its prob down to how well the engine has been looked after in the past. other things to look at would be when was it last servviced ? how many miles has yours ghot compared to your mates? try changeing your deisiel could be bad stuff. you could try making the exhaust a straight through beating out the centre og all the box's cat etc etc would liven it up do a full engine flush engine cleaner , deisel cleaner to clean the injectors etc and oil treatmeant.