Ok. you're going in the right direction. When the creep issue occurred, what was the temperature outside?

 

35F and less. I can check my logs for specific intake air temps, but these temps are the same that the SC6152sp was seeing.

I just inspected the plugs from today with my otoscope, and they look phenomenal. Makes me think what I need to do is put in a 4 bar MAP and see at what boost pressure the creep would even out. . . . . .

 

Alright, my IATs ranged from 40 degrees to 47 degrees (at max boost--lot of meth/water injection). Judging from these temps, and the fact that it's only down to 35F right now, I'm thinking it was probably more than 35 out when I was driving.

Also I see from my logs that I was going leaner than I want to because this turbo is moving a ton of air. I'm out of fuel system at the moment like this, so I guess tomorrow I put in one of my crazy new water/methanol pumps and see what that does. Other than that, I have no choice but to see if a softer spring will bring things back under control for now.

I do think I'll order that 4-bar MAP sensor tomorrow though. 255kPa isn't cutting it.

 

Going with the .82 housing will make the creep issue worse, not better.

This is because the WG placement is obviously less than ideal, and going with a larger a/r turbine housing will just lower pre-turbo backpressure even more, causing more exhaust energy to flow through the turbine rather than out the WG.

Improving exhaust flow into the WG via a teardrop style modification and/or relocation is where I'd focus my energy.

 

Hmmm. . . Well, I can understand your logic on that, so I guess you can theorize it both ways.

Interestingly, Precision, and the Shodan seem to think a .82 turbine housing would reduce creep by reducing back pressure in front of the turbine wheel, and thus spinning it less vigorously. Albeit that neither of them think it will make a significant reduction in the creep.

 

is there any room for an additional smaller wastegate?

 

My advice is based on my experiences. We've dealt with this type of issue many times. Any time a car has a boost creep issue and you lower the backpressure, it will creep worse. Increase backpressure and it will creep less. Exhaust gases will always take the path of least restriction, and decreasing backpressure through the turbine will just cause more exhaust to take that route rather than out the WG - this will cause increased shaft speed and more boost, thus more exhaust generated, thus compounding the problem. Try putting a silencer in your muffler to increase backpressure, and tell me what happens to your boost creep issue - it will disappear.

I don't follow the other line of thinking at all. "Spinning it less vigorously"? A certain shaft speed equates to a certain amount of air being moved by the compressor. 15psi is 15psi, same shaft speed. The WG regulates that speed by diverting exhaust energy away from the turbine. If it can't divert enough, turbine speed will creep up as it revs out. So you have to help the WG do it's job by improving placement, exhaust flow into it, increasing backpressure, or increasing it's size.

 

JFK78, I totally understand what you're saying, but the difference may just boil down to whether the back pressure is created before or after the turbine wheel. I've controlled boost creep myself by going to a slightly more restrictive muffler. That is back pressure AFTER the turbine wheel. The different turbine housing effects the way the exhaust gasses before the turbine wheel interact with the wheel. These things create the pressure differential across the turbine wheel, and that is is why a lower a/r housing spools up faster.

Anyway, please disregard my tales of boost creep for the moment! As The Shodan alluded to, the one thing I changed in addition to the turbo is definitely causing at least part of my problem. The new nylon tube I ran between the new pushlock fittings at the compressor housing and the wastegate has a blowout pinhole in it. . . So I'll fix that and see where I am after that.

It won't be today though, since I'm not finished with the testing of my new water/methanol pumps I need to do before I put things back together.

Soon I hope!

 

While creating backpressure after the turbine wheel via a more restrictive exhaust will also increase backpressure pre-turbo, a smaller a/r housing will also increase backpressure pre-turbo, thus the greater pressure differential and increased spool. Same effect as far as increasing pre-turbine backpressure, no? Maybe TheShodan can further educate me if I'm incorrect.

Creep has always been worse with larger a/r housings on otherwise identical setups in my experience, for what it's worth.

You went to a turbo that flows more air on both sides, so the creep issue is likely just now arising due to the increased exhaust volume at the same boost levels and the fact that the new turbine wheel also flows more and thus reduced backpressure pre-turbine.

Hope that pinhole is your culprit, or at least helps!

 

Hey guys, thanks for all of the input!

It looks like when the Shodan alluded to checking the line to the wastegate, he was right on the money. After spending some hours bench testing water/methanol pumps and nozzle setups, I was able to drive the car to work today without the pin hole in the wastegate boost line. The roads were pretty wet/snowy so I wasn't able to stay all the way into the gas real long, but according to my logs, it looks like boost was stabilizing out at about 17psi. If so, that's less creep than I had with the SC6152sp.

By way of a comment on the turbo, I have only a slightly slower spool with this turbo, and it's requiring conservatively about 8-9% more fuel to maintain similar AFRs. In short, it is pretty awesome! ! !