LS2TT

iTrader: (1)

Hey guys,
I'm looking for some input regarding low boost target and GT30-71R turbos and I was hoping you could help me out. My car has twin internally wastegated GT-30rs on a built 8.4:1cr 403ci LS2. Due to space limitations, the turbos have the .86 A/R 56mm T25 flanged turbine housings.

I'd like to have the capability to run the car at 6psi on 91 octane but I'm having an issue with boost creep. Specifically, the car wants to make 12-15psi with the 6lb springs. We wired the wastegates open and made a dyno pull which still allowed the turbos to slowly spool, hitting 12-15psi by 5500 rpm.

It seems like the 403 is pushing too much exhaust to divert past the turbine wheel and bleed off through the internal WG.

We have a few options.
1) port the internal wastegate housing
2) run the larger 60mm T3 flanged turbine housing .86 or 1.06 A/R (requires significant cutting)
3) shoot for higher target boost (lose flexibility and control on pump gas)

The T3 flange on the 60mm turbine and the separation of the exhaust and wastegate port would likely be benefit the car, but I hate to go through the time and expense if we're still unable to run low boost for the pump. Not sure if porting the housing will net much and running higher boost levels will lose flexibility with the boost controller and make the car hard to control in lower gears.

Any advice, opinions or ideas are welcome. Thanks in advance.

markp03

iTrader: (4)

different turbos....honestly. If they were not internaly gated i would say get a bigger wastgate. I think you have 2 problems. one the internal gate is not big enough and two the turbo with the .86 a/r is too small. I am not sure how much of a difference a bigger a/r would make. It might allow for a little better control. but i am going to guess that the exhaust housing is just to small. Is the turbo choking out at higher rpm's or are you venting enough exhaust to keep that from happening? based on the turbo size do you think the turbo would choke out at a higher boost lvl?

also on a side note. from a graph i looked at. the turbo is not in it's efficenty range untill it hits about 15psi.

onfire

iTrader: (17)

Can you post a pic of the manifold turbo set-up?

Obviously T25 housings are small for your combo but not killer small. LPE uses GT2871R's on their 427 builds with T25 and internal gates and the gate springs hold 6-8psi without a problem....

I'd look at the turbine to exhaust pipe junction where the internal gate dumps the bypassed exhaust gas into the exhaust stream...could be backing up there and sending the energy to the turbine wheel even if the gate is open....or you have a real bad exhaust system.

LS2TT

iTrader: (1)

Unfortunately, the .86 is the largest A/R turbine housing available for the 56mm wheels in these turbos.

http://www.turbobygarrett.com/turbob..._700382_20.htm

Here is the option we’re considering, which will require frame modifications to fit the larger turbine housing with the T3 flange and 60mm turbine wheel.
http://www.turbobygarrett.com/turbob...30/GT3071R.htm

I think we would be fine at a higher boost level but the space constraints have forced some compromises in turbine flow. The smaller T25 .86 A/R housing is definitely not optimal for the higher boost / higher RPM flow but I’m looking for a compromise.

At this point, it looks like we move down to the GT2871Rs or move up to the full GT3071Rs. I just need to weigh the cost/benefit of cutting/fitting the larger 60mm turbine in the car vs. the smaller GT28Rs (limited to ~700rwhp). Any input?

The manifolds/downpipes on the car are identical to what has been proven to work with GT28rs. The exhaust if full 3” through the downpipes, Z06 3” cats and 3” cat back – so I think it should be fine.

What’s ironic is that I considered running the GT2871R’s but UPSIZED the turbine wheels to allow for BETTER exhaust flow with the larger GT3071RWG. The same combination with the GT28s can control boost to 6lbs but upsizing the turbine to the GT30r’s has caused problems with low boost.

This puts me a crossroads
1) Go down to GT28r since they are proven to allow low boost w/ my manifolds
2) Go up to full 1.06A/R GT30rs to improve exhaust flow (unknown)
3) Try to improve flow through these GT28/GT30r hybrids to allow low boost

Haven’t found a way to improve the junction at the exhaust outlet unless we go with a divided wastegate / turbine outlet and that comes only with the larger T3 turbine housing on the full GT3071R.

Thanks again and any ideas are welcome.

JZ 97 SS 1500

iTrader: (2)

If you can fit it, the best way is weld the port shut and put external gates on the manifolds.

onfire

iTrader: (17)

Is this an APS kit on a Vette?

I've tuned three different combos similar to what you're using. A 346ci with GT2871R's had great boost control from 6 to 17psi and made around 700rwhp max in 90F weather on the dyno. A 428ci with Gt2871r's that maxed around 720rwhp.

Both have great boost control and baby T25 turbines.

You installed T30 turbine wheels in a GT2871R?

Did not change the compressor wheel?

Only you can decide the frame hack vs $$ vs hp value.....but a Gt2871R is a great street turbo that will go high 9's with traction...not optimum...but works well...T25 is a bummer but life is a compromise.

The 15psi with the gates open would bug me to no end...I'd solve that problem before I spent any money....I'd drop the down pipes and rig a short pipe to the turbine outlet just for the turbine flow and let the gate vent to the air...cheap way to see if the boost still creeps with the gate wide open...if it does still go to 12-15psi then you know the turbine wheel is the problem.....right now it's too many guesses with routing the gate back into the exhaust pipe, cats, muffs, etc.

If the turbine wheel is proven to be the problem...and it was my car and a nice Corvette, I'd put Gt2871r's on it and be happy with more power than street tires would hold...tune it to 8-10 psi and drive it anywhere.

Good luck.

LS2TT

iTrader: (1)

Not an APS kit, but similar configuration.

Thanks for the advice, this is exactly what I needed to hear. I've been trying to allow the headroom for 800rw on race gas but, considering the compromises involved, I think I would be happier with the GT28Rs tuned for 91 octane and maxed out to ~700rw with race gas. I'm just hoping that the power doesn't hit hard then choke as the exhaust in restricted at upper RPMs. I'm coming to terms with the compromise, but it should make for a killer street car.

Besides, the car IS too nice to cut up for the larger full GT30rs and I'm not 100% confident regarding the net gains from the rework. We will be dynoing once again with fully ported wastegates and see where things fall - then it's time to make the call.

Frost

iTrader: (45)

I tuned a TTi X twin setup on a vette that had this same problem. I'm not sure what size the units were; I can find out tonight. They were ball bearing units. While on the stock motor, without porting the housings to increase gate flow the owner removed his home made very high flowing exhaust and actually put the stock cat back on. This let us hold it to the gate's spring. He wanted to move up to a forged setup to crank things up and while he was swapping engines he ported the housings at the gate and with his high flowing wexhaust, this made all of the difference in the world. Boost would hold steady at the gate's spring. Seems like the opening to the gate was just way too small; exhaust gasses were too much and finding their way out through the unintended path.

paulgto

at 12 psi how much power are u making?

LS2TT

iTrader: (1)

No numbers yet. Without boost control, we've focused efforts on sorting out the turbos. As of today, the internal wastegate housings have been ported and will be back on the car tomorrow. If we can control boost, we'll have dyno numbers. If not, it's decision time.

Swap the current GT3071R's w/ 56mm .86 turbine with
1) GT3071R with 60mm 1.06 A/R turbine (requires cutting)
2) GT2871Rs with 54mm .86 A/R turbine (proven combo)
3) GT3076Rs with 60mm 1.06 A/R turbine (requires cutting)

LS2TT

iTrader: (1)

FYI - We ran the car with the nicely ported internal wastegate openings on the GT30rs and still had no boost control. Like before the porting, wired open wastegates gradually build boost - peaking at 10ish rather than 12-15psi before the porting.

Considering this is a street car and low boost control is priority for car control, I've decided to run the GT28rs instead of upsizing turbine housings to the T3/60mm Full GT-30 unit. The extra efficiency with the increased A/R and turbine flow would be nice, but considering they are untested (on my car) and cannot guarantee low boost (6psi), I'm not gonna cut the frame and take the gamble.

Thanks to everyone for the advice. I'll check in later after we see what these baby turbos can make.

67Firebird455

iTrader: (27)

That's why I was looking into the gt35r's, or 30's without wastegates. Run 48-50mm wastegates and you won't have issues, but you'll pay a pretty penny for them, I don't have to tell you that! Good luck and keep us posted!

Zombie

iTrader: (2)

This isn't likely, but what does the tuner have the timing set at for this testing? If it is overly retarded (less than 10 degrees) and dumping fuel you could be burning the gas in the exhaust adding the issue. It's probably not the case, but thought i'd add it in as another thing to examine. I've head of some tuners starting off turbo cars as low as 5 degrees to be safe.

onfire

iTrader: (17)

Sounds like the 3mm bigger turbine wheel in a 25 housing is backing up the exhaust flow.

You'll be surprised how powerful the car will be with true GT2871r's. Add Mototron 60 injectors and either dual walbros or a single walbro and a KB boost a pump with a front mounted regulator and return line....the car will go 700rwhp on 104 unleaded and have the power to run 9's if you can hook it.....honestly, for the street tune it for 10psi and you'll be surprised how fast it will be ....solid 10 second car.

LS2TT

iTrader: (1)

Quick update: GT-30rs are gone... Car is back together with the GT-28rs. BUT..... It still wants to make 8.5-9.5psi on 6lb springs. Controls steady until ~4k then the boost climbs, stabilizing around 9. At ~9psi, the car made 590rw (91 octane with cats). I was hoping for 6 psi on the springs to allow for better traction in 1st. Considering the ported internal wastegate openings only managed to drop the GT30rs by ~1psi, I might have to drive it like it is and give up on the low boost target. Any thoughts?

Specs: 403ci / 2004 Z06 cam / L92 heads / L76 intake / 3" cats / 3" exhaust

calongo_SS

iTrader: (7)

While I understand the frustration in not getting the boost you want there should be no problem running 12 lbs of boost on pump gas with 8.4:1 compression. 6 lbs of boost is a lot lower than I would be aiming for with the street tune.

Twin76mmTA

iTrader: (1)

when where your cats last checked? any chance they could be clogged and restricting the exhaust flow?

LS2TT

iTrader: (1)

Cats are new 3" C6 Z06 units.

Yeah, I think I can hit 12psi on 91 octane. I was just hoping to have the flexibility to dial in the speed-based boost controller for traction on street tires. Something like 1st gear = 6psi, 2nd gear = 9psi, 3-6th = 12psi. As it sits, I hit 9.5psi in 1st and can ramp up to 12psi. No worries, I'll just have to modulate a little more vs. WOT in the lower gears. I'll be driving the car on Friday and see how it feels at the current boost levels.

Thanks for the input.

ggk

iTrader: (1)

i thought the new z06 cat were kinda restrictive so it would pass in cali.
i may be wrong. but why dont you try to bypass the cats? i dont know if they are welded or bolted in. try that it might be too restrictive and causeing too much back pressure or something

onfire

iTrader: (17)

Are you positive the gates are 6psi? Positive the preload is set correctly with a Mityvac and balanced between both gates? Both gates begin to crack open at 6psi with a Mityvac? Run the gate boost source from the compressor and not the manifold....that will gain you the pressure drop through the IC, plumbing, etc. If all that has been tried then wire the gates open and see if it still creeps....then it's a hot side problem.