Todd,
I'm in a long-winded mood tonight - hope you don't mind my blabbing!
Are you considering all those parts? I'll address a few on your list. As a general rule however, I am very VERY leery of the claim that performance parts are "making your burn more efficient and therefore getting you a cleaner burning car". My experience has shown this is NOT the case. The overall efficiency of a 4-stroke engine is not all that bad to begin with, and most performance parts do not increase it, they simply allow for more flow (meaning more gas and air, and hence a "bigger bang").
Having said that, most parts will not cause you to fail a sniffer test, and if you did, just get some test pipes and throw an extra set of cats on it
or play with timing, or put some of that xylene stuff in the gas - there are a million ways to get around the test, in other words.
Ok, onto the list!
180deg. Thermostat: Mostly useless. The Viper reaches equilibrium above the stock thermostat temperature, so if you are running over, say 195 degress, a 180 degree t-stat won't help unless you increase the cooling ability of the system - like with a different radiator, fans, pump, etc. A 180deg stat will only get you to 180 on the highway on a very cool day. For overall better cooling, you really need to upgrade the system, not change the t-stat.
Billet pulleys. I would not run these. I've seen one Viper with pulleys and the headlights dimmed at idle. At higher speeds, you're certainly OK, but at idle you might be a tad low on ampherage from the alternator. These make a minute difference - not enough to make up for the hassle, IMO.
Plug wires and plugs: Don't get alot of gain over stock items, just be sure your stock items are in good shape!
The internal engine stuff... yes, Yes, YES! This is where the power is at. Smooth tubes and filters are first. Then comes a 3" cat back. Then go with headers. This will get you to the 450RWHP mark pretty consistently, if not quite a bit higher. Beyond this, rockers are good, but get the good ones (T&D), these can give you 10-15HP with the aforementioned mods. Beyond that, you need to decide between 1 of 3 paths. Naturally aspirated (heads/cam or a stroker), forced induction (turbo or supercharger), or nitrous. If you want to go the first way, I would go with heads and a cam, or if you want more power, a stroker. I'd personally go heads and cam.
There are several people on this board who do headwork. Maybe Treynor, Venom Lover, and Fred Kinder could chime in - all have or are getting head work. Seems the BTR heads are quite good, and of course Caldwell are excellent (they make the GTS/R motors), and TNT are very well respected too. A non-stroked heads/cam car with the aforementioned mods can easily hit the >500RWHP range and often quite a bit more. Along the way you might want to look at halfshafts if you intend to use this power.
