Ok, I have the 305 TPI on my 1990 Roc..
baseline horsepower was 210...I added an exhaust system and a better cat- 220-225 horses? Say K+N filters, headers, maybe a chip (if I get the money) and gutted airboxes...how much power am I looking at and what kind of improvement can I expect on my e/t? (Im guessin mid 15's now since its close to stock)

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305IrocZ28- Al
1990 Iroc-z 305 TPI auto, Flowmaster exhaust, 2 Rockford 10'' Punch woofers w/ 160a2 amp, 2.73 rear (1650 rpms @ 60mph)

 

not much more, exhaust systems realistically make about 10-15 HP, and 15% will be lost through the driveline. a removed cat won't make any horsepower, let alone a "better" cat. K&N airfilters are a good investment in my opinion. not because so many people think theyre for performance, but because they can be reused for a lifetime practically. K&N claims performance gains for "as much" as 20 HP. it might give you 5 if youre lucky. a chip is a really good idea. and can give you as much as a true 30-45 HP and installs really easily. headers are a good idea with the exhaust system. it will easily give you 20-30 HP along with the exhaust. TPI manifolds as you probably already know are really small and restrictive. if I were you can wanted to make some inexpensive go fast mods heres what l would do: first, I'd swap the 2.73's for a good set of 3.42's or 3.73's. after that i would install a good shift kit. if youre running 15's you should be pretty well into the 14's with these two mods. after those mods l would get a good set of headers and a good exhaust system. and clearing that end up you would clear up the other end which would be the intake. thats when l would invest in the K&Ns, now since youve got a pretty decent trans and driveline and youre motors breathing a lot more freely a chip would work really well with that motor. depending on what kinda chip you decide to use, a different stall converter might work well with your setup. all this can be done for around 1 grand and youll knock almost 2 full seconds off your ET's by this time you may even need to invest in a set of good street tires

have fun