Boost creep / fuel cut

[YBNormal07]

OK,
I've been answering a lot of question lately about DPs without cats, fuel cut, boost creep, etc. so I thought I'd put a stickey in place. The following is a simplistic explanation. Feel free to respond and correct any errors I've introduced, or provide further clarification. This is to be an instructional stickey, so more information is always better.

Although some EMS claim they can take care of boost creep, in reality, they are adjusting the actuation speed of the wastegate, which actually does provide some protection against boost creep, but this is a "tuning" type fix, and can't fully protect against the "physical" restriction of the wastegate size.

To the explanation (greatly simplified):

Exhaust flow powers the turbine side of the turbo, spooling up the impeller and compressing your intake air to your manifold.

To control boost, you regulate the flow of this exhaust by bypassing exhaust around the turbo once it achieves set pressure. The wastegate is the exhaust bypass around the turbo. As you close the wastegate, this directs more flow through the turbo, increasing boost. As boost rises to set pressure, the wastegate opens to allow exhaust to bypass around the turbo, slowing the turbo down and lowering/maintaining peak boost pressure. In reality, the wastegate is merely a bypass valve though, not a true pressure contol system.

On the stock system, you have backpressure in your exhaust line from the cat, resonator, muffler, and piping sizes. This works to keep the exhaust flow through the turbo to a rate that can be managed in conjunction with the wastegate.

Once you remove these items and free up exhaust flow (reduce backpressure), your turbo now has more exhaust flowing through it at peak throttle than it did before. The wastegate is sized with the stock backpressure in mind. It now can't flow enough exhaust at peak pressure to slow the turbo down, leading to a slow buildup in peak boost pressure, otherwise known as boost creep. This ultimately leads to an overboost condition, usually exhibited by a CEL and fuel cut (your car suddenly lurches and dies or cuts out and back in at full throttle.....a horrible experience believe me!!!!)

The only way around this is to
1. enlarge the wastegate hole (porting the wastegate and some shops call this porting the turbo..so be clear what you are asking/getting)
2. increase the exhaust backpressure (high flow cat or keep the stock catback with a catless DP)
3 or use an external wastegate.

Of the 3, the external WG is probably the best as you can buy one that controls boost in both directions, unlike the internal wastegate the stock turbos come with. Plus the sound it makes is awesome! Of course, this will mean removing the internal wastegate, so this application will usually be found on a larger turbo upgrade. Another good question to ask if considering a larger turbo...does it have an internal wastegate. If you are going external, of course you don't want an internal wastegate.

Personally, I've always erred on the side of caution and used a DP with a HF cat. No boost creep to date with this setup (on my STi and Legacy). Prior to the HF cat though, I did experience boost creep at a track day once. It is not something I enjoyed as I thought my STi had blown it's engine. Luckily for me, it was just overboost followed by fuel cut.

I have not personally verified using a catless DP with the stock catback exhaust to ensure creep does not occur, but it makes sense so I suspect it will work the same as a catless DP. I prefer the HF cat route though, as it is still emissions friendly and precludes having to swap the stock DP back in to pass emissions. The HP loss is negligable so I consider it the best of both worlds.

I've put in a pic (thanks to JSC Speed) of an internal shot of the turbo showing the wastegate. What you can see in the lower right corner of the turbo is the back of the wastegate valve. In shape, it is basically similar to what you would find as a lid on a garbage can, and this pic would be the top of the lid with the lid hinge off to the right.

Comments welcome!

[rexdriver]

Thanks for explaining this, I just installed a vf39 with the utec stage 4 map today. The car was being overboosted I guess since it was doing that thing were it shuts down for second in the middle of excelerating, really sucks, feels like your going to break the tranny! I noticed that the utec stage 4 is set at 18 psi, this kinda sounds like alot doesn't it? I turned the boost down a bit and it got a little better so I turned it down some more and seems to be working great. The car hauls ass!
I have a turboxs turbo back exhaust system that has one cat in the middle, turboxs uppipe, stock intercooler on there right now, walbro 255, sti 565 injectors and a bov. Is it safe to just turn down the boost.
Do you know if there is an external wastegate for the vf39? Whats with the helper springs?
Oh yea the car is a 04 wrx.
Thanks

[YBNormal07]

I'm not sure this is boost creep or fuel cut, as when I experienced it, the car shut completely down.

I thought the Utec stage 4 provided for the higher boost levels. If you are still running a cat, the backpressure should be enough to prevent creep.

Have you checked your datalog maps to be sure your aren't getting fuel starvation. You've got the FP and FI, so it can't be starving from to little fuel, unless you've got a plugged filter.

Did you get any CEL's? That will be key in determinining the issue.

If you've already turned the boost down and it's running good, you may need to map it out cell by cell to raise boost back up. Then again, if it's running great and nothing wrong is happening, why mess with it.

Helper springs allow for faster and earlier WG action. On our stock internal WG, the actuator arm is helped along with spring pressure. Adding a higher force spring or another smaller force spring to the OEM spring "help" the acuator work. There is an external WG for your setup. Tial makes it. Do a search in the engine performance forum (use "tial" or "external+wastegate") and you should be able to hook up with some folks that have installed one.

[Shocker]

lets say for instance someone was running an exhaust system (downpipe, up pipe, and catback) without any cats...maybe just a test pipe. would porting the wastegate work to fix this? and could you/would you port it with something like a dremel tool? how much would you want to open it up? sorry for the crazy questions...i'm a bit of a n00b when it comes to turbos and such.

[YBNormal07]

You'll have to do a search to find out how to port a wastegate. I've never done it as I've always used DPs with high flow cats. Essentially, the answer to your question is yes though. If you have a non restricted exhaust line from your turbo to your tip, and you run a 2.5 platform, you stand a good chance of having boost creep. Based on what I've read, you can succussfully take care of this with porting the wastegate. Personally, I would go with a high flow cat as the small amount of HP you loose with it provides better peace of mind over wondering if the porting was done correctly or not. There are shops that will do this for you. I've seen folks use a dremel to port, but it looks very easy to overport it and thus loose boost.

[bkzshabbaz]

I have a 2005 WRX wagon, no engine mods yet, except a K&N filter. I plan on going stage 2 as my first engine mod (Cobb AP, TurboXS stealthback catless, prodrive axelback). Soon after, I plan on putting in a VF34 turbo along with supporting STi pink injectors (and use Cobb map made for VF34 turbo). Should I go ahead and get an external wastegate as well to prevent boost creep? Or does the internal wastegate on the VF34 suffice?

[rexdriver]

I have a 2005 WRX wagon, no engine mods yet, except a K&N filter. I plan on going stage 2 as my first engine mod (Cobb AP, TurboXS stealthback catless, prodrive axelback). Soon after, I plan on putting in a VF34 turbo along with supporting STi pink injectors (and use Cobb map made for VF34 turbo). Should I go ahead and get an external wastegate as well to prevent boost creep? Or does the internal wastegate on the VF34 suffice?

No you won't need an external wastegate with that setup, but I would recommend a quailty electronic boost controller. I got the Apexi Avcr, it lets you dial in your boost perfectly, it just takes some time to learn all the stuff it does and how to get it to work right. Only complaint about the Avcr is that that the boost is not in PSI it is kg/cm, so I made a conversion chart that I kept in the car to see what PSI I was hitting. It seems the car runs best just around 18 PSI (1.27 kg/cm). Another plus for the Avcr is that you can have boost start early for certain gears, I have it set at plus 1 for first and second, the car pulls so hard through first and second, I just keep thinking I am going to rip the drive train out. I have raced and beaten an 05 STi and an EVO 9 and embarrasad them pretty badly. By the way I am running a VF39. Good Luck!

[rexdriver]

My list of mods are: VF39, STi Pink 565 injectors, Turboxs Uppipe, Turboxs TBE, Walbro 255 fuel pump, one step colder plugs, Crucial racing colder thermostat, Ebay big TMIC, Apexi Avcr, Perrin inlet hose, Turboxs UTEC, (running slightly modified stage 4 map) Ebay special short intake with K&N. My car is an 04 WRX.

[YBNormal07]

I have a 2005 WRX wagon, no engine mods yet, except a K&N filter. I plan on going stage 2 as my first engine mod (Cobb AP, TurboXS stealthback catless, prodrive axelback). Soon after, I plan on putting in a VF34 turbo along with supporting STi pink injectors (and use Cobb map made for VF34 turbo). Should I go ahead and get an external wastegate as well to prevent boost creep? Or does the internal wastegate on the VF34 suffice?

ummm...despite the previous comment, I would recommend you lean towards an external WG with your proposed setup. There is a very high chance you will get boost creep with it. Note the previous poster is driving a 2.0 platform, not a 2.5.

[YBNormal07]

No you won't need an external wastegate with that setup, but I would recommend a quailty electronic boost controller. I got the Apexi Avcr, it lets you dial in your boost perfectly, it just takes some time to learn all the stuff it does and how to get it to work right. Only complaint about the Avcr is that that the boost is not in PSI it is kg/cm, so I made a conversion chart that I kept in the car to see what PSI I was hitting. It seems the car runs best just around 18 PSI (1.27 kg/cm). Another plus for the Avcr is that you can have boost start early for certain gears, I have it set at plus 1 for first and second, the car pulls so hard through first and second, I just keep thinking I am going to rip the drive train out. I have raced and beaten an 05 STi and an EVO 9 and embarrasad them pretty badly. By the way I am running a VF39. Good Luck!

Mind informing us as to where you are getting your information from? A UTEC runs boost a bit different from a Cobb flashed map. Your setup is completely different than the one in question. Also, Cobb isn't meant to be run with a boost controller, although with a VF34 and a wideband lambda input for A/F to support quicker boost response isn't a bad thing. In this case though, he would need to go Street tuner to allow a custom map to support the boost controller.
In other words, please be careful in your recommendations. This is incorrect information.

[WRX88]

As stated, the way to eliminate the risk of boost creep/fuel cut is to increase back pressure in the exhaust(Other than ported wastegate or external wastegate). A thinner diameter exhaust would accomplish this in addition to a high flow cat. These two variables slow down the exhaust flow in order to accomplish higher back pressure. In theory, would a non bellmouth/divorced wastegate downpipe accomplish the same thing? With that extra bit of turbulence exiting the turbo, wouldn't the exhaust gasses slow down in turn raising back pressure? If I am wrong, please tell me. Any input would be greatly appreciated.

[YBNormal07]

It's a decent theory, but he whole purpose of a divorced wastegate style DP is actually to smooth out the flow and remove the turbulance of the wastegate gases being reintroduced into the exhaust gases flowing into the DP. Theoretically, if the divorced section was small enough, it may be enough to indroduce sufficient backpressure to allow proper wastegate control. Only way to find out would be to try it. Alternatively, getting exact measurements of the divorced section diameter may provide the data, but I don't have the math skills to determine this.

[WRX88]

So in your oppinion, would the HKS be a cost effective solution for a downpipe ($70 cheaper than most bellmouths) while in theory, reducing the risk of boost creep?

[YBNormal07]

It is a cost effective DP. As to it being a solution for boost creep, my best guess would be no. As I tried to outline above, a divorced WG DP should flow BETTER, and thus, would likely contribute even more to the creep issue since you would have less backpressure. I can't picture a manufacturer making a DP that is designed to provide more BP. My theory as to if it could help involved a consrictive flow channel for the divorced WG gases. I doubt this does that. Only one way to try though.

[WRX88]

I agree with what you are saying with the divorce wastegate and increased backpressure. I'm not sure if i was clear enough, but since the wastegate is restricted with the HKS dp (flat plate design) would that provide more backpressure?

Maybe i'l just cough up the extra $170 for the helix catted and $50 for a cobb heatsheild.