Seeker1056

hello from Ken
I have read so many threads my eyes are bleary

I am building a stock 6l lq4 twin turbo in a 67 Chevelle with a 4l80e and 2800 rpm stall convertor - 12 bolt with 3:73 gears so I can slalom race a bit, do some arm drop drag racing, and have a stop light bandit.
Motor pcm is tuned efi live for speed density and 80# injectors, with #15 nozzle meth injection

so I am trying to decide what turbos for the street for good spool and the odd track day

I get that turbos in the 62-66mm inducer range would be good?
My quandery is in the exhaust housing A/R ratio that would be best

In the various forums .68 seems the most common, and some approach .83 on a t4 - and divided rather than open housings

Having said that I phoned two turbo suppliers and both told me I am nuts to consider anything less than a .91 housing
this sounds extremely high to me aka laggy as heck?

So help me out here please - what are you guys running on a T4 turbo for A/R for a streetable 800 or so hp

a complete turbo suggestion would also be welcome
thanks for your time

gtfoxy

Have a power idea you would make NA. Then go from there.

Effective A/R is dependant on a number of factors.

The whole idea is to match the required pressure drop, at an engines intake & exhaust energy, across the turbine to drive the compressor wheel as efficiently as possible in a given range of turbo RPM, from one side of the turbine wheel to the other.

Every turbine wheel will have a given efficiency capability, in a given airflow, & enthalpy range, in a given A/R.

Every Compressor wheel has an efficiency range, & peak efficiency, at a given presure differential as well, on a given A/R in an RPM range...

Put those two together in the dynamics the engine can deliver over those operating pressure differentials, and you get an ideal, or as close as possible, for the intnended application.

You can run old school like .81 P-trims & make 800WHP @ like 14psi like 4jenna does with a pair of 67mm turbos.

You can go modern. S366's or 369's & play with A/R's to find what works in your combo.

coltboostin

Twin .81 ar T4...67mm here, Made 870whp out of fuel on a stock LS1 with stock Cam. Spool was great IMO.

Spool with be great with .68's

Seeker1056

unfortunately this is like many - a budget build - so there will be no "playing"

I need to make a one time decision based on available info as the turbos i buy won be swapped out or have various housings tried out - been down that road with my pulling truck - a very expensive proposition

Orr89rocz

iTrader: (13)

80 lb inj, limits you before the meth, so i would consider .68-.81 housings in a 64-68mm turbine inducer. Typical p trim found in on3's master power etc. precision has 66 and turbinetics goes up to 68mm. T4 frame.

I run .81 t4 turbos on my 400 and knocking on 7's. I had .68 master powers on a milder 400" running mid 9's on 14 psi. Spool quick is fun! With your small converter go smaller a/r

Seeker1056

coltboostin - divided or open foot?

Seeker1056

I dont know what a P trim means - I have only ever dealt with A/r numbers
convertor isnt bought yet - what would be better - maybe 3500 on the street?

gtfoxy

Old designation for T4 turbine wheels.
Trim..........Exd.......Ind
O Trim - - 2.296" 2.922"
T4 P Trim - - 2.544" 2.922"
T4 Q Trim - - 2.693" 3.111"

The P-trim is a pretty versatile wheel. It works well on singles with .96 A/R's to twins with .68-.81's. The Qtrim is a tad bigger.

coltboostin

Open. On3 turbos