I'm not exactly new to turbos and I do understand how to read and use compressor maps, but I had a more theoretical question.

Running a 427 right now with GT35's and small .68 a/r.
I only run 7 psi of boost (road race car) and am making about 680 rwhp.

I'm trying to understand the advantage of moving to a larger 1.06 a/r housing. Its obvious to me that running a larger housing means you can make more airflow. If I was trying to run 15psi and 1000hp on this motor that would make sense. But it seems like if I am not losing boost pressure at my top end, and I have large enough wastegates that are releasing excess exhaust pressure, then what would a larger housing help with except to increase spool time ?

I figure it might help efficiency a bit (hard to measure) and decrease housing temps. The normal wisdom is that the small housing is "choking off the exhaust flow" but with 2 40mm wastegates, I don't see why they wouldn't just open and relieve excess flow. If I screw down the the wastegates, it shoots up to 14 psi by 3k easily.

I don't know my manifold backpressure (swain coated I'm loathe to scrape it off to put port in) so I don't know if its excessive.

I just am trying to figure out if spending $800 + fitment labor and potentially increasing my spool time is worht anything when I'm not looking for more power.

Why would I even consider it ? The motor does drop off in power pretty quick above 6k, and I don't know if its an airflow issue somehwere in my combo or related to these small turbos (although again the boost pressure doesn't drop off). I'd like the power band to carry a bit further. This isn't an LS motor so this isn't a question about my combo per se, but about how turbo housings actually affect the system.

 

where is you're power peaking at, if its peaking above 6000rpms, then no point switching to the bigger units.

 

Going from .68 to 1.06 is a huge step. Since the power is dropping off after 6k, it sounds like you could be choked by the small housings, but something around a .81 would probably be a better trade off.

 

That was my thought too, I just see a lot of people on this forum going not just with larger A/R's, but also larger turbos in general on the bigger V8's.

But I have to keep reminding myself they're all trying to run 15+ psi and 1000hp.

yeah I make peak power about 5900 rpm. Its a reasonably mellow 230/230 hydraulic roller cam, so maybe that has a lot more to do with it.

I know lots of people on these forums have made over 1000hp with GT35's, but they don't often mention what a/r they are running and again they are running in a totally different part of the compressor map then I am.

 

members who are making 1000HP with gt35's are running the 1.06 a/r as supplied by APS.

if you peak and 5900, where do you shift at? If you want to make use of ur 7000rpms rev limiter, or maybe a little more, then step up. Other wise stay, but again i bet the power drops just enough to not make u shift at 7000rpms.

Try a different cam, I know jim@speedinc has a camshaft that is specific for backpressured boosted motors, and I run one on my motor.