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 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.
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.
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 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.
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 carefully consider:
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 mind.
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.