the only reliable method for choosing max power setups.
With turbos, the biggest flow
restriction is at the turbine wheel and housing. The use of a bigger housing,
without suffering further turbo lag will give better gains than exhaust
change. Then next most restrictive area is at the manifold between the turbo
and the head, so improvements here will also give good gains.
Problems with Exhaust
Changes. - On
turbo cars the boost can change without changing the boost control.
Exhaust changes can make it
more difficult to control boost. The exhaust flow can be so great that the
internal wastegate is too small to bleed off sufficient exhaust gas to keep
boost down to the desired level. On the GTR the boost level usually changes
from 0.8bar to 1.1bar when changing the exhaust and induction system, without
employing any boost control setup. This is OK on the GTR because it can easily
cope with the additional boost. But other cars, such as the Subaru WRX could
experience reliability problems or even increased detonation because of
uncontrollable boost rises without suitable measures such as remaps to protect
Blower and Nitrous Exhausts. - Tuned exhausts can induce a vacuum which will suck out the gases.
Blower and nitrous engines are
completely different from turbo engines and behave more like NA engines than
turbo engines when in comes to the exhaust.
The exhaust responds more
readily to conventional exhaust tuning techniques using free flow or acoustic
wave tuning principles. We want to make use of wave movement to create a
vacuum which will pull exhaust gas out of the cylinder. This translates into
more powerful and efficient engines.
When the engine has very good
exhaust porting and suitable cam timing, proper exhaust tuning will exert a
vacuum on the cylinders and they will begin to draw in fresh inlet charge even
thought the cylinder is still rising at the completion of the exhaust stroke.
This extends the time available for the cylinders to completely fill with
air/fuel mixture, leading to higher power outputs. We can therefore, achieve
our target horsepower figure without advancing the cam timing as much. This
gives a more flexible engine with a broad power range.
Pumping Losses. - This refers to the power consumed in pumping the exhaust gas out of the
Power is consumed because
exhaust gases have to be pumped out of the engine. As soon as the crank
rotates past BDC, the piston starts its way up the cylinder barrel on the
exhaust stroke. During the exhaust stroke it must ram the exhaust gas out past
the exhaust valve and through the exhaust system. This consumes power provided
by another cylinder on its power stroke. This means that there is less power
available at the flywheel.
There are methods available to
cut these pumping losses to a minimum. One way is to open the exhaust valve
very early. This provides a net gain in the upper half of the powerband but
causes poor fuel economy at cruise.
A better and more considered
route is to ensure that the entire exhaust flow route is free of obstacles and
incorporates any elements that promote positive flow.
Exhaust Pressure Waves. - The
cylinder outlets should be mated to maximise wave resonance tuning.
As mentioned above, wave
resonance tuning is the primary means of increasing flow. This means
connecting cylinders in a particular order using exhaust tubing of a specific
length to take advantage of exhaust pressure waves to pull exhaust gas out of
the cylinders and during valve overlap, suck fresh air/fuel into the
cylinders. When we look at the firing order of the engine, we join cylinders
so that exhaust gas from one cylinder won’t cancel the pressure wave from
Exhaust Header Design. - Design the headers to take advantage of the firing order.
A typical 4cyl engine has a firing order or 1-3-4-2. Number one cylinder will
tend to draw exhaust gas from number three cylinder if all four cylinders are
connected together. Number three will draw from number four which in turn
draws from number two and so on. To overcome this, a tube type header with
branches around 10in long should be used
The design can be a four into
one or a four into two into one. For top end power use the four into one
design and allow plenty of dyno time to get the design right.
Baffle Collector Design. - Two
popular collector designs… baffle and merge collectors. - Baffle collectors produce sharper waves to aid suction from the cylinders. - More suitable for lower powered engines.
Baffle collectors are used to
join two into one. The primary tubes terminate abruptly into an open tapering
chamber that feeds into the tailpipe. When the primaries terminate like this,
a pressure wave moves up one primary as a result of the gas being expelled
from the other primary. If this wave arrives back at the exhaust port at the
right time it aids in the suction of exhaust gas out of the cylinder and also
sucks fresh mix into the cylinder. Depending on primary pipe length, an
opposite pressure wave will arrive just as the exhaust valve is about to close
to reduce the amount of fuel/air that overspills into the exhaust.
This is the most positive
aspect of using the Baffle Collector as a primary to secondary joining method.
The downside is that at certain
rpms the waves arrive at the wrong times and cause the opposite ot happen. Ie
impeding exhaust flow out of the head.
Therefore, this type of
collector is best suited to road engines and mildly tuned race engines.
Merge Collector Design. - More graduated join. - Produces a lower intensity wave. - Engine relies more on exhaust gas momentum to expel the gases. - Better for high power engines.
Merge collectors are better for
heavily modded engines with long duration, big overlap cams.
The primary pipes
do not terminate abruptly but instead merge into the tailpipe. This reduces
the intensity of the pressure waves in the primaries which tunes out the
negative aspects of baffle waves.
Of course they don’t get the maximum effect
of the good feedback waveseither.
However, on max power with high end
engines, the tuner can rely more on exhaust gas momentum or inertia to rid the
cylinders of exhaust gas.
With this design, the primaries
must join at a taper of about 10deg. Some engines respond better to 12deg and
others prefer 8.5 to 9deg.
Tailpipe Design. -Can use a tailpipe or use CATs, baffles and silencers.
After the header, we have to
consider the rest of the exhaust system. On all out competition cars this can
mean just a tail pipe. On road cars to usually means a cat and one or more
silencers. Less tailpipe length will increase top end power and longer tail
pipes will yield more low and mid range power.
This is why some engines will
have a tailpipe not much longer than the collector. However, most tailpipes go
right to the back of the car.
Tailpipe diameter is determined
on the dyno and if a cat is used the diameter after the cat needs to be
A guide to blown and nitrous
80 to 120bhp – 1.875in
110 to 140bhp – 2in
130 to 150bhp – 2.125in
140 to 185bhp – 2.25in
180 to 220bhp – 2.5in
210 to 265bhp – 2.75in
250 to 320bhp – 3.0in
280 to 360bhp – 3.5in
400 to 500bhp – 4.0in
480 to 630bhp – 4.5in
580 to 750bhp – 5in
As always the entire system
must have a minimum of bending and adequate bore size. When ground clearance
is a problem then oval exhaust bores can be used with no penalty in flow. You
can reduce the diameter by ¼ in at the end (beyond
the real wheels) without incurring any flow restrictions.
Silencer Design. - Straight through or reverse flow silencers are best.
A well designed silencer,
placed to the rear of the car won’t drop power by anymore that 3-5%. The
closer to the engine they are fitted, the more flow restricting occurs and the
more power is lost.
Straight through or reverse
flow silencers are the best for power applications. Straight through provides
the best sound deadening and are good for turbo cars. However, on NA cars two
silencers or a silencer and a resonator must be used to stop popping occurring
on overrun. The quietest silencers have an open resonating chamber in the
Reverse flow silencers are
different and don’t contain any sound deadening materials. This makes them
lighter, but poorly designed ones are loud and cause power loss. The main
advantage is that they are good at suppressing popping when you lift off the
CAT Problems. - Don’t use CATs if possible. - Otherwise
use performance CATs with twice the flow of regular CATs.
CATS that melt due to excessive
temperatures caused by combustion in the exhaust will cause a problem by
blocking the exhaust flow.
However, even healthy CATs can
cause problems. A poorly designed CAT can impede exhaust flow due to
turbulence caused by the gas having to enter the honeycomb at 30deg angles or
more and having to exit the honeycomb at the same angle. To overcome this a
performance cat must have a gentle entry and exit taper of about 10deg. A
performance cat will have double the flow rate of a regular cat.