All parts considered for a turbo...

[jibbby]

As Gary mentioned - his worry and mine would be about each twin giving off more boost then the other..Yeah you can have boost gauges for both but still..

My conclusion is the twins will give an easier balance of exhaust pull and flow on each side of the engine....Also you will not have to V the exhaust piping into a single runner with twin turb's like you would with a single turbo setup....With The single turbo you have to combine all the exhaust pipes to meet up with the single exhaust turbo which kinda sucks...8 into 1....

 

[GSMnow]

If the flow maps are matched up properly to the motor, you can achieve the same intake temps with either a single or twin setup. It all comes down to where in the flow range of a turbo you are running. If you make tight short manifold and place a small turbo at each head, then I think a twin setup could spool a little faster. than the longer piping needed to get all 8 ports to one bigger turbo, and it is possible that the total polar moment of inertia might be slightly less on the 2 small turbos for a little extra help in getting into boost. Once you are into boost though, the single larger turbo only has one set of bearings to keep spinning, and will most likely operate at a lower impeller rpm for the same amount of boost and total flow. The tip velocity is greater on a larger diameter wheel. Here is an example

http://www.turboneticsinc.com/cm_e46.htm
Look at the rpm and efficiency of this turbo at 25 lb/min at 2.0 pressure.
88,000 rpm and inside the 76% efficiency line.
Using two would be a good 500 hp setup on a 1UZ twin.

Compare to one of these at 50 lb/min at the same 2.0 pressure ratio.
http://www.turboneticsinc.com/cm_t70.htm
This is not totally fair to the big turbo as it really wants to run higher pressures than just 15 psi.
65,000 rpm and center of the 75% efficiency island.

The T70 is a huge turbo but that is what it takes to huff 50 lb/min of air at low pressures. Just look at the wheels in Vortech blowers. Run it up to 20 PSI of boost (about 2.3 pressure ratio) and the big single looks better still.

I am not surprised at all that many racers pull off twin setups and go with a big single. It may take a little time to get the big wheel turning, but once it is, it makes boost at lower speeds.

Gary M.