KJ Jones
January 2, 2014

This month we’re inspecting new technology for Ford’s latest leaders of the late-model-Mustang band, the ’13-’14 Shelby GT500s. With their big cubes (5.8-liter) and factory-installed superchargers, ’13-up Shelby engines are the baddest yet. It’s capable of making 600 crankshaft horsepower in stock configuration and tons more with mods.

A bead of Permatex anaerobic gasket maker is applied to the blower’s nose housing before it is reattached to the unit. After securing the snout and letting the sealant set for a few minutes, Eddie pumps the oil that was extracted, back into the blower. The excess gasket maker is also wiped away, leaving the small-pulley installation looking factory stealth.

An upgrade that’s quick, affordable, and guaranteed to make a positive difference on the latest GT500 is installing a smaller supercharger pulley. Decreasing the diameter of a blower’s drive pulley increases the unit’s speed, which increases boost. JMS Chip & Performance of Lucedale, Mississippi, has developed such a pulley—a 2.40-inch steel wheel that actually looks just like the stocker it replaces (because it’s made to actual OEM specifications).

Anybody who goes by the name Brotherman has got to be pretty cool. This is the brand-new, completely stock ’13 Shelby GT500 engine that he gave us the OK to modify. In factory-original trim, the supercharged 5.8-liter engine sent an impressive 576 hp and 570 lb-ft of torque to the rear wheels.

The pulley (PN JMS-24-2013-SVT; $139.99) really has our interest, and Brotherman, one of the regulars at Addiction Motorsports in Canoga Park, California, has a brand-new, bone-stock ’13 Shelby that’s been left at our disposal for an experiment with basic boost. We followed Addiction’s Eddie Rios as he performed the standard pulley swap and remapped the GT500’s PCM using SCT tuning software.

The JMS Chip & Performance 2.40-inch Shelby GT500 blower pulley looks like stock equipment on the 5.8’s blower. The bolt-on wheel is not so small that a new/shorter belt is required, nor does the pulley mandate replacing any of the stock 80mm idlers with a larger-diameter (90mm). Such an upgrade is purely optional, and it’s a long-proven method of improving belt wrap for consistent boost.

As a rule of thumb for stock-Shelby owners planning on upgrades, introducing more air—especially boost—makes tuning the engine’s fuel trim crucial. Having such a procedure performed by a pro, like Eddie, on a chassis dyno is one of the best methods for ensuring maximum performance and engine efficiency. However, for DIY enthusiasts, JMS can accommodate your tuning needs with an optional handheld SCT flash tuner loaded with the appropriate custom calibration for the 2.40-inch pulley.


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On the Dyno

The smaller, JMS Chip & Performance blower pulley (PN JMS-24-2013-SVT; $139.99) and the supercharger’s increased speed were good for 2 pounds of peak boost, which generated over 20 additional peak horsepower, and more than 30 lb-ft of torque at the feet of our tester ’13 GT500. Note that our test was done on an absolutely stifling-hot day in Southern California, with ambient temperature in the dyno cell near 100 degrees.

With its required tuning, the stock-looking, 2.40-inch replacement GT500 blower pulley is impressive. However, complementing this upgrade with a larger Cobra Jet throttle body will really unleash a lot more power from an otherwise stock ’13-’14 Shelby.

STOCK JMS DIFFERENCE
RPM Power Torque Boost Power Torque Boost Power Torque Boost
2,500 232.91 489.22 9.66 264.35 555.28 11.46 31.44 66.06 1.80
2,600 253.25 511.54 9.93 278.57 562.75 11.75 25.32 51.21 1.82
2,700 266.16 517.77 10.30 292.01 568.00 11.96 25.85 50.23 1.66
2,800 280.72 526.53 10.69 307.05 575.91 12.18 26.33 49.38 1.49
2,900 294.07 532.55 10.97 321.15 581.61 12.47 27.08 49.06 1.50
3,000 309.69 542.15 11.11 333.33 583.59 12.73 23.64 41.44 1.62
3,100 319.09 540.63 11.14 344.68 583.94 12.87 25.59 43.31 1.73
3,200 333.73 547.75 11.23 356.85 585.70 12.88 23.12 37.95 1.65
3,300 340.48 541.91 11.37 370.95 590.36 12.80 30.47 48.45 1.43
3,400 350.72 541.73 11.60 387.80 599.03 12.76 37.08 57.30 1.16
3,500 364.76 547.36 11.65 398.31 597.74 12.78 33.55 50.38 1.13
3,600 377.57 550.82 11.40 408.05 595.30 12.98 30.48 44.48 1.58
3,700 395.10 560.83 11.27 420.83 597.38 13.30 25.73 36.55 2.03
3,800 407.84 563.67 11.24 431.33 596.14 13.15 23.49 32.47 1.91
3,900 423.36 570.13 11.13 445.78 600.35 12.86 22.42 30.22 1.73
4,000 430.71 565.54 11.08 455.14 597.61 12.72 24.43 32.07 1.64
4,100 438.72 562.03 11.27 461.89 591.69 12.59 23.17 29.66 1.32
4,200 447.93 560.13 11.35 474.58 593.45 12.34 26.65 33.32 0.99
4,300 460.13 562.02 11.06 484.52 591.83 12.18 24.39 29.81 1.12
4,400 466.52 556.87 10.81 495.26 591.14 12.19 28.74 34.27 1.38
4,500 480.19 560.44 10.62 504.13 588.42 12.24 23.94 27.98 1.62
4,600 490.45 559.99 10.79 506.64 578.47 12.27 16.19 18.48 1.48
4,700 498.57 557.14 10.92 521.55 582.83 12.10 22.98 25.69 1.18
4,800 507.68 555.51 10.56 521.48 570.57 11.78 13.80 15.06 1.22
4,900 503.91 540.13 10.42 534.13 572.54 11.50 30.22 32.41 1.08
5,000 516.02 542.03 10.69 536.90 563.98 11.34 20.88 21.95 0.65
5,100 520.13 535.67 10.57 542.08 558.26 11.41 21.95 22.59 0.84
5,200 534.19 539.51 10.34 548.80 554.29 11.41 14.61 14.78 1.07
5,300 533.62 528.82 10.36 552.11 547.14 11.28 18.49 18.32 0.92
5,400 546.58 531.60 10.61 566.82 551.29 11.11 20.24 19.69 0.50
5,500 550.93 526.13 11.05 562.73 537.38 11.09 11.80 11.25 0.04
5,600 557.70 523.04 11.10 569.39 534.02 11.46 11.69 10.98 0.36
5,700 558.30 514.45 11.14 571.96 527.01 11.38 13.66 12.56 0.24
5,800 560.18 507.26 11.22 579.82 525.05 11.13 19.64 17.79 -0.09
5,900 564.06 502.13 11.26 580.95 517.19 11.20 16.89 15.06 -0.06
6,000 565.74 495.21 11.07 583.86 511.09 11.29 18.12 15.88 0.22
6,100 570.62 491.32 10.78 584.31 503.09 11.50 13.69 11.77 0.72
6,200 575.12 487.20 11.03 584.28 494.97 12.01 9.16 7.77 0.98
6,300 575.19 479.52 11.27 592.76 494.16 11.90 17.57 14.64 0.63
6,400 572.72 470.01 11.67 589.43 483.71 11.66 16.71 13.70 -0.01
6,500 573.10 463.07 12.24 589.07 476.01 11.77 15.97 12.94 -0.47
6,600 570.64 454.10 12.23 592.28 471.32 12.11 21.64 17.22 -0.12
6,700 565.40 443.23 11.62 579.05 453.93 12.27 13.65 10.70 0.65
6,800 573.91 443.28 10.58 590.44 456.03 12.61 16.53 12.75 2.03
6,900 561.54 427.50 5.35 579.57 441.15 10.33 18.03 13.65 4.98

Editor's Note: Our test was done on an absolutely stifling-hot day in Southern California.