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High-Stall Torque Converter Questions Question: I recently picked up a '99 Dodge Ram 2500 extended cab with the 5.9L Cummins engine and a four-speed automatic. It's got 3.54 gears and gets 22 mpg running 70 mph (2,000 rpm) without a trailer. One thing I didn't notice when first testdriving the truck was how loose the torque converter was until lockup occurred. I did a little research and my engine's torque peak is at 1,600 rpm. In my truck, the engine rpm easily holds steady way above that if I accelerate rapidly. I'm guessing the factory stall speed is around 2,400 rpm. In my mind, more slippage means more heat in the transmission and running the engine at an rpm that provides less torque will also provide less pulling power. Am I wrong? And if so, why?
Chris Baker Rosamond, California
Answer: We had the exact same thoughts as you until we put a low-stall speed torque converter (BD towing converter) on our otherwise stock '97 Dodge. It stalled at about 1,600 rpm, and the problem with such a low stall speed is the engine would sometimes lose boost during gear changes. When going up a steep grade, there were a couple of times when we got caught between gears at about 55 mph, where the turbo wouldn't light, and we were going too fast for the truck to downshift. We'd have to slow down to 50 mph, let the transmission kick down so the turbo would spool up, and then try to keep the vehicle speed up around 65 mph. With a loose converter, you won't run into this issue, because you'll always be on top of the turbo thanks to the high stall. And if you're just cruising down the freeway, there's always lockup to keep efficiency high. Eventually, the stock converter will slip excessively (or slip the converter clutch), causing huge power losses (upward of 100 hp), but not usually until the truck is making about 100 to 200 hp more than stock.
| As you can see by this dyno graph, a stock torque converter (blue line) actually makes about the same amount of power as the aftermarket converter (red line) until the top of the powerband. A looser converter will slip more but allow the turbo to spool sooner, which actually helps in OEM applications.
Tractor-Engined
Diesel Truck gets 40 mpg?
Question: I found a website called Shadetreeconversions.com that sells a book on how to put a diesel engine from a farm tractor into a truck. The site claims your truck will get 40 to 50 mpg by doing this. My question is, is this actually possible or do they just want the $50 for the book? I would love to see an article on this if it actually works.
Chris Grasty
Spring, Texas
Answer: Sure, they want the money for the book, but what they're doing is also completely possible, and having someone else perform the research and development of a swap is easily worth $50. On the fuel economy tests we've done, we've seen more than 20 mpg while testing trucks that weigh 7,000 pounds. Cut the weight in half and the engine size in half, and doubling the fuel economy isn't magic-it's just physics. We've seen almost 60 mpg in our '82 Chevette diesel under ideal driving conditions, so 40 mpg in an '80s Ford F-150 with a four-cylinder Perkins diesel seems realistic.
Fuel economy is largely dependent on driving style and driving type. Here in Los Angeles, real-world mileage on a daily commute is usually very poor due to the stop-and-go traffic. Our Chevette, which has gotten 55 to 59 mpg on numerous tests, drops to about 37 mpg in the city. So when these guys are claiming "up to 50 mpg," they mean in ideal conditions. Also, a 1/2-ton truck will be pretty darn slow with a 110hp Perkins diesel engine in it. There is virtually no car produced today that has such a poor power-to-weight ratio, and for a good reason.
While there are hop-ups available for the Perkins diesel, those wanting more power would probably be better off swapping in a 3.9L 4BT Cummins engine. These powerplants are found in Grumman stepvans and will most likely be mated to the popular GM TH400 or Allison 545 transmission, which makes them easy to swap.
Finally, remember that in the mid-'80s, Isuzu, Nissan, Toyota, VW, and Chevy all made light-duty diesel trucks that got 30 to 50 mpg, which can now be purchased for $2,000 to $5,000 on the used-truck market. While these pickups won't offer the power or load-handling capabilities of a conversion, they are still a good alternative for those looking for a bed, and mileage, too.
