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How come, the DeLorean's effects don't always start at 88 mph. For example, in Back to the Future 3 the effects started at 65 mph. Also, when Marty escaped the Libyans by going to 1955 the effects started at 86 mph. One other time, was when Doc and Marty were pushing the DeLorean with the train and the effects started at like 82 mph. Why do the time travel effects start at different times?

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The problem is that you are comparing technically different time-machines.

Over the course of the trilogy the DeLorean was changed 2 times.
So we have:

  1. The original powered by plutonium (1985)
  2. Upgraded version powered by Mr. Fusion (2015)
  3. MacGyver version with a vacuum control unit (1955/1885)

Versions 1 and 2 behave the same way:
Reach 88 mph > Flux Capacitor lights up > Flux effects start > Time travel

There do appear to be delays between achieving 88 mph, Flux Capacitor lighting up, and Fux effects, but I would credit them to the 1.21 gigawatts power requirement - it may take the different power sources different times to achieve the 1.21 gigawatts. Or there is perhaps some auto/manual setting (after all Doc would not want to time travel every time he hits 88 mph).

The significant exceptions is when lightning is involved:
1955 to 1985 was instant, once the lightning energy was received; also the 1955 to 1885 time travel - when the lightning hit the car was just hovering but still due to energy overload the time transfer was achieved.

The problems start to appear with Version 3, this Frankenstein's monster is not only old, but many parts were replaced with 1955 technology,...

So it easily understood that there are bugs.
One is that the flux buildup/effects need to start sooner, before 88 mph is achieved.

  • There's also different acceleration rates, whether the vehicle was going forward or backwards in time, and how much time was to be traveled. I don't think you can judge the effects of a machine we know next to nothing about. We've never seen a specific trip in the DeLorean duplicated exactly. The closest we've come is watching the trip from 1955 to 1985 twice, but those were the same instance. – John Sensebe Apr 19 '16 at 19:02
  • I would also add that Doc Brown's unintended trip to 1885 was because the lightning caused the DeLorean to spin rapidly, bringing the flux capacitor up to 88 MPH. That's why the flame trails appeared in a spiral pattern. – John Sensebe Apr 19 '16 at 19:04
  • @John Sensebe, they mostly travel +/-30 years so only the effects would be dependent on direction, but they are not. Also we can compare Einstein's +1 min travel vs Marty's -30 years. As to Doc's trip to 1885: You are mixing things, speed and flux/energy are independent requirements (otherwise Part 1 and 3 would have happened differently). What happened was lighting>explosion>fire tracks. Fire tracks appear in the place the car would have traveled if it stayed in the current time. Yes the explosion could have given the car 88 mph, but we just do not know why and how the speed is important. – Martin Handrlica Apr 19 '16 at 19:54
  • Robert Zemekis himself stated that the DeLorean went into a spin, causing the flux capacitor to reach 88 MPH. – John Sensebe Apr 19 '16 at 21:37
  • @John Sensebe: OK, but it does not correspond to what happened on-screen. A retcon? Still, it is just a minor, unimportant issue. – Martin Handrlica Apr 19 '16 at 23:08
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In the third movie the time circuits were replaced with analog 1955 parts. Presumably this analog time circuit doesn't have the clean "on-off" functionality as the original version built into a microchip. It makes sense that it's function would slowly ramp up. You can also see that the flux capacitor slowly brightens instead of immediately lighting up like in the first movie.

This is one detail that, IMO, they did a very good job on.

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