I agree larger downpipe and larger cutout would be ideal but like mentioned before..its very tight as it is with 3'. The current qtp cutout though its a 3' cutout,the ypipe itself is 3' but the dump valve is definitely not a full 3' exit.The plate for the motor and the valve itself must open on the inside diameter of a 3' pipe which necks down more like 2.75 as an dump exit.My cutout kicks to the side and has a turn down on it also. Its not a priority dump like pictured above.I think the combination of the muffler and the cutout isnt enough flow.Going to a true 4' dump in my mind with priority should give the flow an easier path out and allow the rest of my 3' setup to exhale.Atleast much better than it does currently. Ill try to take some pics to show exactly what my setup looks like.My car is lowered also which limits clearance.3' pipe already scraps on some speed bumps

 

Just for reference here are pics of the exhaust now.Not a complicated system for bends etc.

 

 

 

8 psi POST turbine is too much. I recommend you shorten your downpipe, and or increase the diameter. Not sure how much power you're looking for but I dont see anymore than ~800 on that turbine wheel, though a manual. For comparisons sake, I did a pull on my truck full exhaust 4" pipe with a MBRP 4" muffler and made 880 rwhp, nothing changed in the file, just disconnected the exhaust and put a small 4" 90 off the turbine, laid down 930 rwhp. Pressure drop across a pipe has to be considered.

 

 

I know what you mean for a fender exit on our cars.But around here that would scream pull me over..even tho most of the time I dont have the cutout open on the street. If I can drop more than half of the backpressure would be a huge gain for me at this point.

 

 

Its not about being a show car or not or about hacking the fender.Having a fender exit is an extreme magnet for cops and the hassle it brings in my area I care not to deal. My car gets enough attention as it is lol. I also still have the heater box which makes doing a fender exit fairly easy on our cars.I do drive the car alot and use it during spring/fall with the weather here. I know how much better the turbo reacts to the cutout being open and making power.Ive learned how to drive around with it open and not make it too loud at certain rpm. On the highway its open regardless and cruise in 6th isnt bad at all.

I got an appointment with the shop that fabbed my turbo setup. Once there and the car is up on the hoist again..ill see what wizardry he is able to come up with as a permanent solution. Hell..maybe keep one cutout where it is,add a secondary fender exit on a smaller diameter pipe with a cutout and hope that combined is enough flow lol j/k

 

Mmmk.

 

I can almost not believe what I am reading in this thread. I cant believe so many people have no idea how to size an exhaust. I assumed when you said 3 inch exhaust, OP, that you meant dual 3 inches, like most v8 cars, but am I correct in saying you hav a SINGLE 3" exhaust pipe? And you are trying to push 700-800whp through it? That is insanity, sir. A single 3" exhaust is good for about 350 flywheel hp until it becomes a restriction.

People have been putting twin 3" exhaust on stock standard V8s for decades and making increases in power. On stock NA engines.

The 8psi backpressure is absolutely correct, not an error, and is extremely high.

For future reference here is an online tool I have used for years to calculate exhaust pipe diameter. I find it to be accurate. https://www.mk5cortinaestate.co.uk/calculator5.php
Spoiler: you need 4" at least

 

haha... what ever you say bud.

Dual 3" is overkill for most V8s under 450-550hp. Its been proven for a single 3" exhaust to support 450hp on our cars with mimimal restriction NA. Its also been proven for a single 3" to support 800hp in a turbo application (of course that also depends on the application). I for one have pushed 650hp through my single FULL 3" catback at about 15psi on a 5.3LS and a turbo. Turbo and NA are two different animals, superchargers for one require larger exhausts and more exhaust overlap as they are heavily depended on exhaust flow which hinders boost since its belt driven (air in needs to get out, less restriction out allows a supercharger to build boost easier). On a turbo car you really think the 3" DP is an issue when your rocking a 2" crossover pipe? Sure a larger DP and a larger exhaust would help reduce exhaust pressure/flow losses but in the end a single 3" DP will support 800ish hp. Plenty of 3" DP cars have been in the 9s in the quarter trapping 140+mph. The OP just needs to move the cutout closer to the turbo, I have the same car and there is no frigin way to run anything larger than 3" down the back of the motor/firewall without lots of cutting. Same car, smaller turbo but I run 8.5 - 11psi on the street through a full 3" catback with a large case dynomax ultra flow Fbody muffler and stock 2.5" exhaust tailpipes. Logs show 550hp of fuel at that boost, crank to 15psi and fuel bumps to 650hp through the same exhaust. The cutout adds a few PSI and a bit of spool but its still running through 8ft of 3" pipe with a 45* right off the turbo into two 90* bends like an S, then into a 3" S bend into the cutout which runs straight into a 3" catback.

 

​​​​​​​As with any online calculator... it’s only as good as the information you give it. You can easily make 1300-1400hp on a single 4” system. Yet that calculator states you need 5.8”! This is not the case...

That calculator is for an NA engine and VERY general at best. I agree with what you are saying to a point... but it is not correct for sizing a down pipe on a turbo engine. There are MANY other variables to consider.

 

Here's kind of a fun anecdote about post-turbo exhaust backpressure and how much power it kills.

I have a 370 ci engine with a set of PRC 225cc heads and a moderate cam. I'm using a fairly basic turbo system which uses flipped LS3 Camaro manifolds and a Turbonetics T-Series 7875 on a T4 .96 AR. This is, admittedly, a fairly high backpressure setup in the first place, but my turbo choice was somewhat dictated by cost and packaging concerns.

I initially started out with a 3.5" downpipe into a single 3" exhaust, going through a single high-flow catalyst into a single chambered muffler. This actually worked well with my previous setup (4.2L V6 and a GT35R). With a "12 psi" wastegate spring combination, the engine made mid-500s for WHP. It hit a peak of 10 psi and fell off to 7-8 psi up top:
https://i.imgur.com/SMupAVX.jpg

I then added a boost controller and threw a whole bunch of duty cycle at it to force it to run 12-13 psi. I made about 625 whp. Just for laughs, we stuck a pressure transducer in the downpipe shortly after the turbo. Backpressure was huge--look at the pink line in this graph--it nearly matched boost pressure (the orange line) by 6000 RPM. I hate to think what the actual drive pressure going into the turbo must have been; it must have been quite extreme:
https://i.imgur.com/Jg3grGH.png

We then split the exhaust. The 3.5" downpipe now fed into two 3" pipes, each with a high-flow catalyst and a Dynomax Ultraflo (straight through) muffler. With the same wastegate spring combination, but without a boost controller, it hit over 700 whp at about the same boost pressure as the previous iteration:
https://i.imgur.com/CE5xtVn.jpg (divide torque by 4.10)

There were no setup changes other than what I have described. All of these runs were done on the same dynamometer (Dynapack) with the same correction methodology, under similar weather conditions. At 12-13 psi of boost pressure and the same AFR/timing, switching from that restrictive single pipe to duals was worth over 100 whp on this setup. Now I understand why the more racing-oriented folks just run 4" dump pipes out through a fender or the hood.

 

As this is mainly on exhaust flow just my thought. I pretty much agree with what yo_wattup posted. Of coarse not that you really "need" anything. But we're talking about HP here. Now I won't say every single motor is the same. These LS motors LOVE exhaust flow. They will say thank you if exhaust flow is free/no back pressure. More power. Whether N/A, PD blower, centri, nitrous, or turbo exhaust flow is important for power. Especially getting up there FI/power adder. At 7k rpm the engine is producing 58 exhaust pulses, per cylinder, per second. If exhaust cant get out we lose power. Called pumping losses. Or when the engine has to force/use more power to get it out. Maybe more crankcase pressure from getting by rings. My N/A stroker has 2" primary headers to dual 3" x-pipe, bullets, dumped. Whether it's overkill the motor likes it. As said I'm sure it's hard with space & things. Good luck.

Great info/graphs lemming104. Thanks.

 

 

2 90* bends and a muffler VS a straight pipe half the length... Seems like a goofy test to me.

 

And you'd expect an even bigger difference in power if the 3" with muffler and 2 bends was a huge restriction....vs the almost open 4"

Shows whilst the 3" is some restriction, it isnt as horrific as some might shout, and it is a valid test as per what some might run on their car vs an almost open exhaust, say a cutout or something

 

Sounds fairly consistent with my own case (although I think mine was further worsened by using a very incorrect muffler).

 

i actually cant believe stuff like this. a single 3 inch is good for 350 at the fly? come on. how many people have made over 750hp with a single 3 inch? these people are all over the place. you dont need the biggest borg you can fit with a 5.8 inch exhaust to make 800hp. come on