The Exhaust manifold is one of the most important parts that control the output of an engine. You can spend endless amounts of money on tuning your engine internally, but if the exhaust manifolds design does not complement it, your engine may not produce the power you expect. It may move the torque band into a different place, causing all sorts of problems. For example if the manifold you have chosen to fit on you engine is designed to give an all round mild improvement over the standard item, it would be useless to fit it to a full race engine. A full race engine would be strangled & as the engine begins to come on cam at quite high RPM, the exhaust gases would be totally restricted & probably generate noting like the power expected. If your engine has been designed to produce large amounts of torque at low RPM for a particular type of motor sport, e.g. National Hotrods or Grass track racing. If you were to fit a full race exhaust manifold it would not help the engine at lower revs to produce its full potential, since the manifold is more suited to higher rpm.
We have spent hundreds of hours testing manifold designs on the dynamometer & just by changing the diameters of manifold pipes by as little as a 1/16”, or its length by only ½ “, you can get quite dramatic changes in the characteristics of an engine. If the manifold you have chosen to use is badly made, or of the wrong design, the problems you will experience will increase as you tune your engine further as time goes by. This is because when the engine is only mildly tuned, although the exhaust manifold is still important with mild cams fitted, the manifold is called upon less to help with the charge of fuel rich air being draw in to the engine on the overlap. Overlap is when the exhaust cam is about to close and the inlet cam is just opening. But as the engine is tuned further the amount of overlap is usually increased. As this happens the manifold then becomes more important because it is used more and more to help with the incoming air fuel charge which the engine needs to generate power. If its design does not match the characteristics for which the engine has been designed, the manifold could be at its most efficient at the wrong RPM and have past its optimum when the cams begin to start working. Or maybe the complete opposite, the exhaust manifold may not be at its best until the cams have past the working rev range.