If you can pick up a 1.60DZ for a good price, jump on it. The difference is about 200-300RPM, and is not that big of a deal. If you can't get a good deal on a 1.60DZ, then get the 1.70, but you will have more than enough power with the 1.60DZ. There are a few minor changes other than just the displacement, but power wont be a problem with either motor.

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