oversized exhaust & boost creep rationale

[cjridert1]

put a 3" on the other week and now have mad boost creep. Seems as though the bigger exhaust provides the waste to exit more easily (as well as the main exhaust) thanks to the reduced pressure, however this must not be the case? With more backpressure, does the exhaust path prefer the waste port? Furthermore, an external wastegate uses the same port if its O2 mounted - this only works because the flapper is gone? Do you need to port out the wastegate hole in turbine housing to make it work any better?

 

[1992awdlaser]

This has to be one of the most common questions asked here. If you searched you would have found the information you are looking for.

Back when my car was stock besides a boost controller I had the boost set at 15psi. I put my 3" Buschur turboback on and my Buschur o2 dump and the boost went to 24psi. The exhaust flows so well that it goes right by the wastegate passage and into the turbine wheel which creates more boost(boost creep). It does this because in order for the exhaust gases to get out via the wastegate it has to make a 90 degree turn while in the turbine housing. The exhaust gas looks for the place of least restriction to go which is through the turbine wheel and out of the exhaust.

To fix this you need to port the wastegate passage first of all so the exhaust gas doesn't have to make a 90 degree turn to get out. Now disconnect the wastegate actuator from the wastegate flapper. Then port where the wastegate flapper closes. You will see a ring from where the flapper sits. Port some of the material away while using the ring as a guide as to not go to far. If you go past the ring then your car is going to take longer to spool. Now open the wastegate flapper just like it would open when under boost. You will see that the flapper arm hits the turbine housing which limits how far it will open. Cut away some of that material so the flapper can open more. Doing these things WILL fix your problem.

You can also port the exhaust manifold and turbine inlet which will also help control boost slightly because it should make the turbo take slightly longer to spool because of the increased space that has to be filled before the exhaust gas can get to the turbine wheel and it creates a "lip" at the bottom of the wastegate passage which catches some of the exhaust gases. This will also make your turbo capable of more power.

 

[JayRolla]

The high flow exhaust is causing the air to just fly straight by the wastegate flapper now and not open it completely. Search boost creep, usually 3" or larger piping will cause it. You need to port so the flapper open's 90* and port the housing where the flapper inlet is so the air has an easier path to the wastegate.

 

[MrBoxx]

The high flow exhaust is causing the air to just fly straight by the wastegate flapper now and not open it completely. Search boost creep, usually 3" or larger piping will cause it. You need to port so the flapper open's 90* and port the housing where the flapper inlet is so the air has an easier path to the wastegate.

The exhaust doesn't open the flapper door; the wastegate actuator opens the flapper door. Also, the flapper door won't open anywhere near 90*, regardless of porting; the actuator simply doesn't open it that far, period. Check MrPeepers' MWGA article for pics and explanations of the limits of the stock WGA and flapper door. (The first pic is of the flapper door completely open, to the fullest extent of the WGA armature). I will mention that Matt's design is only for those who are savvy with tools and fabrication, and failure to get the modifications correct can damage the WGA assembly beyond repair. If you go this route, I would contact Matt for more information before starting.

Porting the turbine passage before the flapper is one way of helping to solve boost creep, but it may not make the problem go away completely.

 

[JayRolla]

you are so correct. I sound like an idiot. You want the air to flow easier through the wastegate hole but I didnt mean to say the exhaust opens it, I dont know what I was thinking.

 

[Mr Peepers]

"Turbos operate off a pressure differential(and temperature), so less backpressure= more available energy. The turbo will need to spin about the same speed for a given amount of boost, and with more energy available, it will need to bypass more exhaust around the turbine to maintain that speed. Also, the exhaust will take the path of least resistance, hence why 7cm^2 turbine housings creep more than 6cm^2. The exhaust will have a harder time passing through the turbine, so it will tend to exit through the wastegate hole.
In the case of having less backpressure and more total flow, the air has two options, go through the turbine, or through the wastegate. The flow through the turbine will stay linear depending on the flow, but the wastegate with its 90* turn and half open flapper in the way have exponentially more backpressure as flow rises because of turbulence(air tumbling around the flapper). Laminar flow is linear in regards to drag and turbulent flow is exponential. This will cause the wastegate to flow less compared to the turbine as the volume and/or velocity rises, compounding the problem stated earlier of needing to bypass more exhaust in the first place."

I wrote this a while ago in an attempt to explain why boost creep is more likely with less backpressure behind the turbine, even though a rerouted wastegate has less backpressure behind it as well.

May not be right, but just my take on it. DSM-onster or someone with more in depth knowledge on turbos may be able to give some info. There are many things I'm still trying to understand yet

EDIT: Simplified(perhaps):
It allows more total flow at the same pressure ratio(boost) and more efficient use of the exhaust flow. More exhaust gas will need to be bypassed for that pressure ratio and eventually the flow limits of the wastegate system will be met, resulting in boost creep.

 

[kenamond]

Another way to think about the effect of a 3" exhaust on creep is as follows (with completely fabricated backpressure and PR numbers):

With a stock or medium-upgrade exhaust, you have a certain backpressure at the O2 outlet. The turbine is operating at a certain pressure ratio just like the compressor. That means that you have a factor (PR) that increases the pressure as you go upwind of the turbine (just like pressure goes up as you go downwind of the compressor). That gives you the back pressure in the collector/turbine inlet. So at a PR of 2.0 and DP inlet backpressure of 10psi, the backpressure in the collector would be 2.0*(10+14.7)-14.7 gauge pressure of 31.7psi. If you throw a 3" catless turboback and the backpressure drops to 5psi, the backpressure in the collector (assuming the same PR of 2.0) would be 2.0*(5+14.7)-14.7 gauge pressure of 21.7psi. Now the WG passage in the first case sees 31.7psi on one side and 10psi on the other, so the difference is 21.7psi, and that's what's pushing exhaust out the WG. But with the 3" exhaust, the WG sees 21.7 upwind and 5psi downwind, so the difference is 16.7psi. So the turbine is operating just as before, but the WG pressure difference has dropped from 21.7psi to 16.7psi. So less exhaust goes through it, so more goes through the turbine. That explains why you have to readjust your boost gauge when you add the 3" exhaust, and it explains why you might creep. After putting my 3" exhaust on, my 15psi MBC setting produced over 20psi (I let off). But since I ported my turbine and O2 housings, I was able to dial down the MBC and hold 12psi...but it creeps to 14psi at redline.

I think there's more to it than that, but DP backpressure and turbine PR *will* contribute to creep.

 

[GSixxer]

The exhaust doesn't open the flapper door; the wastegate actuator opens the flapper door. Also, the flapper door won't open anywhere near 90*, regardless of porting; the actuator simply doesn't open it that far, period. Check MrPeepers' MWGA article for pics and explanations of the limits of the stock WGA and flapper door. (The first pic is of the flapper door completely open, to the fullest extent of the WGA armature). I will mention that Matt's design is only for those who are savvy with tools and fabrication, and failure to get the modifications correct can damage the WGA assembly beyond repair. If you go this route, I would contact Matt for more information before starting.

Porting the turbine passage before the flapper is one way of helping to solve boost creep, but it may not make the problem go away completely.

I Think I may have an easier solution than that peeper's mod, I'm working on it now on paper and it all seems to work in theory, however, I am very tired and if this still makes sense tomorrow/sat when I wake up, I will start trying to fabricate it. Worse thing that can happen is I F up my WGA and am forced to use an external that I have laying around