yes there are differences. Especially in the material used to MAKE the pulleys (you dont want something cheap that will bend over time, or rust, ect.), and whether or not the pullies are PIGGYBACK STYLE (YUCK). Piggyback style means the crank underdrive pulley is bolted onto the stock crank pulley. It's weaker, and it's a pretty pain in the ass (i think you have to like, boil, the underdrive crank pulley so it'll fit better, i read that somewhere?? yuck.) Steeda's pulleys ARE NOT piggyback style, and are pretty much the best. MARCH's pulleys are poop, and so are BBK's. and yes, I'm prepared to get flamed for saying that.
I have an 02 GT, and I don't know the differences between the 99 cobra versus 02 GT pullies, but, for me, the install was hard. Why? Because I attempted to do it without the right tools. (i felt like a monkey ****ing a football). Unless you have all the right tools, dont bother, it's going to be more trouble than it's worth (you could just take it to a mechanics shop and have them slap the pully's on no problem for pretty cheap). Once I bought everything needed to install the pullies, though, the install was insanely easy and I had NO problems. It was easy even though I had an auto trans (you can just throw the car into 5th gear, I HAD TO HOLD THE TORQUE CONVERTER!
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Don't attempt to install them without an air compressor, a decently powerful torque wrench (mine's 375ft./lbs) and a pulley puller or harmonic balancer puller (cheap to buy at sears or something). You'll also need high temp. gasket maker to seal the keyway slot in the crank pulley so you dont get a small oil leak. Of course you'll need a standard rachet/sockets, too. If I'm forgetting anything, someone remind me.
As far as differences? I felt nothin'. I'm sure there was some sort of gain, but I didnt feel anything. And I've had no problems with overheating, lights flickering/dimming, ect. And I have run full A/C blast, full stereo blast, with the headlights on. No problem. But, since I didnt feel any gains, and I'll be going back to the stock crank pulley soon enough (for soon coming supercharger) I do feel like i wasted money.
