Exhaust Turbine Clipping

[Enigma_Man]

Every way I've thought about clipping the exhaust turbine sounds like a bad idea to me.

All it serves to do is reduce the efficiency of the exhaust turbine.

People tell me it improves "top end flow" But why would it? If the flow through the turbine and wastegate is restricted in some way, needing higher top-end-flow, the boost would rise (boost creep). So, clipping the exhaust turbine only serves to divert some of the overall exhaust flow from the wastegate to the less-restrictive exhaust turbine.

But the exhaust turbine is supposed to be restrictive. It works off of the heat differential, and being-in-the-way of the exhaust flow to spin the turbine. Providing an escape for extra flow is the job of the wastegate.

I think this is deserving of the Extreme Tuners Forum, if only to be more likely to get any conclusive evidence to the contrary (other than that suppliers offer it as an option, which isn't evidence, it's just supply and demand, consumers want it, so suppliers supply it reguardless of if it's a good modification)

-Jesse

 

[GalantVR41062]

Back pressure is the demise of HP in turbo cars. With a small restrictive turbine that will cause back pressure(to much in most cases), the turbo will spool at low rpm and fall dead up top. The waist gate is there to provide exaust gas deversion from the turbine to slow the turbo down when it has provided the desired boost level. But clipping the turbine in my opinion would then make the turbine shorter, thus redusing back pressure with its horribly small A/R of .48([email protected] for more info on A/R) but I would think this shorter turbine will allow more exaust gas flow with less back pressure. Unlees I am misunderstanding what goes on when you clip a turbine?

~John
bigger A/R more power.

 

[Enigma_Man]

Backpressure is what makes the turbine spin. More backpressure = more spinning. And when the turbine reaches its peak set RPM (peak boost level on the intake side) the wastegate opens up, and any flow problems due to a restrictive turbine are eliminated.

-Jesse

 

[GalantVR41062]

yes but faster spool up due to lots of back pressure means no power what so ever. So if you like full boost by 2,800rpm and no top end flow or hp for that mater then why are you asking about clipping in the first place. I said it would in my mind reduce back pressure increase turbo lag but over all create less back pressure there for increasing flow which is why you get it done, to increase top end power. When the waist gate is open you are waisting exaust gas and enegry that could be used to make power. Or look in the book by Corky Bell, and reserch VATN turbos which spool up twice as fast as "normal" trubos but have constantly changing A/Rs to then slow the turbo down(really big A/R) to provide all the exaust gas flow posible to drive the turbo with the ideal A/R, no compromises. And the waist gate opens befor max boost is acheived "if you were to set your boost at 10psi the waist gate would start opening at 6psi, there for waisting exaust energy to drive the turbo." Ither Aerochargers or Garrett have the variable A/R turbos and for sure cost way to much for the average dsmer.

hope this helps.

 

[DCJ98GST]

Does anybody have links to turbo's with variable A/R's. I have read about them too in Corky's book. I know that they are pretty new and probably pretty expensive. It does sound like something that would revolutionize turbo's.

 

[autronicDSM]

Here's a link for variable a/r turbos http://www.mjmturbos.com/VNT-15.htm

 

[Enigma_Man]

Originally posted by GalantVR41062
waist waisting waist waisting

hope this helps.

Waste!

And, to counterpoint, the high backpressure you seem worried about is dissipated as soon as the wastegate opens, so when the turbo is spooled, and the wastegate is open, you have your high flow that is accomplished by means other than clipping the exhaust turbine.

Sounds like the better solution to me.

-Jesse

 

[draculia]

i've never bought into the clipping idea, it always seemed like a hack idea to me. i remember an article i read about turbine efficiency, and how clipping did little for flow but did a lot for decreasing efficiency.

 

[GalantVR41062]

So having exaust gas flow threw the waist gate makes power? what ever man. I tried telling you why it is sold, if you want power get a proper sized turbo for the aplication.

 

[Enigma_Man]

I'm saying that any restriction in flow is eliminated when the wastegate opens, and now it's a big open system, with all the flow you want. So, until the wastegate opens, yes, it's a little restrictive, but that's good, because it spools the turbo up quickly. And when the whole thing gets going at a good clip, and the wastegate opens, there's all the flow you could ever need... So clipping the exhaust turbine is bad, because all it does is delay the spoolup and nothing else.

-Jesse

 

[GalantVR41062]

So you make power when your exaust gases run threw the waist gate? Explane how...

 

[DrZiplok]

Originally posted by GalantVR41062
So you make power when your exaust gases run threw the waist gate? Explane how...

Mate, you are really confused.

The argument behind clipping is that it "improves flow".

Jesse's point is that opening the wastegate "improves flow" as well.

In either case, the "improved flow" is achieved by not having the turbine wheel interfere with the flow of exhaust gas.

I have to agree that, for a given turbo, I would rather have maximum turbine efficiency up to the point where the turbo is maxed out and then open a wastegate, rather than reduce my turbine efficiency so that I don't need such a large gate.

So I guess the original question stands; does clipping a turbine wheel achieve anything other than the bypassing of some of the exhaust gas that would otherwise be driving it? For all the rhetoric about ripping off customers, things like this usually have a foundation in something that's worked at some point - I'd certainly like to know what it was.

 

[GalantVR41062]

I should have specified reducing back pressure threw the turbine. I was just stating what I thought "clipping" the turbine would do, I have never clipped a turbo I have never seen a clipped turbo, I dont have dyno numbers to prove otherwise. But there is no way to say reducing back pressure thew the turbine (exaust side of the turbo) would not make more power. Look into VATN turbos and there constant changing nozzle area, starts small for spool up then gets bigger to the point where the turbo stays at a constant pressure ratio(its own boost controll with out waisting exaust gas energy), and the exaust has the least amount of back pressure for the given pressure ratio, thus maximizing hp.

 

[DCJ98GST]

This is a good question.

The theory is, why cant we just dump all our unwanted exhaust energy out of a huge wastegate and keep our tiny turbine and tiny A/R exhuast housing. No need to clip. Sounds good right.

Maybe one of the Wisemen would chime in and help out.

 

[Enigma_Man]

Originally posted by DrZiplok

The argument behind clipping is that it "improves flow".

Jesse's point is that opening the wastegate "improves flow" as well.

Again, thank you for clarifying the words

I was also hoping that someone in-the-know would chime in with the info, rather than most of us here just conjecturing about it.

-Jesse

 

[jayntguru]

Originally posted by Enigma_Man
Backpressure is what makes the turbine spin.

Positively incorrect.

The heat of the expanding exhaust gasses causes a pressure differential across the turbine that helps to suck the air through. Backpressure is the resistance to airflow, which is exactly the opposite of what you want.

 

[Enigma_Man]

Originally posted by jayntguru


Positively incorrect.

The heat of the expanding exhaust gasses causes a pressure differential across the turbine that helps to suck the air through. Backpressure is the resistance to airflow, which is exactly the opposite of what you want.

What I meant by "backpressure" is the pressure in the manifold, commonly referred to as backpressure. The pressure in the manifold creates a pressure/heat differential across the turbine. Yes, correct.

-Jesse

 

[02-sir]

just speculation here, but if you look at some turbo specs (Garrett comes to mind) they also show a turbine map (mass flow thru the turbine vs. pressure ratio across the turbine). All of the maps show that the mass flow rate eventually levels off to a maximum value even as the pressure ratio continues to climb. Clipping the turbine wheel may increase the total mass flow rate of exhaust gasses thru the turbine for a given pressure ratio, at the expense of turbine size/efficiency. Having said that, I'm not a big fan of clipping either because it's too easy to do more harm than good.

yaaayyy, my first post

 

[ItsStockOfficer]

effciency is not directly related to how much of your exhaust ir is nessecary to keep a turbo spinning, on any given system, only 25-40% of air goes through the wastege at full throttle at 7k rpm, and thats alot.

Clipping a turbine may increase flow 10% and take 2% more air to keep spooled.

Mitsu turbos respond very very well to clipping. My old tdo6 20g trapped 119 with ease and got full boost at 3500 rpm.

 

[Enigma_Man]

But do you have a comparison trap/et before it was clipped?

You say that only a small-ish amount of the exhaust goes through the wastegate. That still doesn't answer my question though, because if you have say... 25% of your exhaust at WOT going through the wastegate, if you clip the exhaust turbine, you might now have for example only 20% of the exhaust going through the wastegate. So, you haven't increased the total flow of the system at all, you have just diverted some through the exhaust turbine instead of the wastegate, and you have decreased the efficiency of the exhaust turbine wheel. If you had some insane percentage of exhaust flow going through the wastegate, like 80%, then I could see finding a way to make more flow through the exhaust turbine, but you would do that by finding a better matched turbo for the application.

I can imagine a situation where the exhaust turbine wheel is so inefficient to begin with (say a siezed turbine, or a horribly mis-matched turbo for the application, like a diesel tractor turbo on a 2.0) where clipping the turbine would allow for more exhaust flow through the turbine instead of the wastegate, but that is ridiculous of course.

Do you all see what I'm trying to say? The system isn't restricted to begin with, and making the exhaust turbine less restrictive through clipping only serves to divert some of the exhaust energy through the now less-efficient turbine instead of the wastegate.

-Jesse

 

[ItsStockOfficer]

No, we see what your saying, its just not true, because the turbine effciency has very little to do with how much actual air it takes to keep it spooled at full throttle.

We had a tdo6 20g car in socal running 116 mph, clipped it 15 degrees and went 121, with a few other change, c;lipping significantly increase topend

 

[Enigma_Man]

With a few other changes like what?
Show us all some real evidence.

Again, clipping is just going to take some of the participation by the wastegate out of the picture, you don't increase your total amount of exhaust flowed at all.

-Jesse

 

[ItsStockOfficer]

No, your being thick headed because you are not comprehending the fact that TURBINE EFFCIENCY HAS LITTLE TO DO WITH TOTAL AIR VOLUME NEEDED TO KEEP THE TURBO SPOOLED AT FULL BOOST..

But anyways - the reason you get more power from a clipped wheel is
that you get less back pressure on the engine and therefore, higher
volumetric efficiency. Like Scot said, most of the exhaust gas still
goes through the turbine even when the wastegate is wide open.
Generally, you assume somewhere between 25% to 40% max is going thru the wastegate. If you need to size the waste gate for more than 40%, that is a piss poor turbo design.

Turbine efficiency is basically a measurement of how a turbo spools.
So its a transient type of thing. Once its spooled, high turbine
efficiency doesn't buy you much. Clipping the turbine just lets it
flow more, meaning less back pressure. You just need to keep your
turbine big enough (and efficient enough) to provide enough power to
spin your compressor while the compressor is flowing say 40 lb/min at
20 psi (to make 400 flywheel horsepower), which is in the
neighborhood of 40 shaft horsepower (in the turbo). Put on a smaller
turbine, and you end up with a T25 that can't keep the boost at 20
psi all the way to redline.

Jeff

 

[ItsStockOfficer]

Neways, the BR20G is tdo5 cloipped ~ 23 degrees.

My friend had one of the few 20G's with a unclipped TDO5 whels. The car felt disgustingly restrictive on top end. zIt made very poor power for a 20g. Its was just like a very big 16g.

Yur trying to argue with 30 years of proven turbo engineering. Instead you should be asking, what don't I understand.

Not telling us we've all had it wrong.

 

[pneumo]

I get the impression that with high manifold pressure the clipped turbine can act more like a pinwheel blowing in the breeze, which is less restrictive than the convoluted fin shape that comes from trying to make the turbine wheel more efficient at transient pressures. Is that the theory behind it? Clipping trims off some of the convoluted part?