Muscle Mustangs & Fast FordsHow To Drivetrain
Off The Brake! - 4R70W Swap
We swap a 4R70W into our Three-Valve Fox with help from Performance Automatic and Summit Racing Equipment
There’s no doubt that driving a Mustang with a manual transmission on the track is fun. But what do you do when you want to increase your consistency? Use an automatic!
I like to think I’m a decent driver. I have a lot of seat time in a lot of different cars, and I’m happy with what I’ve been able to do in the cars I’ve raced. Our former Mustang, Project Shake ’N’ Bake, was a 3,700-pound Mach 1, and we got that to go 12.40s in road-race trim. Our Three-Valve Fox made less power with a five-speed, but went a best of 12.16 at 115.
While we were ecstatic about the performance, it wasn’t very repeatable. The car would normally perform in the 12.30 and 12.40 range, with occasional dips into the 0.20s and 0.10s when we did everything right. To race this car a little more competitively, we decided to swap out the five-speed for an automatic.
Our Three-Valve Fox is a mash-up of different and rather odd components. The base of the project is a Three-Valve 4.6L, the trans was a TR3650, and the rear suspension is based around a torque arm from Maximum Motorsports, so what is the right auto for it? We turned to Performance Automatic and Summit Racing Equipment for a trans with the strength to handle all the power we can throw at it on the track and street. The answer was a PA AODE/4R70W Competition Transmission (PN PA45101C-46L, $3,653.23).
We chose this trans for a few reasons. First and foremost, the 4R70W can handle a ton of power. It has overdrive, which makes street driving much more tolerable than a non-overdrive trans. And PA offers the 4R70W with a full manual valvebody and a transbrake. This means we will not need a transmission controller to handle shifting, and hard launches are as easy as letting go of a button. Plus, this keeps a good amount of the driver interaction, and driving the car isn’t boring.
|4R70W Gear Ratios|
|Fourth gear (Overdrive)||0.70:1|
When it comes to an automatic transmission, the torque converter is just as important as the trans, and we wanted a top-notch piece for this project. A call to Chris Sehorn at Circle D Specialties got the ball rolling on a custom billet converter tailored to our project from one of its 4R70W Pro Series Stage III 245mm billet converters (PN 30-13-04-5C, $1,100). The weight, torque, and horsepower, along with our plans for the car, dictated the Circle D build, and the stall is set at 4,000 rpm. Circle D designs and manufactures the billet components on its in-house CNC machine, so you get an extremely durable setup that is made in the USA.
After ordering the trans and converter, we ordered all of the supporting parts from Summit Racing Equipment. Our friendly parts supplier sent us a Hurst Quarter-Stick shifter (for a C4), a new transmission crossmember mount, a massive trans cooler, AN lines and fittings to connect it all, and the correct flexplate from PA.