Twin turbo A/R sizing, too small? 750 hp

[Big T]

Hey guys I'm having trouble deciding on which way I need to go here. I have a new set of twins I put together on my 12 valve. This is all built from scratch with a Bullseye S475 on the bottom and a 62mm comp ball bearing on top. Both are T4's. My problem is my 62 65 .88 turbo. It never quite catches up to my fuel curve now. The pump is benched about 500cc's. I have sucked all the fuel everywhere I can and still have a heavy back haze all the way up. I need the air curve to pass my fuel curve during normal driving to clean up the truck a little on the street. I have the options of a .81 housing or a .70 housing from Comp. The .70 is a divided the .81 is open. The .88 I'm running now is open also. I have 44mm Tials on both turbos now so what I'm wondering is if I go down to the .81 is it going to make much difference or is it just too close to my .88 to do much. If I do the .70 divided will the 44mm gate still be able to control drive pressures and boost. I do run a shot of spray on the dyno. The truck is running 97 psi right now but I am dialing it down to about 85 on the street. The truck has hit 722 in the past and I want to maintain that ballpark still. With the .70 will I have surge issues? out of control barking? this is a driver and a heavy tower with a gooseneck trailer. Thanks for the help.

 

[RonA]

When you say you have sucked all the fuel away you can, I assume you mean that you have ditched the full cuts, and have a stiff spring in the afc so you don't see any movement untill at least 15psi or so?

 

[Big T]

No I still have the full cuts in. I have pulled all the fuel I can from the pump. I dont mind a little smoke when I pull out but I want the turbo to clear it up just a little more once it catches up during normal driving. Right now it never quite catches up. I have a valet on the pump so I can disable the afc from moving. So I know we are working with the fuel below that. The screw on the back center of the afc plate is backed out all the way, I run a 100 plate, the foot is set as low as possible on the 100 plate. I drive the truck on the street between idle and 1/8 throttle normally, that's it.

 

[LReiff]

The open turbine housing sure isn't helping you and those 44mm gates are a bit large. Put the .70 divided housing on it, set the gate on the manifold to open at 35-40 lb boost which and the gate on the 75 to open up higher in the rpm range, only to alleviate drive pressure.

 

[Big T]

Thats what I was thinking and hoping for but was afraid the .70 with a 44mm gate would choke at 4000 rpm and 750+ hp.

 

[LReiff]

Being you have the 44mm gates switch to a .70 housing and see if you can get the gates tuned to work. With a 44mm gate, the gate on the manifold should only gate 3 cylinders. You'll need the other 3 to keep the small charger spooling.