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May 2011 Nuts & Bolts

Transmission
Fred Williams
| Brand Manager, Petersen’s 4Wheel & Off Road
Posted May 1, 2011

This Months Readers Tech Questions

New vs. Old
Q I have a '77 Chevy Stepside with a 400 small-block, a TH350 transmission, and an NP203 transfer case. I'm thinking about putting a 6.0L LS motor with a 700R4 transmission in my truck. Do you think it's worth the swap, and if so, what would I need to buy along with the motor, transmission, and computer?
Scott
Via email

A Yes, it is worth the effort to do to the swap. The throttle response, ease of use, and general performance of the 6.0L LS series engine far surpass your old 400 in my view. I would contact Turnkey Engine Supply (760.941.2741, www.turnkeyenginesupply.com) for a set of old-to-new-style motor mounts. You will also need a wiring harness and a fuel system that includes a high-pressure fuel pump. You can get these through Pacific Fabrication (408.250.1772, www.pacificfab.net). Both Turnkey and Pacific Fab can also supply a new engine if you need one. There may also be an issue with your radiator and plumbing the coolant lines. Finally, I would recommend a 4L80E transmission instead of the 700R4 due to its added strength. You can find these rebuilt from Bowtie Overdrive (760.947.5240, www.bowtieoverdrives.com).

Megatired
Q I have an '06 Dodge MegaCab with a 5.9L Cummins Diesel. I upgraded the OEM tire size to 35-inch BFG All-Terrains. Do I need to change my gearing from stock (3.73:1) to something higher? I do mostly towing, and the terrain is all flat.
Tony D
Windsor, Ontario, Canada

A Changing gearing for taller tires is a smart idea most of the time, but your situation may be an exception. Getting taller tires moving from a stop is tougher than with stock-size tires, which in your case was probably 31 inches. The difference is that you have a lot of torque with the Cummins diesel to overcome the added leverage of the 35s. Your engine should have no problem turning the 35s, and the 3.73 gears should be strong enough, but your clutch or transmission will end up having to work harder to turn the bigger tires.

In our Project White Truck, a '10 Ram 3500 with the Cummins, we also went to a 35-inch-tall tire, but we stayed with the stock 3.73 gears. The six-speed manual in the White Truck has a low First gear that we rarely ever use when driving around town empty, but we do use it when starting out with a trailer.

In my opinion you should be fine with the 3.73s even if you are towing. Possibly adding wear to your clutch or transmission by staying with the 3.73s is a concern, but I'm not sure I would go through the trouble and cost of upgrading to 4.10s just for 35s, unless the transmission or clutch have a ton of miles on them already. If so, you're going to eventually spend money either on a clutch, auto transmission upgrades, or with a gear swap. Plus, you may notice that the taller tires allow your engine not to spin as fast at highway speed as it did prior. This is due to the tires acting like a taller gear. There is even the possibility that your mileage will increase over certain terrain. Also, the taller tires will cause your speedometer to read slower than your actual speed. You can get a programmer to correct this or get/borrow a GPS and track your actual speed to what your speedometer says.

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