Turbo Manifold Design

The Basics

The goal of any turbo manifold is to direct the exhaust gasses from each cylinder into the turbine, while wasting as little energy as possible. Unfortunately, however, it must do so day after day in 1500 degree plus temperatures without cracking, warping or simply melting away. There exist all sorts of manifolds for all sorts of applications, ranging from a basic log style manifold to an individual runner equal length unit. Each has its own distinct advantages and disadvantages, which will be discussed on the following pages.

Manifold Style

Virtually any manifold can be classified has either log style or tubular. A log style unit routes all the runners directly into one central area from which the high-pressure in the manifold forces the gasses through the turbo and out the exhaust. A tubular unit provides each cylinder with its own runner unit the collector directly before the turbine. This type of design allows for near even flow in each runner and greater efficiency due to lower backpressure. This can be taken one step further with an equal length runner design, which though nearly eliminating pulse overlap in the turbo improves spool time and overall efficiency.

Material

Mild Steel

Due to its low degree to thermal expansion, Mild steel is an excellent choice for flanges. Stainless steel, although stronger and more corrosion resistant, is known to expand under heat and rip off the bolts or studs holding the exhaust system together. However, its resilience to high temperatures and corrosion are hardly those of higher-grade steels, and is generally a poor choice for the runners of a manifold.

Stainless Steel

Stainless steel offers excellent corrosion and high temperature properties, and while not appropriate for flanges is a great choice for runners. 321 stainless steel’s coefficient of thermal conductivity is nearly 1/14^th that of mild steel’s, significantly reducing heat loss.

Cast Iron

Arguably the easiest material for mass production, when produced in volume cast iron makes a good deal of sense. However, due to the necessity of molds for its production, it is not appropriate for both custom applications and complex designs. Unfortunately, due to its high temperature properties, it has a tendency to crack after continuous (street) use.

Cast Stainless Steel

For mass production of street manifolds, this is the ideal material to use. Virtually impervious to cracking and corrosion, and easy to make, there should be more using it in production.

Heat Loss

Much of the energy that drives the turbo is in the form of heat. As exhaust gas heat energy is lost, the gasses slow down and consequently hit the turbine hit less force. Constructing a manifold that will allow for as much heat retention as possible is an obvious goal, and it can be achieved in several ways.

Surface Area

Heat must escape through a medium, and in a turbo manifold that medium is the metal of the manifold itself. Keeping the surface area of the manifold to a minimum will similarly keep heat loss to a minimum. Using smaller diameter runners will do much to lower the overall surface of the unit by a reasonable proportion, and at the same time promote higher exhaust gas velocities. However, using too runners that are too small will increase backpressure to needlessly high levels. For Honda motors, 1.5” inner diameter piping tends to be the norm.

Another method of limiting surface area is simply using higher gauge (thinner) piping. However, depending on the metal used and manifold design, this can lead to a structurally unsound unit, and is generally not a good method of increasing manifold efficiency.

Heat Wrap

Using heat wrap to provide a barrier between the manifold and the atmosphere will certainly reduce heat loss, but will also promote corrosion and often overheating to dangerous levels. Due to the high temperature / high pressure nature of Turbo manifolds, they are subject to quite a bit more stress then a normal naturally aspirated unit, so heat wrap is generally not a very good idea either.

Plating and Coating

Chrome plating or ceramic coating can do quite a bit to bolster a manifold’s corrosion resistance and reduce its heat loss, not to mention the attractive look. These are almost always beneficial processes, limited only by cost.

Other Design Considerations

Wastegate Integration

The placement of the wastegate on the manifold plays a large role in boost control. The exhaust gasses from each cylinder should experience equal or near equal resistance in traveling to either the turbo or the wastegate, or else the resulting uneven flow will lead to either an overworked turbo or an overworked wastegate. In manifolds where the wastegate is only easily accessible by one or two cylinders, such as the Drag unit, boost creep can quickly become an engine-killing problem.

Collectors

As with any exhaust system, the collector of a turbo manifold plays a large part in improving the flow and backpressure.

Expansion Slots

Even with a properly selected material, heat expansion can still cause flanges to warp and/or destroy the fasteners holding them on. Cutting simple slots along the flanges can do much to provide them with sufficient room to safely expand without doing any damage.

Bracing

Many manifolds, particularly long runner units built with poorly selected materials require extra support to avoid cracking. Usually a simple brace from the head flange to the turbo flange is sufficient, but for some applications a more extensive, more flexible support system is needed.

Gaskets

Due to the extreme high temperatures and pressures seen in turbo manifold, turbo to manifold gaskets are unreliable and best if done without. Precision machined surfaces and proper fastening seal just as well and are always less likely to produce headaches later on down the road.

Fasteners

Obviously, only high temperature nuts, bolts, studs and washers should be used in this sort of application. Stainless steel is by far the material of choice, and lock washers will do much to prevent unwanted loosening under the vibration of a high reving motor.