Everything you need to know about boost creep

[wgknestrick]

With the countless threads about boost creep problems with turbobacks on the 2.5l STI, I think this thread should be helpful as a quick summary to those planning on moddifying their STI.

Q) What is boost creep?
A) Boost creep occurs when the wastegate is physically unable to bypass enough exhaust around the turbine on the turbo.

Q) Is boost creep harmful?
A) Yes, boost creep is one of the worst problems you can have with a turbo engine. Boost creep means that there is NO way for the boost pressure to be controlled. Boost pressures in excess of 20psi!!! have been recorded on STIs with TBE before the ECU cuts fuel.

Q) What causes boost creep?
A) Installing a turbo back exhaust whether aftermarket catted or not will in almost all cases cause boost creep to a certain extent.

Q) Are there any other options to prevent boost creep besides porting the wastegate?
A) Yes, Installation of an exhaust that either keeps the stock OEM muffler or one of the stock cats should not cause boost creep.

Q) How do I know if I have boost creep?
A) If you have a full TBE, then you probably have it. A boost gauge will tell you what pressure you are at as well. Fuel cut occurs when the ECU senses boost in excess of 17.5psi for 2-3s. Fuel cut in my experience is a very violent "stutter" from the engine that resembles extreme knock, followed by CEL.

Q) What is the overboost CEL?
A) (P0244) Wastegate malfunction

Q) Is there anything else I can do besides porting the wastegate to prevent the creeping....MBC, EBC, engine management etc?
A) This problem is isolated to the wastegate only. The wastegate is a mechanical system. There are no current workarounds or "fixes" besides this. The 2.5l just flows too much exhaust through the turbo once a turbo back has been added for the wastegate to control boost. If you think of it as a hierarchy, then it should make sense.

level 1) wastegate controls boost pressure
level 2) boost solenoid controls wastegate
level 3) MBC/EBC/ECU controls boost solenoid

Using this logic, if the problem resides on the 1st level, no matter what you do on top of it, you will still have the problem. Whatever (MBC, EBC, UTEC, ECUTEK) is controlling the wastegate CANNOT prevent boost creep if the WASTEGATE is UNABLE TO FLOW ENOUGH EXHAUST to prevent boost creep. A fuel cut defender will not eliminate the boost creep problem, but it will stop the fuel cut. Your boost will still creep up to 20psi, but the ECU won't be able to tell due to the altered signal from the FCD.

Q) My car seems to boost creep when it is cold out. Why is that?
A) Ambient temperature plays a huge role in how well your engine runs. In general, the cooler ambient temperature, the better your engine runs due to the laws of thermodynamics. The colder air causes your engine to make more HP (more work under the curve with lower temps), therefore more energetic exhaust gases, therefore your car is more prone to boost creep when it is cold out. If you live in an area that does not see colder temperatures (AZ for example) too often, then boost creep may never become a problem.

Q) I want to port my wastegate. What do I do?
A) Can you remove and install a turbo? If yes, there are a couple of options. You can send it to Deadbolt (www.deadboltspeed.com) for porting, have a local machine shop port it, or do it yourself with a "Dremmel like" tool.

Some other info on porting:
http://rx7.freeservers.com/modhb/boostcontrol.htm
http://www.fwdmopar.com/sites/dennis/wgateport.html
http://forums.nasioc.com/forums/show...hreadid=300296
http://www.allpar.com/mopar/boost-creep.html
http://members.tripod.com/alexander_grabau/id18.htm




(edited---to fix links, add links, and include CEL code)
(edited #2---to include temperature question)

I hope this is helpful to some people and please let me know if anything needs to be corrected.

Bill

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[Sev]

nice thanks for the laying it all out....



areg

[ANZAC_1915]

I hope I never meet the boost creep.

[DanzBorin]

you need to fix the nasioc thread... put this link...

http://forums.nasioc.com/forums/show...hreadid=300296

[Unabomber]

The scoobynet link is messed up too. Very nice post you got there, reminds me of my FAQ series! Good Job!! Since this is a super common issue with the STi, maybe this should be stickied??

[T3RMIN4L]

lol@Glenn... nice writeup wgknestrick. Anyone who has met the boost creep and capped the flow with their stock axleback care to comment on the performance loss compared to the full 3"+ back? Personally I think its more than noticable on the butt dyno.

[wgknestrick]

Can anyone help me out with detailed, pictorial instructions for turbo removal/installation. getting the turbo installed back in the car was more difficult than I ever thought it would be. Instructions would be nice for the community.

[hatchy]

ECU cuts fuel at above 17.5psi, I think, not 20psi.

And fuel cut feels more like hitting the rev limiter.

[Neil Fowlie]

oh good I am not an idiot
I listen to all the people talking about waiting for the utec
to eliminate boost creep, and wondered how it could possibly do that. Same thing happens on eagle talons, but there it raises boost from 10 to about 13 psi, still safe and nowhere near fuel cut.

The only way to get rid of creep is to port the wastegate.
unless you are planning to use the utec, to actually tune the car
based on the 20psi you will be running.

another thing to consider before going for a tbe
when you see manufactures claims of hp increase
most of it is due to the increase in boost.

for example my old fwd eagle talon dynos at 145 whp stock (10psi) after TBE (with cat) 170whp (13 psi)
after i had my turbo ported, I tried setting the boost back to stock(10psi) with the TBE, dynoed at 151. Then same day turned the boost up to 13psi power back up to 170whp

So basically the TBE gave minimal gains on its own
all the claims from the aftermarket shops were kinda
misleading. Taking the credit from boost creep
in their hp increase claims.

I know these are totally different cars
but when I see a company claiming their exhaust makes an extra 27 whp for the sti, the first thing that i think is, oh 20 hp for the extra boost, 7 for the exhaust

[qbbraveheart]

thanks for clearing everything up

[XT6Wagon]

Actualy A wastegate doesn't NEED to flow enough exaust to control boost, it only needs to be able to control the pressure drop ACROSS the turbine. So the stock wastegate setup in a VF39 works great because while it doesn't flow worth a damn, its great and murdering the flow out of the turbine with the stock downpipe in place. In fact I bet if you could measure the flow through the wastegate for both the stock downpipe and a aftermarket bellmouth, you would find that the flow goes up alot with the aftermarket setup... yet can't control the boost as well thanks to the flow not being able to affect the flow out of the turbine anywhere near as well as with the stock downpipe

[BU308]

Does a stronger wastegate acuator such as the one from HKS cure the boost creep problem? I am having the boost creep problem on my JDM STi7 wagon after installing the Cusco downpipe. Thanks.

[carguy19]

Originally posted by Neil Fowlie
for example my old fwd eagle talon dynos at 145 whp stock (10psi) after TBE (with cat) 170whp (13 psi)
after i had my turbo ported, I tried setting the boost back to stock(10psi) with the TBE, dynoed at 151. Then same day turned the boost up to 13psi power back up to 170whp

So basically the TBE gave minimal gains on its own
all the claims from the aftermarket shops were kinda
misleading. Taking the credit from boost creep
in their hp increase claims.

the gains on a turbo car with an exhaust are typically 7-10% from my experience, even on cars that don't spike.

[wgknestrick]

Wastegate diameter is pretty much the only variable that is directly linked to boost creep. A spring will do nothing to help creep. The boosts "creeps" because the wastegate is completely open yet still cannot flow enough exhaust through it to control the boost. Whether it "flows" enough exhaust or "is large enough" to equal the pressure differences is a moot point because this is the same thing, just worded differently.


Downpipes can also contribute (read HKS and stock designs)

[jagcars26]

Originally posted by T3RMIN4L
lol@Glenn... nice writeup wgknestrick. Anyone who has met the boost creep and capped the flow with their stock axleback care to comment on the performance loss compared to the full 3"+ back? Personally I think its more than noticable on the butt dyno.

I had to cap mine with the stock axleback after getting boost creep with a Bosal downpipe,cat eliminator pipe,stock mid section and a Prodrive tailpiece.
It didnt seem to hurt the lowend,,but the car definately does NOT pull as well topend.My butt dyno says,,if i got a 30%increase in power with boost creep, I figure i lost 15% by putting the stock axleback back on.
But hey ,it's safe.

Rudy

ps excellent writeup wgknestrick

[T3RMIN4L]

yeah Id agree the power gains were amazing will the full turbo back and Im sooo upset about having to settle for less power and seemingly worse throttle response.

[XTC]

What if you get a downpipe such as the B&B downpipe or the BMP twin dump downpipe? They have split collectors for that separate turbine and wastegate gases. Will that help eliminate boost creep?

[jagcars26]

Like I said earlier,,I have the Bosal(twin dump) downpipe,,I still got boost creep.


Rudy

[Zornorph]

XTC - you will still get the boost creep.

[complexx]

can anyone link to a diagram that draws out the flow of gases through this system, I want to make sure I'm understanding this concept properly.

[slapmyfro]

so one can eliminate boost creep by simply keeping the stock muffler?

will boost creep still be eliminated even if one upgrades all exhaust piping except for the muffler?

sorry im a newbie...............never had a turbo car

[wgknestrick]

As requested:

Section View of a wastegated turbo (wastegate is called "valve assembly" here):




And thanks to Matt, the diagram of WRX exhaust (STI is minus uppipe cat):

[Red Rocket]

Originally posted by wgknestrick
Wastegate diameter is pretty much the only variable that is directly linked to boost creep. A spring will do nothing to help creep. The boosts "creeps" because the wastegate is completely open yet still cannot flow enough exhaust through it to control the boost. Whether it "flows" enough exhaust or "is large enough" to equal the pressure differences is a moot point because this is the same thing, just worded differently.


Downpipes can also contribute (read HKS and stock designs)

Wastegate shape is also a very large factor. The IHI housings have a sharp 90 degree turn for the gas flow to the wategate hole, this is no good. It helps to round off the edge internally. The AVO turbine housing on my XS500 turbo has a rounded wastegate path already.

I will also step up and remind everybody that I was the first person on NASIOC to port a IHI turbo wategate, on a VF29, on Feb. 15th, 2003.

Kevin

[wgknestrick]

I will also step up and remind everybody that I was the first person on NASIOC to port a IHI turbo wategate, on a VF29, on Feb. 15th, 2003.

Do you want a cookie?

[JRock]

I wish I had a cookie, I'm kinda hungry.