Seeing as that was your first post on here it seems,let me welcome to the show of Ls1tech.With that formality out of the way let me say that your assumption may have some merit if my car was N/A and i wanted to make anywhere from 350-500whp that having a true dual exhaust would net me some gains.Regardless of that when my car was in fact N/A and just a heads/cam setup it had long tube headers..into a 3" Y PIPE/3" highflow cat/my current gmmg muffler/current 3" qtp cutout was making 400whp. The exhaust size and flow for That particular setup was not being limited from Too small single 3" exhaust. Having my cutout open only net me 15whp and that was after a cool down dyno run just to bang the number I wanted.

My car has a single turbo,meaning I cannot run a true dual exhaust because each manifold/header must merg..or come to a Y pipe to feed exhaust into the 1 turbo.And then exit said turbo from its single output which is a 3" vband flange.

So iirc Customblackbird has a very near identical setup to me in the same car but when i first set out to try turbocharging my car as mentioned I was making 400whp.My goal was to add 100-150whp..maybe hit 600whp on a cool dyno run. I did this on the cheap before cxracing offered a turbo kit for our cars that has more potential than my custom setup and heres why I think so. Im using flipped truck manifolds that are welded to a 2.5 merge pipe vs the cxracing turbo headers that have a better flow out of the heads into the turbo.This only complicates my exhaust issues for backpressure on the high boost side of things.At the sametime..the truck manifolds with the 2.5 merge does help spool quicker than a larger header.there are pros and cons to this.

I can easily just run the car at 12psi and not even worry about things and leave all else alone and have my existing cutout open and make low 600whp.I just cant go back down to that power level after tasting 15 ..18 20psi in my car as it sits..wanting to make more power safely if it could diarrhea **** itself fast enough! All i can do is try to aid the existing down pipe with another bypass much closer to the turbo itself and have a 2.5" fender exit on a loudvalve and then the rest of my exhaust setup and cutouts handle everything else.After that..if I the car still isnt happy enough I could try changing mufflers but I feel that the muffler is so far down the chain that with 2 cutouts in a very straight piped system before the overaxle isnt affecting much.

The main concern isnt power for me at this stage guys,thats not the point of my thread..its the concern of how bad the backpressure if I continually run high boost having 8psi backpressure is shortening the life of my motor power..which in affect is robbing me of extra power i havent felt yet.I was very content at making 650whp on 94 pump gas at 15psi till that motor popped at the track due to my neglect at the track on a hot day forgot the turn on meth and made a high boost first pass out the gate.

The car goes in the shop next week for the fender exit. Which Im doing a few add ons like having a lip welded ontop the tb so my charge pipe stops slipping off,upgrading to a newer faster cutout,install line lock,and then start using 30/70 water/meth at 20psi and see what the dyno tells me and what the backpresure equates to.If there is any backpressure still..slowly back off the boost bwteen 15-20 where the backpressure is in a tolerable range and Ill call it good and thats my final high boost setting from here on out

All else fails..Ill have a boost activated fender exit exhaust to shoot flames at cars while at a light hitting the 2step and just have fun with the car like I normally do on the streets and highways.I never intended the car to make over 600whp and its now costing me to make everything else on the car handle what its putting out.

 

I keep re reading this thinking that I missed something, and I still see the guy saying he has 8psi of backpressure AFTER the turbo. I can't imagine how much HP is getting lost in that arrangement.

 

The cutout probably only netted you 15 hp because you were NA at the time and making less power. You're trying to flow nearly twice as much now. This is the same thing I had with my own setup, in a way--I was pushing 12 psi into a little V6 that didn't flow all that well, and my exhaust wasn't too much of a restriction at 435 whp (edit: even though, if you look at my graphs, it seems likely it was making 7-8 psi of backpressure even then), but it absolutely was a restriction when I tried to make 600+. If you look at the second graph I posted, you can see how the pink line (exhaust backpressure after turbo) follows along with the blue line (wheel horsepower). I managed to get over 100 whp at the same boost pressure by reducing that restriction and doing very little else.

Are you trying to make "high boost" because you like big numbers on a boost gauge, or are you trying to make horsepower? You may find that reducing exhaust restriction allows you to make the same power at lower boost pressure, which is almost certainly going to mean lower EGTs, lower pre-turbo backpressure, lower IATs without methanol, faster boost response, etc. Even if you are using a turbo with a 3" V-band, the fact that you are seeing 8 psi post-turbo would tend to indicate that there is something you can do to reduce restriction after the turbo. You may find that doing a short dump or cutout right off the turbo may make a noticeable difference. I didn't want to do that, so I went with a setup where the downpipe spreads out to 3.5" as quickly as possible and then splits into dual 3" as soon as there is room. Overkill, probably, but it clearly worked.

 

I hear you and understand what your saying. Im not exactly trying to make more horsepower..Im quite happy with 650-700whp.I just want to be rid of as much backpressure as possible to that i can continually run 15+psi and not prematurely blow the bottomend up again.The feeling of the car at 18psi with the cutout open and meth kit full tilt is hard to down from.If I never felt the car on higher boost levels and neverfound out how much backpressure it is making.I would be back here again posting about how i somehow blew my 3rd motor on a mild build with mild boost with pumpgas and asking where i went wrong this time around in such a short period of time with low timing.

 

 

A simple word of advice before the flames get too hot. Read a little more and drive a lot more before jumping into this sort of thread. The numbers you are spouting would be a lot more applicable if we were talking about late 80s or early 90s NA builds, but are almost meaningless in the current discussion. A simple search of the stickies in the forum, or actually driving something would convince you of this.

 

We tried a 3" and for how tight it would be to get 3" pipe infront of the heater box and out the fender..major cutting around the strut tower/fender would be needed. 2.5 fits nicely and i can heat wrap the crap out of it to protect everything in that area including the main engine harness that comes from that corner.Mixed with the existing exhaust and more direct 4" dump before the over axle Im hoping is enough to cutdown alot of the pressure as a whole. Ill report back next weekend once its complete.Some highway pulls will tell me right away if its still struggling as much or not.Then hit the dyno to verify the backpressure and see if the motor has taken any damage since then.

 

*Update*

So the exhaust shop that fabbed my turbo kit did a solid job on making my fender exit loudvalve.Its on a vband and flex pipe and fits quite perfectly.I got sent this pic on Thursday when the show owner called me to confirm if I wanted a bezel around the hole or not.And that everything was done very cleanly and if i was going to hook up the lines and wiring myself.



I was excited to see it in person,get it home and hookup the loudvalve.I knew if I had it working on the drive home when I picked it up..id get into trouble since its a hr drive and lots of law wouldnt be happy to see or hear that thing in action.After picking up the car and talking to the shop owner and other customers there asking about my ratty looking bird that has a crazy setup underneathe.I cruised home,got some vacuum line and hooked up just the loudvalve,didnt jack the car up yet to wire in the new 4' e-cutout and see how it was re-worked for better flow.



I backed out the driveway and started driving,I could hear the turbo spool up quicker and crack the valve open around 3psi and it was snappy. i did a few short pulls and it was lighting up tires and generally felt much happier. I found a spot and wanted to see for ***** n giggles if the 2 step had enough boost pressure to crack open the valve and maybe shoot flames..but it was having it. My tuner is anxiuous to see when we can strap the car back down on the dyno and measure any gains in power..and what the backpressure reads with the cutout and loudvalve helping release.butt dyno says its loving the ability to exhale. Check out the video and hear the turbo response.

 

 

Lol the whole hot side was made by the same guy and its ss...I just sprayed it black and wrapped it all to help reduce heat.I will be doing the same on the loudvalve.Its a shame because his work is amazing.
I went with the 2.5 due to packaging and honestly its perfect size to snake in the way he did. If I had the cash,with the wheel liner remove..he offered to make the exhuast come out of the factory fender vent and make it completely stealth.That could be a future iteration.

The test runs I can hear it cracking open around 3psi. If i feel that I want it to open later then I would just use a manual boost controller to set it higher for maybe 10psi. The electric cutout is positioned better as a primary dump while open and its a larger diameter.Having both open and going for a high boost run I definitely feel it running harder.Very curious to see if it made a significant increase in power or just holds a steady topend. Time will tell.