This article was first published in 2005.
If you fit a turbo to a naturally aspirated engine, or decide to
upgrade a turbo to the extent that it no longer fits the
standard manifold, you’ll need to make a new manifold. But if you’re on a
budget, how do you go about achieving that? Workshops charge an arm and a leg
for custom manifolds, but if you can use a few power tools and can access the
services of a welder, it’s not hard to do it yourself.
Two different approaches can be taken to the fabrication of a bespoke exhaust
manifold. The most common approach is to make the manifold from heavy gauge,
mild steel “steam” pipe. Alternatively, thin stainless steel bends can be used.
The heavy gauge pipe is more durable and in many ways is easier to ‘work’ – eg
it can be held securely in a vice.
In addition to the pipe, flanges will also be needed at the head and the
manifold. These flanges must be thick so that they do not distort under heat and
pressure, so allowing leaks. A flange that is 10mm thick after it has been faced
is appropriate for most applications. (More on facing in a moment.) To allow for
this machining, it’s good to start with 12mm plate.
While known to everyone into modified cars as “steam pipe”, the sorts of
bends used to make manifolds are not actually called that at places where you
can buy them! Instead, they’re called “buttweld fittings” and as the name
suggests, are designed to be welded together. The buttweld fittings of most
interest to manifold constructors are the bends. Most commonly used are 45 and
90-degree bends, which are available in short- and long-radius forms. Most
manifolds are made from long radius bends which as their name suggests, are less
Buttweld bends are available in a variety of sizes. Typically, the quoted
size is for the inner diameter (ID). For example, buttweld bends can be bought
in 13.9, 18.9, 24.2, 32.4, 38.1 and 49.3mm inside diameters. The wall thickness
depends on whether the pipe is standard weight or extra strong; in each case the
wall thickness also depends on the diameter of the pipe. For example, in 32mm
fittings (ie 1¼ inch), extra strong wall thickness is 4.85mm and standard weight
wall thickness is 3.56mm.
The diameter of the bends is normally matched to the diameter of the ports
(or with non-round ports, the cross-sectional area). However, if the ports are
very large, a smaller diameter fitting may be used with a transitional cone
either incorporated into the first section of pipe or formed within the manifold
Buttweld fittings are available at engineering supply houses. In
has a good range of buttweld bends and fittings, including T-pieces.
Normal mild steel is able to be used to make the mounting flanges for the
manifold and the turbo. In most cases, flat bar can be bought – it’s readily
available in a 100 x 12mm size that is suitable for most turbo manifold
construction. If bar is not available, flat plate can be used instead.
Buttweld bends are able to be cut with a hacksaw. This is made easier because
of their strength – they can be tightly clamped in a vice without unduly
distorting. By using a combination of 45 and 90-degree bends, and cutting and
shaping their ends, it’s possible to create all sorts of patterns of pipe
direction. Buttweld fittings come with a chamfer ground on their ends, allowing
good penetration of the weld bead. When the bends are cut off short or are
otherwise modified, the chamfer should be re-ground.
It’s important that you have a good selection of bends available. Even though
it may appear on paper that (say) four 90-degree and two 45-dgree bends are all
that will be needed, it’s is wise to buy twice as many bends and so have a
variety of options available. The bends shown here cost about AUD$8 each from
The manifold and turbo mounting flanges can be cut from the flat bar in a
number of ways. These include laser cutting, water jet cutting and oxy-acetylene
cutting. For a one-off job, oxy cutting is the simplest: it’s gives the poorest
edge (it must be cleaned-up with an angle grinder and/or filing) but any
workshop or welder with oxy equipment will be able to do it.
The welding can be done by MIG or stick. Stick (arc) welding allows the
selection of a high-strength electrode to suit the application – in the case of
the manifold shown here low hydrogen rods were used which are both very strong and also
reduce the chance of hydrogen embrittlement.
Before any welding is done, it’s wise to play around with the bends and
manifold plate until the best possible outcome is realised. In addition to gas
flow considerations (we’ll come to those in a moment), you should also very
to the studs or bolt holes for attaching the manifold plate to the head (it’s
very easy to place a pipe so that you won’t be able to get a nut on a
to stud or bolt holes for attaching the turbo to the manifold
of the manifold pipe runs to obstacles within the engine bay
of the manifold pipe runs to the oil feed, oil drain and water pipe fittings on
the turbo centre housing
location of the turbo, especially with regard to the plumbing runs (inlet from
the airfilter, outlet to the intercooler, inlet from the manifold, outlet to the
the turbo centre bearing will be correctly placed so that its oil inlet is at
the top and its outlet at the bottom (in some turbos you can rotate the exhaust
and/or the inlet housings to allow this to be varied, but in others you may have
much less flexibility!)
It’s extremely easy to overlook one or other of these so it makes sense to
write a checklist and each time you think that you’ve come up with the optimal
turbo manifold shape, go back over each of them. Good manifolds are made in the
planning, and the easiest way of doing that is with a bunch of bends on the
bench in front of you.
If you look at many custom manifolds you’ll see what can only be described as
works of art. Enormous pains have been taken to achieve equal-length runners and
the bends are superbly gentle. And if you have a huge amount of time and want to
achieve the very possible outcome, that’s great! But we suspect that turbo
manifold construction is one of those areas where the law of diminishing returns
very strongly applies.
Factory cast-iron turbo manifolds seldom conform to the
long-runner-gentle-bend school of design but there are plenty of stories of
excellent power being gained from factory exhaust manifolds running much larger
than standard turbos. So, if in the real world, the factory exhaust manifold appears to perform much better than many suggest, a fabricated manifold
with similar short-length branchings is also likely to be quite adequate in
nearly all applications. (We’ll cover more on the theory of turbo manifold
design next week.)
When constructing a manifold there are a number of things to keep in
you are going to grind back the welds so that the manifold appears to be made in
one piece, it’s likely that you’ll need to do some of this grinding at intervals
through the construction process. That is, you can’t expect to take all the bits
to a welder and have them welded together in one go – and still be able to fit
an angle grinder in all the required places! Instead, the manifold will normally
be constructed in stages, with the grinding done at the end of each
outside appearance might be nice but it’s the inside of the pipes where the gas
flows! As with the external grinding, the inside of the pipe will need to be
smoothed during the construction process – eg where a weld has penetrated the
pipe. The best way of doing this is with a carbide burr in an air-grinder – if
the air tool is not available, high speed electric grinders designed to work
with carbide burs are available. (Metal-cutting carbide burrs are available from
professional tool suppliers – not usually from hardware stores.)
sure during construction that all scale and small particles of metal are removed
from within the manifold runners. If these remain, they are likely to become
detached when the manifold gets really hot – and then go through the turbine... In
addition to mechanical means (using a screwdriver, chisel, wire brush, carbide
burr), one way to remove these is to place the manifold overnight in a hot bath
of caustic soda (available from supermarkets and hardware stores).
the manifold flanges are cut out with an oxy, you’ll need to drill the mounting
holes yourself. (The oxy cutting isn’t accurate enough to make the holes.) If
this is the case, you must use a drill-press so that the holes are at
right-angles to the plates.
exhaust gasket is extremely useful as a template for laying out the manifold
pipes and showing the required shape of the head mounting plate and the position
of the bolt holes. If a gasket for the turbo exists, it can be used in the same
way to determine the shape and hole positions of the turbo mounting
and turbo mounting studs can be replaced with bolts, or bolts can be replaced
with studs. This change of mounting procedure can open-up lots of space that
might appear to be lacking. In really tight engines, a mounting plate can be
bolted to the head with countersunk Allen-key fasteners and then the turbo plate
bolted to tapped holes within the mounting plate. This allows much larger
diameter runners that otherwise would have fouled the head-studs.
manifold (and probably the turbo) mounting flanges will warp when welded. To
ensure a gas-tight seal, these plates need to faced – that is, to be machined
flat. A cylinder head specialist or small engineering shop will be able to do
this for you.
A simple exhaust manifold for a turbo is not nearly as hard to make as some
people suggest. Sure, there’re hours of cutting and grinding and filing, but
unlike (say) making a supercharger bracket, the location of a turbo is unlikely
to have to be millimetre-accurate. Especially if you have access to a friendly
welder, it’s a straightforward process which can save you at least half the cost
of buying one made by a high performance workshop.
Next week, we’ll show the step-by-step construction of a four cylinder turbo