The secrets of the McLaren-Honda package

The public debut of the rekindled McLaren-Honda Formula 1 partnership last month offered the first glimpses of the Japanese manufacturer's new engine.

From what we've seen at the post-season Abu Dhabi F1 test and the team's filming day at Silverstone prior to that, the Honda power unit appears to be at least partly inspired by developments we saw from the other manufacturers in 2014.

Although the Abu Dhabi test was not a successful one, due to electrical problems preventing the car running anywhere near a full programme, the two outings Honda already has under its belt with this package puts it far ahead of where its rivals were at the same time last year.

Aside from changes to accommodate the Honda engine, the MP4-29H IXI (H for Honda and IXI for the engine specification), externally the car remains identical to the late-season MP4-29 racer.

Honda revealed the first image of its power unit before the Japanese GP

But underneath the bodywork the car is very different as the Honda power unit differs in many ways from the outgoing Mercedes package.

In the week of the Japanese Grand Prix Honda produced renders of its engine. Although much of the detail was edited or hidden, we now know that elements of these images were accurate from the limited view we had of the installation on the test car.

It appears that the key difference is the turbo layout. As widely expected, the Honda engine features a turbo split between the front and rear of the engine, as per the 2014 Mercedes motor.

Images of the test car appear to show the airbox ('1' on the main image, below) leads towards the front of the engine and not the rear.

The split turbo concept has benefits in packing by reducing heat transfer from the exhaust-driven turbine side of the turbo to the compressor, shortening the turbo's pipework for less turbo lag, and critically allowing space for a larger compressor.

A rear-mounted turbo is limited in diameter by the fixed gearbox mounts and space to package the MGU-H inside the V of the engine.

Having a larger compressor creates more power, but this can be compromised by the larger turbine being slower to spin up at low revs, which creates turbo lag. Mercedes appears to have countered the lag issue with use of the MGU-H and electronic control of a wastegate and pop-off valve.

In the McLaren test car and Honda's publicity images, we can tell that the turbo's charge air is cooled by an air-to-air cooler ('2' in the main image) mounted in the right sidepod. This is a mirror of McLaren's 2014 Mercedes installation where the cooler was in the left sidepod.

There's no apparent benefit to the cooler being on a particular side, so it's most likely the turbo location and the mounting of ancillaries that defines which side the outlet appears on.

The air-to-air intercooler is a bulkier but lighter solution, compared with a water-to-air cooler. It also lowers the temperature of the air entering the engine, meaning it can create more power.

Unusually for an F1 engine, the plenum that forms the intake above the engine is made from aluminium, rather than carbon fibre. This may be a solution purely for testing in order to allow access to the variable inlet trumpets, which are now allowed in 2015. The pipe exiting the intercooler can be seen running back up to the top of the engine to connect the plenum.

As the intercooler is on the right of the car, the other coolers for engine water/oil and ERS are now in the left sidepod. To ensure these cooled the power unit enough in testing, the team created large slots in the sidepod in order to vent the heat.

McLaren could opt for Red Bull-style sidepods with its 2015 car © XPB

These openings are not a legal part of the regulations for races, but in testing such solutions are allowed. As with McLaren's 2014 race car the gearbox oil cooler is mounted above the gearbox and fed by a duct from the roll-hoop.

Mounted low down at the sides of the engine are large heat-shielded items ('3' on the main image) - these are likely to be the exhausts feeding the turbo. Unlike the majority of 2014 exhaust installations they are very low - mounted at floor level.

Presumably they tuck in around the back of the engine and lead up to the turbo, which sits in the front section of the gearbox.

The only other team to run such low exhausts in 2014 was Red Bull, which mounted a very long cooler package in the sidepods, with the intercoolers sited directly above the heat-shielded exhaust. This allowed much larger coolers within slimmer sidepods for better aerodynamics. Perhaps McLaren's 2015 MP4-30 will follow Red Bull's long and narrow sidepod philosophy?

Since pre-season testing does not begin until the start of February, McLaren and Honda now have three months to work through the issues found at Silverstone and Yas Marina, and complete the 12 days of pre-season testing, ahead of the first race of the season in Australia on March 15.