Nick Chester concedes not all Lotus' problems are because of Renault and is "pretty confident" they won't suffer the same fate next season.
Last season Lotus were fighting for race wins, this year the team is struggling to get close to the points.
With just two top-ten finishes from 11 grands prix, the team has eight points and is eighth in the Championship.
Many of Lotus' woes have been blamed on the team's Renault engine, which has lacked both power and reliability.
Chester, however, concedes not all the blame belongs to Renault.
Asked by the official F1 website if the team's struggles were 'tied entirely to problems with the power unit', Chester said: "I suppose not entirely.
"But much more so than it used to be: with the old V8s everybody had engines that were pretty similar, maybe within 20bhp. t is a lot bigger than that now - you can tell that from looking at the results.
"Probably this year it is more of a power unit formula than aero, when in the past it has always been aero by some margin. It has turned it around quite a lot.
"From the top to the bottom of the grid that deficit is probably more aero than it is power unit, but in the middle of the grid the power unit is making quite a big difference."
As for regulations aimed at the downforce on the cars, the Lotus technical director says downforce levels have not dropped as much as some have highlighted.
"It is a little down, but not much. When we started developing for the 2014 regs we were a long way down and then we had a very good build-up theory and we got to a level that we were reasonably pleased with.
"It is never enough - you always want to put more on the car - but it has improved to a pretty sensible level. If we had exhaust blowing, you would probably be equal, if not better."
He added that the loss of exhaust blowing had changed the dynamics of the cars. "It has, yes.
"A lot of drivers complain about traction this year, and I think some of that is down to having more torque, and some of it is that the cars have lost exhaust blowing, and I think that is making it harder for the drivers. The rear isn't as pinned on corner exit and it's harder to drive."
Looking ahead to next season, Chester is confident that Lotus won't be in for a repeat of 2014.
"Pretty confident," he said when asked if the team had learnt lessons for next season. "We had an awful lot of unknowns coming into 2014. There were some things we were really worried about that turned out not to be problems, and then some things we hadn't expected at all.
"So there was an awful lot of learning, and the early testing was really hard work, the hardest we have had with a new car since I have been at Enstone. There were whole systems on the car that we just had very little of experience of. But we learnt an awful lot from it."