I'd like to limit this discussion as much as possible to either a) hypothetical/theoretical discussion of how changing the A/R might effect creep, or b) actual experiences on this point -- i.e. you changed to a different A/R turbine housing and noticed a change in boost creep. Thanks in advance for efforts to keep the discussion on point!

So here is the background that may be necessary to frame the discussion a bit: I'm currently running a Precision SC6265sp with a .63 T3 v-band turbine housing, on a 2.0L Neon. The motor has a ported head, a high-ish flowing intake manifold with short runners, and Crane "18" cams. The cams are probably not real big by Honda-Tech standards, but they're fairly healthy for this motor.

The turbo manifold is a decent quality tubular one, but the packaging in the car makes it hard to get any sort of wastegate priority built into the wastegate tube exit from the collector. To the extent that TiAl would probably have a fit over it! LoL....

I'm running a TiAl 44mm on it with a spring that seems to open at about 16psi. From there the boost creeps up to the range of about 24-25psi at 8,500ish, depending on the gear.

What I'm pondering, and the reason for this thread, is that I wonder whether my boost creep would be lessened by switching to a .82 housing, or whether it might even become worse??? I can see it both ways I guess, in theory.

Theory number 1: Changing to a turbine housing that presents less of a flow bottleneck might build up less pressure in front of the turbine wheel, and the resultant smaller pressure delta across the turbine wheel might cause for the production of less wheel speed/boost.

Theory number 2: Changing to a turbine housing that presents less f a flow bottleneck might build up less pressure in front of the turbine wheel, and in so doing, the reduced pressure in the collector might actually provide less encouragement for the flow to exit the collector via the wastegate and dump. In so doing, this reduction in flow through the wastegate might not reduce the wheel speed/boost, and it could even cause there to be greater wheel speed/ boost produced....

I really think scenario number one is the most likely one, and thus I'm considering spending the money to find out. However, if the theory is seriously flawed, or if people's experience indicates that it doesn't work that way in practice, I would rather keep the money or spend it on other things like tires... LoL.

Please let the discussion begin!

 

I would have to agree with you. By switching to a bigger a/r on the exhaust side you will have less back pressure and in turn less creep. Even if their is less pressure in the collector your wg will still be opening at 16psi and the exhaust will not need pressure or encouragement to leave the collector that's physics 101. It will take the path with least resistance so it will exit instead of turning that wheel. If ur having creep issues I would look into other things such as wg spring setting and boost controller before opting for a new turbo. Creep usually comes from a wg not being able to flow enough.

 

Thanks for the contribution!

I'm not sure that all of the information on the board agrees with the assessment that pressure in the collector always makes for a standardized function of the wastegate though... There seems to be quite a bit of evidence and thought that the wastegate exit angle from the collector is very important in determining how much flow there will be out the wastegate. As in, there seems to be a lot of flow momentum involved, as opposed to treating the collector like it may function in a manifold that is more log style.

On the other hand though, some of the people who have posted up CFG (?) models of collectors and wastegate exits that are less than optimal may not have been factoring much collector pressure, and I think that may be a super important factor.

Potentially there is a point where the boost level is high enough to cause sufficient pressure gain in the collector to even out the effects of the less than optimal flow path. However, that much pressure in the collector is going to be really spinning the hell out of the turbine....

And of course, it could also be like you said, that a single 44 isn't enough gate. I hope not, because in this situation fitting another one is going to be a big challenge.

 

In theory I can see that both ways as well with the wg needing to be mounted a certain way, however theoreticly it might not make the big of difference in an external situation. Being that the wastegate opens and operates based on static pressure the exhaust pulse is what drives the exhaust out u could also think about a twin scroll setup. That could also help boost creep since the pressure is separated and pulses between the cylinders are kept separate.a bigger a/r could potentially cause more boost creep due to losing velocity. The smaller a hole the higher the velocity but the higher the pressure may be.

 

Usual rule of thumb seems to be, the better the system flows, the more prone it is to creeping. So a bigger turbine housing would probably be more prone to creep than a small one.

Reasoning in simple, if you back up the system before the turbine not only will you not be able to spin the turbine as fast, but the wastegate will also become the path of least resistance and will evacuate exhaust gases easier and faster. This backup also causes boost loss on the other side of the spectrum, the AR is so small that everything backs up that it chokes the wheel and gives primary to the wastegate.

Increasing, although primarily it will take more energy to spin the turbine, you will also have opened up the exhaust gases to go through the turbine instead of the wastegate (especially with poor wastegate placement) and up high you will more than likely begin to creep. From my experience, creep is generally equal no matter what target psi you're running.

This is also the same reason why a lot of guys who switch from recirculated dumptubes to open dumps run into sudden boost creep issues. If your system flows very well and you don't have a good wastegate setup, you're probably going to creep.

The only way I see a bigger turbine housing helping out is if you go so big that you can't generate enough energy to spin it out of control.

 

larger a/r will help alleviate boost creep. I have done this on 2 different setups to help with boost creep.

However, you now have me worried about my setup that I am putting together. Its a 2.0L with a billet 6262BB .63 a/r revving to 8000. Hope it doesnt creep like your experiencing.

 

Are you saying here that the reduction of exhaust volume in the main exhaust increases the likelihood of creep because it basically reduces the overall flow of the system? If so, that makes sense. If not, please explain further!

If you could site some specific examples with some comments on the reductions in creep you saw that would be GREAT!

 

Yes, having a recirc reduces the overall volume of the entire exhaust system, which equates to more pressure at the manifold and the turbo. This will keep turbine speeds down and help keep creep under control.

 

You guys have left out the compressor out of the equation

Turbine RPM and boost goes hand in hand, and boost is based on how much air the engine can ingest and what the compressor is delivering at a certain pressure ratio.

By going larger A/R, assuming that the wastegate priority and its path remains unchanged, it slows down the turbine wheel. We'll have leave out exhaust velocity because it gets too complicated; the exhaust flow out of the wastegate depends on the angle of bend, area, etc.. of the path, but the flow gets affected based on velocity. The higher the velocity, then the same bend or turn becomes a bigger hinderance.

If you have a compressor that is not efficient at lower wheel speeds, then going with a larger A/R turbine hsg will actually reduce boost creep. Slowing the turbo down and increasing turbine flow (larger A/R) can land the compressor at a lower efficiency range, thus, also lowering airflow (which in turn, lowers boost).

I have had this occur on my IS300 w/ Tial 38mm gate and a T04B/T4 turbo and a tubular manifold. The car originally was creeping to 14-15 PSI on a 8 PSI spring and a 0.69 A/R housing. After stepping up to a 0.96 A/R housing, my overall boost became lower with a larger A/R housing with the same wastegate, same spring and everything else identical. The engine managed to ingest all the extra air the compressor has to offer at a certain turbine RPM, and ended up running close to base spring pressure (8-9 PSI) instead.

I think the cause of boost creep versus turbine size really depends on the turbo itself. I bet all the newer billet wheels with such a wide efficiency range, will always creep more if you threw in a larger A/R housing. The compressor is constantly efficient, and lowering turbine speeds does not lower airflow by much on a a billet wheel.

If you have a small compressor though, running larger turbine housing can make the compressor run closer to choke line, thus, also reducing the tendency to creep.

 

So what you're saying is that it could go either way depending on a whole bunch of variables, huh?

 

u could also change your wg spring to a smaller size. It would be alot cheaper

 

??????

 

Yup, so picture a turbo that has a big compressor in relation to the turbine size. For example, a GT3076R, or a GT4094R, etc... If you are getting creep on this setup caused by either too small of a gate, or no wategate priority on the manifold, etc.. upsizing the turbine would probably promote even more creep.

By slowing down the turbine speed on such turbo, the compressor is still spinning fast enough to feed the motor with plenty of boost and airflow, but the reduction of exhaust pressures is encouraging more exhaust going through the turbo.

So it really boils down to what turbo you have, and how much you are using up its airflow.

 

yea its really a relationship between manifold design, turbine size, and wastegate size.

if you have good wastegate priority and a PROPERLY sized turbo, and you're still getting creep, its apparent that your wastegate sizing is incorrect and needs to be increased...

90% of the boost regulation falls to the wastegate... after all its job is to divert exhaust gasses/pressure around the turbine wheel to regulate boost.

things such as manifold design, and turbine size will play a role but not as key as wategate and wastegate placement.