For starters here is a good thread with some great info on vortec heads:

http://www.nastyz28.com/forum/showthread.php?t=56505


So, the vortec was built for smaller lift cams with a lot of flow under the curve from about .300 to .500 lift. Our TPI cars with 1.6 rockers would have about .430-.440 lift. Why would you want much more lift without modifying the heads? Seems to me this is a very good ECM friendly street combination. OK, so I have the vortec lower, SLP runners with the plenum port matched and stock TB. I also have a modified CAI. The real question is the exhaust. I know these cars have restrictive exhaust but I was wondering how much would one gain, in this application, with headers over cast manifolds with all else being equal in the exhaust system (good y pipe, cat and cat back system). Are we talking about losing 10HP or more like 20-30 with cast manifolds vs headers. My understanding is that the stock cast manifolds in the 350TPI applications aren't bad. Certainly will not flow as much as headers but do they need to in this particular application? I'm thinking it's all about the combination and not necessarily the largest lift or flow etc. A well thought out mild combo is capable of out performing a lot of poor combination "higher" performance cars, particularly on the street. Thoughts?

 

Your correct about combination being the key. I would say from experience that a medium length header in the 1 5/8" tube diameter range with a high flow cat and a 3" cat-back exhaust would do your combination wonders. I think if you consider that headers improve performance through out the power range, not just the top end...you'll see that efficiency is the key. A combination of 10 to 15 horsepower tricks add up quickly. If money is an issue, I would certainly optimize the combination you have.

My son and I are doing a science fair experiment where we are looking to gain as much efficiency as we can by cutting frictional loads, tuning for efficiency, and concentrating on the rpm range most used. The subject is a 5500 lb Suburban. Vortec heads and TPI highlight the combination. Were looking for mid 20 mpgs on the highway. It's not a budget effort. We'll be exploring some rather pricey combinations to achieve our goal.