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.
How Much Horsepower Can a Stock Allison Handle?
Question: I have an ’08 Chevy with the 6.6L Duramax engine, and I’m looking to upgrade my truck with more power. I have a question about your March ’10 issue, in which it says the six-speed Allison can safely handle an additional 120 hp. I was wondering if this was referring to horsepower at the engine’s crank, or rear-wheel horsepower?
Garrett Stramel
Carthage, Missouri
| september 2011 Top Tech Questions allison Transmission
Answer: Predicting what kind of power the Allison transmission will handle in stock form is sort of like predicting the weather—there are lots of variables. We’ve seen light trucks with careful tuning run almost 12s with a stock Allison, while other drivers who get stuck in the snow (or do burnouts to impress their buddies) trash a transmission with stock engines. One of the main things to remember about the Allison is that it is a learning transmission, and it will try to tailor itself to the driver’s style. While this leads to a very smooth driving vehicle, it can also have its downsides.
If all you do is putter around town for a month and then suddenly you hook to a 10,000-pound trailer and go full-throttle with 120 additional horsepower, your transmission will have learned the lower power setting and sedate driving style, and you could be in trouble with more power and a ton of weight. Consistency is the key here, so if you do add more power, make sure not to go gonzo right away—do some half-throttle and three-quarter-throttle acceleration runs for a day or two to let the transmission learn the new power level.
So in closing, yes we’ve seen stock Allisons handle an extra 120 rwhp, but only through some very careful tuning and driving. If you’re the type of guy who doesn’t want to chance it, we’d recommend staying in the 60 to 80 extra-rear-wheel-horsepower range.
Lock it Up
Question: On project Triple Threat (your ’95 Dodge) did you install a manual torque converter lockup switch? I’m thinking of installing one on my truck, but if I use it for drag racing, will I risk snapping the transmission’s input shaft?
Colby Stonham
Medicine Hat, Alberta, Canada
| We’re hoping this little switch causing the converter to lock early will put nearly 80 more horsepower to the ground in our ’95 Dodge known as Project Triple Threat.
Answer: You know, we’re just getting around to putting a manual lockup switch in our ’95 Dodge, although we wouldn’t recommend it unless you have a billet-steel input shaft, aftermarket flexplate, and an aftermarket torque converter. Shifting with the converter locked is extremely hard on the transmission, and we’ve seen stuff break at just slightly more than stock power levels. Usually either the flexplate gets the center ripped out of it, or the input shaft breaks off in the converter, which means a new input shaft and a rebuilt torque converter.
Now that we’ve scared you, we’ll tell you about the benefits. Even on a mildly modified truck with 400 hp at the wheels, you’ll only get 300 to 350 of that horsepower to the wheels without the torque converter locked. So a lockup switch allows more power to get to the ground by eliminating converter slippage, which can be as much as 20 to 30 percent in some diesels. A converter that is locked up will also generate less heat, which is easier on the transmission (except during shifts). If you’re the racing type, it can also have a dramatic effect on 0-to-60-mph and quarter-mile times.
If you want to install a lockup switch to keep the converter solid while towing and don’t plan to change gears (say for instance, pulling a trailer up a hill in Third with the converter locked), then installing a lockup switch with stock parts is probably OK. Just make sure to unlock the converter before the vehicle comes to a complete stop, or the truck may stall. And don’t make shifts lock-to-lock without upgrades. If you do, and something breaks, don’t say we didn’t warn you!
A Diesel Rotary?
Question: Is it possible to build a diesel rotary engine?
Alan Huber
Simi Valley, California
| A diesel rotary-engine-powered Mazda RX-8? Sounds like a cool idea, but we’re not holding our breath for diesel rotaries to make their debut any time soon.
Answer: We get a few outside-the-box questions, but this one really had us thinking. Rotary engines offer some of the best power-to-weight ratios of any internal-combustion engine and are extremely simple, with very few moving parts. They do have their drawbacks, though: mainly fuel consumption, sealing, and emissions issues.
Another reason rotary (or Wankel) engines weren’t popular in diesel applications is they don’t lend themselves to compression ratios of more than 9:1, which is perfect for turbo or supercharged gasoline-powered stuff but would make for one hard-starting diesel engine. Back in the ’60s, Curtiss-Wright fiddled around with spark-ignited diesel engines ranging in size from 4.3 cubic inches to 1,920 cubic inches but never found a great amount of success when compared to the traditional piston engine design. Rolls-Royce was another company that took a stab at it in the ’70s, and its solution was to use two engines, one (the larger) as simply an air pump to raise the effective compression up to diesel levels, and a second smaller engine to ignite the ultra-compressed air. From our research, it wasn’t clear whether this engine actually ran, or whether it just made it to the prototype stage.
OK, so we’re another 50 years down the road now. Does that mean we’re ready for a diesel rotary yet? Well, maybe. There are several companies, such as Reg Technologies and Quasiturbine, that are working on rotary-like designs that just may work. In fact, Reg Technologies says it’ll soon come out with an engine that is 6x6 inches and can make 42 hp, with 24 combustion strokes per revolution. Although we’d all love to have an engine with that type of efficiency, price, emissions, and availability will probably keep diesel rotaries out of mainstream markets—at least for the foreseeable future.
