The DUKW is normally equipped with a GMC 270 cubic engine and a Clark un-synchronized five speed transmission. With the standard engine the DUKW is somewhat under powered. One solution is to change the engine and transmission. A Chevy 350 cubic inch engine with a Turbo 400 automatic transmission is a much better power plant for the DUKW.
My brother and I made this modification for one of the commercial tour companies. The performance is much better on the road and in the water. Just put it in drive and go. No more double clutching which was required with the original transmission. Now anyone can drive it without the fear of grinding gears and burning up clutches. Plus replacement parts are much easier to get. With the original engine few auto parts stores had replacement parts in stock, which means that you had to wait for them to order what you needed. With the 350 you can get parts just about anywhere.
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