Brettus

Been reading about this topic on here and see there seems to be the view that T4 turbine housings flow better than T3 ones with the same AR. I think there are cases where this may appear to happen but it is caused by other factors , not by the ability of the housing to flow.

Firstly , check any manufacturers flow diagrams for turbines. No mention is ever made of T size , it's only ever AR and turbine wheel size that are discussed .

Secondly , have a look at the actual shape of the volute for both a T3 and a T4 of same AR and turbine size. Sure the area of the T4 starts out bigger , but both neck down to the same size very quickly .

Obviously manufacturers use larger T sizes for bigger turbos as the pipes feeding the turbine need to blend into a flange that doesn't reduce the area suddenly. But as turbos get bigger , so do turbine housing aspect ratios. As long as the flange isn't smaller than the area of the volute that follows it .... it wont restrict flow.

So ...where people have experienced higher flows from going from a T3 to a T4 , I believe this has more to do with the size of the pipes feeding the manifold and the ease with which you can blend the pipes into the flange than it does the 'T' size of the turbine housing .

Could be relevant to those with lower power goals wanting to run smaller pipes (smaller than 40mm ID) to improve spool ....maybe.

RXBeetle

I don't think the actual numbers agree. With the same A/R a T4 will always outflow a T3 because, by definition, the volute has a larger cross-sectional area.
The R is the nominal radius of the volute (turbine nozzle) and the area is the cross-sectional area at that radius. The T3 and T4 are just a frame size with bigger turbines to match. The bigger turbine inherently requires a bigger radius volute so the R of the A/R is bigger. So for the same A/R, say 1.0, the cross-sectional area would be bigger for the bigger frame turbo. This is super clear by the flow numbers if you compare turbines and housing combos on Borgwarner's matchbot.
Holset and Mitsubishi clear up all this confusion and just define the turbine housing by nozzle area in cm^2.

Not trying to be contradictory, just suggesting you dig a little deeper into the geometry. A well deigned system will not have an expansion area, just neck down in the turbine housing to act as a nozzle at the turbine inducer.

Brettus

Thanks RxBeetle ...was hoping this would gets some comments and some discussion going because I knew that there is strong evidence to suggest a T4 does flow more . I'm not disputing that it happens , just why it happens.
I think you will find where the volute starts (by definition) is at the largest area that is at right angles to the center of the turbine . And AR is the same at any point from there on all around the circumference of the turbine . That area will be the same on any turbo of the same frame size. What we are discussing is the area before that , at the flange opening . It's that area only that is larger on a T4 than a T3 .
Check out Matchbot , there is no mention of T size anywhere . BW tend to use T4 everywhere in the range though so hard to compare a T4 with a T3.
Better one to look at is Garrett :




These maps are all for the 68mm inducer turbine and most of the A/Rs have both T4 and T3 variants . Yet there are no separate flow maps.
Here are the different options for the GTX35r and the only flow maps are those you see above :

Brettus

Another thing I see bandied about here is how larger turbine wheels always flow so much better than smaller ones . This can be misleading IMO because It's the combination of wheel size plus AR that determines flow . Check out the matchbot maps .... a 74mm 0.92AR combo flows the same as a 70mm 1.22AR and
74mm 0.83AR combo flows the same as a 70mm1.05AR (although there is a mistake in matchbot and they call it something else)

RXBeetle

So are you referring to T3/T4 flanged or T3/T4 framed turbos? The GT35 is a T3 framed turbo regardless of the turbine flange. BW doesn't mention T-size because they have a different frame designation scheme. B1 or B2 frame and S### turbine wheel inducer range.
Your point "74mm 0.92AR combo flows the same as a 70mm 1.22AR" is a good one. This makes me wonder how much the smaller S200 turbine (lower inertia) would compare for response and spool with the equally flowing A/R S300 housing.
Going back to your earlier comment about runner size matching the flange; the T3 flange (twin scroll) is so small that you would end up with 1-1/4 sched. 10 pipe (36.6mm ID) to match the flange area. That's barely bigger than the port area and much smaller than the exhaust sleeve/flange ID. Maybe a tapered runner? If you are shooting <2500rpm spool and instant response that's probably the recipe. Those choices limit you to a compressor options that are out of surge at low rpm (probably smaller than 76mm) limiting top end pretty significantly.

Brettus

My understanding is that T3/T4 etc is not a frame size at all - GT35 , GT40 etc are the frame sizes for Garrett. And with BW , have a look at all the turbine options for the entire EFR range . T4 flanges are used right from the smallest (6258) through to the biggest (9180) so the 'T' definitely doesn't refer to frame size there either.
Edit: you have to go back to when turbos were actually called T3s T4s and T3/T4s etc. Back then the 'T' meant something to do with the size of the actual turbo. I suspect they abandoned that nomenclature when it became too confusing as people combined all sorts of flange sizes with all sorts of wheel size etc. But even back before all that swapping around started happening,I suspect that what a T3 turbo was in reality was 'a turbo with a T3 flange that's sized to suit a certain output range'.

By my calculation the hole size for a divided T3 corresponds to a 38mm ID pipe . I'm guessing somewhat here but I would imagine you could go up to something like a 42mm ID pipe before it becomes impractical to transition into the hole of a T3 and that becomes something that would reduce overall flow . Given that no matter what the pipe size is , it still has to go through a smaller orifice once it gets to the volute, that transition is where you get the gain between a T3 and a T4 ....IMO . Of course you also get gains in flow just by having a larger pipe but there is a point where a smaller pipe doesn't unduly affect flow. So the case for a T3 flange on a 13b becomes about whp goals and spoolup.
Something else to consider ....... as the wastegate typically takes 30-40% of the total flow , if you can manage to get the wastegate close to the ports you can use smaller diameter pipe to the turbo without any penalty to flow potential.

BLUE TII

iTrader: (9)

Open volute turbo exhaust housings also flow more than divided housings since they have lower frictional losses due to surface area. Especially so the v-band open volume turbo exhaust housings.

A BW T3 V-band open volute housing will flow more than the same A/R dividend T4 housing with the same size exhaust wheel.

They call it "swallowing capacity" or some such because peak flow is really just important for calculating if the exhaust side will choke out at a certain HP.