Bigger Exhaust and Lost Peak Boost?

[Torchy]

Hey guys, I put a 3 inch exhaust on my car and literally changed nothing on the boost controller.

I went from stock OAP and Corsa Xtremes to SW 3 inch catback. Basically it now sees 5.5 peak psi when it was seeing 6.9 peak on old set up.

It is because of less back pressure?

13 TT GT




Side note: Turned the boost back up to 6.9 and a huge difference now. Pulls hard! Logged while doing it and tune still looks good.

 

[UnleashedBeast]

Boost is a measurement of restriction, not power. The freer flowing exhaust reduced backpressure, therefore reducing boost.

Power did not reduce, and having less boost with the same or more power is a GOOD thing.

 

[Robertt305]

boost is a measurement of restriction, not power. The freer flowing exhaust reduced backpressure, therefore reducing boost.

Power did not reduce, and having less boost with the same or more power is a good thing.

+1

 

[D.T.R]

What unleashed said.
You should definitely be making more power now.
This is the reason why i will be running open O/R midpipe with mine here shortly.
The turbos want and need a free flowing exhaust.

 

[manolith]

Why I agree with blackbeast about boost being a restriction it does not apply to how a turbo works. The wastegate will open at whatever psi the spring or boost controller is set to. It does not matter how efficient the motor is flowing. If the turbo is sized properly manifold pressure will be constant regardless of exhaust. Spooling the turbo might change but peak boost will be the same. I think op accidentally touched the boost controler because there is no reason for boost pressure to change that much.

 

[CPRsm]

Ehhhhh, If you dropped back pressure, it's possible to drop boost. Not because anything is now wrong. A turbo will speed up to regain boost, so it's not like a blower in that aspect. Back pressure is a funny thing. It can make a wg flow more when it's higher. When it's lowered sometimes the gate just needs a bit more help to keep shaft speed up. Just up the duty cycle, you're fine.

 

[Torchy]

Why I agree with blackbeast about boost being a restriction it does not apply to how a turbo works. The wastegate will open at whatever psi the spring or boost controller is set to. It does not matter how efficient the motor is flowing. If the turbo is sized properly manifold pressure will be constant regardless of exhaust. Spooling the turbo might change but peak boost will be the same. I think op accidentally touched the boost controler because there is no reason for boost pressure to change that much.

Well I triple checked to make sure everything stayed the same on the BC or I wouldn't of made this thread. Also I went through each individual setting to see if any of them changed (even though I know they wouldn't of changed) just to make sure and every setting was where it always has been.

Yeah CPR after making many pulls over a week of daily driving I said screw it and upped the duty cycle. Also after going to the bigger exhaust I noticed boost came in slower than what it used to given equal BC settings so I upped the gain too.

Kinda want to get it back on the dyno. She feels closer to 700whp now.

 

[Derek@Lethal]

That's weird that you would see less boost and slower spool. Like manolith said, boost should stay constant depending on BC settings or WG springs being used, but spool should speed up when you remove restriction post turbo on the exhaust side. To have it actually get slower is a little odd.

 

[UnleashedBeast]

Yeah, when I made my post, I was thinking PB blower car theory.

Ugh!

 

[Torchy]

That's weird that you would see less boost and slower spool. Like manolith said, boost should stay constant depending on BC settings or WG springs being used, but spool should speed up when you remove restriction post turbo on the exhaust side. To have it actually get slower is a little odd.

Well this is pretty much what I thought hence why I made the thread. I don't think anything is wrong (nor do I think anyone here is saying that) I just thought this was a interesting result to a change.

Maybe someone has a answer.

I can say that it def feels stronger at the same peak boost lvl.

Surely there is a correlation between lower peak boost and spool time?

 

[CPRsm]

The SW could be more restrictive? The muffler maybe? It would make sense if it was. Would explains lower spool also.

Changing back pressure can screw w boost setting pretty easily. Most springs are rated based on about a 1.6 back pressure ratio. Boost will vary depending on kit design and back pressure. So a simple change in exhuast can definitely change boost settings involuntarily

 

[MRSUPRA]

I would think boost would go up in most situations with lower back pressure behind the turbine wheel('s). Basicly, you'r allowing the turbine wheel to spin more freely, which turns the compressor wheel faster (with more boost). I remember several years ago I replaced my CAT with a test pipe on my Supra, and the boost went from 18 to 20psi. Had to turn down the duty cycle on Apexi EBC.

 

[Torchy]

The SW could be more restrictive? The muffler maybe? It would make sense if it was. Would explains lower spool also.

Changing back pressure can screw w boost setting pretty easily. Most springs are rated based on about a 1.6 back pressure ratio. Boost will vary depending on kit design and back pressure. So a simple change in exhuast can definitely change boost settings involuntarily

It's the SW retro catback 3inch. It looks more free flowing than my old set up but in actuality it might not be.The inside of their resonators look like some off a SAW movie.


DTR fwiw I started my car open X and sounded horrible lol. Prob had something to do with it dumping directly under the car. I'm sure with a pipe making it exit outside the car would of helped it... a little

 

[CPRsm]

The tubing probably is better, but if the mufler is no free flowing,......
Could drop the mufflers and see what happens.

 

[Torchy]

Yep I could see this being true. Only problem is that this exhaust has nothing to keep it off the ground if the mufflers aren't there.

 

[CPRsm]

Oh yeah. I keep forgetting. I've got hangars at the tranny mount so I can do that lol. W the mufflers swap to the other ones you had before?

 

[Torchy]

Oh yeah. I keep forgetting. I've got hangars at the tranny mount so I can do that lol. W the mufflers swap to the other ones you had before?

Hmmm how'd ya do that? Have any pics?

 

[04sleeper]

Funny, I picked up boost going from stock resonators and GT500 mufflers to the 3" Maganflow Competition Cat-Back. :shrug:

 

[WIPLASH1]

I would think boost would go up in most situations with lower back pressure behind the turbine wheel('s). Basicly, you'r allowing the turbine wheel to spin more freely, which turns the compressor wheel faster (with more boost). I remember several years ago I replaced my CAT with a test pipe on my Supra, and the boost went from 18 to 20psi. Had to turn down the duty cycle on Apexi EBC.

I agree with this. With a wastegate on spring pressure or a mechanical boost controller, the resulting manifold pressure is just a relationship between the amount of (backpressure pushing the valve open + manifold pressure pushing against the diaphram in the gate) and the spring rate of the spring in the gate. By reducing backpressure it SHOULD increase manifold pressure since there's less backpressure trying to open the valve (more wheel rpm).

When they label a spring with a "pressure rating", it's just a ballpark guess with how much backpressure an average setup has. I'm pretty sure there's setups that could make 9psi with a 6psi spring and mani pressure only on the bottom of the gate.

 

[OveRBoreD]

I'll be finishing up my mbrp v2 race catback tomorrow, complete 3in. Got rid of the factory 2.75 neck downs. I'm hitting 6.5 psi now with the stock h-pipe and the race catback (long story short couldn't get the x-pipe to fit so I had to modify it). I'll let you know what mine does without touching the BC. I have a feeling boost will increase and spool time will definitely decrease. I ran open downpipe for a couple days and it definitely felt like spool was quicker so I couldn't see why or how this would make it worse. Didn't get a chance to run her out open DP though to see boost increase.