Speed

It seems that every 6 months or so, someone always asks about how to get more road speed out of a CCKW, DUKW or WW2 Chevy. This article hopefully will answer all of your questions and list all your options.

The FIRST thing to remember is that these are Lo-o-ong stroke engines. These engines will NOT accept over revving for any sustained or cumulative period of time so get that out of your mind right now. You will hear people state that you can go 60 mph in a CCKW. Why yes you can for a little while! Yes we did produce engines in WW2 like the B-29 18 cylinder radial engine that generated 200 hp every time a cylinder fired. An exceptional engine! The WW2 GMC 270 however is not one of them! The 270’s were made with the metallurgy from the WW2 era. They are much more advanced than most, but they still have their limitations.

If you want to go faster, there are basically 4 choices. More if you combine some of them:

1. Get an engine that can turn faster then 2750 rpm for extended periods of time.
2. Get “rubber gears” i.e. larger diameter tires
3. Replace the transfer case in the CCKW or DUKW with a Chevy transfercase.
4. Replace the ring and pinion gears with a numerically smaller ratio.

Options A,B,C, and D all apply to vehicles with Banjo axles. Only options A and B apply to CCKW’s with Split axles.

A.  The easiest and best way to increase the over the road speed of a CCKW or DUKW is to install a civilian 1950’s GMC 302 engine. You will move the top end up to 3250 from 2750 rpms and go from 91.5 horsepower to 160 horsepower! This will give you an 18% increase in speed from 45 mph to 53 mph. Because the 1950’s GMC engines will bolt right up and depending upon which model drop right in unmodified, they are you best and easiest choice. The 1950’s GMC 270 is another choice and is rated at 3250 rpms and 145 horsepower. They made both of these engines up through 1962.

B.  Next on the list is “rubber gears”. If you install 9.00 x 20 wheels and tires say from an M35, you will increase the speed dramatically! Your speedometer will be quite far off and you will need to modify the steering stop on the driver’s side to prevent cutting the inside of that tire, but it is easy! According to the information at Coker Tire, the difference in circumference between a 7.50 s 20 and a 9.00 x 20 NDT tire is about 10%. This will translate to 49.5 mph versus 45 mph with the smaller tires

C.  You can also replace the transfer case (Banjo axle’d trucks only) with a WW2 Chevy transfer case. This will bolt right in and only needs to have the front driveshaft lengthened. The Hi-low lever will now work backwards and you will need to cut the interlocking stop off of the front axle lever in the cab. With this Chevy transfer case, low range will not be as low (1.94 x 1 as opposed to 2.63 x 1) but high range will be 1 to 1 as opposed to 1 to .7 in the banjo CCKW and DUKW transfer case. This will give you a 42% increase in speed now up to 64 mph from the stock 45 mph.

D.  Finally you can replace all the ring and pinions (banjo axle’d trucks) with lower numerical ratio civilian 1.5 ton truck gears or with the M211 or M135 gears. The M211 and M135 gear ratio is 6.16 to 1 as opposed to the WW2 GM ratio of 6.66 to 1. The difference is only 8% or 48 mph versus 45 mph stock. To me it is not worth all that effort. If you change to the civilian 5.43 to 1 ratio (times 3 differentials), you will get a 25% increase or about 56 mph versus 45 mph stock.

There are a couple other items to consider: As you try to go faster, you will need more horsepower. This is an unfortunate law of physics. In addition, you will periodically want to stop…..

In Conclusion, if you mixed the civilian 302, with a Chevy transfer case and 5.43 gears, let’s see you could get (theoretically) a 210% increase. That is 94 mph!!!!!!!!!!!!!!!! AND you could still put bigger tires on it and get it over 100mph!!!!!!!!!