ON THE ROLLERS
QUESTION: I am getting ready to put my 6.6L Duramax-powered ’07 GM pickup on a local performance shop’s Mustang chassis dyno. But before I do, is there a way to lock the Allison 1000 automatic transmission in Fourth gear so it doesn’t downshift when it’s being tested? I’ve been playing with it on the road, using the Overdrive button and Tow/Haul mode, but it still keeps downshifting when I nail the throttle. It has a Stage 1 Garrett turbocharger and Edge Attitude with Juice programmer.
ANSWER: Our suggestion when it comes to making pulls on any dyno is to let the dyno operator do all the “driving.” They know their dyno and usually have a lot of experience getting different trucks’ automatic transmissions to function in the best way possible for repeatable numbers. The key is to turn Overdrive “Off” and roll just hard enough into the throttle to keep the transmission from downshifting. That takes practice. (Tow/Haul mode just changes the TCM algorithms to a more aggressive level where shift points happen 300 to 400 rpm higher, and the torque converter locks in Second gear at around 25 mph, which helps reduce EGT.) As you noted, your rig’s Allison has the Overdrive Disable button on the end of the shift stalk; pressing and holding it in until “O/D Off” appears on the dash keeps the transmission from shifting into Fifth gear. This is the perfect scenario for making a dyno run: The transmission is locked in whatever gear gives a 1:1 engine speed to transmission speed ratio during testing and remains in that gear with no downshift through the entire sampling window (typically rpm-based). John Lambert, the head of the tuning department at Hypertech, says he sets up the company’s in-house dyno so it replicates the wind drag and curb weight of a test truck as if it were running unloaded on the road. On GM rigs, he turns the Allison transmission’s Overdrive Off, and then accelerates until the gearbox shifts into Fourth gear (around 50 mph), backs off the throttle to get rpm down around 1,500, then rolls slowly back into it until it’s matted, and the wheel speed is up to around the equivalent of 100 mph. The object is to keep the transmission in Fourth from about 1,500 rpm to 3,100 rpm, or whatever the upper-rpm level is for that engine, while getting maximum boost from the turbo as early as possible. “This process produces accurate, repeatable numbers under realistic loads for the street,” John says.