long time lurker I just created an account today.
I have a ’69 Chevelle with a 6.0L l96 stock intake and throttle body, PNP heads, and a Stage One twin-turbo cam (221/.598, 231/.601, 113+4). It has a tight Circle D 265mm, 3200-3400 stall converter (foot brakes to 2100 RPM and flashes to 2500), and a 6L90 transmission with 3.25 gears. I’m using a Huron Speed T6 hot side kit with a Force Performance 8588 UHF turbo. It makes 810 HP on 13 psi, but the problem is that I don’t make any boost until around 4200 RPM, where it hits 1 psi, and I’m in full boost by 4600 RPM way too much lagggg for a fun street car. I’d like to replace the turbo with something more street-friendly that has minimal lag, like the 7875, but my hot side kit is already built for an S400. I’m wondering if an S478/87 from FIS would be equivalent to the 7875. Any input would be greatly appreciated, as I’m on the verge of ordering a turbo. I just don’t want to make the same mistake I made before by ordering too large or small a turbo.

here is the turbos I'm looking at: TDI S478/83 https://tdi.forcedinductions.com/tdi-billet-s47883
S478/87 FIS https://shop.forcedinductions.com/fis-etr-s47887



Here’s your photo from a few weeks ago Redwood rally

Here’s a photo of the engine bay currently.

 

I'd stick with the turbo you have and focus on the tune and converter.
That converter sounds awfully tight and there may be some better spool to be gained by getting more aggressive with the timing out of boost.
I run an S484 on my LS1 and it comes on line faster than your bigger 6.0 with a smaller turbo.
I don't think the problem is with your turbo.

 

I have a vsr g45-1500 clone (76/89) in a t4 configuration. built 346 with ported heads and a turbo cam. also an auto car. it has almost 0 lag. I believe you can get a similar sized version in a t6. depending on your power goals I'm sure even a s475 would fit the bill.

 

believe me the tune has been worked and reworked and worked some more to attempt to eliminate lag.

I was looking at that exact turbo what’s its hp limit? unfortunately to avoid fabricating new down pipe and switching to t4 I think I’m stuck with the T6 s400.
My current setup has clipped exhaust wheel Force Performance call a UHF ultra hi flow. I believe this is a major contributing factor to my turbo lagg

I wonder if anyone has experience with the FIS S478/87 or TDI S478/83

 

Supposedly they rate it to 1350 flywheel hp. I’ve talked to 2 people that had them on slightly larger motors than mine. One being a 383 and one being a 408 if i remember correctly. The 383 car made 1150whp my car is on 20-22 psi and i don’t have numbers but it pulls from 3400-6700 with ease. I’d probably guess I’m around 800 range. My car hangs right with my buddy’s 1050hp supra from 50-140

 

Those are great numbers to me. If I can have a street car that makes 800 horsepower with little lag, I feel like it would be a blast to drive or fun street car.

 

BORG does their rear wheel diameters backwards when listing the specs. The T4 S475 is basically a 75/75 Though its listed as a 75/83. The exh wheel is really an 83/75. So same size exh wheel as the 78/75. (better design though) You can get S400 units with 75-88mm compressors. The basic Borg T4 S475 outperforms the 78/75 VSR base models in my experience by a decent amount. Better turbo all around I think. I have not worked with the "top of the line" VSR 78/75. But I can tell you they won't out spool what you already have.

Your issue is all in that converter, not the turbo. I don't understand how they sell those as "3200-3400 stall converters". Similar to what you say, I've never seen one stall anywhere near that. 2500 flash is pretty good for that unit, means you are making decent power NA. But that RPM isn't gonna cut it. Get a decent stall speed and it will be night day. Have circle D re-stall that one, or buy a PTC. A 19-5 stator 9.5 PTC will spool that turbo like nothing.

Only other solution is to use a 50-100 shot for a second or so to spool the turbo initially.

 

a converter like this? https://www.summitracing.com/parts/tqe-s0022 or
https://ftiperformance.com/products/...80e-6l90e.html

your thoughts are a smaller turbo will not solve the issue of turbo lag?
my issue is 100% converter.

I've been considering 100 shot, but I don’t feel that that will sustain on the street rather just eliminate the lag.

 

Really depends on what you are wanting to do. If you are willing to have 2:1+ back pressure ratios and run relatively low boost/power then a smaller turbo can help, yes.

You have what sounds like a “drag setup”. That turbo is sized well to a 6.0 in that application. What that combination is lacking is stall RPM. I don’t know what that summit converter you linked would stall to behind your setup. I’d avoid shelf converters and contact PTC or a similar “performance” shop that makes custom converters for your specific setup.

For summit to call that a 3500-3800 converter is misleading. You need to know what the flash stall speed is behind your specific combo. You already see a “3400” rated stall converter is only flashing to 2500… if your lucky. Who’s to say what that converter you linked will stall to behind your engine. I know from experience what the PTC converters stall to behind basic “LS” combos. A trans brake would be a night/day difference as well. I spool twin 78/75’s in less than 2 seconds with a cam only 5.3. But I have a trans brake and my 2 step is set to 4400rpm. When I release the brake it flashes up to 5600ish at 7-8psi and this hits peak boost in about 1 second.

Best bet for your money would be to contact Circle D. tell then their 3400 rpm converter only flashes to 2500 in your setup and you want it restalled to flash closer to 3800-4000. Which will most likely tell you isn’t possible. So you have them put the loosest stator combo they can fit in your converter and go from there.

 

great info here! converter does play a big part! if I were in your shoes id maybe try to restall your current converter before changing the turbo set up. I'm a cheapo though and tend to lean towards whatever will cost me less in the long run.

 

I've spoke to Circle D they stated I needed a smaller converter 9.5 from there PRO line $2000+. Circle D recommended a stall of 4000-4200 for my set up. Its just a waist to replace brand new parts with other brand new parts. I hate spending on the same thing. That why I'm here seeking advice I do appreciate all the help.
Its my understanding that the Circle D 265MM is designed for heavier cars and trucks.
A few individuals have recommended a better converter I figured it would be best to ask for advise before purchasing anything.

Also based on what you had said a Borg S478/83 with a TW of 83/75 would make it a 7875 technically.
Then I could do T6 AR 1.10
Hmmmmm.

I'm looking for a fun street car setup with little lag. Maybe 700-800whp.

 

You are a little oversized turbo wise then. A box stock S475 with a 1.10 T6 would drop right in and spool much better and meet those power goals. How much better would it spool though, I couldn’t say. Likely still won’t make you happy with that tight of a converter.

Could pull and sell your converter, they seem popular enough. Then call up PTC and tell them what you stall at now and what you want it to stall at. Also Could send your current converter into a performance shop and have it restalled if Circle D isn’t being reasonable. Might call PTC or jakes performance. Tell them what core you have and what its doing. They may tell you something different than Circle D did.

Nitrous is an option many are afraid of. But you can piece together a dry kit for around $300 and have it installed a couple hours. Also if you only use it to spool the turbo its generally only active for 1-2 seconds. So bottle will last a LONG time. I sprayed a 60 shot for a whole season and still had half a bottle at the end of the year. But it was literally active for less than 1 second on the transbrake only.

The tune up could also be part of the issue. Are you on a stand-alone ECU? Who tuned it?

 

I run a stock e38 pcm it’s been professionally tuned by reputable tuners. It’s had 3 tuners look it over on 3 occasions.

it sounds like no matter what I need a converter and a turbo

I oversized the turbo!
and I undersized the stall.