Turbo hotside question

[Alin]

So, there are different sizes for the hotside of a turbo. The part of the hotside that connects to the uppipe. The two common ones that I see are 8cm and 10cm. What is the size of the stock uppipe and VF-39?

VF39 TB, PZ18 ex.h/47.9mm c.h/9b c.w/0.63 A/R - US MY04-06 2.5L STi


I am finding different answers for how big the stock hotside is. Same goes with the uppipe... The reason I ask is because the hotside on the turbo I have is 10cm. Now, I don't know what that means exactly. What correlation does the size of the hotside make in terms of flow and power?

I'm assuming 10cm is bigger than the stock... What would have to be done to mate up at 10cm hotside to an uppipe (and then headers)? -Can I keep the stock headers and just get a different uppipe? -Do I have to replace the headers and uppipe? Can I PnP the uppipe, headers, or both?

What are options and suggestions for this part?

 

[Alin]

Man, 3 days of research. Hours upon hours. I cannot find a definite answer on the size of the stock P18 hotside housing. I keep seeing guesses at 5,6,7cm^2, but not one definite answer. That goes for all of the subaru IHI turbos, as a matter of fact... :tdown: I've been through almost 20 pages of google searches and countless different forums, yet no answer. :angry: :banghead:

I did however, find the correlation the 8cm and 10cm hotsides make. :tup: The higher the cm of the hotside housing, the more top end the power band is. For 10cm, I'm seeing a steady 4-7k top end range for the turbo and higher flow, which in return equates to more power. There are exceptions that I have found out and they are dependent on the supporting mods and tune.

I also found out the stock STi uppipe diameter and that is 1.75" on the inside diameter. Now, I have been looking at 2.5" aftermarket uppipes as possible replacements for the stock one to help accommodate the turbo.

A very intriguing thing that I have found out is that the early 2004 STi models should have had a P17 hotside housing. The kick here is that on the inside of these housings, is a stamp that says P18 even though the outside stamping says P17. From what I have read, and this is from threads that go back to 2003, is that there never was a P17 housing associated with our cars.

 

[Spamby]

You might try asking grimmspeed or sixstarspeed about the hot side size. Since they deal with turbo work they might know.
Also remember your pipe will only flow as much as the smallest diameter within that run of pipe. Meaning, you slap on a 2.5 up pipe but your headers are 1.75" then your still only able to flow at the rate of a 1.75 pipe.

 

[Alin]

You might try asking grimmspeed or sixstarspeed about the hot side size. Since they deal with turbo work they might know.
Also remember your pipe will only flow as much as the smallest diameter within that run of pipe. Meaning, you slap on a 2.5 up pipe but your headers are 1.75" then your still only able to flow at the rate of a 1.75 pipe.

Ok... That changes things! :lol:

George recommended I go with this: http://www.grimmspeed.com/exhaust-m...ru-02-05-and-08-14-wrx-2004-2014-sti-all-lgt/

Now, if the uppipe diameter, 1.75", is the same exact diameter as the stock exhaust manifold, then Grimmspeed PnP it to a 2". Please confirm or deny that the stock exhaust manifold is 1.75" on the inside diameter. I did a quick search with no success on an answer. :tdown:

 

[Spamby]

The idea of the pnp is to smooth the transition from one are to the next, ridding the manifold of its casting marks and ridges to reduce or eliminate gas turbulence. They will either be porting to gasket match or to the diameter of the other area, such as the head, or both. The entire tube should not be ported and only at or near or as far down as possible from the flange area.
I believe the exhaust ports on the head to be the same diameter as the exh manifold. I also understand the exhaust gasket between the two to be oversized. (Confirm or deny the before) When you bolt this up it creates a small little gap that creates turbulence in the exhaust flow. When you port or oversize the manifold to match the gasket, your effectively eliminating that small little kink and the heads have a smoother path to dump the exhaust gasses. Ideally you would match the head to the manifold and to gasket but since most don't remove the head just for this, typically just the manifold is done.

 

[Alin]

Ok, still have a lot to research and learn about. :ty: