Nick Heidfeld has revealed that one of the reasons he has struggled to match his team-mate Kamui Kobayashi is because he is in a 'worst position' with regard to engines.

Heidfeld, who was drafted in to replace Pedro de la Rosa after the Italian Grand Prix, has been out-qualified twice so far by his team-mate and has yet to beat him in a race - although encouraging both finished in the top ten and scored points in Korea and Japan.

"Interestingly I did always run exactly the same downforce setting as Kamui," Heidfeld told his personal website when asked by one of his fans why he is always down on Kobayashi in the speed traps.

"But I might be in the worst position in regard to the engine situation. The team has lost three of just eight allowed engines on my car this season. To make matters worse this happened at an early stage where the engines were pretty fresh.

"Therefore the remaining engines have to cover a lot of extra miles. This unfortunately leads to an engine power loss owed to the fact of having 'older' engines, which is the reason for losing out on acceleration and top speeds," added the Sauber man ahead of this weekend's F1 2010 Brazilian Grand Prix at Interlagos.