Project S550 - 2015 Mustang GT Introduction

by Nathan Brummer

Ford followed up their hugely successful S197 Mustang with a completely redesigned platform. The 2015 model year known to Mustang enthusiasts by its chassis designation S550 is now coveted around the world (even outselling the Porsche 911 in Germany this past March) and boasts two major differences from its predecessor, the S197. While aesthetics are subjective – and always hotly debated - the first, and most obvious, difference is how much sleeker this new model looks. Secondly, it now sports an independent rear suspension, meaning the new Mustang no longer suffers from the live rear axle, which has been widely panned as a critical shortcoming for many years. While always a formidable straight-line drag racer, it has never been considered a great handling car. Those days are now long gone, making the S550 a perfect candidate for a performance-oriented daily driver project.

The goal of this project is to build a very respectable street car that is first-and-foremost reliable, while still being able to both hold its own at the local motorsports events and turn a few heads at the local car meet. We’ll address this project in several stages. First, we’ll tackle the suspension in preparation for building more power. Next, we’ll dress it up with some cosmetic improvements. And finally, we’ll look to generate even bigger power numbers.

Now that we have a solid plan in place, lets review the car’s actual specifics. Project S550 is an Ingot Silver 2015 GT with the 5.0L V8. Given this car will be our daily driver, we opted for the premium package, which gives us some nice creature comforts including the large touchscreen with Sync and a 9-speaker sound system. Also included are some interior improvements like heated and cooled leather seats and sharp-looking aluminum aircraft-inspired toggle switches. Lastly, this package also provides a more aggressive front and rear fascia over the base model GT. We also couldn’t resist the GT performance package. At the time of purchase, the performance pack was a $2000 option and, in our opinion, a no brainer. It includes:

  • Strut-tower Brace
  • Larger Radiator
  • Upsized Rear Sway Bar
  • Heavy-Duty Front Springs
  • K-Brace
  • Brembo™ 6-Piston Front Brake Calipers with Larger Rotors
  • 19”x9” (F) and 19”x9.5” (R) Ebony Black-Painted Aluminum Wheels
  • Unique Stability Control, EPAS & ABS Tuning
  • 3.73 TORSEN® Rear Axle
  • Gauge Pack (Oil Pressure and Vacuum)
  • Spoiler Delete

While ultimately our upgrade plans will replace many of these parts, the Brembo brakes, 3.73 gears and larger radiator alone more than make up for the cost of this upgrade.


The Performance Package radiator measures in at 1.44” compared to the stock radiator’s 1”. Providing an additional 168 in3 of volume (552 in3 vs. 384 in3.) Given this car will be daily driven in the hot Arizona Summer any additional cooling is welcomed.
Many people consider the vacuum gauge a place holder for when boost is added. In fact the 2.3L Ecoboost Mustang’s stock boost gauge is a direct replacement, and a popular upgrade for GT owners that have gone the supercharger route. Both of which are upgrades being planned for this project car.
The upgraded 3.73 gears offer better off the line performance and more torque when compared to the Regular GT model’s 3.55 gears. We also feel for this project, the 3.73 gears are in the sweet spot of balancing top speed, mpg, off the line performance and torque. While more drag racing focused builds may opt for a 4.10 or 4.30 gears, both top speed and mpg can suffer with gears this aggressive.
Page 1 of 4 Next Page
Bookmark and Share
Wednesday, July 06, 2016 8:57 AM
It's really amazing how much technology is packed into a modern performance vehicle with selectable drive modes and all. I test drove an Ecoboost Mustang and I want to say it had oil temp even as one of the optional displayed items. Ford also has done a great job with performace package options straight off the showroom floor. Also amazing how all modern engines are trending to 0w-20, or 5w-20 in the case of this engine.
Wednesday, July 06, 2016 11:20 AM
Excellent start! Shoot me an email, I'd love to send you out a couple parts to help with the project. Kolton@maximummotorsports.com
Wednesday, July 06, 2016 2:56 PM
Kolton, thanks we'll be in touch.
Wednesday, July 06, 2016 3:37 PM
Nicely detailed and well put together article. Nice reading this and getting a different perspective on such an iconic car.
Wednesday, July 06, 2016 6:39 PM
Any pics of the inside of the Steeda oil separator?
Wednesday, July 06, 2016 8:41 PM
With the IRS equipped Fox Bodies sn95 etc, road racers almost always swapped back to the stick axle for better handling. Is the new model irs good enough to avoid this fate?
Wednesday, July 06, 2016 9:59 PM
The old Cobra fox body with the IRS was a compromised design. The engineers were told to stuff and IRS where the original solid axle went without major design changes to the pickup points. So it was a band-aid job.

This design is clean sheet. I've drive the previous gen Mustang and this new gen and they are worlds different when the road gets twisty and bumpy. The previous gen Boss 302 is great when the surface was smooth, but couldn't handle the bumpy stuff. The new Mustang can.
Thursday, July 07, 2016 1:59 PM
having owned an S197 Mustang with the Coyote engine and 3.73 gears I have to disagree that 3.73 gears are ideal. the 5.0 engine has enough torque where the 3.73's are too short in my opinion. the 3.55 option is better for everything except drag racing with drag radials or better tires. A lot of owners that track their cars swap to even 3.31 gears since the engine has enough torque to keep them from losing speed due to gearing but saves some time with reducing shifting.

Monday, July 11, 2016 4:30 PM
You forgot to mention how much fatter the 550 is compared to the 197. Both visually, especially in the ass end, and on the scales. Major weight gain. My track pack S197 is a good 300 lbs lighter than these big boys are. I'll trade the IRS for lighter weight any time. I have 3.73's and I find them just right with the massively overdriven 6th gear. Ford needed to do what GM did with the new Camaro and put it on a major diet. 15/16ths scale would have been good, 7/8ths even better.
Monday, July 11, 2016 5:07 PM
I'm looking forward to this series. As a current muscle car and truck tuner, and a former SR20DET owner.... I love when MotoIQ digs into some American iron!

Nathan, it will be interesting to see what you think of the 3.73s. Our shop has found the same as mentioned by warmmilk above: 3.73s and big power can be troublesome, because a shift to fifth is required right before the end of a 1/4 mile run. A tall tire would help, but 3.31s go real well with a blower and 600+ rwhp :)
Wednesday, July 13, 2016 3:14 PM
my comments on the gears were mostly for road course use, not street driving or drag racing. although I'd prefer 3.55 or even 3.31 for street driving also. In my opinion even with the 3.73 you need a downshift to 5th at highway speeds if you wanna pass anyone quick

as far weight vs IRS, I'll take the IRS anyday. the main reason I ended up selling my S197 is cause I got sick of the live axle. sure you can "make it handle", but its not really designed for it. according to Vorshlag a bone stock S550 GT was faster around a track then a S197 with a bunch of handling mods, even with the extra weight.

but its ok, I'll go and enjoy my severely underpowered NC Miata for now... engine swap next year for some ok power...
Thursday, July 14, 2016 7:24 AM
At the risk of beating a dead horse, Fair of Vorshlag elected NOT to do the S550 and the weight issue was the biggest factor in that decision, his initial enthusiasm died quickly when the reality of that extra weight set in.

6th gear for sure is not a passing gear no matter what rear gear you've got and likely the car would reach a higher top speed in 5th anyway. It's strictly a highway cruising gear, and with the 3.73s we're turning about 2700 rpm cruising at 130 km/h on the 4 lanes, getting 9 L/100 Km to boot, if anyone cares about fuel economy these days that is.

I'm not looking for maximum cornering performance, its about the overall package and that includes aesthetics. I really believe the S197 is a much more faithful rendition of the original fastback Mustang and that to me means a lot. IRS would be nice, but I just don't like the overall look of the S550 as much. I like that old school rough and tumble feel of the S197 as well, makes me remember what muscle cars were like when I was 20 years old, 45 years ago. Though having said all that, Ford's price right now on a S550 GT stripper (no premium package) with just the GT performance pack and Recaros is mighty mighty tempting.
Thursday, July 14, 2016 8:25 AM
From a road course use point of view, the 3.73 gears top out at ~152mph while the 3.31 pushes up to ~167mph. I don't know of many road courses where a car at this power is going to go over 152mph, so the 3.73 does add a lot of usable power under the curve vs. the 3.31.

@warmmilk, K-swap?
Thursday, July 14, 2016 1:20 PM
its not about where they top out at, its about how much shifting you have to do at speeds below that. the coyote makes enough power that its not really gaining anything by adding mechanical (shorter gears) power, you just waste more time shifting

no k-swap, 2.5 out of any 09+ Ford or Mazda. with some cams and a bump in compression its good for about 220whp on a dynojet and 200wtq from idle. given that its a direct bolt in with all oem parts it comes out a good bit cheaper than a k swap even with the engine rebuild.
Thursday, July 14, 2016 1:32 PM
also, local track, Pacific Raceways, if you have the balls for it, 152 might become a limiting factor on the front straight... which isn't really a straight... hence the balls comment
Thursday, July 14, 2016 6:58 PM
Yeah, I get what you're saying, though it is a track dependent scenario. 4th gear tops out at ~114mph and 128mph respectively. So if there's a track where you're hitting maybe 118mph, then the longer gears might be advantageous to avoid a shift. But as a generalization, the shorter gears should be faster in most situations.

2.5 sounds fun! Should be easy to come by too plus cheaper spares. I like it!
Post Comment Login or register to post a comment.

MotoIQ Proudly Presents Our Partners:

© 2018 MotoIQ.com