Weird boost creep problem - ongoing issue

[20pwns]

I posted a thread about a week ago regarding boost creep issues in my stock CT-26. A few suggested that it could be my GReddy T.V.V.C. MBC so I disconnected it and tried shimming the wastegate instead – same thing. Others suggested that it might be a leak in the IC piping so I checked that over, too – no problems. So, I’m back here with more details on this annoying problem in hopes that someone might be able to shine a light into this benign mystery. I am running a wide-mouth downpipe with no cats and a straight-thru 3” exhaust system. Intake is a K&N FIPK but otherwise the car is pretty much stock. I’m doing most of my test drives in 3rd gear and here is what’s happening with my car:

Running with an MBC:

• I mash the gas around 2000 RPM and wait for the turbo to spool.
• Around 2800 RPM, the turbo is fully spooled and I see the needle at 0.45 BAR.
• It holds it at 0.45 BAR with NO signs of creeping until the revs hit 4000 RPM.
• Reaching 4000 RPM and beyond, the boost starts to creep to 0.7 to 0.8 BAR depending on where I’ve set the MBC to.
• It holds 0.7 to 0.8 BAR to 6000 RPM and I shift.

This is already very strange because theoretically, it should achieve 0.7 to 0.8 BAR or my desired boost setting at 2800 RPM. However, it doesn’t seem to matter where I set the MBC to; it’s always 0.45 BAR until 4000 RPM.


Running with a shimmed wastegate:
Thinking that the MBC was the culprit I tried shimming the wastegate, but the results were also very strange and similar.

• When using 1 washer, I now see the turbo spool to 0.55 BAR at 2800 RPM.
• But again, once it nears 4000 RPM the boost takes a leap to 0.7 to 0.8 BAR. =(
• Now, by using 2 washers, I’m now able to hit close to 0.65 BAR at 2800 RPM and the car picks up very fast. But things start looking a bit scary around 4000 RPM.
• Entering 4000 RPM, the boost creeps all the way to 0.8 to 0.9 BAR and I really don’t like that.

I haven’t hit fuel cut yet but I’m sure I would in 4th or 5th gear at this rate.

It seems that by using the washers I am able to attain a higher boost pressure initially. But both MBC and washers exhibit an increase in boost once the engine hits 4000 RPM. This leads me to believe that neither the MBC nor washers are at fault. So, if the problem isn’t the MBC, washers or even the IC pipes – which I double checked – what could possibly be causing this? I’m really lost. Please help. :1zhelp: Sorry about the lengthy post. :icon_conf
Thanks,

 

[Joel W.]

Mine creeps a few pounds like you described once I installed the 3" exhaust. Especially when it was really cold outside(30 degrees F).. The 3" elbow-less dp exhaust just lets the turbo flow better I think. :dunno:

I was going to try a DrJones MBC since I have heard so many good things about them.

 

[SMP142]

well to be honest that doesnt really make much sense lol. but i have been through a couple weird things before too. do you know how long it has done this? when did it start? did you just notice it when you put in your guage? also, can you feel the difference when it goes from .45 to .8?

 

[NDBoost]

smp wtf is that a pic ofi n your sig? a jet engine?

 

[SMP142]

sure is, its the compressor section with the top half of the case off. engine is a TF34 off an A-10 worthog....

 

[NDBoost]

cool so you can kill people and no one would know eh.. kind of like a big meat grinder on steroids...

 

[SMP142]

it happens more than people think. especially on big engines. cant get too close if ya know what i mean! :aigo:

 

[20pwns]

well to be honest that doesnt really make much sense lol. but i have been through a couple weird things before too. do you know how long it has done this? when did it start? did you just notice it when you put in your guage? also, can you feel the difference when it goes from .45 to .8?

LOL, yes it's not making sense to me either. Nevertheless, it's happening and it's driving me crazy. As for the difference, yes, I do feel a good boost in power. It's just that now I have a proper boost gauge and I'm able to see the change visually with accurate readings of boost pressure.

I have a question to further clarify things: the wastegate starts opening up as soon as the actuator sees a set amount of boost, correct? So then, as boost increases does the actuator push the gate to open more and more or is it a fixed position? Because if the opening is "dynamic" then I'm beginning to think that the culprit is the actuator. However, if the opening is static then my car is either making a surge of power at 4 grand - perhaps because of the exhaust setup - or making not enough at 2800 RPM. But I somehow doubt that it's the latter because my engine was rebuilt 2 months ago and it's both running and sounding great. By now you must be thinking "then it MUST be the actuator!" right? But I somehow doubt that, too. I bought one - although it was used - not too long ago and I did check to see if it had a good, strong spring. It did and it still does, unless it's not supposed to be THAT strong. But it is pretty stiff, I must say. So, what do you guys think? Should I try another actuator? I feel like I'm on a wild goose chase...

 

[Supra]

LOL, yes it's not making sense to me either. Nevertheless, it's happening and it's driving me crazy. As for the difference, yes, I do feel a good boost in power. It's just that now I have a proper boost gauge and I'm able to see the change visually with accurate readings of boost pressure.

I have a question to further clarify things: the wastegate starts opening up as soon as the actuator sees a set amount of boost, correct? So then, as boost increases does the actuator push the gate to open more and more or is it a fixed position? Because if the opening is "dynamic" then I'm beginning to think that the culprit is the actuator. However, if the opening is static then my car is either making a surge of power at 4 grand - perhaps because of the exhaust setup - or making not enough at 2800 RPM. But I somehow doubt that it's the latter because my engine was rebuilt 2 months ago and it's both running and sounding great. By now you must be thinking "then it MUST be the actuator!" right? But I somehow doubt that, too. I bought one - although it was used - not too long ago and I did check to see if it had a good, strong spring. It did and it still does, unless it's not supposed to be THAT strong. But it is pretty stiff, I must say. So, what do you guys think? Should I try another actuator? I feel like I'm on a wild goose chase...

The actuator is spring loaded with a diaphram. It will stay closed until you reach a certain PSI, probably 3-4psi, and then it will gradually open with the pressure increase from the pres. port. The MBC bleeds presure from the actuator tricking it into staying closed longer because it thinks you are running less boost than normal.

I was told years ago by someone much wiser than I.....
When the wastegate is fully open, it is designed to vent a certain amount of air. When you increase your boost into the motor, you are increasing the amount of air coming out again. You are also heating the air more which expands it more than it normally would be, and we all know heat is what spins the turbine.

From 0-4,000RPM you are within the ability of the wastegate to vent what it needs to, and the turbo hold a steady level of boost. When you hit 5,000RPM at increased boost levels, you are over capacity of the wastegate and hot air is forced to find an alternate route, through the exducer. This forces the turbo to boost creep. As you over boost, you are increasing airflow through the motor and the cycle continues. Compound that with more heat at higher RPM's, and higher heat generated by over-boosting a stock CT verses a 57-trim.

I have also been told that there is a possiblity of turbulance causing a restriction above the wastegate door. Similar to the idea above except turbulance is creating the restriction rather than the size of the wastegate.

I don't know for fact that either of these are true, but they made sense enough for me to remember them.

 

[20pwns]

The actuator is spring loaded with a diaphram. It will stay closed until you reach a certain PSI, probably 3-4psi, and then it will gradually open with the pressure increase from the pres. port. The MBC bleeds presure from the actuator tricking it into staying closed longer because it thinks you are running less boost than normal.

I was told years ago by someone much wiser than I.....
When the wastegate is fully open, it is designed to vent a certain amount of air. When you increase your boost into the motor, you are increasing the amount of air coming out again. You are also heating the air more which expands it more than it normally would be, and we all know heat is what spins the turbine.

From 0-4,000RPM you are within the ability of the wastegate to vent what it needs to, and the turbo hold a steady level of boost. When you hit 5,000RPM at increased boost levels, you are over capacity of the wastegate and hot air is forced to find an alternate route, through the exducer. This forces the turbo to boost creep. As you over boost, you are increasing airflow through the motor and the cycle continues. Compound that with more heat at higher RPM's, and higher heat generated by over-boosting a stock CT verses a 57-trim.

I have also been told that there is a possiblity of turbulance causing a restriction above the wastegate door. Similar to the idea above except turbulance is creating the restriction rather than the size of the wastegate.

I don't know for fact that either of these are true, but they made sense enough for me to remember them.

Very insightful, thanks. I will keep all the above in mind while I continue to search for a definite cause of the problem. But in the end, I just might need to go with an upgraded CT-26 to rid of the problem altogether. Thanks again.

 

[IHI-RHC7]

Hmm.
I would have to somewhat agree with Rich.
I would really suggest you get a good EBC. one that has, say, rpm based boost, gear based boost, and a spool aid.
The only one that comes to mind is the AVC-R, which is the only boost controller I would ever recomend to anyone other than going standalone.
It sounds to me like the MBC is not conected properly.
What happens when you just connect the actuator to the turbo like stock?
Boost should be linear. If it's not, then swap the actuator. make sure the wastegate itself moves freely, as there was a binding issue with most of ITS' cast ct turbine housings. This issue is less common on stock cts, but it could happen.
Keep us posted.

 

[BTZ]

I hit 15-16 psi on my 57 trim with ported wg with no boost controller. I would not worry about it. just turn the boost controller to .8 bar and live with it. Put the stock dp back on and tell me what happenes.

 

[gilberjj]

My friend had this same problem, what he eventually had to do was use a grinder on the exhaust houseing to direct more exhaust towards the wastegate, worked like a charm.