Dyno Testing GT-500 Shelby Bolt-Ons

One look at the new Shelby GT500 can have you thinking it’s 1969 all over again. Now, before you are lost in the past — flash back into the present. It may look like a S197 Mustang with a body kit, but this wolf in sheep’s clothing is vastly different from the GT’s that roll off the lot next to it. Built on the same passion as the first, the killer thing about the Shelby GT500 is the potential that lies under the hood of the supercharged 5.4L. We’ve heard of some wild power claims, so we decided to dyno-verify some of the most basic bolt-on’s: a high-flow air intake, cat-back exhaust, and a ECU tuner.

We here at StangTV like to think we have a little bit of hot rod ability in the powerTV shop. However, it doesn’t take a good wrench to make big power with a GT500. Our goal was simple – adjust fuel and timing, add more air, and get it out more efficiently.

Not only did we write this article, but we produced a video blog.

Here was our plan to upgrade the GT500 and test it on our Dynojet:

  • Steeda GT500 Cold Air Intake — high-flow intake elbow combined with a new mass air flow sensor, velocity ring, stainless steel air box, and high flow Steeda air filter.
  • Gibson Cat-back Exhaust — high-flow Convergent Flow Technology Gibson mufflers combine with mandrel-bent stainless steel construction to speed exhaust flow and provide a great sound
  • SCT Flash Tuner — required with the Steeda Cold Air, the SCT Tuner optimizes the air/fuel ratio and timing curves to produce better throttle response, more power, and use less fuel.

The Baseline

The new GT500 is built not only to honor its history, but to create some. Just pulling in the GT500 to the powerTV garage brought all of the office guys out into the shop to grab a look at this modern day muscle car. It’s that type of car. Before we got the car up on the lift, we wanted to make a baseline run on the Dynojet.

A crowd quietly assembled. The 2007 Shelby put 450.19 hp to the wheels on the best run, and 450.48 foot-pounds of torque.

Here was the baseline Dynojet run

After the dyno runs, it was time to figure out our strategy. Bolt-on parts like a high-flow intake or exhaust may not always yield the same power as a smaller supercharger pulley, but to us, they are the building blocks of a performance engine. Sure, more boost equals more power, but why not improve the basic breathing of the engine before adding extra pressure into the equation. With the GT500, it responds well to both.

Cold Air Intake: Steeda GT500

There are a number of high-quality, high-flow air intakes on the market to choose from. The one we picked for this project is the Steeda GT500 cold-air which includes some fantastic features such as the velocity ring, the very nicely built stainless air box, and is available with a custom SCT tune-up that is already built for the Steeda cold air.

Steeda has been making parts for Mustangs for as long as we can remember, and the GT500 is no exception to their history of quality. The new cold air intake is made from a high temperature resistant plastic in a straight flow design, that leaves the incoming air a low-resistance path into the supercharger. Also included is one of Steeda’s lifetime air filters that can be easily cleaned and re-oiled to make maintenance a breeze. To top things off, Steeda also supplied a stainless steel air box with their built in Billet Super Velocity Stack that is designed to help increase air flow by smoothing out the incoming air.

Exhaust: Gibson CFT Mufflers

The question then came to exhaust. Again – many different choices in the marketplace. We picked a Gibson Exhaust for the GT500 because, frankly, Gibson has recently introduced a line of American Muscle exhaust systems and we wanted to evaluate their power and performance. Gibson is a premier name in the truck and SUV markets, and makes a very high quality product.

The GT500 kit we install was a full stainless steel kit for the GT500 — and included all of the exhaust from the x-pipe back, using their Superflow CFT Performance Mufflers. “CFT” is what Gibson refers to as Convergent Flow Technology, which produces a deep, powerful sound via a Chambered and Louvered design that doesn’t use packing or baffles. The CFT mufflers use an unique internal angular design that Gibson says creates a venturi effect inside the exhaust, building pressure in key places that actually pulls gasses through the system.

The stock GT500 exhaust isn’t bad. In fact, the Gibson GT500 tailpipes we installed looked similar to the stock kit, to the untrained eye. A closer inspection found that the piping was not only made from a better T-304 stainless material, but was a better flowing design as well with no crimps at all. In places where the stock system restricted air flow, the Gibson exhaust was mandrel bent. Also, we had the option of a new off-road X-pipe as well from Gibson, but the owner decided to leave the GT500 in “smog” condition.

SCT Tuner: GT500/Steeda Spec

When it comes to the tune required for a GT500 cold-air kit, many experts disagree. Some will say that you need to re-tune the GT500 when you replace the intake or exhaust with a high-flow version. Others, will tell you that you don’t need to do anything and that the Shelby will run fine. We aren’t taking any sides in this debate other than the fact that Steeda told us that with their cold-air, a tune-up is mandatory for maximum power and street manners.

In any case, it’s never a bad thing to install an aftermarket tuner. With today’s OBD-2 flash technology, a tuner brings you more than just the ability to download a new tune file into your computer. The basic benefit is the revised air/fuel ratio and timing curves that SCT Flash gave us with the SF3 device, programmed with a custom tune spec’d for the Steeda Cold-Air and Gibson exhaust – but in addition — we also got a bevy of tuning tools at our finger tips with the device.

Some of the user adjustable features include wide open throttle air/fuel adjustment, timing adjustment, axle ratios, cooling fan temps, rev limiters, and even on/off traction control selection!

The Installation:

Steeda Intake – Tips and Tricks
Removing the stock intake was simple: after un-strapping it from the throttle body, and removing a few bolts — the entire system was sitting on the shop floor. Before we junked it: we had to grab the MAF sensor and two rubber grommets. Using a Torx wrench, we carefully removed the mass air sensor that requires a crafty touch. If it is dropped, or if you even touch the sensor wire with your hand, you could damage it.

Removing the MAF Sensor from the factory intake.

With the old system sitting on the shop floor, it was time to install the new Steeda Kit. Installation is extremely simple, and the first thing was the new heat resistant high-flow intake tube. After, the heat Shield was the next item to be installed, which required sliding the end of the tube over the back side of the Billet Super Velocity Stack. Finally, we secured the bottom of the heat shield to the two rubber grommets from the old intake in the frame, and then topped it all off with the Steeda high-flow air filter.

Overall, the kit not only reduces the restriction in the intake tract for the supercharger, but it improved the overall look of the engine under the hood. There was a small increase in noise from the intake, not anything excessive, just enough to tell the driver and any bystanders that this was a Shelby that wasn’t not stock.

Screwing in the Steeda “blue” air filter to the high-flow intake tube was as complex as this installation got.

Gibson Cat-Back Exhaust – Tips & Tricks

We put the Shelby on our 4-post lift to make it easy to get at the exhaust. Once we got under the car, we took care of few bolts and clamps, and the old mufflers slid right out leaving plenty of room for the new system to be bolted in.

Installing the Gibson Performance GT500 kit was exactly the reverse of removing the old stock system – in a word – easy. The Gibson CFT Mufflers fit perfectly in the hangers, aligning the tips perfectly in the center of the rear bumper cutouts. The hardest part of the Gibson install of the exhaust was snaking (no pun intended) the exhaust over the rear end into place. Even that just took two minutes to do. With everything in place we made a few final adjustments to the piping and bolted everything down.

At that was left was to fire her up, put the SCT flash tune in, and go. Once we twisted the key, the Gibson sound was really strong, and unique. The CFT mufflers have a unique low growl that only screams when you rev the engine. With just the cat-back, the exhaust was on the slightly more civilized side, so your neighbors are more likely to high-five you than stick you with a knife at the local BBQ.

SCT Flash: An Easy Install

As I mentioned before, SCT has a dyno-tested tune for the GT500 using a Steeda cold air intake and aftermarket exhaust. This made it so simple to install. We plugged the SF3 into the OBD-II port under the dash, and the tuner came right to life ready to unleash the power locked inside of the 5.4L supercharged engine.

After confirming our vehicle type – we were ready to download the stock file in the ECU over the SCT Flash device, and then re-flash the ECU with our tune file. We simply followed the on-screen options to select the tune for use with the Steeda cold air intake and aftermarket exhaust to account for the Gibson kit. We wish you we could give you a bunch of tricks on how to do this, but it’s so simple your 5-year old kid brother could do it.

It’s GO Time: Dynojet testing the GT500 Bolt-ons

With the tune installed, we pulled the GT500 off the lift, and got ready to get the Dynojet rolling. You could already tell there was power to be had, the Steeda cold-air sounded raspy as it gulped air, the SCT tune up sharpened the engine’s throttle response, and the Gibson exhaust made it all work together with a sweet exhaust note.

Not only did it sound better at idle, but the more we got into the throttle the wider our smiles got. And we were just in neutral. By now, the owner of the car had returned to see the progress on his ride, and he was happy to see his car not only running, but spinning onto the dyno. We made quick first pull and the numbers were promising: 508.19 rear wheel horsepower and 483.43 foot-pounds of torque at 4,900 rpm.

We were excited at the gains, but we wanted more. We noticed the air/fuel ratio was slightly fat at 11.7:1, so we picked up the SCT SF3 flash device and decided to lean the car out a little bit at wide open throttle to see if we could squeeze a few more hp out of the car.

We tweaked the WOT fuel trims by subtracting 1% of fuel. Sounding a little more pissed off, the GT500 attacked the rollers with a fury. The final run produced a mind blowing 510.35 rwhp, and 486.03 foot-lbs of torque. This is an improvement of over 60 rear wheel horsepower from only the most minor of bolt-ons.

Needless to say, the owner of the Shelby was ecstatic. The only thing that made this project extra cool was how easy the install was: mods such as the Steeda cold-air intake and Gibson exhaust are quick at-home bolt-on mods that build the foundation for a strong engine and a later pulley change for more boost. And the SCT Tuner took less than 10 minutes from unboxing to flashing.

The GT500 is a winner, and has been since 2006 when it was introduced. Take a look at the dyno curve below, and start thinking about how you are going to pick up one used and do exactly what we did, 3 hours of work and pick up 60+ wheel horsepower!

Article Sources

About the author

Tom Bobolts

Tom started working for Power Automedia in early 2008 at the young age of 20. Starting off as an intern spinning wrenches in the PowerTV garage, Tom cut his teeth helping us build the very project cars we feature. Since moving inside the office, most of his time is spent writing and shooting installs - but he still finds time to get out in the shop. Outside of work, Tom enjoys a variety of different motorsports from Street Bikes, Muscle Cars and just about anything that demands high amounts of horsepower.
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