Do under drive pulleys really add any extra horsepower? I also heard you run a risk of snapping your crank due to the extra vibration from the lighter and smaller pulley. any help on this?
I ran a set of Mac pulleys 25,000 + miles on my old 89 5.0 several years ago and had cooling and battery charging issues, but they did add a little hp.
I ran a set of Steeda pulleys for ~50,000 miles on my 2000 GT with no issues. I only removed them to install the supercharger kit I have now which wasn't compatible with the pulleys.
There was a noticeable improvement in hp and the engine seemed to run smoother and wind out faster with the Steeda pulleys.
I think the crank problems you mention are from "piggyback" style pulleys which mount the crank pulley out further and cause torsional stress, or so I have also heard.
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"Ignore that whistle, it's just a vacuum leak."
2000 Mustang GT:
Saleen Series IV Twin Screw, Chicane I/C, AFCO Extreme H/E, 2.8" pulley, FMS 42# injectors, Dual Catted 2 1/2" UPR X Pipe, UPR LCAs/UCAs, 2 1/2" Flowmaster American Thunder kit, 90mm LMAF, 112" Goodyear Gatorback, Kenne Bell BAP, Mosaleen idler, T-56, Detroit TrueTrac w/ 373s, Demolet 100mm CAI, SCT XCal2 with JDM Engineering 93 Octane "Race" tune.
underdrive pulleys 
