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 doesnt 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 isnt large enough.
#2 the air path after the waste gate isnt easier to flow through than the turbine
#3 the waste gate isnt 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 doesnt 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
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 doesnt 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 isnt large enough.
#2 the air path after the waste gate isnt easier to flow through than the turbine
#3 the waste gate isnt 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 doesnt 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