kenton

I have noticed that the guys on here are some of the best to ask questions about turbos.

I am confused about something that is probably very simple. People generaly use a larger turbine housing so that there is less exhaust back pressure and you get a better VE, which means more power.

What I dont understand is why cant you have a very small turbine housing with a very large wastegate? It seems that if you were having any more back pressure before the turbo on one setup, it would cause a boost spike. Am I correct or way off?

My question in a simplified form is this - Why cant you run a very small turbine housing for quick spool and a very large wastegate and get the same VE as you would on a larger turbine housing?

Kenton

fdracer

u can't run as much boost on a small turbine housing before getting tons of back pressure. u can just use a smaller turbo altogether if you want quick spool and less hp. your question doesn't make any sense.

kenton

Why would you get more backpressure if you have a large enough wastegate to bleed it off?

I think my question makes plenty of sense. A lot of people will use a turbine housing that is larger to get less backpressure and increased power but at the expense of spool times.

Why not get a small housing with a large wastegate? Or two wastegates? That way you have less backpressure?

fdracer

u bleed off boost when u open the wg. u can bleed off as much exhaust out the wg as you want, but that just means you're boosting less. hence, like i said before u can't run as much boost w/ a smaller exhaust housing.

Marcel Burkett

The turbines a/r ratio determines , to a greater extent your engines POWER BAND. If you want to reduce back pressure ,it would be more effective to utalize a larger turbine wheel.

kenton

I'm not looking for just a statement like "this is the way it is". I am asking for an explanation.

You say you cant run a smaller housing with the same about of boost without getting higher back pressure. Assuming that both turbo's have the same compressor and just different turbine housings, why would the one with the smaller turbing housing have more backpressure the the bigger turbine housing if the wastegate was able to control the boost.

Does the smaller housing require more exhaust power to turn the compressor that the larger one making it require more backpressure to spin the turbo at the same speed? It seems that the smaller housing would spin faster with the same amount of exhaust back pressure. Is that wrong?

Wargasm

Here's the easiest way to think about it.

If you run a small A/R and a huge wastegate, it will spool up very fast, but since you're bypassing most of your air through that huge wastegate and it is not going through your turbine... the maximum amount of work the compressor side can do to squash air in there is reduced.

In oder to flow a LOT of air on the compressor side, you need to harness a LOT of power on the exhaust side.... the super small A/R with a large wastegate doesn't let you harness a lot of power - it just bleeds it out through the wastegate.

kenton

So are you saying that the small turbine housing just doesn't generate the power to spin the compressor at the right speed without more backpressure that a larger housing and turbine wheel?

Wargasm

you could definitely make a turbine/wastegate combo that had a tiny A/R with a giant wastegate... but I'm saying that you'd be making almost NO power to spin the compressor, cuz it's all going out the wastegate instead of passing through the turbine.

SPOautos

Exhaust is energy....the less of it you have going thru the turbine housing the less energy you have to spin the turbo.

teeter

humm...I'm not sure I buy this.

Thinking out loud. the exhaust wheels are the same. Its just the free space around the wheel where more exhaust gasses can flow by and you'll have more mass flowing by the wheel...but its always going to spin the same speed because the intake side is where the intake air is being compressed...but that speed will be rpm/air flow dependent...

Hum...

Thinking more out loud...So...you're saying that at a low rpm, the turbo will spin up to high rpms fast to the required set boost PSI and then vent off the rest of the exhaust gases through the wastegate... but as the engine RPM rises and thus total engine air flow and turbo speed RPM...increases (even with constant boost PSI)...it will take more energy to spin the turbo to compress the ever increasing need for air flow the engine is requiring...so more and more exhaust gasses will have to be diverted to the exhaust turbine to produce the necessary torque to keep spinning the turbo wheel at an ever increasing rpm. And if you had too little of an exhaust turbine AR...you're going to have to cram the same amount of exhaust gasses through there to get the required torque to spin the turbo at the required speed....

So with a small exhaust turbine and big wastegate...you'd get fast spool...but as RPMs rose, you'd have to close the wastegate more and more till theoretically it was completely closed...cramming more (all) the exhaust air through the small exhaust turbine to get the required speed out of the turbo. And because you're cramming more air through the small turbine...you have a lower VE. And you know how bad rotaries respond to that...

With a big exhaust turbine you'll have lots of air sneak by the blades at low exhaust quantities...but at higher RPMs when you're cramming lots of air through there to get the high turbo speeds...then you'll have higher VE...and thus have the ability to cram even more air through the motor and make more power.

I think I understand.

I still think there is a balance I like fast spool...and a big wastegate always makes the boost spikes more manageable.

What that balance is...I think can only be determined case by case...for me 0.84 and 46mm wastegate on a stock port motor works sweet.

john

kenton

^That is the kind of reply I was looking for! Thanks teeter.

RETed

No, that's not correct.
The housing are machined to basically identical tolerance once the turbine housing necks down to the outlet.
It's a bad assumption that you get more exhaust gases bypassing the turbine wheel due to extra clearances.


No, not true.
The turbo (performance) is more dependent on the turbine section, which implies it's the exhaust gases that dictate turbo performance.
The compressor wheel almost becomes a "multiplier" of the effect on the turbine wheel spining.

That is how we would like the turbo system to ideally perform.

I've reread this several times, and it still confused me.
But, the smaller A/R requires less exhaust gases to spin it identically to a larger A/R turbine in terms of *turbo RPM*.

No, the small A/R will eventually not be able to handle the amount of exhaust gases the engine it pumping at it.
Exhaust gas pressure will skyrocket.
This surpresses the engine's ability to make power.
This is why turbos with too small an A/R choke at higher RPM's.


No, throw away the notion of air "sneaking" past the blades - that's a bad misconception.
The big A/R is less restrictive.
The big A/R is less restrictive due to the larger physically turbine section.
Exhaust gases need to fill and pressurize this larger section which adds "lag".
This is how larger A/R turbine sections shift the power band higher in the RPM band.


-Ted

kenton

If this is true, then why cant you just bleed off the rest of the exhaust gas through the wastegate? It seems to me that if this were true, you would need less exhaust gas to go through the turbo and therefore could get a better VE by bleeding off more of the exhaust gas and pressure.

What it seems to me is that it takes less exhaust gas to spin the smaller turbine, but it requires more pressure. This would mean that while you could bleed off all that extra gas, you would still have to maintain the turbine inlet pressure which is the bad part.

RETed

Ah, yes, a recent customer of mine was asking this same question...

The problem with the smaller A/R is that you do not utilize the full capacity of the turbo (compressor wheel).
So technically, my answer is not quite correct - I just stated it that way just to avoid this debate.

I couldn't figure out the answer until looking at the Ray Hall GT-Series turbo listing...
http://www.turbofast.com.au/GTseries.html
Notice how identical compressor section with different turbine A/R's make different power levels.
So larger turbine A/R's will make better use of the compressor wheel capacity.

So just to summarize, smaller turbine A/R will spool faster, but it will not make as much power as a larger turbine A/R...assuming identical compressor sections.


-Ted

fdracer

of course that's the reply you're looking for, he's as clueless as you are. no offense guys, but kenton and teeter, you have no idea what you're talking about. everyone's already tried their best to explain, but it to you kenton, but you just don't have the capacity to understand. just get a turbo with a mid sized compressor and turbine and be done with it.

teeter

[QUOTE=RETed]No, that's not correct.
The housing are machined to basically identical tolerance once the turbine housing necks down to the outlet.
It's a bad assumption that you get more exhaust gases bypassing the turbine wheel due to extra clearances.

Thats not true. The cross sectional area that the gasses pass over the turbine wheel is THE critical difference in the ARs. Mitsubishi spec's their turbos in 7 cm^2 or 8 cm^2 type numbers...the number is not the volume of the exhaust housing, it is the cross sectional area that the exhaust gasses have to pass by the wheel. The increased volume is an artifact of the need to make the transition from the exhaust manifold to the correct cross section that flows over the turbine.


I've reread this several times, and it still confused me.
But, the smaller A/R requires less exhaust gases to spin it identically to a larger A/R turbine in terms of *turbo RPM*.

Its too bad you couldn't follow it, because it explains exactly how the system works.


No, the small A/R will eventually not be able to handle the amount of exhaust gases the engine it pumping at it.
Exhaust gas pressure will skyrocket.
This surpresses the engine's ability to make power.
This is why turbos with too small an A/R choke at higher RPM's.

Simple statements is not what Kento was asking for. He was asking for a physical description of why a small A/R chokes off exhaust gas flow.

No, throw away the notion of air "sneaking" past the blades - that's a bad misconception.
The big A/R is less restrictive.
The big A/R is less restrictive due to the larger physically turbine section.
Exhaust gases need to fill and pressurize this larger section which adds "lag".
This is how larger A/R turbine sections shift the power band higher in the RPM band.

this is REALLY wrong. The lag is not due to the physical volume of the housing. Gimme a break...the exhaust gasses fill that volume in 0.0025 seconds...but they can't spin the wheel faster if they're not all directed directly over the blades at a higher velocity. Smaller cross sectional area for the same mass flow means higher velocity (and higher back pressure).

The confusion a lot of people have is that they say...10 psi at 3k rpm is as hard for the turbo as 10 psi at 8.5k rpm. Obviously thats not true...I hope you realize. 10 psi at 3k rpm...the engine is sucking through lets say 100 air flow units and the compressor had to spin at 100 rpm. At and engine speed of 8.5k rpm...at a very simplistic statement its sucking through 250 air flow units. That is not assuming any engine efficiencies, or turbo efficencies...etc... So the turbo will have to spin 2.5 times faster (so 250 rpm) to pump out the same amount of air flow. To spin 2.5 times as fast...it will need 2.5 times as much exhaust gasses to accelerate the turbo wheel to a faster rpm. If the cross sectional (described by AR) area that the exhaust gasses flow by the exhaust turbine wheel is too small...the turbo back pressures will increase...and your engine will not be as efficient at pumping air...and so you'll make less hp.

John

teeter

You're right. My degree in chemical engineering where you study mass transport using gas turbines for 6 months of your life probably doesn't have any relavance in a turbo application.

http://hotrod.com/techarticles/engin...turbo_13_s.jpg

thats a pretty good picture describing A/R.

Because we're talking about the same turbo with just different A/R housings...R is constant...so the CROSS SECTIONAL AREA (not volume) that the exhaust gasses must flow over the turbine is smaller for a smaller A/R.

And again...because at higher engine rpms...you must spin the turbo faster to flow the required air...you have to push more exhaust gasses over the exhaust turbine...and if the cross sectional area that those gasses must flow through is smaller...then it is less efficent...and you have a higher exhaust back pressure...which is not good for high engine performance.

john

kenton

Thats the "kind" of reply I was looking for. I was looking for an explanation of why. Not just a cut and dry answer like "this is the way it is so stop asking".

Please dont try and insult me because I dont have quite a full understanding of something that you may or may not have. I wouldn't have asked the question if I though I knew what I was talking about.

fdracer

wtf is an air flow unit? did they teach you that one in chem. engineering school? a specific turbo has to spin at a certain rpm to create a specific amount of bust, totally independent of engine rpm. turbo does not have to spin any faster or slower with engine rpm.

kenton what more of an "explanation" do you need, read my first post, i already explained it. it's a very simple answer to a very simple question. if you want to use a tiny a/r and divert all the exhaust through the wg, fine, but you won't build much boost. if you want to boost more you have to send more exhaust through the turbine to spin the turbo faster, but since your a/r is so small you start to get excessive backpressure. so what's the point of getting a big compressor when the exhaust side can't even support the boost it's designed for. you just increase lag unecessarily, because now you have to spin a big heavy compressor, yet you don't get any top end benefit because the exhaust gets choked prematurely. you just end up with the worst of both worlds.

RacerXtreme7

iTrader: (2)

OMG!

QUOTE: {"wtf is an air flow unit? did they teach you that one in chem. engineering school? a specific turbo has to spin at a certain rpm to create a specific amount of bust, totally independent of engine rpm. turbo does not have to spin any faster or slower with engine rpm.
"}

unit of air he's describing is generic just so people can understand what hes trying to explain. A measured value (though not actual for his explaining purpose) of air volume the motor is ingesting. and if you believe your ......."a specific turbo has to spin at a certain rpm to create a specific amount of bust, totally independent of engine rpm. turbo does not have to spin any faster or slower with engine rpm."... statement to be true you have a lot to learn and need to back down now. He explained it well, I was going to explain it but decided against it cause it would of been too lengthy.

If you want 10 lbs of boost at 4000 rpm and want to maintain that boost level all the way to 7800 rpm the turbo WILL indeed spin faster to maintain the same amount of boost :rollseyes: have you ever even looked at a compressor map?????? its boost pressure vrs. flow rate in relation to compressor rpm. you want to maintain boost pressure as your engine revs higher its taking in more air therefore in order to maintain that boost more flow is needed.. hense the turbo spinning faster.

~Mike.............

kenton

Even me knowing nothing knows that this is wrong. If it were true, then just adding a bigger wastegate would work for increasing your VE for a smaller A/R housing. You would get the turbo up to speed and that would be that, no more need for extra exhaust flow through the turbine.

If you look at a compressor map, you will notice that the lines that show compressor RPM are curved downward. This is because as your airflow increases, you will have a lower boost ratio at the same compressor RPM. That is simple.

What I was looking for was someone to explain to me why it was that why. Give me proof. Not just say "smaller A/R means better spool but more backpressure". I already know that this is true...I want to know why it is true though.

I have a better understanding now that some other people have taken the time to explain this to me. Thanks guys.

Although, I am sure some others will chime in too.

RETed

So is there a correlation between Mitsubishi's standard of using cross-sectional area versus Garrett's standard of using A/R?
So is the turbine housing outlet just before the exhaust gases hit the turbine wheel the primary variable in determining turbo performance versus RPM's?
If this is the case, then why bother messing with larger turbine housings?
Why not keep the turbine housing as small as possible (for best spool-up?) and then adjust the orifice of the turbine housing right before it hits the turbine wheel?


So we now introduce exhaust gas vector or is this purely a velocity variable?


Bernoulli's Theorem?


Not a problem...
Turbo makes boost dependent on engine load.
Engine load is dependent on the differential between exhaust gas output versus intake charge input.


-Ted

Zero R

iTrader: (1)

This is a decent answer to your question, just bleeding off more exhaust is not going to eliminate the fact that you can only cram so much air into the turbine housing, thus performance will drop off.