Given that Stansted is Ground Zero for the Low Cost Airline business in Europe, why on earth are you flying to Heathrow?

Have a look at www.whichbudget.com & www.skyscanner.net to see if there are any LCA's that fly from where you are in Europe to Stansted.

As for getting between the 2, the coach is, in theory, about 1h25m, getting out of LHR itself is a minimum of 1 hr (unless you are citizen of the EU and haven't any checked luggage) plus the time it takes for you to reach a gate from landing.

Checkin at STN will, at the latest, close 30 minutes before the flight - but as it's to the US it's probably going to be longer.