OK, I'll bite. How is it that you can get away with it in an 87 but not an 84?
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Because w/ the Rabbits there's a digit in the VIN# that differentiates between a 1.5/1.6/1.7 and a 1.8. There's nothing in the '87 VIN# that differentiates between an 8v and a 16v (both 1.8 motors). That's why.

And not all A2 Golfs came as GTI's either (w/ all the GTI goodies like rear discs and a close-ratio box), yet you can build any one of those cars into a GTI. You could even build a 4-dr version, if you were so inclined.