Exhaust Turbine Clipping

[Enigma_Man]

Originally posted by 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..

The word is YOU'RE. Learn to spell. And lay off the caps.

-Jesse

 

[02-sir]

I don't want to step into an argument here, but I thought I may be able to explain the way I see it.

Jesse, I understand your argument, but the main purpose of clipping is not to increase the overall flow capacity of the exhaust system. The main purpose is to increase mass flow rate through the turbine for a given exhaust pressure. Think of this scenario:

You want to produce 20 psi boost all the way up to 7000 rpm. 20psi at "x" CFM requires "y" horsepower to be generated by the turbine and transferred to the compressor wheel (lets ignore all efficiencies for now). In our example, lets say that this horsepower requirement is constant. The horsepower that the turbine generates is determined by the following equation:

Theoretical hp generated by turbine = (h1 - h2) x mass flow rate

"h" (Enthalpy) is simply the energy per unit mass of the exhaust gas, and is pretty much a function of temperature and pressure. Gasses at higher temperatures and pressures inherently have more energy. h1 is the enthalpy just before the turbine, and h2 is the enthalpy just after.

Now, if you believe my argument that clipping the wheel increases the mass flow rate for a given exhaust pressure, you can see from the equation that an increased mass flow rate can generate the same turbine shaft power with a smaller h1. This means less pressure is required in the exhaust manifold to generate your 20psi, resulting in more engine power.

The only problem here is turbine efficiency. Turbine efficiency is defined by:

actual turbine hp generated / theoretical turbine hp generated (equation above)

The efficiency of the turbine wheel is similar to that of the compressor wheel, with islands of efficiencies at various operating conditions. I assume clipping a turbine will generally decrease it's maximum efficiency, but more it importantly it will shift the efficiency "islands" up on the scale. This will surely kill the efficiency at low flows, but if it's done right, may actually improve the efficiency at higher exhaust flows. Even if it doesn't increase the efficiency at higher flows, the benefit of the increased mass flow may outweigh the decrease in turbine efficiency. The real problem is that there is no way to determine how clipping the turbine wheel will effect performance without testing it.

I hope this helps. I should mention that I have no real life data to support this explanation. This is simply the conclusion I have come to after analyzing the problem from a scientific point of view.

cheers

 

[hoffman]

or you can only flow so much gas through the turbine area before the wheel just tops out. it just cant get any faster. so by removing some of the material off the turbine you delay this allowing it to continue spin to a slightly higher rpm than it could unclipped.

it may make it slightly more laggy, but it will allow it (if properly balanced) to hold its max potential (or close to it) longer than it normally could.

hope that helps alittle.

 

[DCJ98GST]

I know that clipping is sometimes beneficial or nobody would do it. But I still dont understand something:

Say you have two turbos that are exactly the same but one has a bigger turbine that flows say 25% more than the smaller at the same rpm. So during spool up, the bigger turbine will have an easier time expelling the exaust energy because the only path the exhaust energy has to take is through the exhaust turbine wheel (wg is closed).

But once the turbo is spooled the exaust energy has two paths to take (turbine and wg). Therefore any extra exaust that can not go through the restrictive turbine will go through the wg. Right!

If it cant go through the wg, the turbine would speed up and you would get more boost, thus opening the gate more or boost creep. For arguments sake, lets say that you have a wg that is sufficient enough to overcome boost creep, and therefore re-route the exaust energy around the restrive turbine.

So now you have one turbine that flow more and at full spool say 85% goes through the turbine and 15% goes through the WG. The other turbo has 75% go through the turbine (more restricive) and 25% goes through the wg.

Both of these conditions have relieved the exhaust manifold pressure enough to control boost. So why would one have a higher Volumetric Efficiency (VE) than the other? As long as the gate is properly mounted that it flows enough to relieve the pressure the VE should stay high.

 

[Enigma_Man]

Originally posted by hoffman
or you can only flow so much gas through the turbine area before the wheel just tops out. it just cant get any faster. so by removing some of the material off the turbine you delay this allowing it to continue spin to a slightly higher rpm than it could unclipped.

it may make it slightly more laggy, but it will allow it (if properly balanced) to hold its max potential (or close to it) longer than it normally could.

hope that helps alittle.

You're forgetting something, we don't ever make the turbo spin as fast as it possibly can. We want it to spin at a certain rate, hence the Wastegate. The rate of spin is (mostly) determined by how much boost we want to run. So, making the exhaust turbine spin faster than it already can isn't good at all, because we already use the wastegate to prevent that from happening.

O2-sir. Thank you for the well thought-out explanation, but I'm willing to bet money that clipping the wheel, while it may increase some things beneficially, decreases the efficiency of the turbine enough to cancel out any other benefits. Imagine where you clip 100% of the wheel away, you've just increased the flow capability through the turbine 1000 fold, but look at what you just did. Flow through the turbine isn't that important, because the turbine only has to spin at a certain speed anyway, and then once we reach that speed, the wastegate opens, and any flow issues we have go out the drain.

-Jesse

 

[Enigma_Man]

Originally posted by DCJ98GST
I know that clipping is sometimes beneficial or nobody would do it.

Go to the nearest car-stuff store, and look at all the snake oils, and intake "vornados" and other useless "performance" crap they sell there.

Then tell me again that your reasoning for it being beneficial is because other people do it.

Just because people do it, doesn't mean it's good. There are plenty of oil-additives out there that are completely worthless, and yet they sell enough to at least keep in business. The shops that do it just do it because the customers ask for it, it's an easy job to do, and they stand to make a few extra bucks out of it. It's not a bad thing, it's just how supply/demand works. Clipping the turbines is the same way.

The turbo manufacturers never clip the turbines (that I've heard of, don't confuse that with shops that assemble turbos please), and they probably know a hell of a lot more about all this than any of us do. It's just like the oil manufacturers say that additives to their oil isn't reccomended, because they've already got the oil the way it should be.

 

[jayntguru]

Originally posted by Enigma_Man


Go to the nearest car-stuff store, and look at all the snake oils, and intake "vornados" and other useless "performance" crap they sell there.

Go and look at www.dsmtimes.org

Look at the turbo section, then search for BR20G.

The results should be obvious.

 

[02-sir]

Originally posted by DCJ98GST
I know that clipping is sometimes beneficial or nobody would do it. But I still dont understand something:

Say you have two turbos that are exactly the same but one has a bigger turbine that flows say 25% more than the smaller at the same rpm. So during spool up, the bigger turbine will have an easier time expelling the exaust energy because the only path the exhaust energy has to take is through the exhaust turbine wheel (wg is closed).

But once the turbo is spooled the exaust energy has two paths to take (turbine and wg). Therefore any extra exaust that can not go through the restrictive turbine will go through the wg. Right!

If it cant go through the wg, the turbine would speed up and you would get more boost, thus opening the gate more or boost creep. For arguments sake, lets say that you have a wg that is sufficient enough to overcome boost creep, and therefore re-route the exaust energy around the restrive turbine.

So now you have one turbine that flow more and at full spool say 85% goes through the turbine and 15% goes through the WG. The other turbo has 75% go through the turbine (more restricive) and 25% goes through the wg.

Both of these conditions have relieved the exhaust manifold pressure enough to control boost. So why would one have a higher Volumetric Efficiency (VE) than the other? As long as the gate is properly mounted that it flows enough to relieve the pressure the VE should stay high.

I think this is exactly the same scenario as with the clipped turbine (efficiency issues aside). The power that both of these turbines need to produce is the same, and since the larger turbine has a higher mass flow rate, it can produce this power with less exhaust manifold pressure (resulting in more engine power) For the car with the smaller turbine, the increased exhaust manifold pressure is not a result of the turbo being more restrictive, the waste gate is purposefully holding the manifold pressure higher in order to produce the desired boost. In other words, if you opened the waste gate further to reduce the manifold pressure to the same level as the large turbine car, it would no longer be able to generate the desired boost.

Originally posted by Enigma_Man
Imagine where you clip 100% of the wheel away, you've just increased the flow capability through the turbine 1000 fold, but look at what you just did.



The turbo manufacturers never clip the turbines (that I've heard of, don't confuse that with shops that assemble turbos please), and they probably know a hell of a lot more about all this than any of us do.

Agreed, but looking at the extremes is not fair. If you put an enormous 12" dia turbine housing on your TD05, the result will be the same. While manufactures don't clip turbines, they do offer various A/R housings to achieve the same result (without sacrificing turbine efficiency). If it was easy to make a custom turbine housing with larger A/R ratios, than speed shops would probably do that instead of clipping the wheel.

 

[ItsStockOfficer]

until you understand that turbine effciency is transient and has nothing to do with the percentage of air needed at full spool to keep the turbo spinning you will not understand it. Clipping cam make a turbo require 1% more air and be 15% less restrictive.

Clipping is not a snake oil. Clipping a tdo5 20g increase it maximum power protential from the 400 hp to the 500 hp range.

learn to understand turbine effciency before you try to understand clipping.

 

[DCJ98GST]

Originally posted by 02-sir


The power that both of these turbines need to produce is the same, and since the larger turbine has a higher mass flow rate, it can produce this power with less exhaust manifold pressure (resulting in more engine power) For the car with the smaller turbine, the increased exhaust manifold pressure is not a result of the turbo being more restrictive, the waste gate is purposefully holding the manifold pressure higher in order to produce the desired boost. In other words, if you opened the waste gate further to reduce the manifold pressure to the same level as the large turbine car, it would no longer be able to generate the desired boost.

Good point, I get it now

Thanks

 

[02-sir]

Originally posted by DCJ98GST


Good point, I get it now

Thanks

you're welcome

 

[Enigma_Man]

Originally posted by 02-sir
Agreed, but looking at the extremes is not fair. If you put an enormous 12" dia turbine housing on your TD05, the result will be the same. While manufactures don't clip turbines, they do offer various A/R housings to achieve the same result (without sacrificing turbine efficiency). If it was easy to make a custom turbine housing with larger A/R ratios, than speed shops would probably do that instead of clipping the wheel.

This is the best explanation so far, thanks

So, clipping is not the best way to do it, but a good-enough second best to changing the turbine housings.

And ItsStock, no I don't understand the efficiency stuff, which is why I was asking, and explaining things from my layman pov.

-Jesse

 

[ItsStockOfficer]

depends, put a garret and mitsu turbine next to each other and you'll notice that the garrest is cast with a bit more factory "clip". Mitsu turbine respons to clipping very very well.

 

[JayHass]

This is directly from a real world Garrett engineer who is on the SE-R mailing list. Rob is a lead engineer with a M Engineering. He is the smart of the smart over there.

>What the hell does it mean when a turbo is 'clipped'?

Basically, you just cut back the blades at the outlet of the turbine. The
angle at which you perform this cut is called the degree of clipping.
Clipping 10 degrees, for example, means you cut the turbine blades at the
turbine outlet back at a 10 degree angle.

Clipping is done to increase the flow through the turbine. It is generally
bad for turbine performance (from an efficiency standpoint). You're better
off to go to a larger trim turbine or a different A/R housing, but often in
the aftermarket, you've got to work with what you've got, so you clip.

Rob

 

[ItsStockOfficer]

Im sure we were all aware of what clipping is, just not why it worked. what i qouted was from a garret engineer,

 

[02-sir]

Originally posted by Enigma_Man



So, clipping is not the best way to do it, but a good-enough second best to changing the turbine housings.

I think so. Besides describing what clipping is, I think Rob's email also suggests the same.

 

[Batty200]

So this is why a 10cm Super 20G can flow more than a regular 20G. I have one and I also had good spool characteristics. I also bet the "more wastegate" theory has some relevance since the JUN Supra used 2 HKS GT wastegates and made more power (1300 or so claimed hp) than any other T88 application I have ever seen.


Later

 

[JayHass]

Originally posted by ItsStockOfficer
Im sure we were all aware of what clipping is, just not why it worked. what i qouted was from a garret engineer,

Really? What's his name. I'll see if Rob knows who he is.

Besides describing what clipping is, I think Rob's email also suggests the same.

Exactly. Thank you.

 

[kutschca]

I thought JayHass' response was very pertinent and cleared up many questions. Basically clipping is an alternative to changing the a/r of the turbine housing when new housings are not available (or at least not for a reasonable price)

thanks for the response JayHass, and not to thread whore, but those wheels are freakin HOT! Are those RS Limited Akunin's?

 

[JayHass]

Originally posted by kutschca
Are those RS Limited Akunin's?

Thanks man...I have no idea what they are, I bought them for $500 with tires in almost new condition. One mans loss is another mans gain.

They do have RS on the centercap though. Maybe you just told me what they are.

 

[GrocMax]

Originally posted by Enigma_Man



The turbo manufacturers never clip the turbines (that I've heard of, don't confuse that with shops that assemble turbos please), and they probably know a hell of a lot more about all this than any of us do.

Many Garrett (and other) turbine wheels are as-cast with a clip right from the factory.

 

[pneumo]

How does a bigger turbine housing flow more than a smaller housing given that the exhaust wheel is the same size? I know the inlet and outlet are bigger, but the hole the turbine fits into has to be the same size. Where does the extra flow come from? Or are you saying a bigger housing AND a bigger turbine wheel flows better than a small clipped wheel?

 

[JayHass]

Originally posted by pneumo
How does a bigger turbine housing flow more than a smaller housing given that the exhaust wheel is the same size? I know the inlet and outlet are bigger, but the hole the turbine fits into has to be the same size. Where does the extra flow come from? Or are you saying a bigger housing AND a bigger turbine wheel flows better than a small clipped wheel?

You ever actually look inside of a turbine housing at the scroll outlet?

They are inter-related. Changing either one or both affects how the turbo flows.

 

[pneumo]

My experience is limited to comparing a Mitsu 6 and 7 cm housings, and a clipped turbine compared to a stock turbine. From what I'm looking at, these turbos have the housing as close as possible to the blades on the turbine. Do you know the part I'm trying to describe? It's not the scroll, not the outlet, it's just the part between the inducer and the exducer. The part that makes every bit of exhaust go through the turbine wheel. The hole the turbine fits into has to be the same size, right? Sure, the housing is bigger everywhere else, but how does that help the exhaust get through the turbine wheel? I'm aware of the benefits of a higher pressure differential before and after the turbine, but as a post above stated, a turbine can only flow so much before it chokes off. Once that plateau is reached, then what?

From reading the relevant posts I come to the conclusion that a clipped Mitsu wheel flows better than an unclipped Mitsu wheel, and that a bigger wheel would flow better, too, but with better efficiency.

So which is better, HRC 20G or BR 20G? big housing vs. clipped wheel?

 

[GRNDSM]

I think that you guys need to look into why clipping was so popular in DSM world. For years we had one basic center section: TD05, with 2 popular compressor sides upgrades: 16G and 20G. There was only one reasonable exhaust housing option, 7cm^2.

There were very few other things that you could do with a turbo other than clipping the exhaust wheel. That why just about everyone had a clipped turbo.

8cm^2 housing, TD06 exhaust wheels and Garret turbos did not come until much later (late 90’s). Now that we have all of those options, clipping the turbo does not make much sense, unless you are trying to duplicate a very well established combination (like a “20G”).

Personally, I think that TD05-16G with 7cm^2 housing is “just right” the way it is (no clipping). This comes from my own personal experience after racing with 16G for several years (first unclipped and then clipped). 20G has proven to work better with 10-20deg clip. Aside from that, I would not spend much time worrying about this subject… Does anyone want to talk about rebuilding carburetors while we are at it?

Leon
RR