This is a question that has come up before. I say it is 2.4, BUT some others disagree with that, and some others agree. I did some research and I am fairly sure that I am right, BUT I will not stake my reputation on it because everyone makes mistakes.
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"Come on, ALL the cool kids are doing it...."
A freshly charged DigiQ car registers 2.7v (that's a nicely peaked 2.4v battery).
In contrast, a freshly charged iRacer comes in at 1.38v...HALF the voltage of the DigiQ (for the folks that don't know what that means...With higher voltage, the DigiQ can go SIGNIFICANTLY faster than an iRacer...Especially if you have the Digi Propo Special controller to adjust the speed to the MAX)