Advantage/Disadvantgage of Higher A/R?

[Hot Air]

What are pros and cons of a high (0.82) vs low (0.63) A/R on the turbine side of a CPT66 turbo on a 109 block equipped with Champion Iron head, Champion matched intake, Scorpion 1.6 RR, Comp Roller Cam (212/212), 72# Injectors, Aeromotive A1000 fuel pump w/sumped tank, CAS V1 FMIC, Art Carr 9.5" LU torque converter, FAST Bank-To-Bank, ATR 3" DP w/ Deltagate, ATR dual 3" Cat-back, High Flow Cat, etc.

Would the low A/R add back pressure to the exhaust? Would it affect the spool time? I primarily drive it on the street to car cruises and shows. It will occassionally see a drag strip at "test-n-tune" events with my Buick/Ty/Sy friends.

Conrad

 

[RUQWKNF]

Here are a couple of thoughts that come to my mind.

Advantages of .63 A/R.
- Quickest spool up of the two turbine housings.
- Less stall speed requirements for your converter.
- Less drivetrain loss due to running a tighter converter.
- Easier to launch the car.

Disadvantage of the .63 A/R.
- More back pressure at high boost levels.
- Depending on camshaft size and cubic inch size, it could become a restriction at high boost/high hp levels, as well as a possible restriction on larger CI engines. .

Advantage of .82 A/R.
- Less back pressure results in slightly higher peak hp capabilities.
- Engine will operate more efficiently as the boost is turned up.
- Turbo will also operate more efficiently, due to not having to work so hard to make positive boost pressure.
- Recommended for motors that are larger than 235 CI, or have a higher RPM band than a stock engine.

Disadvantages of .82 A/R.
- More stall speed requirements on your torque converter.
- It will move your power curve higher into the RPM band.
- Drivetrain losses will be higher due to running a looser converter.
- Due to slower spool up associated with the larger A/R, you will have to launch the car at higher boost levels and rpm levels to achieve the same 60fts as you would with the .63 A/R. Meaning if the car launches good at 8psi with the .63 A/R, you might have to launch the car at 10-12psi to achieve the same 60ft times.

On your combination, with more street driving and less track visits and if the CI is 235 or smaller, I would stick with the .63 A/R housing for more responsive spool up. Converter selection will be very critical if this is a journal bearing turbo.
HTH

Patrick

 

[Hot Air]

Patrick,
Thanks so much for the input. Turbo will be a ball bearing unit. Sounds like 0.62 is the way to go for me.
Conrad