Low 11-second quarter-mile times are pretty impressive for the first outing with any carùespecially if the only serious power mods are an exhaust system and a supercharger kit. The maiden dragstrip voyage of our AMSOIL Mustang GT was a huge success. The car went a best e.t. of 11.22 at 124 mph from A to B with no drama. Considering the GT is equipped with a 3.15 rear gear and stock converter, breaking into the 10s should be a breeze, right?
1 Aside from its smaller...
1 Aside from its smaller size, our new converter has a higher flash stall than the stock unit. The OEM converter flashes at about 1,900 rpm under WOT, and our new piece will flash at a much higher 3,200 rpm.
With any performance-based, automatic-equipped vehicle, performance gains can be found with replacing the stock torque converter with an aftermarket unit. After our first trip to the track, we knew the stock converter was leaving a lot of e.t. on the table. Stock converters are tight, meaning they don't let the engine rev up quickly to maximize acceleration. With the stock converter, it was nearly impossible to powerbrake above 1,500 rpm, which made for less than desirable 60-foot times. Blend that with some less-than-aggressive transmission tuning, and performance for us was not optimized--especially considering the power level of the engine.
In this installment of our AMSOIL GT buildup, we turn to Performance Automatic (PA) of Gaithersburg, Maryland, to enhance performance. The stock torque converter, flexplate, and two-piece driveshaft are being put aside in favor of stronger, lighter components, which are designed to perform on the street and track. At the same time, we are ditching the stock 3.15 gears in favor of a 3.73 ring and pinion from Ford Racing Performance Parts. We also hit up BMR Suspension for one of its driveshaft safety loops.
The installation of these components, although fairly straightforward, is time consuming. We turned to AntiVenom EFI of Seffner, Florida, to turn the wrenches, and before we knew it, Greg Lovell and Kyle Miller had the 6R80 six-speed transmission out of the car for the swap.
The stock torque converter measures about 11 inches, which is based on the diameter of the internal turbine. The stocker flash-stalls at about 1,900 rpm, but when you break-torque (powerbrake) the car on the starting line, you begin to overpower the brakes just above idle. This results in the car pushing forward, reducing rollout or causing a redlight.
The new converter is built by Circle D Specialties for PA, and is constructed from billet aluminum with a 9.75-inch turbine wheel, and is designed to flash-stall at about 3,200 rpm. The smaller design means less rotating weight, which will allow it to spin up quicker, and the higher stall of will let the engine get into the meat of the torque curve quicker.
"A stock torque converter is designed for fuel efficiency, and not for max performance," explains Harvey Baker of PA. "It doesn't allow the engine to get into its optimal rpm range quickly. A higher stall speed lets the engine reach it's peak torque faster, which is more efficient and gives you better performance.
"Another benefit of a higher stall speed is during shifts. With a stock converter, the rpm will drop significantly with each shift. A higher stall will keep the revs higher so the engine stays in its power band."
In addition to the billet torque converter, PA sent us one of its SFI-approved flexplates so we can be NHRA-legal. An SFI-approved flexplate is required when you reach 9.99-seconds or 135 mph.
Moving down the driveline, Performance Automatic also sent us an aluminum one-piece driveshaft to replace the stock steel shaft. The stock shaft has a joint in the center with a casing and mount that bolts in the trans tunnel, which could potentially be a weak link when big power is being transferred.
The stock two-piece driveshaft weighs in at a hefty 41 pounds. PA's new aluminum shaft checks in at a much lighter 22 pounds, which gives us a 19-pound weight reduction. In addition, the weight we lost is all rotational weight, which will help in the performance department.
2 PA also supplied us with...
2 PA also supplied us with one of its SFI- approved flexplates for the Coyote-equipped with the 6R80 six-speed automatic transmission.
3a The next piece in our...
3a The next piece in our drivetrain to go was the stock two-piece driveshaft. We replaced it with a one-piece aluminum shaft from PA...
3b ...The new driveshaft...
3b ...The new driveshaft is designed to bolt to the 6R80 auto, and comes with the correct adapter for the rearend.