ON THE ROLLERS

Diesel Power - - Top Tech Questions - Chip Rodgers via e-mail

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.

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