Turbine Housing

[Nigel]

I'm not looking for performance improvements at this point, just some discussion if anybody has some information.

For the turbine housing A/R calculation, the area used is the area right at the tongue entering the volute. Also looking at the housing, it has what I would call a “radial nozzle” (not sure what it is actually called) , it is a passage that runs around the entire circumference except for the tongue. Is this area equal, greater, or less than the area used for the A/R calculation? It is understandable how the A in the A/R impacts gas velocity and the R impacts swirl for a given housing type. These items will affect the velocity impacting the wheel. In turn affecting spool up and turbine flow capability. How is the “radial nozzle” area set. Also, since our typical housings have no guide vanes, the features referenced above seems to be the major factors the housing contributes to the turbo’s performance, are there others?

As the housing limits are approached, at what gas velocity as a ratio to sonic velocity, does the housing start to limit performance?

Anybody know ideally how much of the pressure drop is taken through the housing until it enters the wheel and how much is taken through the wheel on a proportional basis? The wheel is typically a combination of impulse and reaction. A 100% impulse wheel would use only velocity to drive it(very little pressure drop) and a 100% reaction wheel would use pressure drop to drive. In most cases, the single stage centrifugal turbine we use has a combination. In a well matched wheel and housing will the housing choke first?

Since the housing has a cast surface finish, any benefit to having the housing polished? Since you are using wasted energy, and in many cases exhaust is being bypassed, my guess is not too much. But it could possibly provide some if pushing hard.

 

[Mr.Spool]

A lot of variables with your statements.the exhaust housing is a tricky thing because it depends greatly on what the motor can or will do.the rpm window in a turbo car is where your options grow or are limited.

 

[dank GN]

I can tell you that with a .63 AR exsaust housing that the back pressure is crazy high . On a mildly modded motor I would definitely sudgest a .82 or .85 . And if you are going for a race motor or stage motor a 4 bolt is definitely the way to go . I have a giant 281CID and am runing a 3 bolt .85 ar on my 6871 it spools crazy fast but my back pressure is Alil over 1.5 :1 . Hope this helps you

 

[Nigel]

A lot of variables with your statements.the exhaust housing is a tricky thing because it depends greatly on what the motor can or will do.the rpm window in a turbo car is where your options grow or are limited.

Definitely follow, looking to open up some dialog and learn from what others know.

 

[Nigel]

I can tell you that with a .63 AR exsaust housing that the back pressure is crazy high . On a mildly modded motor I would definitely sudgest a .82 or .85 . And if you are going for a race motor or stage motor a 4 bolt is definitely the way to go . I have a giant 281CID and am runing a 3 bolt .85 ar on my 6871 it spools crazy fast but my back pressure is Alil over 1.5 :1 . Hope this helps you

Thanks for the feedback.

 

[Mr.Spool]

Definitely follow, looking to open up some dialog and learn from what others know.

If you run into backpressure the car will not make boost.you can run a lot of power through the 3bolt. understand if you oversize the housing or turbo it will be harder to get into boost and make max boost.this is why most guys slow down when going to larger turbos plus not enough rpm range and compression.converter plays a large roll as well.looking at the diameter of most ex housings its mathematically not impressive (2.5 inch opening)yet many go fast with the right upgrades so it seems the turbine wheel just needs to be fed properly.

 

[Mr.Spool]

I can tell you that with a .63 AR exsaust housing that the back pressure is crazy high . On a mildly modded motor I would definitely sudgest a .82 or .85 . And if you are going for a race motor or stage motor a 4 bolt is definitely the way to go . I have a giant 281CID and am runing a 3 bolt .85 ar on my 6871 it spools crazy fast but my back pressure is Alil over 1.5 :1 . Hope this helps you

i have had .63 housings run faster and make more mph than .85 as long as the rpm doesn't get to high.i like the 85 when the rpm gets up there.

 

[Nigel]

If you run into backpressure the car will not make boost.you can run a lot of power through the 3bolt. understand if you oversize the housing or turbo it will be harder to get into boost and make max boost.this is why most guys slow down when going to larger turbos plus not enough rpm range and compression.converter plays a large roll as well.looking at the diameter of most ex housings its mathematically not impressive (2.5 inch opening)yet many go fast with the right upgrades so it seems the turbine wheel just needs to be fed properly.

Definitely appreciate your experience, you have to match the parts to get good performance. Would be curious to find out what a designer considers and what kind of test equipment is used to validate the designs of turbos and the components. The turbine hosing (like the compressor housing) is a simple looking piece of hardware but has significant impact on how the engine is going to perform. One of the things I was hoping to discover was how the area of the radial nozzle is set and how important is it as compared to the area at the tongue used to set the A/R.

 

[JDEstill]

You asked how much pressure drop there is across the turbine. Hard to say, there's several factors that go into it.

First, understand that the turbine drives the compressor. The amount of hp required to drive the compressor can be determined from the air flow rate and the boost it's making. That might be 50 hp, might be 75 hp, might be 125 hp. Whatever it is, the turbine has to provide that power.

The power extracted from the exhaust gas by the turbine depends on the exhaust gas flow (through the turbine only! Can't count any exhaust going through the wastegate!), exhaust temperature, turbine efficiency, and the *ratio* of turbine inlet pressure to outlet pressure. Note that it isn't a set pressure drop, it is the ratio of inlet to outlet pressure that is important.

If the pressure ratio required is 2.1, and the turbine outlet pressure is 5 psig, then the inlet pressure is (5+14.7) x 2.1 = 41.4 psia - 14.7 = 26.7 psig. So there would be a 21.7 psi drop across the turbine in that case. Now suppose you get rid of the catalytic converter and plugged up mufflers and the turbine outlet pressure drops to 2 psig. Nothing else changes. Now the inlet pressure is (2+14.7) x 2.1 = 35.1 psia = 20.4 psig. The turbine inlet pressure dropped by 6.3 psi, and the pressure drop across the turbine was reduced to 18.4 psi, or 3.3 psi less than before.

See how changing the exhaust system after the turbine has an effect on the pressure in the headers (the pressure that the engine actually sees) and the pressure drop across the turbine? That is why a free flowing exhaust adds power.

When you get to A/R ratios, a smaller A/R turbine will make the same power as a bigger A/R turbine with less flow through it. Suppose the smaller A/R makes the power required by the compressor with 60% of the total exhaust flow, and 40% of the exhaust goes through the wastegate, whereas the bigger A/R housing needs 85% of the exhaust flow to make that same amount of power with the leftover 15% going through the wastegate. The smaller A/R housing should spool quicker, because it can provide the power to the compressor faster with less exhaust. BUT... with less flow rate, it needs a bigger pressure ratio to get that power. A bigger inlet pressure/outlet pressure. Less flow with bigger pressure ratio = same power as more flow with smaller pressure ratio. And you have to supply that same amount of power to the compressor either way, because that is what the compressor needs.

Suppose once everything is all spooled up that the small A/R housing needs a pressure of 2.3, whereas the big A/R housing needs a pressure of just 1.7. If the turbine outlet pressure is 3 psig with either housing, then the turbine inlet pressure (i.e. header pressure) with the small A/R is (3+14.7) x 2.3 = 40.7 psia = 26.0 psig while the turbine inlet pressure with the big A/R is (3+14.7) x 1.7 = 30.1 psia = 15.4 psig. The big A/R in this case results in the headers running 10 psi lower pressure than the small A/R turbine housing. Yeah spoolup might be slower, but the engine is going to make a lot more power with that lower backpressure it is seeing. Less backpressure is a Good Thing, means less exhaust gets left in the cylinders, which means more room for fresh air and fuel, which means more power.

So it's a rob Peter to pay Paul kinda thing - you want max top end power, or you want quick responsive spoolup and low end? Your turbine selection is an important factor in achieving what you want.

One of these days I'm going to get around to logging my turbine inlet pressure... I've always been curious. Some folks here have done it. One metric I have seen used is the ratio of boost pressure to turbine inlet pressure, i.e. if you are making 20 psig boost and have 30 psig turbine inlet pressure then you have a ratio of 1.5. Lower ratio should make more power, and as I pointed out above, you can lower the turbine inlet pressure with better, more free flowing exhaust, and also with turbine wheel and turbine housing changes.

Hope that adds to your confusion! I mean, hope that helps!

John

 

[Nigel]

Thanks for the detailed response.

I understand that the whole thing is a system from the compressor inlet to the turbine exhaust. You make power by increasing mass flow. The operation is not a steady state, but transient. Pumping losses, residual exhaust and cylinder fill are significant issues, I am just zeroing on the turbine housing. Mainly curious.

Some of my thoughts follow.

If you push the flow capabilities of the engine, and the turbine/turbine housing becomes the limiting feature, I would like to simply start understanding what part(with regard to the turbine/housing) is limiting and under what conditions. The turbine can only flow so much volume before either A. flow becomes choked, or B. the relationship between the wheel speed and the gas velocity vector won’t produce any more appreciable shaft power. If A is the problem, and choked or near choked flow is encountered, more flow area is needed. If B is the problem, then I would guess you may have a mismatch of compressor wheel to turbine wheel. I know there is more to it, but just trying to look at the basics.

If you end up choking the housing before the wheel becomes limiting, than a housing change should improve your top end performance.

If you have a small wheel, and the wheel is the problem, then going to a larger housing won’t help, especially if you are choking the exit.

Once an area becomes choked, velocity will be sonic or close to it, and reducing downstream pressure will not change flow(bigger post turbine exhaust will be irrelevant). The only thing you can do is increase upstream pressure to push more flow, but you will not get any performance gain, power will fall off. Mainly because you hit a wall on velocity. Easy to picture if choking upstream of the wheel, not sure how the impact would be if it was the wheel exit that was choked(if that is even possible given the design and geometry). You have a much larger exit area than inlet area, but you also have a much larger specific volume of the exhaust gas at the outlet.

Having the ability to read wheel speed, inlet pressure, pressure in the volute, and pressure at the wheel exit would be very telling, along with temperature before and after the turbo. Having the money and time to change and test various combinations would obviously be telling too. I’ll have to stick to speculation at this point, unless somebody else has some data to share.

I know there is a lot of experience here, especially the work that Bison has done, determining performance ranges for various turbos. I am not trying to figure out anything new with regard to performance, just what makes the housing do what it does.

 

[Mr.Spool]

Thanks for the detailed response.

I understand that the whole thing is a system from the compressor inlet to the turbine exhaust. You make power by increasing mass flow. The operation is not a steady state, but transient. Pumping losses, residual exhaust and cylinder fill are significant issues, I am just zeroing on the turbine housing. Mainly curious.

Some of my thoughts follow.

If you push the flow capabilities of the engine, and the turbine/turbine housing becomes the limiting feature, I would like to simply start understanding what part(with regard to the turbine/housing) is limiting and under what conditions. The turbine can only flow so much volume before either A. flow becomes choked, or B. the relationship between the wheel speed and the gas velocity vector won’t produce any more appreciable shaft power. If A is the problem, and choked or near choked flow is encountered, more flow area is needed. If B is the problem, then I would guess you may have a mismatch of compressor wheel to turbine wheel. I know there is more to it, but just trying to look at the basics.

If you end up choking the housing before the wheel becomes limiting, than a housing change should improve your top end performance.

If you have a small wheel, and the wheel is the problem, then going to a larger housing won’t help, especially if you are choking the exit.

Once an area becomes choked, velocity will be sonic or close to it, and reducing downstream pressure will not change flow(bigger post turbine exhaust will be irrelevant). The only thing you can do is increase upstream pressure to push more flow, but you will not get any performance gain, power will fall off. Mainly because you hit a wall on velocity. Easy to picture if choking upstream of the wheel, not sure how the impact would be if it was the wheel exit that was choked(if that is even possible given the design and geometry). You have a much larger exit area than inlet area, but you also have a much larger specific volume of the exhaust gas at the outlet.

Having the ability to read wheel speed, inlet pressure, pressure in the volute, and pressure at the wheel exit would be very telling, along with temperature before and after the turbo. Having the money and time to change and test various combinations would obviously be telling too. I’ll have to stick to speculation at this point, unless somebody else has some data to share.

I know there is a lot of experience here, especially the work that Bison has done, determining performance ranges for various turbos. I am not trying to figure out anything new with regard to performance, just what makes the housing do what it does.

The velocity and volume of the ex gas will be the limiting factor on the housing as the size can only handle a specific amount.overspeeding a comp wheel is really tough on the turbo and shortens turbo life.losing turbo speed by oversizing makes for a real turtle of a ride and I will overspeed a turbo any day of the week to avoid that.

 

[ATL_87GN]

i have had .63 housings run faster and make more mph than .85 as long as the rpm doesn't get to high.i like the 85 when the rpm gets up there.

At what 1/8 and 1/4 mile times/mph did you experience this? What was the rpm shifts and through the traps at also?

 

[Mr.Spool]

At what 1/8 and 1/4 mile times/mph did you experience this? What was the rpm shifts and through the traps at also?

On that particular combo,137+mph @3800lbs shift rpm was below 6000.

 

[ATL_87GN]

On that particular combo,137+mph @3800lbs shift rpm was below 6000.

Wow, so a precision .63 will take you pretty far. I’ve got a 8.8:1cr 249ci stroker with ported irons and a 215/220 cam. I shift between 6000 and 6300. Is the .85 too big on my application with the current 6766cea ball bearing? (PTC 9.5” 17 blade/3.42 gears) It’ll flash to 5100-5200 off the line and the shifts drop from 5800-6100 to 4900 and seems to unlock. I have the springs set to 160lbs seat and 420lbs open.

 

[Mr.Spool]

Wow, so a precision .63 will take you pretty far. I’ve got a 8.8:1cr 249ci stroker with ported irons and a 215/220 cam. I shift between 6000 and 6300. Is the .85 too big on my application with the current 6766cea ball bearing? (PTC 9.5” 17 blade/3.42 gears) It’ll flash to 5100-5200 off the line and the shifts drop from 5800-6100 to 4900 and seems to unlock. I have the springs set to 160lbs seat and 420lbs open.

How much boost can you make and is the car responsive?

 

[dank GN]

I added a 60psi transducer to my headers . I will have my power logger record it . My transducer went bad so I’m in the process of ordering a new one . I’m currently runing a 7168 on my stage motor. It spools crazy fast with my A/R being a .85 3 bolt . I want to capture the back pressure data with this exhaust housing before I switch to a 4 bolt .96 A/R turbine housing. Here is a pic of where I mounted the pressure sensor

 

[ATL_87GN]

How much boost can you make and is the car responsive?

It’s pretty responsive, but rolling I can go from 3-30psi in about 1.1sec. It’s usually around a 10.5@128 on 23-25psi. (10.5afr/17*) I’ve leaned it out to 11.0afr and bumped it to 19* with 30psi, but the tracks are closed now to test.

 

[Mr.Spool]

I want to capture the back pressure data with this exhaust housing before I switch to a 4 bolt .96 A/R turbine housing.

Red a made 900+rwhp with his stage motor and had a 1.5.1 bp with a 3 bolt .85 ex housing.nice motor by the way.like to see your results with your combo and when you 4 bolt it

 

[Mr.Spool]

but rolling I can go from 3-30psi in about 1.1sec.

That can be cut way down.

 

[dank GN]

Red a made 900+rwhp with his stage motor and had a 1.5.1 bp with a 3 bolt .85 ex housing.nice motor by the way.like to see your results with your combo and when you 4 bolt it

thank you yes my turbo is on the small side I would like to up it to a 7275