Welcome to Top Tech Questions. Oftentimes, readers contact us with questions about articles, or to praise us on what a good job we are doing. But our favorite form of reader communication is tech questions. Our Top Tech section is a place where you ask what’s on your mind, and we answer it. Got a trouble code? Wondering how to get your engine to make more power? Send us an email at jason.sands@sorc.com and we’ll do our best to answer it.
Towing MPG
Question: In a past issue, Diesel Power stated that a 7.3L Power Stroke would only be capable of 12 to 14 mpg while towing a 4-ton camper. That number seems a bit low to me. I worked for a grain elevator and made a few 350-mile trips to get seed and feed in my 6.5L-powered Chevy. The GCVW of the pickup on a certified scale averaged 21,000 pounds over a series of four trips. I averaged (combined between all trips) 16.4 mpg. Though my pickup isn’t stock, it’s very surprising to me that an indirect-injection engine can get better mileage towing than a direct-injection engine.
Mark Cargill
-via email
| Tests like our Colorado “King of the Hill” comparison are an awesome way to measure the durability and peak performance of diesel trucks, but the tradeoff is that fuel economy suffers during these full-throttle torture tests.
Answer: One thing that actually plays in favor of older IDI engines getting very good fuel economy is the fact that they have a much higher compression ratio than most modern diesels. In part-throttle situations with low boost levels (if the engine is turbocharged), the older IDIs are very efficient. Unfortunately, they don’t have the ultimate power potential of the new engines, but they are good on mileage.
Just like regular driving, your towing mileage depends vastly on your speed and how you drive. Driving down a country road at 57 mph probably isn’t going to achieve the same mileage results as cannonballing down the freeway at 80 mph. In addition to speed, gearing, tire size, vehicle weight, and aerodynamics all come into play.
During our tests, we’ve gotten a variety of towing mileages, most of which have been somewhere between 8 mpg and 15 mpg. Since most of our tests involve really steep hills (California’s Grapevine, Colorado’s Eisenhower tunnel), our mileage isn’t as good as it could have been had we been cruising on flat ground.
A Stronger ZF-6
Question: I am looking at swapping out the 6.0L Power Stroke in my Ford for a 5.9L or 6.7L common-rail Cummins and would like to make a true 600 “towing” horsepower. In addition to maintaining that power level, I’d also like to bulletproof the transmission, but it doesn’t seem like there is that much out there for the ZF-6 manual, other than upgraded clutches. Is there anything else to do to make the transmission stronger?
John S.
Sneads Ferry, North Carolina
| Zane Koch’s 871hp, 10-second Power Stroke lived a happy dragstrip life for years, on a set of cryogenically treated stock transmission shafts, which makes us believe there really is something to this process.
Answer: OK John, first thing’s first. Most trucks in the 600 to 700hp range can only use about 350 to 450 of that horsepower to actually tow before they overheat, their exhaust gas temperature gets too hot, or their transmission heats up too much. Since you have a ZF-6, you actually have an advantage in that it should be more reliable at a true 600 towing horsepower. The engine, on the other hand, will need some very precise tuning and a small set of very responsive compound turbos—something like an HE351 and 75mm S400 should do the trick. To keep heat down, you’re also going to need to run the truck virtually smokeless, which means something along the lines of EFILive tuning or even a Zeus standalone controller (available from Destroked).
As for the transmission, the ZF-6 is one of the strongest light-duty transmissions ever made, which is why there aren’t very many aftermarket parts available for it. One thing you can do, however, is cryogenically treat the input shaft, which is usually one of the greatest stress points. Cryogenic treating aligns all the molecules in the material by superfreezing it at -300 degrees, which results in an ultra-strong grain pattern and a more uniform piece throughout. As long as you’re treating the input shaft, it might not be a bad idea to cryo the gearbox’s internals, along with the ring and pinion in the rearend, as the hardened surface can last two to three times longer than an untreated ring and pinion. To learn more about cryogenics, check out www.300below.com.
I’d Like a Third Option
Question: I may be a little old school, but it seems to me that Ram keeps pairing up strong engines with weak automatic transmissions, and that the manuals are dying out. Why won’t the company offer more manual trans choices, or beef up their automatic offerings?
Johnny Hancock
San Antonio, Texas
| With power already being derated in Ram’s manual transmission trucks, we wouldn’t be surprised if the company eventually discontinues them just as GM and Ford have.
Answer: Unfortunately, there are less and less “old school” guys out there looking for a manual transmission. While Ram does still offer them in the company’s line of ¾- and 1-ton diesels, it’s the last of the Big Three to do so, with Ford discontinuing its manuals for the ’10 model, and GM for the ’06. Since traditionally automatic transmissions outsell manuals at a rate of nearly 10:1, it’s easy to see why Ford and GM decided to discontinue the line.
Automatics have also needed to be continually upgraded to match the increase in diesel torque and horsepower outputs, and speeds have been added to keep the emissions-laden engines in their powerbands. Since 2007, Ram has switched from the 48RE to the 68RFE to the Aisin A69RC as the torque of the 6.7L Cummins inline-six has increased. The new Aisin transmission is dimensionally similar to GM’s Allison 1000 and Ford’s 6R140, so the stout Aisin will definitely shore up Ram’s transmission line.
We’re not sure what the future holds, but we’ve heard of automatics in development that have integrated clutch systems instead of torque converters, or even electric motor-assisted autos. While you may not ever have the gear-banging feel of the old-school manual transmissions, you might be surprised at what’s coming down the pike.
