MrSmokey
MrSmokey Reader
9/10/20 8:37 p.m.

I may be asking a completely dumb question here but... someone explain in six year old terms.

how exactly does a bigger turbo push more air?

what I mean is I think my turbo on my 98 dodge Cummins is 58mm, I am somehow getting almost 44 psi out of it max... how then would a bigger turbo size, let's say 66mm or something like that (I just picked a random number), push more air at the same psi? Am I completely off my rocker? What am I missing?

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
9/10/20 8:48 p.m.

It's a bigger fan.

Like the difference between a small jet engine on a private jet and a large jet engine on an airliner- they work exactly the same, maybe even at the same pressures, but the larger fan moves more air- which is needed to move the larger plane.

I'm sure someone will come up with the actual equations- but flow is proportional to pressure and area (not exactly linearly, if I recall correctly), but if pressure is kept constant, and area is increased, the flow will go up with area.

codrus (Forum Supporter)
codrus (Forum Supporter) UberDork
9/10/20 9:27 p.m.

You make more power by burning more fuel, but to burn that fuel you need more oxygen, and that oxygen needs to be in the cylinders, not in the intake manifold.  "boost" is a measure of pressure in the intake manifold, and it basically tells you how much is piling up because it can't get through into the cylinders yet.

Bigger turbos make more power for two reasons.  One is that the turbine sits in the exhaust flow and creates backpressure which prevents the cylinders from getting rid of all of the exhaust gas.  That means there's less room for intake air to come in meaning you can't burn as much fuel.  Bigger turbos have bigger turbines, and thus less backpressure.  This means you can have more air in the cylinders with the same pressure in the intake manifold because the engine is more efficient at getting rid of exhaust gas and pulling in intake gas.

The second reason is that a bigger turbo also has a bigger compressor and (all else being equal) that bigger compressor can compress a larger volume of air even if it's going to the same target pressure.  Generally speaking a turbo compressor maxes out at a certain amount of horsepower and trying to drive it beyond that point just results in hotter gas, rather than more of it.  Looking back to high school chemistry, "PV = nRT", or pressure times volume equals number of molecules times temperature (times a constant that we can ignore).  "V" is fixed (volume of intake mainfold), so if you want to raise the pressure you can either do it by increasing n (more air) or by increasing T (more temperature).  You want more n, but after that max point what you get is more T.

That said, you can't really describe turbo "size" with just a single number.  There are lots of parameters (A/R ratio, inducer/exducer diameters on both wheels, etc) which all typically need to be in a certain ranges related to the others in order to function well.  Then the design of the wheels themselves makes them work better at different pressure ratios and flow volumes.  Turbos need to be properly sized for both the desired horsepower and the size of the engine they're feeding.  Two "400 hp" turbos will be quite different if one of them is just doing a small amount of boost on a relatively low-revving 5-ish liter V8, while the other is doing a ton of boost on a high-revving 1.8L.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/10/20 10:45 p.m.

Big turbos need less exhaust manifold pressure for mass flows at the big end of smaller turbos.  Less exhaust pressure, and more efficient compressor function, means more power.

 

Turbochargers are not fans, they are compressors.  Look at a compressor map: Turbo speed affects pressure ratio, not so much mass flow.  Spinning faster makes for more pressure,  but more flow will need a larger turbo.

Mr_Asa
Mr_Asa Dork
9/10/20 10:57 p.m.

Six year old terms:  Turbos are air pumps.  If you increase the size, but maintain the same pressure you have increased the volume that passes through the turbo.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/10/20 11:03 p.m.

In reply to Mr_Asa :

But turbos aren't air pumps.   They're air compressors that rely on flow to be able to compress air.  The difference is subtle but enormous.
 

They sling air at a curved wall and use centrifugal force to compress the air.  If you ever went to an amusement park and rode in the Rotor, you feel what it feels like to be air after a compressor wheel throws it into the compressor housing.  At  very low boost.

Mr_Asa
Mr_Asa SuperDork
9/10/20 11:08 p.m.

In reply to Pete. (l33t FS) :

To a six year old, subtle doesn't generally exist.

I get what you're saying, I'm just trying to stick to basic terms

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/10/20 11:15 p.m.

In reply to Mr_Asa :

The first step in explaining things to six year olds is to not lie.  They are young enough to believe whatever bullE36 M3 you spit out and will actively rebel against better explanations when they are older.  It's a bug in the ol' brain software, unfortunately one that gets heavily exploited.

MrSmokey
MrSmokey Reader
9/11/20 7:17 a.m.

Lol guys laugh ... thanks a lot tho, I should've known all that but anyway 

dean1484
dean1484 MegaDork
9/11/20 7:24 a.m.

6 year old terms?   Bigger is better. Like ice cream cones or pizza or chocolate candy's . That is why a bigger turbo is better. 

jharry3
jharry3 HalfDork
9/11/20 7:26 a.m.
dean1484 said:

6 year old terms?   Bigger is better. Like ice cream cones or pizza or chocolate candy's . That is why a bigger turbo is better. 

Up to the point the 6 year gets a belly ache and throws up.  Or, in the case of a bigger turbo, the engine adds new windows to see the crank better.

Fueled by Caffeine
Fueled by Caffeine MegaDork
9/11/20 8:36 a.m.

yeet it.. yeet it all day.  that's what makes it push more air.

 

Seriously.. Garrett has always had a great webpage and a good source to learn the basics from.. https://www.garrettmotion.com/racing-and-performance/choosing-a-turbocharger/

jimbbski
jimbbski SuperDork
9/11/20 9:59 a.m.

The question asked is "turbo boost".

Added to what's been said already is "More boost is good until it isn't!"

More boost goes to boom at some point.

wspohn
wspohn Dork
9/11/20 10:39 a.m.

Just to confuse things. let's debunk the popular idea that more boost = more power.

The boost pressure depends on what the turbo is pushing against. If you do some intake tract work, maybe throw on a high flow CAT and improve the efficiency of the engine, you can produce more power using the same or even lower boost.  Many/most owners don't seem to grasp that and look on a bigger turbo as a panacea that gives them the keys to the kingdom, instead of first optimizing the exhaust, intake etc.

I've also seen a lot of people blow their engines pushing boost pressures too far, getting into even momentary detonation and trashing the engine.  Less today with fuel injection and computer controlled ignition and fuel, but the old attitude of 'go for as much boost as you have the balls for' isn't exactly a sensible metric.

frenchyd
frenchyd PowerDork
9/11/20 11:29 a.m.

In reply to wspohn :

Somes  good 

Mores better 

Too much is just getting fun. 
 

I go around before the race chanting that mantra and then during the race I let them fly by me early on only to pass them when they blow up or have to cool it in order to limp home. 
 

What I don't say is to finish first, first you've got to finish. That's my mantra. 

MadScientistMatt
MadScientistMatt PowerDork
9/11/20 12:40 p.m.
wspohn said:

Just to confuse things. let's debunk the popular idea that more boost = more power.

The boost pressure depends on what the turbo is pushing against. If you do some intake tract work, maybe throw on a high flow CAT and improve the efficiency of the engine, you can produce more power using the same or even lower boost.  Many/most owners don't seem to grasp that and look on a bigger turbo as a panacea that gives them the keys to the kingdom, instead of first optimizing the exhaust, intake etc.

I've also seen a lot of people blow their engines pushing boost pressures too far, getting into even momentary detonation and trashing the engine.  Less today with fuel injection and computer controlled ignition and fuel, but the old attitude of 'go for as much boost as you have the balls for' isn't exactly a sensible metric.

Also, due to how the controls operate, this is way more noticable on a supercharger than with a turbo. A normal supercharger will run on pulleys and its RPM is locked to engine speed, while turbo controls work off pressure. So if you increase the airflow in a supercharged engine without any other changes, the boost is likely to drop. Doing the same on a turbo engine and the turbo controls will target the same pressure (although they may not quite reach it).

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
9/11/20 12:42 p.m.
Pete. (l33t FS) said:

In reply to Mr_Asa :

The first step in explaining things to six year olds is to not lie.  They are young enough to believe whatever bullE36 M3 you spit out and will actively rebel against better explanations when they are older.  It's a bug in the ol' brain software, unfortunately one that gets heavily exploited.

Nobody is lying.   A turbo is an air pump- one that uses exhaust energy to spin a shaft to turn a compressor to pump air.  Air at a pressure and at a flow- which is dependent on many things- one of them being turbine size (what drives it) and compressor size (what is being driven).  

As you suggest, it's a pretty complex thing.  But there are some simple explanations to simple questions- and while they don't explain the whole thing, they do accurately answer the question.

It's not a question on how to properly size a turbo, or how to use the one that you have, or how to plumb it in, or what's the best cam for the job.  Just why does bigger at the same pressure mean more flow.

MadScientistMatt
MadScientistMatt PowerDork
9/11/20 12:54 p.m.

When you get down to it, drawing a line between a compressor and a fan isn't as clear cut as it may seem. The main distinction is how big a pressure rise you get across the device. There are blowers that use a centrifugal design, and axial compressors.

Although you could make a case that anything that forces the air into a smaller volume when it moves the air (Judson, screw type, and scroll type superchargers, for example) should always be called compressors.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/11/20 1:50 p.m.

In reply to MadScientistMatt :

There's the rub, and where I draw the line in the sand between an air mover and a compressor.  A compressor will compress the air as a function of its design.  Centrifugal or centripetal (yup, on some WWII engines) or axial.  Or, I guess, scroll.  Or piston.

Blowers just move a fixed volume of air, and any boost pressure is down to the volumetric efficiency difference between the blower and the engine it is feeding.  There is no compression in the unit.

This is why screw blowers are so nice: they are designed in such a way that they are also compressing the air as it moves from the inlet to the outlet.

 

 

A 401 CJ
A 401 CJ Dork
9/11/20 2:26 p.m.
codrus (Forum Supporter) said:

You make more power by burning more fuel, but to burn that fuel you need more oxygen, and that oxygen needs to be in the cylinders, not in the intake manifold.  "boost" is a measure of pressure in the intake manifold, and it basically tells you how much is piling up because it can't get through into the cylinders yet.

Bigger turbos make more power for two reasons.  One is that the turbine sits in the exhaust flow and creates backpressure which prevents the cylinders from getting rid of all of the exhaust gas.  That means there's less room for intake air to come in meaning you can't burn as much fuel.  Bigger turbos have bigger turbines, and thus less backpressure.  This means you can have more air in the cylinders with the same pressure in the intake manifold because the engine is more efficient at getting rid of exhaust gas and pulling in intake gas.

The second reason is that a bigger turbo also has a bigger compressor and (all else being equal) that bigger compressor can compress a larger volume of air even if it's going to the same target pressure.  Generally speaking a turbo compressor maxes out at a certain amount of horsepower and trying to drive it beyond that point just results in hotter gas, rather than more of it.  Looking back to high school chemistry, "PV = nRT", or pressure times volume equals number of molecules times temperature (times a constant that we can ignore).  "V" is fixed (volume of intake mainfold), so if you want to raise the pressure you can either do it by increasing n (more air) or by increasing T (more temperature).  You want more n, but after that max point what you get is more T.

That said, you can't really describe turbo "size" with just a single number.  There are lots of parameters (A/R ratio, inducer/exducer diameters on both wheels, etc) which all typically need to be in a certain ranges related to the others in order to function well.  Then the design of the wheels themselves makes them work better at different pressure ratios and flow volumes.  Turbos need to be properly sized for both the desired horsepower and the size of the engine they're feeding.  Two "400 hp" turbos will be quite different if one of them is just doing a small amount of boost on a relatively low-revving 5-ish liter V8, while the other is doing a ton of boost on a high-revving 1.8L.

This.  + 10

 

I get so sick of people saying things about how much more efficient their smaller "turbo" engine is because it makes the same or more hp than a similarly architected larger engine.  It does because it burns more fuel!  

A 401 CJ
A 401 CJ Dork
9/11/20 2:30 p.m.

Now, this Volvo heavy duty truck turbo actually does increase efficiency.  But I've never seen anything like it on the street.  

 

dean1484
dean1484 MegaDork
9/11/20 2:46 p.m.
jharry3 said:
dean1484 said:

6 year old terms?   Bigger is better. Like ice cream cones or pizza or chocolate candy's . That is why a bigger turbo is better. 

Up to the point the 6 year gets a belly ache and throws up.  Or, in the case of a bigger turbo, the engine adds new windows to see the crank better.

Ok Dad. . ..  ;-)  

wspohn
wspohn Dork
9/12/20 12:04 p.m.
A 401 CJ said:
I get so sick of people saying things about how much more efficient their smaller "turbo" engine is because it makes the same or more hp than a similarly architected larger engine.  It does because it burns more fuel!  

 

True, and that is often overlooked, but to be fair, a turbo engine that is smaller than the same output NA engines will normally get far better fuel mileage when not in boost, so it is like having a switch - if you are just puttering around you get great mileage, but if you hit that 'switch'/accelerator  you get huge payback at the expense of higher fuel consumption.  Sounds pretty good to me....

Best personal example I know are the GM Kappas (Solstice, Sky). The turbo 2.0 gets a higher average fuel mileage (producing up to 290 bhp on a factory tune) than the NA 2.4 engine can manage (output 177 bhp).

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