Toro Rosso technical director James Key says the switch from Renault to Honda Formula 1 engines for 2018 is not straightforward because of the way the two power units are designed.
The different locations of the turbo compressors mean that fitting the Honda is not a straightforward swap into a car that was originally being designed around the French engine. McLaren faces the same challenge in reverse.
"They are completely different," Key told Autosport. "It's a very nicely packaged engine, but the whole power unit is a different architecture.
"It doesn't drop into the same space. There's quite a bit of car layout work that has to be done to adapt to it."
Key says it was important not to waste the aero work that had already been done for 2018.
"We try and stick to a rule that, if a car has already been in a development process for a while, not to upset any major items such as aero surfaces and that sort of thing, so we're not starting from scratch in too many areas," he said.
"We've adapted the car under the skin as best we can, and that's led to quite a different approach to the chassis design, to the way the gearbox works, and so on.
"We've carried over the concepts and developed them further from this year's car."
The gearbox, which is designed by Toro Rosso but uses internals from Red Bull Technology, will also be new, and shorter than its predecessor.
"Its layout and its size is different for this engine, compared to what we have this year," Key added.
"I think the concept is the same, but the architecture is different.
"Some internals will carry over, some will be changed, some will be bespoke to the STR box. Otherwise it will be pretty close to what we've been running."
Key insists that the Faenza team does much more of its own development than is generally believed.
"There's a lot more going on at STR than people think," he said. "People don't know STR, it's really frustrating.
"Although it makes a lot of sense to join together [with Red Bull] where you can, we've got our own aero department in Bicester, with our own windtunnel, and that's entirely independent because it has to be legally.
"The entire design of the car is done in Faenza - the only bit of the car that comes from RBT is the gearbox internals which are jointly designed, because there are often things that we specifically need."