The same rules regarding the 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
A log-type exhaust header
In our guide to exhaust header design, we did not mention log-type headers 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 have a negative effect on 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.