Tom Wilson
October 5, 2006
Just one pulley--the crank pulley--is underdriven on V-6 underdrive pulleykits. The one-piece pulley/harmonic damper assembly is exchanged withthe stock damper, along with a shorter serpentine drive belt. GTR sellsthe ASP Racing pulley/damper for 4.0-liter V-6s. ASP says this 25percent underdrive pulley provides charging voltage at only 850 rpm.

And then there was the underdrive pulley. The 4.0-liter, like the3.8-liter that preceded it, is one of those engines where the underdrivepulley is only the crank pulley.

Again like the 3.8-liter, the 4.0-liter crank pulley is integral--onepiece--with the harmonic damper. Installing "underdrive pulleys" on a V-6Mustang, only changes the harmonic damper. But pulling that damper isgenerally a difficult job.

It would have been pretty simple, except the bolt supplied with thedamper to mate with the harmonic damper puller was too short. All sortsof time-wasting effort resulted, with the final fix being thefabrication of our own bolt. This wasn't the easiest thing, as thecombination of thread series, diameter and length is oddball, but it gotmuch easier when we realized a modular V-8 head bolt was correct, savefor the length. As GTR has old modular head bolts on hand, after a pairof cuts with the chop saw and some quality time with the smoke wrenchand we were in business.

We have to say, after hours of messing around all the effort provedworth it. The power jumped to 204 rwhp, then backed up at 203 rwhp, then203 rwhp again and we figured that must be what it runs, but we hit itagain just to see what it would do when totally heat soaked. It maxed at204.5 rwhp.

RPM

Pulleys

Total Gain

POWER

TORQUE

A/F

POWER

TORQUE

2,400 n/a n/a n/a n/a n/a
2,500 n/a n/a n/a n/a n/a
2,600 n/a n/a n/a n/a n/a
2,700 n/a n/a n/a n/a n/a
2,800 118.69 230.78 13.94 10.72 20.76
2,900 123.22 231.13 13.7 7.72 14.5
3,000 127.78 231.41 13.59 7.11 12.88
3,100 132.63 232.2 13.58 7.5 13.14
3,200 136.66 231.54 13.62 6.9 11.7
3,300 140.97 231.37 13.65 7.53 12.35
3,400 145.28 231.22 13.68 7.81 12.43
3,500 150.22 232.05 13.7 8.24 12.73
3,600 154.21 231.41 13.67 8.46 12.69
3,700 158.12 230.69 13.65 8.89 12.97
3,800 162.2 230.25 13.61 9.35 13.27
3,900 166.56 230.21 13.51 10.58 14.62
4,000 170.97 230.24 13.48 11.1 14.95
4,100 174.13 228.63 13.46 10.76 14.11
4,200 177.9 227.89 13.38 11.19 14.33
4,300 181.29 226.71 13.31 11.98 14.99
4,400 184.9 225.84 13.26 12.52 15.29
4,500 187.97 224.38 13.23 13.44 16.04
4,600 191.05 222.99 13.24 13.51 15.77
4,700 193.3 220.71 13.26 12.89 14.73
4,800 194.91 217.81 13.27 13.05 14.58
4,900 198.51 217.21 13.24 15.28 16.72
5,000 199.41 213.74 13.21 14.96 16.04
5,100 201.19 211.34 13.24 15.73 16.53
5,200 202.41 208.45 13.31 15.75 16.22
5,300 202.75 204.78 13.35 16.28 16.44
5,400 203.93 202.09 13.39 17.81 17.65
5,500 204.44 198.84 13.41 18.68 18.16
5,600 204.27 195.06 13.42 19.34 18.46
5,700 203.65 191 13.45 20.18 18.92
5,800 202.44 186.54 13.46 20.68 19.06

Thanks to the cold-air intake and underdrive pulley, we gained a solid20 hp at the V-6's wheels. The muffler didn't hurt and might help with ablower, while higher octane fuel was a waste until more ignition timingarrived due to electronic tuning. All told, it was a 9.8 percent gainin horsepower, and that's just right for a daily driver. It's enoughpower to feel, but not enough to ruin good manners.

For our tastes, we'd go with the cold air and underdrive pulley and saveour muffler money for when forced induction is fitted. Considering we'dhave a V-6 Mustang around for economical, yet fun, daily transport, thedefinite noise increase from the muffler isn't in keeping with the car'suse to begin with. Without a big power gain, why bother? But we're usand you're you, and if more sound makes it more fun, then there's youranswer.

Another tactic would be to fit a true dual exhaust, as V-6 enginesprefer two separate three-cylinder exhaust systems with no H or Xcrossovers. While more expensive, such systems can definitely aid powerwith forced induction and often sound more V-8-like than rasp-can giventhe Mustang's generous six-cylinder displacement. It just depends onwhat you're listening for.

In any case, bolt-ons and the 4.0-liter V-6 are a good thing.