The BTB exhaust configurator will guide you through some of the choices you will have to make when deciding the specification of your exhaust manifold. The results of those choices will speed up the quotation process for you, and give a visual indication as to what the finished manifold will look like.
1. Exhaust port shape Oval or round.
2. Manifold Type 4-1 or 4-2-1.
3. Collector Type Box, spaced, spliced.
4. Step or No step.
5. Flange type Bar or individual.
6. Material 304, 321, Inconel, Polished SS.
1 Exhaust Port Shape. The exhaust port shape at the face which the exhaust manifold mates to the cylinder head, determines the shape that the exhaust primary pipe has to be formed into to match. A simple round shape is much easier to form from tube than an elongated oval, which must be carefully flowed to avoid sudden transitions that will cause turbulence. Care must also be taken to ensure correct sizing at this point, especially if the cylinder head has been modified. Hand ported cylinder heads can have variations which must be matched by the exhaust manifold for optimum performance.
2 Manifold Type. A 4-1 manifold tends to produce best power at high rpm, at the expense of mid rpm torque. 4-2-1 Manifolds can produce the best spread of torque throughout the rev range, and if specified correctly can give good high rpm power too. Cylinders must be paired correctly to give the desired result with a 4-2-1, and even with a 4-1 the firing order must be taken into consideration too.
3 Collector type. A box collector is folded from flat sheet that is welded around the pipes and is the simplest type to fabricate. A spliced collector is made from bent tubes that are cut in such a way as to merge with each other and gives a more attractive flowed look. If done correctly it can be used as an aid to cylinder scavenging by inducing a low pressure zone at the end of each joining pipe. The performance gains may be small but the skills required to make them (particularly 4-1s) are considerable. Spliced collectors can be made with swaged sleeves that allow the primaries to slip inside them, which makes complex manifolds easier to assemble. Spaced collectors leave a gap between each incoming sleeve to allow for clamps or heat wrapping, these are the most difficult to fabricate and therefore are the costliest option.
4 Step or No step. Manifold primaries (and secondaries) can be specified with a step change in diameter at a predetermined length. Generally an abrupt and significant step induces a pressure wave that can be used as pulse tuning aid in very highly-tuned engines with a high degree of valve overlap. The position of this step in the pipe is critical and must be carefully calculated. Sometimes a series of less pronounced steps can be used as an approximation of gradually tapered pipe, which can increase flow through otherwise compromised cylinder head porting. Ideally all steps must be matched across all the pipes in the manifold for optimum results. Stepped manifolds don’t lend themselves to automated tube bending operations and require a lot more fabrication and welding.
5 Flange type. A bar flange is a one piece item containing all the mounting holes with the correct spacing to match the studs on the cylinder head. There must therefore be sufficient space to be able to fit the whole manifold as an assembly. Individual flanges will only have the mounting holes relevant to that particular primary, and can be used with slip on collectors to facilitate assembly. If the primaries are to be welded to the collectors, then care must be taken to ensure that individual flanges maintain their correct spacing and flatness allowing for the distortion that welding creates..
6 Material. 304 is the standard first choice for stainless steel exhausts, being a good compromise between cost, corrosion resistance and ease of fabrication/welding. 321 is a titanium stabilised stainless alloy, with better resistance to heat, and is therefore an ideal choice for manifolds. “Inconel” is a trade name for a high nickel content alloy with extremely heat resistant properties. It is a common misconception that Inconel is lighter than stainless steel, in fact it is slightly denser. However given its unique ability to withstand very high temperatures, it can be specified with a much thinner wall thickness (or gauge) allowing considerably lighter components to be made without compromising their reliability. All forms of stainless steel contain chromium and can therefore be polished to a mirror finish if required.