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Making Turbo Manifolds, Part 1

Getting the exhaust gas flow to the turbine

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Heavy gauge bends
  • Manifold flanges
  • Planning
  • Construction hints
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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.

Materials Overview

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.

The Bends

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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 sharp.

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.

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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 flange thickness.

Buttweld fittings are available at engineering supply houses. In Australia, Blackwoods (www.blackwoods.com.au) has a good range of buttweld bends and fittings, including T-pieces.

The Flanges

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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.

Construction

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.

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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 Blackwoods.

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.

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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.

Planning

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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 carefully consider:

  • Access 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 stud!)
  • Access to stud or bolt holes for attaching the turbo to the manifold
  • Clearance of the manifold pipe runs to obstacles within the engine bay
  • Clearance of the manifold pipe runs to the oil feed, oil drain and water pipe fittings on the turbo centre housing
  • The 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 exhaust)
  • Whether 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.

Gasflow

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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.

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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.)

Construction Tricks

When constructing a manifold there are a number of things to keep in mind.

  • If 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 stage.
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  • The 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.)
  • Make 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).
  • If 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.
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  • The 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 flange.
  • Head 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.
  • The 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.

Conclusion

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 manifold.

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