Jenson Button believes McLaren will endure a "very difficult" Mexican Grand Prix weekend given the demands on the engines.
The team expects this to be the toughest weekend of the year for the Honda package, and has in effect sacrificed grid positions knowing that it is likely to be near the back anyway.
Both Fernando Alonso and Button will have fresh V6s to the latest spec, with the Briton expected to use two during practice. McLaren sources suggest Button could incur a penalty as big as 60 spots on the grid.
"The biggest problem for me in the simulator is it's such low grip," Button said when asked by Motorsport.com. "Because you run in basically Monza downforce here, because of the air pressure.
"The wings aren't working, because the air flow is different. So it's really tricky.
"You would normally run a lot of downforce here, because there are a lot of slow speed corners, and also for the engines it's tough as well, for the compressor.
"For us it's going to be very difficult. Because of our compressor it will be the toughest race for us, possibly, which is tough."
Brakes will also be a challenge, according to Button, "because of cooling".
"There are going to be a lot of unknowns, which makes it interesting I guess."
Meanwhile, Button said that, along with Hungary, the last race in Austin was the best executed by McLaren in 2015 in terms of strategy.
"I think Hungary was good. It was a mess for me because I couldn't pit under the safety car, because Fernando pitted and he had a problem with his brake ducts, and they couldn't bring me in.
"So that messed me up a little bit in that race. But I think strategy-wise we've done pretty well in Hungary and in Austin.
"But we weren't quick enough in Austin, it was sheer strategy, and also with the changing conditions we were able to make a difference there. In the wet and in the dry we weren't quick enough.
"Still quick enough to have some fun and have some battles. I really enjoyed it, it was nice to be in the mix. We did everything right, which was good.
"There were so many chats on the radio behind the safety car. 'We want you to stay out, you're fifth,' 'The tyres have grained, I think we should come in,' There were lots of good chats and we made the right calls in the end."