Bazman

Just upgraded from LS1 rear mount to a new LS3 on a rebuilt STS ball bearing 6275 T4 0.81AR turbo and found out why we can't get decent power out of it. The turbine is so small it is creating 22psi of back pressure!

Awesome bottom end and mid-range LOL

But it falls flat up top. Exhaust system is high flow headers into single 3" stainless no tight bends and no restrictions other than 2 x 3" high flow cats - there is no improvement to be had there in flow, it's all in the turbine restriction.

Engine is LS3 6.2L with 218/222 0.561 on 114/116 cam, springs/pushrod upgrades, 040 Cometic gaskets, ARP bolts etc, rpms to 6500rpm max, runs 725cc injectors and twin Bosch 044 pumps.

What would be cool would be to get 2lb by 2500rpm, 5-6psi by 3000rpm and all in by 4000rpm, yet flow enough to make 800hp at 10psi boost.

What sizing do I need guys? The problem I see is getting the exhaust AR right to speed up the spool with a bigger unit yet still reduce back pressure massively.

$50k later and still going

for anyone that can get me to a reliable and good solution here

Bazman

I'd almost settle for a 75hp shot to help spool and size the turbo for good power per psi of boost, never run drugs in my car before tho

lol

ren987

iTrader: (4)

what is the manifold pressure when you are seeing the 22psi of back pressure?

Johnv

Bigger turbo and move it up front will solve all your problems....
But if you want to persist with the rear mount, ditch the headers and fit factory cast manifolds, heat wrap the hell out of the exhaust system pre turbo, this will improve heat retention and exhaust velocity improving turbo spool.

Replace the turbo with something bigger that will flow enough for your power goals, maybe a turbonetics TC 76 or thier new 7675 using a devided exhaust housing, and add a quick spool valve.

foose04

iTrader: (4)

22psi BP isn't bad. You have something else off, likely that 62mm compressor feeding a 6.2L or tune. Also if your wanting 800 wheel/roughly 1000HP flywheel (if auto) at 10psi you better keep spending/wasting money.

Bazman

Just spoke to tuner as he's still working on it. Bottom end and mid range is very good and a huge imporvement over the bigger cammed LS1.

Pressure to turbine is 25psi while the manifold is seeing 6-8psi. That explains why it's been so hard to get boost down to a lower setting even with a big wastegate valve. We're trying to get a low setting of 5psi max but the srping is seeing 25.

The turbo peaks at 12psi in mid range then falls off to 10lb on top end.

Bazman

Everyone says that about headers but I can't get my head around how adding more restriction will help power, it has to hurt top end power. I can see it help spool up, and I guess that is your point - make the power with a bigger turbo then speed its spool by a more restrictive exhaust. Fair enough.

When I upgrade the turbo I am going to wrap the entire exhaust first and hope the latest turbos like the 7675 you suggested spool good enough. If not, I guess time to lose the headers, and that'll make me consider moving the turbo up front.... that'll waste a lot of money spent so far lol

Bazman

Sorry, meant 800 engine hp. With an auto and 2800 converter that is around 650rwhp....

The current turbo is only making 510rwhp at 9-10lb boost.

Bazman

Thanks guys

PUNISHER TA

iTrader: (13)

Grab a fluid dynamics book and read up on port velocity.

Having large ports with slow moving air is not good on a turbo car. Most builds on here get far too large of hotside tubing. and then complain about spool times, its getting annoying.

Im at work, where yellowbullet is blocked and ls1tech isn't. go figure. so I can't do all your work.

but go search on yellowbullet. Renengade has a post where he breaks down and explains the importance of port velocity.

So, with your "lower restriction" you are hurting your port velocity(ignoring thermal events). Also, the larger surface area of the larger diameter tubing with thinner wall, lowers EGTs which also lowers your port velocity...

Don't expect to have a well functioning rear mount setup by going with "what feels right to me" and ignoring physics and the more experienced.





Besides that. a 62mm turbo is going to pretty maxxed out at 650whp, which is a good thing, but everything has to be working together.






Yes, 22psi/6-8psi ie 4:1 backpressure ratio is very high, but I can't imagine it being your turbo, its not big enough to create those issues. I think its just too small, where are you on the compressor map?

Bazman

That small turbo with that turbine housing will get no where near 650rwhp no matter what boost we try to pull, it's pretty much maxed out at 510rwhp on my setup.

While I respect your viewpoint, spool is not a problem on my set up with that turbo. It could provide positive pressure from a dig even on the old LS1 with nothing but a light brake and a little load to stall the converter briefly. On normal take off it would boost from a stop in under a second - no wrap, totally exposed stainless exhaust. Spool may become an issue with a larger turbo

THe exhaust headers and 2 x 3" system blend via a 2nd collector into a 3" single pipe that makes up the longest section of exhaust, thus speeding up gases and restricting the pipe size to around the same size as the T4 inlet.

THe LS3 spools far quicker and makes so much torque in the mid-range it is actually twisting the trans down and forcing the driveshaft into the loop, so we are having to get stronger mounts and may need to drop the loop slightly. So we are making massive torque, just can't get enough air up top to keep the power climbing.

As you said, the compressor is too small.

I've been recommended an Airwerks S300 BW turbo but would have thought a S400 or BW EFR9180 would be better?

NO idea about my current compressor map - STS did all that figuring out and don't share that info. I can't really read those charts to size right anyway

ren987

iTrader: (4)

are you sure on the exhaust side of that turbo being 75 and not 65? I know a 75 turbine is still small for an ls3 but I wouldn't think back pressure would be 4:1 if it really had a 75 wheel.

smokeshow

iTrader: (17)

Sounds like the turbo is just too small. You're so far to the extreme right of the compressor map that it cannot continue to flow air. You're missing at least 150hp at 10psi simply due to a lack of compressor.

Bazman

It's some kind of Asian made hybrid that STS has made for them. This is the only detail I have per their invoice:

TTS-UNI-SINGL... Tuner Turbo System Single Universal T4 E-Cover
Turbo 805.00
TC-STS-75-T4E... TURBO STS 75 T4E/T4V P-TRIM BB (67) NO
TURBINE HOUSING 1,745.00
TH-STS-T4V-PT... Turbine Housing STS T4 P-TRIM .81 A/R with
Coating 240.00

I've tried emailing STS but they do not respond, even when asking about buying a bigger unit

Bazman

Wow,

well if I could achieve that without losing too much bottom end I'd be very happy.

In the meantime I'll enjoy what I've got given the cost to get this far and the turbo just got rebuilt.

Bazman

Interstingly, the tech at Borg Warners EFR said their EFR 9180 would do exactly that - add 150rwhp at 10psi so your math and theirs agree

They also claim it will spool as quick as the small unit I now have

smokeshow

iTrader: (17)

Sounds like you have a turbo to purchase. That 9180 is an interesting beast...67mm is normally pretty small for a 6.2, but that compressor map has a really fat low end and it is ball bearing... I am definitely interested in the results if that is the turbo you go with.

Bazman

Will do, this is what I'm planning to do, just want to get some mileage out of my newly built old one.

Interestingly for those that think rear mounts don't have bottom end, my dyno graph comparisons (all same dyno and tuner who is conservative) tell the truth.

Being a 4L60e all pulls are in 3rd gear as it is 1:1

The stock LS1 with headers and cold air intake and tuned ecu made just 100rwhp at 80kph (50mph) - about 2600rpm.

Adding a 222/226 cam and stall converter this moved up to 115rwhp at 50mph.

The same turbo I now have on that LS1 jumped to 210 rwhp - that's an 83% jump!

The LS3 however makes 260rwhp at just 50mph and only 3mph later cracks 300rwhp on same turbo.

Show me a blower doing that on a stock 6.2 motor (well I have very mild 218/222 cam).

Jenson

iTrader: (32)

You have a water cooled rear mount now? or want one with that efr turbo? Search for zombie's rear mount build on here, he's already figured all this out. I want to build an 800whp rear mount ls1 next year, so this is very intriguing

Bazman

No I have yet to do the water cooling, the current setup doesn't get that hot usually. That might change next week with about 600 semi-race miles on closed roads lol