Honda tries 'spec 3' Formula 1 engine in Baku practice with Alonso

Although Honda had been coy about its plans for Baku ahead of track running, Autosport has learned that Alonso's McLaren was fitted with an updated internal combustion unit for the Friday sessions.

The new parts do not feature all of the upgrade elements that Honda has been working on over recent weeks, but the changes that were fitted were enough to leave Honda engineers encouraged.

Despite Alonso's day ending early due to a gearbox problem, and him not getting a completely clean run in the yellow-flag-interrupted afternoon session, straightline speed data has produced some positive signs.

With Alonso and team-mate Stoffel Vandoorne running more similar downforce levels in practice two, the main speed trap in Baku showed Alonso's new engine at 203mph, with Vandoorne getting just up to 195mph with the 'Spec 2' version.

There remains plenty of room for improvement though, with the fastest speed recorded through the trap being 220mph by Esteban Ocon's Force India-Mercedes.

Despite the clear benefit the 'spec 3' engine has delivered, Alonso will revert to the old version from Saturday.

The decision has been prompted by the fact that Alonso is already taking a 15-place grid drop for a new MGU-H and turbocharger, so Honda felt it made no sense to put extra mileage on the new unit unnecessarily on a weekend when McLaren will be at the back.

Switching engines will also allow Honda to check on any potential reliability issues that may have come up after initial running, with the 'spec 3' then expected to return at the Austrian GP in a fortnight.