Fernando Alonso is confident his upgraded Honda power unit is a step forward over its predecessor despite newfound reliability issues preventing him from completing more than 18 laps so far this weekend.

The Spaniard was always destined to start the Belgian Grand Prix from the back of the grid after a power unit change following FP1 resulted in 30 positions worth of penalties, but then more problems struck with his new engine in FP3.

Opting to send Alonso out in qualifying regardless, the Spaniard managed just 500m before his McLaren came to a halt at the top of Eau Rouge.

Despite the limited mileage harking back to the reliability issues that hampered his 2015 season, Alonso was nonetheless upbeat about the upgraded Honda's performance gains based on the result of ninth-placed team-mate Jenson Button.

"We saw some anomalies with the engine in FP3 and we didn't have time to do a proper check, so we started qualifying not knowing what will happen and unfortunately after 500m we knew what happened!" the double world champion sighed.

"We already start last so it is a difficult weekend for me and a little bit unlucky because Jenson has the same power unit with zero problems.

"Let's see... it is part of our learning process unfortunately. I think [it's better]. On the dyno, it is better and, on Jenson's side, it is delivering the power. On my side too, but I had four laps today. I will tell you tomorrow when I am fighting side by side with someone!"