The same rules regarding exhaust header design that apply to naturally aspirated engines also apply to turbocharged engines but with a few rather significant differences. If you haven't yet read our introduction to performance exhaust systems and our guide to effective exhaust header design, then do so before reading this section as this section builds on the information in our previous pages on exhaust systems.
Turbo Exhaust Headers
In our guide to exhaust header design, we did not discuss log-type headers in much detail as these headers are always less effective than exhaust headers with equal length primary pipes that joint together in a collector. However, on a turbocharged engine, you may not have enough space for an equal length header and a turbocharger. This space limitation would necessitate the use of a log-type header. In addition, the primary pipes of the exhaust manifold must come together at the collector before it feeds into the turbocharger and the size of the collector will be determined by the size of the turbocharger's turbine inlet.
In some cases you may even need to retain the stock cast-iron exhaust manifold. If this is the case, you should examine the stock exhaust manifold closely for imperfections that could restrict exhaust gas flow, much the same as you would do when porting the cylinder head. The aim would be to make the internal surface of the exhaust manifold as smooth as possible while keeping the shape and size of the primary pipes as uniform as possible, without weakening the manifold. By smoothing down the internal surface, you would not only improve exhaust gas flow, which would be crucial to reducing turbo lag, but you'd also reduce carbon build-up. Remember, however, that widening the primary pipes would reduce exhaust gas velocity and would result in a thinner manifold wall, both of which would result in an increase in turbo lag! A thinner manifold wall would have greater exhaust heat loss, which would mean a reduction in the heat energy that is used to drive the turbine.
When designing your own turbo exhaust header, you need to ensure that your header is strong enough to support the weight of the turbocharger, and that is can withstand the heat buildup caused by the turbocharger. This means that you have a choice of two materials when designing the exhaust header: steam pipe and bends, or stainless steel tubing. Stainless steel tubing may be easier to bend and shape, and would require less welding and grinding but you should ensure that the bends are formed in a mandrel bender that does not deform the inner radius of the bends.
Integrating the Wastegate
A major difference in the design of the exhaust manifold for a turbo exhaust system is the integration of the turbocharger's wastegate. As we've mentioned in our section on turbocharger boost control, the wastegate is used to control boost pressure created by the turbocharger, and to prevent the turbocharger from creating too much boost pressure. For this reason, the wastegate should be integrated into the exhaust manifold in such a way that it is exposed to as much of the pressure in the exhaust manifold as possible. This means that the wastegate should be located either between the collector where all the primary pipes join together and the turbocharger, or between the last exhaust port on a log-type manifold and the turbocharger. The wastegate should also be located at an angle that neither restricts nor interferes with exhaust gas flow as efficient exhaust gas flow is required to reduce turbo lag. In other words, the exhaust gas must be able to flow to the wastegate so that the wastegate can experience the correct exhaust pressure in the system without interrupting the exhaust gas flow.
There are also a few important aspects of a turbo engine that you must take into account with regards to your tail pipe. Firstly, the turbo increases the amount of air/fuel mixture that is fed into the combustion chamber and consequently increases the amount of exhaust gas that must be expelled from the engine. Secondly, the exhaust gasses of the turbo engine are much higher than a naturally aspirated engine; therefore the exhaust on a turbo engine will be more prone to heat expansion. The flange that is attached to the turbine outlet can experience temperatures of up to 1500°F! For this reason the flange should be beefed up and a minimum flange thickness off a ½ inch with additional bracing is recommended. The rest of the exhaust system needs to make allowance for heat expansion and should incorporate swaged joints.
The size of the tailpipe or diameter is also complicated by the size of your turbo and the boost you are running. Some tuners recommend a tail pipe that is 10% larger than the turbine outlet. This takes turbo size into account but not boost pressure! I personally prefer basing my tail pipe size on the amount of power produced by the engine. The diagram on the right is a good starting point for selecting a tail pipe diameter for your turbocharged engine. As with naturally aspirated cars, arriving at the ideal tail pipe diameter, as well as the ideal primary pipe diameter and length, will require some time on the dyno-tuner.