A turbine wheel blade has two main components: -

1 A flat paddle wheel like area
2. A trailing curved area that changes blade direction approx 75 degrees to give rotational thrust to the turbine wheel as the gases exit the turbine.

Both these areas act in different ways.

After the gas packets have entered the turbine housing and travel into the snail, the direction of travel is aligned with the feed throat (slot ). The slot directs the gas packets onto the flat sections of the turbine blades. The slot extends a full 360 degrees around the turbine wheel allowing gas packets to be acting on the wheel over its full circumference.

As the packets enter the slot, the flat blade area receives a direct push at 90 degrees to the blade surface from the gas packet while it is at its highest blade contact pressure. This is why the blade is flat at this point to recover maximum energy.

After contacting the flat area of the turbine blade, the gas packet continues to expand into the tapered trailing section of the blade towards atmosphere. In this tapered stage as the gases continue to change direction they also continuously exert thrust on the blade (thrust varying as the Sine of the blade angle relative to the slot) until exit from the blades.

Back cutting the turbine is done to increase the size of the trailing orifice and change the discharge angle of gas. This loses some low speed efficiency and increases spool up time but provides gains from increased top end performance due to less restriction at higher gas flows.

The main benefit of back cutting is where the blade discharge orifice is restrictive at higher engine speeds or where the turbo is incorrectly sized and too small for the application. It can help improve performance without having to upsize the turbo.


Does this help or just confuse the issue?

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Booooring
C'mon,... you can do better than that. Bore it up me.
Where's the hate, where's the abuse, the name calling, the hysterics, the idiotic pictures.
Judging from the e-mails i get, it's a source of great humor & i have a bet riding on it, so let 'er rip.

We all get a good laugh from you.

Am I supposed to say BOO here?

Someones pretty bright to work out what that said. I thought it was to well disguised. But I guess I couldnt fool Doogie Howser himself. It might have been boring but even if it said namegdodyrotsaemllet it still wouldnt have mattered. It was sarcasm for here's the only hidden message in what I am writing.

I can't wait for the story to follow this!

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I am the devil you are all going to die one day???
True,...but BOOOOOOOORING

Why don't we wait to see how it works out for 1200RC & save all the rhetoric till then.

L18_B110
Save your breath, your argument will never be allowed to prevail, regardless of merrit or otherwise.
Now 5 will get you 10 that i will have scorn heaped upon me for having the temerity to even comment, just wait & see.

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So now I am saying things in posts without even knowing it. Even if I read through them again and again I still don't see it. But lucky your here to find the subliminal messages in my posts.
Yadenoeidotgniogllaerauoylivedehtmai.

So now you do agree that a turbine harnesses energy from these Gas Packets, plain and simply a turbine is an object in the pathway of the exhaust gases flow. But yet before you couldnt see how a turbine is a "Restriction" in the pathway of this flow and in doing so it steals energy from these Gas Packets, which effectively slows them down.
So why cant you answer this question?Quote:

Now your last parragraph;
So what you are saying is that most factory turbo cars have a turbo which is on the smallish side so that it has 'minimal lag', which in itself ( as stated many other times )ends up reduceing top end performance. And a way to better top end preformance is to "Back cut" the turbine.
Now I don't have a turbo book to read' so I can't exactly say what "Back cutting" is and what it involves and nor could I be bothered looking it up on the net. But I am guessing its a process of reshaping the turbines blades so as to increase the turbines ability to flow an increased quantity of exhaust gases for a given time frame, but still have maximum surface area for the turbine to absorb energy from these exhaust gases.
But like I said I am guessing, I could be wrong.

It's hard to answer your other 5 questions without getting into a discussion on the whole theory of turbine design, which none of us want I'm sure!

But in a nutshell its a compromise between harnessing as much energy as you can from the exhaust gasses while allowing the gasses to flow out as easily as possible. A larger turbine (in a given size housing) will absorb more of the available energy from the gasses, but too large a turbine will restrict the flow of exhaust enough to reduce its breathing efficiency and performance.

The problem of exhaust manifold pressure on turbo cars is mainly found with factory setups. These are typically designed for good low end response, and as a result they are slightly on the restrictive side of the compromise. In cases where people are trying to extract more power from a factory setup, one of the tricks of the trade is backcutting the turbine, making it less restrictive... Explain that one away in your model of how turbos work!

you said it in the quote in my last post (leaving aside the oversimplification of the exhaust stroke - we already have enough problems working this part out!)...You may not have understood that's what you said, but nevertheless...If it didn't have more energy, the pressure in the manifold would actually force gas back into the cylinder when the valve opened instead the exhaust forcing its way out. Like you said - strongest wins. Why not use the strongest force on the turbine?

As for your 5 other questions, I need an extension on that assignment - the 2 fingers I use for typing are sore enough already from this thread...

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Now you have lost me. When did I say that?
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Firstly, the exhaust gasses are formed and travel in pulses. The statements I made and you quoted above are entirely consistent with each other. Your confusion about wether its gas or a pulse is in your own comprehension. Sure, as revs increase the pulses are much closer together (look at my 1st or 2nd post on the subject), and can be simplified to a constant flow, but the gasses are still present in pulses

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Mate, you are almost there! You conceed that the exhaust pulses coming out of the chamber have more energy than what is in the stored pressure in the manifold. And you accept a turbo is driven by the energy in the exhaust gas. So why do you think turbos work best by reducing the amount of available energy in that gas before letting it work on the turbine?

Hey that really was funny!
Its all good