Why Is the GMC Twin Six V12 the Biggest Gas Monster Nobody Talks About?

GMC quietly built one of the most extreme engines in U.S. history: the GMC Twin Six 702 V12. This massive 11.5-liter gasoline V12 was designed for brutal, nonstop work.

In this deep-dive automotive documentary, we uncover the shocking truth behind theGMC Twin Six: a 1,500-pound V12 capable of producing massive low-RPM torque and running up to 15,000 hours continuously. Built by essentially combining two heavy-duty GMC V6 engines into a single block, the Twin Six powered firetrucks, irrigation pumps, generators, and industrial equipment where failure was not an option.

We also follow the Twin Six’s unexpected comeback in the custom car world, including its role in extreme builds and modern restorations capable of producing over 400–600 horsepower. From forgotten workhorse to mechanical legend, this is the full story of one of Detroit’s most audacious engineering experiments.

If you love forgotten engines, rare V12s, classic trucks, fire apparatus history, and deep automotive engineering stories, this video is for you.
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Credit:

Special thanks to Waylon Wire Old Iron @1963impala2drwaylonwire for their original footage, which contributed to the making of this video.

Check out their amazing work:
   / @1963impala2drwaylonwire  

We also acknowledge and appreciate other creators whose footage may appear in this video.

If you are the rightful owner of any material used in this video and have a concern about its use, please contact me at cardocumentary@gmail.com and I will address it promptly.

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