The Briton said that while the rate of development in aerodynamics during the course of the season would be steep, it would no longer be the dominant factor.
"The importance of aerodynamics to the championship is going to be at least as important as the differences in power levels between the various engine manufacturers," Allison told the Ferrari website (www.ferrari.com).
"However, if I had to choose the thing that was likely to be the dominant factor for the whole season, I would choose neither the level of power nor the aerodynamic development.
"I would say that this year reliability is going to be absolutely fundamental."
The new power unit, a V6 turbocharged engine with energy recovery systems, represents a huge challenge for engineers. Drivers are now limited to 100kg of fuel in the race, instead of 140kg, and are allocated five engines per year instead of eight.
Former Ferrari champion Niki Lauda, now non-executive chairman of the Mercedes team, also highlighted reliability on a visit to his old team for discussions with Ferrari president Luca Di Montezemolo about the sport in general.
"The combination of new car and installing the car and engine in a way that (means) you are reliable, this is the biggest challenge," he told the website.
"Whoever has less failures this year will be world champion."
Teams start pre-season testing in Jerez, southern Spain, on Tuesday although Mercedes completed 40km on a filming day at Silverstone on Friday.
Ferrari team principal Stefano Domenicali expected the first test to be tough for everyone.
"The technical challenge is, as far as I can remember, the biggest one that we have seen in the last decade of F1," he said.
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