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What I don't get about Back to the Future Part II is how was Old Biff able to come back to 2015 from 1955 after he gave his 1955 self the Sports Almanac.

If you recall from Part I, the chance lightning strike over the clock tower was absolutely essential to getting Marty back to 1985 from 1955 because there are not enough megawatts in 1955 to get the flux capacitor to operate (or something like that).

So how was Old Biff able to come back? He couldn't have been able to set up that wire from the clock tower the way Doc did. There's no indication he even knew about wattage limitations in 1955.

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    I may be remembering wrong but by then doc had the future engine that would recycle junk into fuel right? so as we see at the end of the first one, no lightning needed.
    – GµårÐïåñ
    Commented Aug 1, 2019 at 2:27
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    Gigawatts. Not enough gigawatts in 1955. :) Welcome to the site!
    – KlaymenDK
    Commented Aug 1, 2019 at 14:11
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    @KlaymenDK don't you mean Jigger-whats?
    – HorusKol
    Commented Aug 1, 2019 at 23:41

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Biff didn't actually have to use the same plutonium-driven Delorean that Marty had to use in the 1st part. He used the Delorean that Doc had modified based on 2015 technology, which didn't need the high energy of plutonium to operate anymore rather than just garbage (nor did it need any roads either). We see this in the beginning of the film, where Doc just puts some garbage into the tank and is done with it.

In the same vein it's also likely that it was a lot more fuel efficient too, so Biff didn't actually have to go scanvenging dust bins either. The Delorean simply had enough fuel to go from 2015 to 1955 and back again. (Alternatively, he was aware of how the Mr. Fusion Home Energy Reactor worked from his time, too, and that you just need to put some trash into it anyway.)

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    Doc was planning to carry extra plutonium fuel with him in the original version of the machine so he could come back from the future, but the Libyans interrupted him before he loaded the car. So even with the “mark I” it’s possible to come back merely by bringing extra pellets.
    – Todd Wilcox
    Commented Aug 1, 2019 at 14:52
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    Bigger question is how did he know that driving 88 mph was necessary to operate the machine? Even if he knew it was a time machine, that's a hell of a guess. Did Doc leave the operating manual in the glove box?
    – Darrel Hoffman
    Commented Aug 1, 2019 at 19:18
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    @DarrelHoffman He doesn't have to know to get to exactly 88 mph. All he needs to know is that he has to accelerate to some fairly fast speed, and just keep accelerating until he hits it. And to know that, he only has to have seen it in action at some point. Alternatively, he just stumbled upon the ability after just driving it really fast after having messed with some buttons. We don't have any idea how much time passed for him from stealing the Delorean to its return.
    – trlkly
    Commented Aug 1, 2019 at 20:37
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    Even bigger question is how did he end up in the same 2015 he left, instead of traveling down the alternative path he created with his travel, i.e., the place where he is rich and powerful But that's a completely different question.
    – Spiros
    Commented Aug 1, 2019 at 22:55
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    @Spiros That question is already here
    – RedBaron
    Commented Aug 2, 2019 at 10:21

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