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After doing my usual perusings on Nabisco, I saw an interesting post on boost creep.

SO, who here is experiencing it and what are your mods??

I am going to put the full 3" exhaust on soon and this is something I would like to avoid.

apparently the bellmouth downpipes tend to allow quite a bit of boost creep. The CES design, also copied by perrin(the divorced wastegate connects closer to the turbo on the perrin though)helps with this quite a bit

i get 1 psi worth of creep at wot in 4th around 125mph

hmm,seems like we don't get it compared to the 2.5T people on Nabisco. INTERESTING!!!

I know that Trevor from CES was working on a boost creep fix for the EJ257 STI. They were having minor creep issues even with the CES twin dump, which is one of the most stable DP's on the market. They were having customers, with only the CES TBE for mods, hitting fuel cut in 4th and 5th occasionally.

I can't remember exactly what Jeff was telling me but I think it has something to do with the VF39's wastegate and the OEM boost controller.

azn2nr did you have any more boost creep with the VF-39?

I'm not sold on the divorced DP being better for creep. If you are getting creep you need a bigger wastgate or an external wastegate. A good 3" free flowing exhaust lowers the pressure in your DP plus you are going to add more boost or you wouldn't be doing mods, so you end up with too much exhaust for the small wastgate hole.

Port the wastgate or drill it out and get a new flapper welded in or use an external WG, quit bitching about the DP. Subaru's are not the only turbo cars that have problems with creep.

The problem with the VF-39 is that it has a rather large compressor but a rather small wastegate. The large volume of air fed into the engine creates a large volume of exhaust gas, and the wastegate can't flow enough to stop turbine speed from rising.

Jeff explained to me that Subaru's solution to this was to put a little dimple in the STi's downpipe flange. What happens is the gas leaving the wastegate bounces off the dimple and goes backwards into the turbine. So, the wastegate not only bleeds exhaust gas away from the turbine inlet, but also blows exhaust gas backwards into the turbine outlet. That makes it much more effective at controlling turbine speed.

To some degree, this is also why Subaru uses a flat flange on the WRX. Bellmouth and divorced-wastegate downpipes reduce the ability of the wastegate to actively slow the turbine.

Perrin's downpipe is not a copy of the CES downpipe. The length of the wastegate pipe is way shorter and it joins in at a sharper angle. The CES design gives more space before joining the two flows and also provides a shallower angle where they join, and the precise value of each of these parameters is based on a huge amount of testing. The CES one is supposed to be used with a turbine-specific plate that extends into the turbine housing.

http://www.perrinperformance.com/Produc ... n_Pipe.htm
http://www.cesracing.com.au/

Deadbolt offers a wastegate modification for the VF39 called the Black Hole. They enlarge the wastegate hole and install a larger flapper. This is supposed to eliminate the boost creep problem by simulating what IHI probably should have done in the first place.

http://www.deadboltspeed.com/wastegate.htm

vrg3 wrote: Perrin's downpipe is not a copy of the CES downpipe. The length of the wastegate pipe is way shorter and it joins in at a sharper angle.

LaureltheQueen wrote:CES design, also copied by perrin(the divorced wastegate connects closer to the turbo on the perrin though)

I am stable for as much boost as I've ever cared to run on a ported/polished 16g and the CES DP03.

I didn't think I was being redundant... I just meant that it wasn't like CES invented the twin dump and then Perrin copied and modified it. They're just two different designs.

BPM, on the other hand, does make a downpipe that's a copy of the CES one except with poorer quality construction.

Boost creep = boost spread between gears? Are we thinking this is more a function of wastegate and downpipe design as opposed to boost control?

I have a Dawes Devices $25 "basic" MBC (ball-and-spring style), with a hybrid t-bird/vf11 turbo and a muffler shop custom 3" downpipe. Current setting on the controller results in 13psi in 1st, pretty much to redline. But in 5th, I get almost 17psi. Used to be a bit worse with the old TurboXS bleed-style MBC....


Vikash - do you think that if I were to take my downpipe off and make a dimple in the pipe where the wastegate gasses exit, it might help to better regulate my boost? That sounds like it adds better control over the turbine speed. Or did I read your post about that wrong?


Interesting.

Boost creep just means a failure of the wastegate to limit boost. Usually it manifests itself as boost rising with engine speed. That's different from boost spread between gears.

If your car was suffering from boost creep, you might floor it in 4th gear at 2000 RPM and see 12 psi, but as you held it and engine speed rose, you might see 16 psi by the time you got to redline, still in 4th gear.

It is possible that making the wastegate more effective at stopping the turbine would reduce boost spread; I don't know for sure since I still do not have an understanding of what causes boost spread.

Boost creep, a problem I have. I have to set my boost fairly low because no matter where I set it, it tends to hit fuel cut unless it's at stock boost. I usually never get good boost levels. I'm usually no higher than 8 psi unless I'm going uphill or on the higway at cruising speed. But most often, I'm boosting at 4 or 5 psi.

ahh... I see. I have the opposite problem, as far as rising engine speed is concerned. RPM++ => Boost--

hmm....