Replacing my stock engine. I lost a good bit of power and have quite a bit of blow by now.. was at a little over 600hp when it happend. So I'm looking for opinions on what to rebuild it with.

Car is a 1999 C5 with the six-speed T56.
Has the new STS twin turbo kit on it from Holley which I actually like a lot and intend to keep.

As for other mods ive got a twin turbo cam from comp, 60lb injectors, racetronix fuel pump, meth injection, dual disc clutch and that's about it.

I'm being told it will cost the same for a forged short block regardless of whether it's a 347 383 or 408.
Car is 100% street / highway pulls.
I'd like to keep it spooling fast and not have to stretch it out to 6,500 for a good pull if I don't have to.
I would like to reuse my heads and twin turbo cam with the build if possible. If not then 347 It is I guess.
I'm willing to spend the money to get a short block with good forged internals, but I'm not trying to go buy all new everything. I do all my own tuning so that is not a concern.

So comparing the three. (347 / 383 / 408) is there anything making one better than the other besides more off boost power?
The stock 347 before it blew ran awesome at 12psi With the TT cam and meth.

I'm only looking for 700 maybe one day 800hp max.
I know if I go bigger than the 383 I will have to change out my turbos so that will add to the cost.

I know my turbos are probably too smal as is but being it is a rear mount my twin 40mm T3/T4 highbrids with a .36 housing spool fast and make great torque in the mid range so its perfect for a fun street car.

I know I'd be happy just geting the forged 347 short block with around 9.5 compression from Texas speed and making mid 600's without changing anything except doing a re tune. I just want to explore my options before I decide and see if
Theres Something I'm missing here.

Is there any reason a 383 or bigger would be better for me?
If I only went up to a 383 could I possibly still use the small turbos I have without it causing problems?

Cam specs and dyno sheet attatched.
Dyno pull was done on only 9lbs back when it still had the stock cam.


Opinions anyone??

 

Bigger cubes would help with spool and power out of boost, but you will need to up the turbo size on the twins. Your heads and cam would work with any of the motors, but the turbos really need to be sized properly for the engine and given that it's a rear mount application, I'm just getting my first rear mount setup up and running. Do you have full specs on the current turbos?

 

All the specs for the turbos are on Holly's website.
STS just revised and released their new kits back in December. I was the guinea pig / or I should say my car was for dialing in the caned tune that comes with their full package deal.

I do have some larger exhaust housings for them, (.48AR) I tried them and my top end held on a little higher, till 6,300 or so vs dieing off after 5,600 with the .36 housings.. But the spool up wasent nearly as good and for a street car they were just no fun.

I do know the turbos are rated for 750hp @ engine in stock form with the .36 housings but I'm sure that's pushing them pretty hard.. they spool loud though! Loud as if I roll into it part throttle the spool sounds like a diesel trucks turbos spooling up. Deffnetly turns heads.

 

Specs from Holley website for the turbos.

AR .36
T04B Compressor Housing.
Diameter 48.4 mm Compressor Housing Compressor Housing Inlet 2.75 Compressor Housing Outlet 2 inch
Turbine Housing T3
Turbine Housing Diameter 36 mm
Trim T-3
Bearing Style Journal Bearing.

Says Max boost is 8-9 PSI and they are rated for 760 crank horsepower.

Really wish they would provide a compressor map for them..

 

I guess it just depends on what you're willing to trade off. You'll likely need to either push those turbos really hard or move up, but sacrifice spool. Larger motor, larger turbo. I'm not sure how a 347 could be the same price as a 383/408 since 1 of them uses a stock crank and the others dont. If you're happy with the 347, then just turn the boost up and see where it takes you.