What is changing between the Merlin 1D engine and the uprated Merlin 1D+ engine?

Question

SpaceX is apparently uprating the thrust of the Merlin 1D engine used on the Falcon 9, by about 15-20%.

What are they changing (in the engine, in the fuel) to achieve this new thrust level?

It has been suggested they will super cool the RP-1 to decrease its volume, thus increasing it's density. What else?

SES is willing to forgo a launch slot in order to let someone else be first to fly on the new uprated engines in Spring of 2015.

You know SpaceX is introducing into their manifest ... a modification of the current engine, with about a 20 percent increase in thrust," said Martin Halliwell, SES’s chief technical officer. "We’re making a decision internally as to whether we want to be the first to fly it.

It makes sense to do if possible, since it helps them recover payload capability they lost by reusing a stage.

Back in 2013 we heard Elon say:

With respect to the future potential for the rocket, I do think we've got.. I'm really happy with this rocket design. It's an incredibly capable vehicle. It's actually one of the biggest rockets in the world, it's worth noting, at about 1.3 million pounds of thrust, and we're only actually operating the engines at about 85% of their potential. Down the road, in future missions, we anticipate being able to crank them up to their full thrust capability, which would give about 165,000 pounds of sea-level thrust per engine. Anyway, it really is something that is, I think, going to serve really well for the commercial launch market, for government satellites and for Dragon, both crew and cargo. I believe its inherent reliability potential is better than any other rocket in the world. It will be up to us to show that it lives up to that reliability potential.

Answer 1

There isn't a lot of publicly available information.

I think they are planning to subcool the LOX in addition to the fuel. The increased density results in an increased propellant mass flow rate for the same turbopump speed. This will give increased chamber pressure and thus higher thrust and probably higher Isp. Hardware changes to the engine might include stronger thrust chamber structure - or they might use up some of the margin built into the existing structure. Likewise, perhaps the regenerative cooling will need to be improved. They might or might not increase the turbopump speed also, to further increase propellant mass flow rate.

Because the thrust increase was always planned for, some or all of these changes may have been anticipated at the time of the original engine design so they might not need to be made.