Board Thread:Game Discussion/@comment-33905330-20180103074424/@comment-26829117-20180103082840

This isn't the simple question it seems but I'll give you some options and a lot more information:
 * You can unlock EK quite quickly through the career series if you focus on that as a goal and the Porsche 919 Hybrid (2014) only needs 258 to upgrade to get to 10 lapper.
 * EC is harder than EK because of rolling starts but you don't need to unlock the 10 lapper (there are 8 and 9 lappers which almost as good). The Audi R18 e-tron quattro (2015) is the cheapest car to upgrade but it costs 490 gold to unlock 10 lapper. You don't need to spend all of that though to reach the 8 and 9 lappers.

You can always unlock all races in a series with the best car upgraded to the right PR. The only races you can't do are those that require a specific car.

You need PR 95 to unlock the 10 lapper in EK and the Porsche is the most cost effective by over 600 gold.

Costs and PR requirements are on the Upgrades Strategy For 100% Completion and if you look at the series then you can judge see what PR is required for other options.

Recommendation? Personally, I'd run the 4 lapper in Classic Ferrari Showdown until I had enough gold to open the 10 lappers safely. It pays more fame per minute than the 10 lappers and requires no outlay beyond what is required in career to open up series.