macca replied:

g/day Ian. I fitted genie extractors and a larger 2.5 inch exhaust system to my 4.2 diesel a few years back.And they have made a vast improvement in fuel economy and torque. The cruiser hangs on longer in top gear when climbing long hills.I when for the genie system. Rather than the HM headers because they followed the same line as the original exhaust system. The HM headers hange down under the chassis rail and i could see them being damaged in the bush. Hope this is of some help to you regards macca

Peter replied:

G'Day Ian
I also fitted a set of headers to my HZ1 recently. Engine is certainly more powerful right through the rev range, and it can now rev far higher than before. It also runs more smoothly throughout the range. Not had much change in fuel economy but I think I need the setting adjusted anyway. Had the full exhaust system replaced at the same time, as the original silencer had started to leak. They fitted a performance muffler and no expansion box. The noise was terrible. I ran it around for a couple of days like that but had to take it back as it was giving me a headache. They then fitted what they called a Hot-Dog to the end of the pipe which made a bigdifference to the noise but doesn't appear to have effected the performance at all. Hope this helps. Cheers Peter