Steve Turner
Former Editor, 5.0 Mustang & Super Fords
October 1, 2009
Photos By: Stever Turner
Out back, the three-piece rear wing steals the show, as its wraparound design really softens the factory styling a bit, and greatly improves our least favorite view of the '10 'Stang. The lower rear valance is an all-new Roush piece built from OEM-spec ThermoPlastic Olefin, and while it is more aggressive in its visage, it still dominates the rear of the car like the stocker. The Roush exhaust is installed post-title at the dealership.

Balance is the essence of the Roush 427R. It packs equivalent levels of power, handling, and braking and keeps the ride quality and comfort even on the other side of the seesaw. Sure a bit more power would always be nice, but at the end of the day, the 427R is more than enough Mustang to rock your world.

On The Dyno
I absolutely love chassis dynos as they take me out of the testing equation. When it comes to driving, I'm anything but repeatable while the chassis dyno is known for repeatability. Once the car was strapped down to the Mustang Dyno at Moore Tuned in Lakeland, Florida, we learned that Roush really has the tuning dialed in on the '10 427R. After a normal morning drive, I let the car sit for 15 minutes before making a pull on the dyno. Our corrected baseline numbers were 339.6 hp and 316.3 lb-ft of torque.

Those familiar with other chassis dynos-like the inertial Dynojet models-already know they often result in slightly higher numbers than the eddy-current Mustang units, which load the vehicle based on its weight. We once compared the two, and our testing showed about a 13-percent difference between the Mustang and Dynojet, so our Mustang 339.6 hp pull would be about 383.7 hp on a 'Jet, which sounds about right for a car with 435 at the flywheel.

Thanks to a big leap in the quality of the stock interior, there isn't much sprucing needed to really doll things up. Headlining the improvements in the 427R is the optional Roush Sport leather seating, which amplifies the stock seats with increased padding and a vibrant, body color inset. Of course, the Roush-logo'd release button on the armrest rounds out a flurry of logos from the floormats to the pedal covers, all confirming the 427R is no mere Mustang. As always, the familiar high-leverage Roush retro shifter adds a bit of visceral personality to the driving experience.

After working the car on the dyno to build up heat, Allen made a second heatsoaked pull, and power only dropped off to 336.8 hp; torque actually jumped up a bit to 318.3 lb-ft. "The Roush low-temp radiator gain also isn't captured in a formal SAE net power run," says Craig Barker, powertrain development manager at Roush. "The larger water capacity can absorb more heat, and the larger frontal area of the low-temperature radiator (a.k.a. heat exchanger) will remove the heat faster. This will be most noticeable in lower average water temps during a road-racing situation, more so than a short drag race, and not noticed at all in a formal dyno run." So while the new system can produce a lower average air-charge temp in driving situations, it seems the tuning doesn't aggressively roll back timing based on air-charge temperature, as is common factory practice.

Roush engineers are pretty keen on the factory airbox, but caution that its grille-fed induction doesn't come into play on the dyno. "The net effect of the new 2010 air induction increases as vehicle speed decreases, a gain not captured during a formal SAE power run on dyno because induction air is always controlled to 77 degree Fahrenheit dry air on a SAE dyno run," Craig explains. "Vehicle speed does not play a role in the formal power rating on dyno."

That said, we couldn't help but try and see what the induction system would do without sucking through the factory filter and airbox. Allen simply popped the lid off the stock airbox, and propped it up so there was a small gap for the inlet to breathe through. On the earlier cars, the filter played a vital role straightening the air for the mass-air sensor, and they would run poorly without the filter. Not so with this '10. Though running the car this way leaned out the air/fuel mixture slightly, it wasn't to a dangerous level, and the power really jumped up. Our open-airbox run resulted in a corrected 351.3 hp and 322.5 lb-ft of torque at the wheels, so there may just be some power from a freer-flowing airbox, especially when you start dialing up the boost.