I'm putting a 6 Litre Vortec engine in my 1966 GTO and I want to put a twin turbo set up on it. I'm not sure what size turbo's to use. I bought Corkey Bell's book, Maximum Boost, and did the math myself to pick the turbo's but the people on Ebay that sell turbo setups think my turbo's are way to big. I calculate that at 15 PSI or a pressure ratio of 2 at 6000 RPM I need to push 1062 CFM. When you use 2 turbo's that's 531 CFM per turbo. I picked a Garrett model GT4088. I thought 2 of them would be great. My question is what size turbo's should I use for this 6 litre to lay down about 800 horsepower? Any ideas?  

with a single turbo that size you can easily make 800hp,a friend has a single 104mm turbo on his sb chevy and with 10lbs of boost it makes 1000hp,and the car can take 18lbs.  

A single turbo will work, but large turbos tend to be lazy at low r.p.m. (unless they have expensive, lightweight ceramic turbine wheels) This is less of a factor in drag racing if a "spool-up" system is used (such as loading the turbo on the converter and using a low MSD rev limiter chip before launch to combust excess, unburned fuel in the turbine).

Large singles also tend to yield a "peaky" torque curve similar to centrifugal superchargers (albeit with more "area under the curve" due to higher impeller speeds at lower r.p.m. and less parasitic load on the engine to drive the compressor)

For the street, properly-sized twins are more responsive and look better (no ugly y-pipes and crossovers in the pre-turbo exhaust tract).

531 CFM/turbo looks to be about right in the center of the compressor map for the GT4088 (at about 78% efficiency).

Of course, some of us purists would rather see a real old-school Pontiac plant in that Tin Indian tiger (such as Paul Johnson's twin turbo 455 Trans Am (http://www.highperformancepontiac.com/events/hppp_0706_firebird_lawn_show/awards.html))