Tom Wilson
January 1, 2003

Horse Sense: Embarrassing but true, everyone at the first dyno test of Mark Brown's Bullitt was momentarily stumped when we couldn't get the car to rev past idle on the rollers. Stabbing the traction-control defeat button cured everything but the red on our faces.

Mark Brown's Bullitt is a looker as well as a goer, thanks to several small but well-placed details. From the front, a sharp eye can catch a few of those modifications, including GT foglights from Velocity Mustang, a stainless steel running-horse grille emblem from stallionmedallion.com in front of mwspeedshop.com's grille-delete kit, and a Banshee Performance SR71 hood.

Ballistically speaking, hot rodding cartridges with more powder or more efficient bullets is known as wildcatting. It's done by handloaders who start with factory brass, then trim and neck it to accept the powder and bullet they think will work better. The concept is something you might recognize in your own garage, especially if your bullet is spelled with an i and two t's. When it comes to wildcatting that sort of Bullitt, the time-tested staples of supercharging and increased breathing have once again proven themselves.

The wildcatter in this story is Mark Brown. He hadn't pulled the trigger many times on his '01 Bullitt before thoughts of increased cylinder pressure and higher velocities began to take hold. The result of this thinking is a nicely modified daily driver with 369 rear-wheel horsepower and a tasteful collection of appearance and performance baubles. Luckily, we were able to peer over Mark's shoulder while he was wildcatting his Bullitt, and what we learned will be interesting to all late-model hot rodders.

We first came across Mark and his already Vortech-supercharged-and-aftercooled Bullitt as part of a review of Denso Iridium spark plugs. These are not your garden-variety sparklers, but rather sophisticated, new-technology spark plugs designed to flourish in the toughest performance environments. Denso's breakthrough was developing a process for welding tiny bits of Iridium-a tough-but-difficult-to-handle (weld) metal ideal for spark plug electrodes. The costly Iridium is quite heat resistant and is an effective spark plug electrode because it functions in a miniscule 0.4mm-diameter dot on the plug tip. This smaller tip reportedly unshrouds the spark, unlocking otherwise lost horsepower.

Mark's Mustang Bullitt GT is one of the last '01 models built, being Bullitt number 5,368 of approximately 5,500 total. It's Dark Highland Green, but what you can't see underneath in this rear view are the 4.10 Ford Racing Performance Parts rear gears.

We've witnessed several dyno tests where the Denso Iridiums justified their princely $12-per-plug retail price by returning huge power gains. As with any ignition-system improvement, the Iridium plugs really shine when there are ignition problems at hand. Typically this is on a supercharged engine, where increasing rpm and cylinder pressure begin blowing out the spark. The resulting misfiring can begin subtly, enough so that the engine feels OK, and the only indication that something is amiss(ing) is the power is a tad low.

Enter the Denso Iridium spark plug on a white horse. Strapped to the Wes-tech Superflow chassis dyno, Mark's car picked up an amazing 15.5 hp when the Densos were screwed in place of the stock plugs. Admittedly, this is precisely the set of circumstances where these plugs look good-and they really looked good. We'll also repeat our usual plug-testing caveat, that any out-of-the-box-fresh spark plug will give a hair more power over a plug with even a few miles on it, but not 15 hp. Thus, even the curmudgeonly cynical must agree there's at least 12 hp from the Densos. It's the easiest, least expensive 12-15 hp Mark could have found.

While traditionalists may be all agog over the stock Bullitt's five-spokes, Mark is clearly enamoured with a flashier, more contemporary, street-scene look. The American Racing Hopster 18x8-inch front and 18x10-inch rear wheels play that trick to the max. Currently, the brakes are stock, but coming next are Baer two-piece Eradispeed Plus rotors for a little extra stopping power and fade resistance.

Next, Mark elected to fit a free-flowing MagnaFlow exhaust system. This was done at MagnaFlow's proto-type shop, where a combination of off-the-shelf and prototype parts were installed. The after-cat is PN 15673 from MagnaFlow's MagnaPack ready-to-wear collection for '99-'02 Mustangs. It uses 2.5-inch stainless steel tubing; 4x14-inch round, straight-through mufflers; and a pair of Souza-horn-sized, 3.5-inch polished tips. As with all MagnaFlow systems, it's a showy, well-built item. The MagnaFlow system delivered six otherwise missing horsepower. Six horsepower seems a bit low for an exhaust system on a blown car, especially since we've had great experiences with the MagnaFlow gear in the past. But, please keep in mind this is our first dynoing experience on Westech's Superflow dual-eddy-current chassis dyno, which we expected to deliver numbers a bit lower than a Dynojet. So our gains of 6.1 hp and 10 lb-ft of torque were respectable. But, after installing the system, MagnaFlow tested the system on a Dynojet and recorded big gains. In the end, our final numbers with the exhaust weren't that far off, with the Dynojet reading 6.9 more horsepower and 2.3 more lb-ft of torque.

The final power addition was done during our last dyno session at Westech. We asked John Mihovetz of Accufab and 4.6 drag-racing fame if he would install one of his billet throttle bodies on Mark's car. Always ready to ham it up for the camera, John was happy to oblige, and after a bit of grinding to open the upper intake runner, the dyno showed a 10.5hp gain from the larger throttle body.

More MGW billet trickery is found on the oil-fill cap; Mark has been pouring Castrol 10W-30 Syntec synthetic oil through it. Also unexpected was a premium-quality Denso oil filter. It's a new product from a company best known for spark plugs and oxygen sensors. The Denso filter's unique feature is a "torque-stopper" device to prevent under- or over-torquing the filter.

For those not keeping score as we've gone along, the Denso spark plugs, MagnaFlow exhaust, and Accufab throttle body combined to deliver an extra 31.5 hp, for a total of 369 hp at the rear tires. Torque rose a commensurate 31 lb-ft from the supercharged baseline to the final throttle-body installation. In other words, numbers any wildcatter would be proud to load.

Dyno Tales
While the modifications to Mark's car were performed over several weeks time, we were able to test the modifications part-by-part on the same Westech chassis dyno. Weather changes were also minimal between the dyno sessions, which grouped the baseline and spark plug tests on one day, with the exhaust and throttle body test on another. As the dyno sheets show, the final gains of 32 hp and 31 lb-ft of torque were nicely spread across the powerband, which makes them desirable for a smooth-running street car such as a Bullitt.






Actually bolting on the Accufab throttle body is simple stuff. Don't forget to set the Throttle Position Switch with a digital voltmeter, otherwise you may end up with funny throttle response, a high idle speed, and other irregularities. Because the large single-blade throttle of the Accufab design opens so much more area more quickly than the stock two-blade throttle body, off-idle driveability is a bit touchier. John says some Mustangs accept the big throttle body without complaint, while others seem to take a fair amount of fiddling and time for the computer to readjust before the idle drops that last 150 rpm or so. Cobras are finicky in this regard, and the Bullitts are worse, according to John.

Baseline Spark Plugs Exhaust Throttle Body
RPM POWER TORQUE RPM POWER TORQUE RPM POWER TORQUE RPM POWER TORQUE
3,478 203.5 307.3 n/a n/a n/a n/a n/a n/a n/a n/a n/a
3,575 212.2 311.7 n/a n/a n/a n/a n/a n/a n/a n/a n/a
3,674 222.3 317.9 3,477 213.7 322.7 n/a n/a n/a 3,466 219.6 332.8
3,776 231.5 322.1 3,582 223.0 326.9 3,446 192.2 292.9 3,574 233.5 343.0
3,877 242.1 328.0 3,689 233.4 332.4 3,552 214.3 316.9 3,686 245.7 350.2
3,982 252.9 333.5 3,798 243.7 337.0 3,659 230.7 331.1 3,798 257.7 356.4
4,089 263.0 337.9 3,908 255.2 343.0 3,768 243.9 339.9 3,912 269.8 362.2
4,143 268.7 340.7 4,020 266.7 348.5 3,880 256.2 346.8 4,030 283.3 369.3
4,251 280.2 346.9 4,134 279.6 355.2 3,992 267.3 351.8 4,149 296.5 375.4
4,361 292.0 351.7 4,250 292.4 361.4 4,106 280.3 358.5 4,268 309.4 380.7
4,469 {{{300}}}.7 353.4 4,361 303.8 365.9 4,216 292.7 364.6 4,390 320.3 383.1
4,572 307.0 352.7 4,477 313.3 367.6 4,332 303.6 368.2 4,508 330.0 384.4
4,686 313.2 351.0 4,592 321.0 367.1 4,447 313.8 370.6 4,628 338.6 384.2
4,789 319.9 350.8 4,708 328.7 366.7 4,561 321.4 370.1 4,748 346.6 383.5
4,896 326.6 350.4 4,{{{825}}} 336.5 366.3 4,677 329.1 369.6 4,866 354.5 382.6
5,004 332.6 349.2 4,920 343.2 364.8 4,792 338.1 370.5 4,982 361.8 381.4
5,108 335.7 345.1 5,053 347.2 360.9 4,906 345.1 369.5 5,097 366.5 377.6
5,211 335.7 338.4 5,163 349.6 355.6 5,018 351.3 367.7 5,210 367.7 370.7
5,312 336.1 332.4 5,272 350.9 349.6 5,130 356.6 365.1 5,319 369.0 364.4
5,409 337.3 327.5 5,378 352.8 344.5 5,238 358.9 359.9 5,426 369.4 357.5
5,504 334.1 318.8 5,479 346.1 331.8 5,342 358.5 352.5 5,528 368.0 349.6
5,594 327.0 307.0 5,577 340.3 320.4 5,443 357.7 345.1 5,625 362.9 338.8
5,682 {{{323}}}.1 298.7 5,674 340.6 315.3 5,540 354.8 336.4 5,724 356.6 327.2
5,768 319.5 290.9 5,771 341.9 311.1 5,637 349.8 325.9 5,824 361.0 325.6
5,855 321.3 288.2 5,869 346.5 310.1 5,729 349.5 320.4 5,923 370.2 328.3
5,828 297.3 267.9 5,872 341.8 305.7 5,815 350.0 316.1 5,961 356.2 313.8

Baseline: Internally stock Bullitt 4.6 GT with Vortech supercharger, Vortech aftercooler, and 8 pounds of boost Spark Plugs: Engine as in Baseline, but with Denso IT20 Iridium spark plugs Exhaust: Engine as in Spark Plugs, but with MagnaFlow exhaust system Throttle Body: Engine as in Exhaust, but with Accufab throttle body

Because the Accufab throttle body offers an unobstructed air path, but the stock Bullitt upper intake tube does not, it is necessary to mark the intake, unbolt the upper tube from the rest of the manifold (in back, near the firewall), grind out the aluminum in the center, then refit the intake tube. The marking is easy-just hold up the Accufab throttle body to the intake and draw its outline on the mating flange. Reaching the four bolts holding the intake tube to the rest of the intake requires more effort, as John Mihovetz is about to demonstrate here. Once again, 11/44-inch drive sockets and extensions come in handy for this job. Note also that FRPP intakes, which are similar to Bullitt intakes, dive more between the throttle bores, and they cannot be ground out as deeply.