I'm looking at the 2 turbos I have attached. One has a very small turbine wheel and the other has a larger one. I was thinking the larger wheel would have more leverage to spin the compressor wheel, but since I'm no expert on turbo sizing, thought I'd start a discussion so I could learn. Am I completely offbase? Both are the same ar housing, but the spread is wider between exducer and inducer on the bigger wheel. #ad
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I think the larger wheel is only going to be a benefit if your pushing enough gas to feed it, you should be able to run a lower pressure ratio. The smaller wheel should spool faster, but will have a higher pressure ratio. What are these for?

 

My 2.5L. I'm more interested in understanding the relationship between the turbine size and how that affects the system.

 

Ican't find the formula off hand but you can use the matchbot calculator to play with this. Ultimately you'll have to have displacement, rpm and volumetric efficiency nailed down to know exactly what you need, or trial and error will work too.

From my limited first hand experience:
Larger wheel =lower overall pressure ratio, and will create a lazier system, this could be desirable on a street car or a tire limited race application because the boost hit from a dig will happen over a broader RPM range (ramps up over 2k rpm rather than 500rpm), its less likely to blast the tires off and is likely easier on parts.

A smaller wheel will light faster, and tends to have higher pressure ratios. They tend to feel more like a light switch, but if your talking about a seriously underpowered vehicle or an all out drag car this can work really well. Most OEM are using undersized turbos (turbine and compressor) because they want to get rid of the laggy feel and get maximum economy and drivability.

I think you'll get a lower maximum boost and less gain per PSI with the smaller exhaust. It just comes down to how you want the system to behave.

 

Ok, now I have to decide what I want. Like to hear some more opinions as well.

 

I would also like to see more.

 

Small wheel does not equal faster spool. A larger wheel will apply more leverage to the shaft. The AR of the housing has more to do with spool than the wheel. I am not saying that there is no place for smaller turbine wheels, because if there wasn't, there would not be as many sizes.

 

That was the logic path I was following. More leverage = more potential for boost and spool time. With both ar being equal, yet such a large disparity in wheel sizes, what will I see from each? I was thinking the 60mm wheel in the .82 housing would be pretty laggy.

 

PV=nRT
n, R, T dont change in this case, assuming the engine and turbine position stays the same If you restrict the flow volume by decreasing the wheel and outlet size the pressure must increase (increased PR). That leads me to believe a smaller wheel will spool to a given RPM with less gas, IE earlier. This is backed by using a smaller turbine on a smaller engine.

If you model the gaseous flow as liquid flow you could use bernoulli which I believe would suggest that if your seeing a pressure change less than the IGL states that you must be seeing an increase in velocity. Which also means an increase in wheel speed and faster spool, potential over speed of a too small turbo, which also seems accurate. It's been a while since I did fluid dynamics so I need to sit down with the equations when I have time.

 

Ok, I get that. I'm not arguing flow, I'm saying the longer lever arm will move the compressor wheel with less effort, albeit restricting flow more. With less wheel occupying the space, the velocity of the gas will be higher than in the same ar area with the larger wheel. So I think we are thinking about different aspects of the same problem?

 

My assumption that may be incorrect is that a larger wheel will have a larger outlet? It could be I'm off base.

 

You need to add wheel trim to the mix as well.

Some of your decision should be based on what the engine wants as well... a more na like build will like more turbine flow, whereas something with an old school turbo grind and a low compression ratio would probably be slow to spool the same turbine.

 

Comparing those two turbines the "larger" one in flow terms is the second one - 68/62.3 mm. For turbines the exducer diameter (62 mm in this case) determines flow capacity - this is the size of the "hole" the exhaust gas must pass through to exit. The other diameter - 68 mm is the blade tip diameter. This is much smaller than the first turbo (84 mm). The smaller tip diameter will reduce inertia - reducing lag. It will also increase turbine power at low rotor speeds (low PR) - good for lag and earlier spool. The downside is lower turbine power at high speed/PR, so less top end power and steady state performance (higher back pressure).

The compressors look the same. Turbine A/R is the same at 0.82.

All else being equal, the second turbo should spool earlier with less lag and would be a better street turbo. There won't be much in it because the larger flow capacity will have the opposite effect.

 

I just built a new Double BB turbo which is pretty close to your second turbo. 58x76 comp with a 62x68 turbine. Mine is a T4 turbine housing w /.82 . My engine is a 2387cc . If it does not spool as quickly as I think it will then I will be going to a P trim turbine ( 64x74 ) with everything else the same . My old turbo was a TO4B H trim (58x71)compressor with a P trim exhaust w/.68 . It made 400 hp at 15psi .