To get back to the original question, after all the discussion. It seems to me unlikely that the exhaust system is causing Andrew's poor performance, although it may contribute to it. Maybe he will let us know what difference it makes when it is fitted. My view is that the exhaust system has an effect on engine performance as follows. As a general rule, maximum power from an engine (in this case a normally aspirated diesel) will be when it processes the maximum volume of air, assuming that the maximum possible amount of fuel is supplied, and ignoring losses. At first glance, this would seem to suggest that maximum power would be with no exhaust system at all. However, it is possible to arrange for the pulses of gas in the exhaust to set up a standing wave with a low pressure at the exhaust port at a particular rpm. If this is arranged to be at the maximum rpm, it will give maximum power, and this can done to get more power than would be gained from having a completely open exhaust. For most practical vehicle engines, however, this exhaust tuning is used to increase the power at less than maximum rpm, usually to spread out the torque curve. The same sort of thing can be done with the intake. More crudely, simply having back pressure (and a restricted intake, lumped together by Ricardo as "fluid pumping losses), also spreads out the torque curve, despite limiting maximum power. With multiple cylinders, the whole business gets very complicated. JD