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Finally going to explain boost creep

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crankbender

20+ Year Contributor
1,840
11
Apr 15, 2002
houston, Texas
I will try and keep the math out of this.

Ok there seems to be a million people asking about boost creep and how to fix it. This has prompted me to write this to help everybody get on the same page and ask the right questions. To properly understand this lets do a little examination of what a turbo is, how it works, and how it is controlled.

What is a turbo?
The turbo in your car is basically an air pump.

How does a turbo work?
Air begins its trip through the engine at the air filter. It then flows from the air filter to the turbo where it is spun. Literally the turbo works by spinning the air very quickly (up to 50 times faster than your engine (100,000+ RPMs). Now think of a weight at the end of a string. If you spin the weight fast enough it will be very hard to hold it in and eventually it will break the string. This is exactly how the turbo works. It spins the air and forces it to flow into the intake very quickly (remember stagnation pressure has velocity in it!!!). Now the number of air molecules that flow through the engine is a function of intake and exhaust pressure (at a given rpm and constant engine parameters). This means that if the turbo is flowing a larger number of molecules than this the intake pressure increases, if it is flowing fewer then the intake pressure decreases. But how does the turbo spin?

How does a turbo spin?
The turbo is actually powered by the exhaust gasses coming out of the engine. After combustion the piston comes up and forces the gasses out into the exhaust. These gasses then flow to the turbo and are forced through a nozzle. The air then impacts the turbine blades and forces them to move…this is much like air blowing leaves in the wind (just really fast wind). The turbine wheel is connected to the compressor wheel and they spin together. Through this coupling energy is transferred to the intake charge and how much is transferred is largely based on the speed of the shaft.

What controls how fast the turbo is spinning?
As we can see the turbo is harnessing energy from the exhaust and transferring it to the intake charge. But what happens when we transfer too much energy? If this occurs we begin to move more air molecules through the turbo that the engine will swallow at this intake pressure. Because of this the intake pressure begins to rise. Consequently the engine swallows more air and sends more energy into the exhaust. The turbo harnesses this air energy and sends it to the intake….Here it is apparent that we have to limit how much energy is transferred or the intake pressure will CREEP up. This is done through a waste gate.

What does the waste gate do?
The waste gate is a simple device that regardless of the design has only 1 purpose, to divert energy (exhaust gas) away from the turbine of the turbo. It accomplishes this by basically opening up another passage for the air to flow through. A balance must be made where the energy gained by the turbine must equal the energy transferred to the intake air.

I have a waste gate but I still have boost creep.
This is because the waste gate is failing. The waste gate might be operating correctly but it just doesn’t do its job. This can be thought of as trying to use a 1 quart baggie to hold a gallon of water. The baggie will operate correctly and hold the quart but it just CAN NOT hold the gallon no matter what. So your waste gate is failing to divert enough air away from the turbine even if it is opening.

So what is going on here and why is the waste gate failing to divert enough air?
Well now here is where we come to the problem. To approach this we need to know exactly why the waste gate is not diverting enough air away from the turbine. Now we can not make a blanket statement here on why but we can look at some possible problems.
#1 the waste gate simply isn’t large enough.
#2 the air path after the waste gate isn’t easier to flow through than the turbine
#3 the waste gate isn’t opening

So how do we fix it?
To fix this problem we should use a systematic approach.
#1 is the waste gate opening? Make sure the waste gate is actually opening when you are under boost. To do this hook the vacume line that runs to the waste gate from the intake manifold up to a source of the desired pressure (an air tank or bike pump works). Does the waste gate open? If it does now try it with your boost controller…if it doesn’t open now your boost controller is crapped out. Reassemble the car without the boost controller and see if the problem is fixed.

It operates but I still have boost creep!!!!
Well the next thing we can do is try and make the opening bigger. To do this port it out and possibly get a larger flapper door mod (check the faq on this). If the waste gate is external try and get a bigger one.

The door is as big as I can go and it still creeps!!!!!
Now we have to attack the flow path and try to get more air through there. First make sure the walls of the turbo waste gate flow path are polished. Next do the same for the O2 housing. Make sure the 2 paths have the same size openings as a step can cause a resistance to flow. Check the faq on how to do this.

Ok I have ported everything and have the biggest door possible and still have creep.
At this point you are going to do something to make the flow through the waste gate path less restrictive or add restriction to the main flow path. Nobody wants to add a restriction so lets approach decreasing resistance along the waste gate flow path. This path really only consists of the waste gate and O2 housing on a stock style setup. If the turbo is ported to flow as well as possible you have to look at the O2 housing. There are many good products out there to solve this problem.

I have a new O2 housing and still have boost creep!!!!
The next step would be to go with a waste gate that either dumps to the atmosphere or one with a longer path before it reenters the main exhaust. The former is much easier and is almost guaranteed to fix your problem at this point. Also you may want to look at going for an external wastegate.


I hope this helps everybody. please comment and let me know what you think

Peter
 
Excellent post. Anyway we can make this a sticky? Boost creep questions are put up just about every day.

Matt.
 
Yeah, and the answers are all in the archives. I know I have done this several times.

That said, commend the original poster. Well done.
 
Thank you guys!

School sure does make people think weird, doesn't it?

Actually it does. We all had a big kick out of this: Today I was speaking with some other engineers about wrecking my bike long ago. We were discussing how I fell on my keys and that made me slide longer because the steel had a lower CF than my clothes.....Sombody had to pipe up and mention that the conversation would be radically if we were not engineers.....

Damn understanding physics it screws with your head big time. But it really is enlightening to understand what is going on around you. I suggest to anybody to learn physics and basic engineering concepts because it just opens up so much for you to understand and doesn't take all that long....IMHO it is worth it even if you are just a tv reporter.
 
I definately have a different outlook on everyday life now that I am in my third year of my Mechanical Engineering degree.

My fiancee even has a decent handle on quite a few engineering concepts/fabrication concerns because I CANNOT watch junkyard wars/monster garage/american choppers without giving it a thorough analysis :D .
 
Originally posted by nine5raptor
I definately have a different outlook on everyday life now that I am in my third year of my Mechanical Engineering degree.

My fiancee even has a decent handle on quite a few engineering concepts/fabrication concerns because I CANNOT watch junkyard wars/monster garage/american choppers without giving it a thorough analysis :D .

I love watching the people on junkyard wars flop. I mean the show is setup so they can find whatever they need and they still can't build a ghetto boat.....


Yeah my gf gets mad at me when i loose her...
 
most nubs wont read that and or you simplifed it too much and things dont really make sence or flow together well. and You left out why heat is a factor in how a turbo works


Your hole essay could have been said in a paragraph. But good job though.

also the first few paragraphs could have been linked to at www.howstuffworks.com
 
You should also realize that there is pressure built up in the exhaust manifold. With this pressure, the exhaust will want to exit the path of least resistance to the lower pressure (the O2 sensor housing). The wastegate isn't exactly large and it's port on the stock O2 housing is not a monster hole either. Sometimes this means the turbine route has the least amount of resistance after the max airflow is overflowing the wastegate.

Follow crackbender's tips. It'll help out a bunch.
 
Originally posted by TimG
You should also realize that there is pressure built up in the exhaust manifold. With this pressure, the exhaust will want to exit the path of least resistance to the lower pressure (the O2 sensor housing). The wastegate isn't exactly large and it's port on the stock O2 housing is not a monster hole either. Sometimes this means the turbine route has the least amount of resistance after the max airflow is overflowing the wastegate.

Follow crackbender's tips. It'll help out a bunch.

I agree. This is why I say that the greatest cause of creep ( other than something like a broken wastegate or similar problem ) is an improperly sized exhaust for that turbine housing flow. Exhaust gas flowing through a 14b turbine is going to want to flow out of that 3 inch no restriction exhaust, and that little wastegate isn't going to do a thing against that kind of flow. Turbo's love zero backpressure, but wastegates don't. If the turbine exit is no-restriction, the wastegate is stuck trying to create a path of least resistence it could never achieve. The turbo is a system, not a single component. You have to build around it and for it.
 
Originally posted by Groomz
I agree. This is why I say that the greatest cause of creep ( other than something like a broken wastegate or similar problem ) is an improperly sized exhaust for that turbine housing flow. Exhaust gas flowing through a 14b turbine is going to want to flow out of that 3 inch no restriction exhaust, and that little wastegate isn't going to do a thing against that kind of flow. Turbo's love zero backpressure, but wastegates don't. If the turbine exit is no-restriction, the wastegate is stuck trying to create a path of least resistence it could never achieve. The turbo is a system, not a single component. You have to build around it and for it.


"Turbos love zero backpressure but wastegates don't"

This is flat out wrong.

A wastegate "loves" zero backpressure just as much as a turbo does. As a matter of fact, a wastegate's effectiveness will decrease, as exhaust backpressure increases. Just like a turbo.

A turbine will never be no restriction, so that's not even something that should be considered.

The reason stock wastegate setups creep is very simple; they simply cannot flow enough exhaust. The exhaust has to flow around a couple of corners, and then through a little tiny crack of an open wastegate.

The reason a better exhaust makes this worse, it not because it lowers the pressure after the turbine and not the wastegate (which simply is not true). The reason is this: The flow restriction of the turbine wheel and that section of the housing never changes. Thus, the only flow variable flow restriction to the exhaust gasses NOT going through the wastegate is the exhaust.

However, the gasses going through the wastegate are not restricted by the exhaust, or at least not to as great an extent. The design of the factory wastegate is such that as the exhaust restriction decreases, the restriction to the gasses going through the turbine decreases at a FASTER RATE than the restriction of the gasses going through the wastegate.

The bottleneck for the turbine wheel is the turbine itself, and the exhaust. When you install a bigger exhaust, this bottleneck is much decreased, and the flow is better.

The restriction for the wastegate, on the other hand, is mostly the shape and size of the flapper and wastegate passage. This restriction doesn't go away when you change the exhaust flow.

The amount of creep is also inherently related to the flow of the motor, because that determines the amout of exhaust gas volume. When we add a bigger exhaust, this flow increases, whether we like it or not.
 
Originally posted by kpt4321

However, the gasses going through the wastegate are not restricted by the exhaust, or at least not to as great an extent. The design of the factory wastegate is such that as the exhaust restriction decreases, the restriction to the gasses going through the turbine decreases at a FASTER RATE than the restriction of the gasses going through the wastegate.

So how is a larger exhaust not the culprit in the wastegate not performing? Changing the wording does not change the fact that the exhaust is now accelerating through the turbine passage faster, and making it more difficult for the wastegate to flow the necessary amount to function. Yes, the wastegate can be made to function with a large exhaust, but a significant amount of work is necessary. If you balance the flow restrictions around the turbo, you can have a well working setup. Bring the restriciont down too low for the turbine, and now the wastegate has to be brought to the same level.
 
Originally posted by Groomz
So how is a larger exhaust not the culprit in the wastegate not performing? Changing the wording does not change the fact that the exhaust is now accelerating through the turbine passage faster, and making it more difficult for the wastegate to flow the necessary amount to function. Yes, the wastegate can be made to function with a large exhaust, but a significant amount of work is necessary. If you balance the flow restrictions around the turbo, you can have a well working setup. Bring the restriciont down too low for the turbine, and now the wastegate has to be brought to the same level.

You said, and I quote:

"Turbo's love zero backpressure, but wastegates don't."

This is wrong. A wastegate is just as dependant on presure ratio as a turbine is, and therefore it will benefit from a large pressure drop to the same extent.

The rest of my post was simply information to prove my point, I am sorry if it was misleading.

I never said that the exhaust is not the cause of creep, although it isn't exactly. Creep is caused by a wastegate's inability to flow enough, that simple. The exhaust just creates a situtaion where you actually notice this inability.

Also, another point that needs to be made is that adding backpressure is NOT an acceptable answer to the problem of boost creep, not in my book.
 
Also, another point that needs to be made is that adding backpressure is NOT an acceptable answer to the problem of boost creep, not in my book.

Agreed. Doing that is like putting in a smaller less powerfull motor because you have crappy tires.
 
I would agree that the wastegate should be modified to eliminate boost creep. Adding backpressure is bad, but understanding why the wastegate is not function is important. I would say a smaller sized exhaust is a solution, not the solution. The best fix is a well flowing wastegate, either internal or preferably external.

If I feared boost creep from adding a 3 inch exhaust, would it be a smart idea to go to a 2.5 inch to prevent this, or to focus on porting the wastegate passage? I have modest power goals and believe 2.5 inch is all I need, and I also think it would help avoid boost creep ( it would be a full O2 eliminator exhaust ). What do you guys think?
 
A properly built (not restrictive) 2.5 inch exhaust will creep too.

You simpyl can't buy a performance exhaust worth the money and not boost creep on the stock wastegate. You're going to have to port like crazy either way.
 
Originally posted by kpt4321
A properly built (not restrictive) 2.5 inch exhaust will creep too.

You simpyl can't buy a performance exhaust worth the money and not boost creep on the stock wastegate. You're going to have to port like crazy either way.

Assuming both the main exhaust and wastegate dump are non-restrictive as possible, would as much attention need to be paid to porting? Which side is more important, the exhaust side or the turbine side ( before the gases hit the turbine blades )? I would think that with a free flowing exhaust and wastegate passage, it should function correctly.
 
So basically, it's flow which is the problem (cfm), not pressure(psi)? More flow in the exhaust manifold, regardless of the pressure, will cause more air to run into the compressor housing. As it tries to flow past the wastegate, air flow is at max and it runs through the turbine instead. This speeds the turbine up and creates more boost?

If this is so, what dictates pressure in the exhaust manifold?
 
Originally posted by Groomz
Assuming both the main exhaust and wastegate dump are non-restrictive as possible, would as much attention need to be paid to porting? Which side is more important, the exhaust side or the turbine side ( before the gases hit the turbine blades )? I would think that with a free flowing exhaust and wastegate passage, it should function correctly.

The problem is that the stock wastegate itself simply doesn't flow enough. It's not a matter of exhaust restrictiveness.

You need to port the wastegate inlet and oulet areas, a lot.
 
Originally posted by TimG
So basically, it's flow which is the problem (cfm), not pressure(psi)? More flow in the exhaust manifold, regardless of the pressure, will cause more air to run into the compressor housing. As it tries to flow past the wastegate, air flow is at max and it runs through the turbine instead. This speeds the turbine up and creates more boost?

If this is so, what dictates pressure in the exhaust manifold?


Pressure in the exhaust manifold is dictated by several things; for each setup and power level, you have a certain pressure differential across the turbine wheel. Therefore, depending on the exhaust backpressure, you have a certain manifold pressure as well.

I'm not sure what you mean by pressure and/or flow causing the creep. The problem is simply that not enough of the exhaust gas is going through the wastegate. This can be caused by too small of a pressure differential across the wastegate, as well as a lack of volume flow.
 
Originally posted by kpt4321
Pressure in the exhaust manifold is dictated by several things; for each setup and power level, you have a certain pressure differential across the turbine wheel. Therefore, depending on the exhaust backpressure, you have a certain manifold pressure as well.

I'm not sure what you mean by pressure and/or flow causing the creep. The problem is simply that not enough of the exhaust gas is going through the wastegate. This can be caused by too small of a pressure differential across the wastegate, as well as a lack of volume flow.

Understood.
 
instead of porting wouldnt an external wastegate help?
 
school is amazing, i attend new england tech in RI and god damn automotives have more than i thought to it. i am not some ricer just incase you guys thought well duh. but the eleltronics on cars now a days is amazing. but back to the boost creep issue i give you an A+ on your thinking
 
kpt4321 said:
You said, and I quote:

"Turbo's love zero backpressure, but wastegates don't."

This is wrong. A wastegate is just as dependant on presure ratio as a turbine is, and therefore it will benefit from a large pressure drop to the same extent.

The rest of my post was simply information to prove my point, I am sorry if it was misleading.

I never said that the exhaust is not the cause of creep, although it isn't exactly. Creep is caused by a wastegate's inability to flow enough, that simple. The exhaust just creates a situtaion where you actually notice this inability.

Also, another point that needs to be made is that adding backpressure is NOT an acceptable answer to the problem of boost creep, not in my book.

The way I understood it was that if the turbine has no restriction, and the wastegate isn't ported, or flowing well enough, it won't like the lack of backpressure, because that's exactly it, you need your wastegate to be capable of literally flowing as much or more air than the turbine, not that it ever will, but it should be capable, then, and only then will zero backpressure be mutually beneficial to your turbine and wastegate.
 
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