series5 turbos........ they bleed boost towards redline...



is this because of the compressor wheel or the turbine wheel?

 

Both.

 

so, i had my turbo off yesterday porting the wastegate, and noticed that the turbine wheel is not clipped... yet i have a t04b compressor wheel (not sure which trim) machine to fit the stock housing...


should it drop boost?

 

From what?

 

from high rpms. we'll say ~12psi

 

So... you're asking if you have something wrong when you boost pressure is dropping from 12 psi to some lower value in higher gears, despite having a TO4 compressor wheel upgrade?

If so, why not just ask this?

 

if the wastegate is working like you want it to, boost shouldn't fall off at high rpms. the wastegate is bleeding off too much i guess.

 

the dropping boost has nothing to do with the wastegate.


damnit, all i'm trying to figure out here is:

which is the bigger problem with the stock turbos... the compressor wheel or the turbine wheel?

 

compressor wheel...no wait, the turbine wheel.......well, no, the compressor wheel.......

my stock turbo holds 17+ psi all the way to redline. so i can't help you.

 

is it an s4 turbo?

 

yes,
but apparently i was unaware that the S5 turbos have different turbine and compressor wheels......

 

No, the S4 and S5 turbos are identical apart from the exaust housing and manifold. Both wheels are the same.

Remember the S5 has factory electronic boost control. Manufacturers often program boost curves that drop off toward redline, as this improves reliability, econmy, emissions, etc. Replacing the stock EBC with an aftermarket one should help.

 

every person that i have ever known to have a s5 turbo (with all emissions and solenoids removed) has had bleeding boost.

 

Both!
Compressor wheel efficiency should be matched to the turbine wheel efficiency. If you change the efficiency of one, you have to change the efficiency of another for them to work well together... by, say, clipping the turbine wheel.

 

I've read it happening to s4 turbos often as well as s5.

I've also heard of plenty of s4 and s5 turbo guys that don't have much boost dropoff near redline.

 

I know it wasn't planned, but this is a kinda funny thread. Proceed.

 

my stock s5 turbo couldn't hold full boost.

my hybrid (stock hotside wheel) does.

 

aren't turbochargers supposed to spin at the same speed the entire time after it reaches full boost, i noticed in my stock s4 turbo i think the boost gauge would drop a little as it went past 6800-7000 rpms. Well if the turbo is spinning at the same speed even up that high of an rpm, is the engine just sucking in the boost to fast? i don't really understand.

 

No,
they will continue to spin faster as RPM gets higher however their efficiency is at it's max so the compressor cannot compress anymore.

Anyways your bleeding boost has to do with the fact that the stock S5 setup had electronic boost controller built in with the ECU, now you replace that with an aftermarket Boost Controller (electronic) you will end up with far superior boost control.

As for efficiency as scathcart said you change one half of the efficiency you should change the other by clipping the turbine wheel, or by changing the wheel entirely, But it has been done where the stock wheel was never clipped and to4b or whatever has been put on and the only real problem was that they had trouble with boot creep. So be aware of boost creep.

 

turbine wheel was not clipped?