What is boost?

[sergei]

Increasing molecular density is the technical term for pressure.

Its not as simple as that.
Put a cold gas in a container with a pressure sensor, seal it, then heat it. Pressure goes up, but density is the same, since no extra gas has entered the container and the container hasn't changed size.

The engine is not a closed system. The real question must be asked is following:
How a compressor increases the pressure? Be that centrifugal or positive displacement type.

The answer is following: the compressor primarily increases the pressure by adding more mass from outside of the system (changing 'n' in PV = nRT).


In case of internal combustion, the boost is the difference between flow rates of engine and compressor. When you have boost pressure the compressor supplies more air than engine can pump through. Engine is just a fancy air pump.

Why charged engines run a substantial boost pressure (ie not just keeping it at 0, where VE approaches 100%)? Because generally by increasing P the 'n' increases due to extra molecules added by the compressor.

In modern cars the Mass Airflow Meter measures 'n' directly, while the MAP sensor measures 'P' (and 'n' is indirectly inferred from PV=nRT).

What happens in combustion chamber of charged engine vs N/A?
The only difference is that the starting pressure will be boost pressure (vs slighly below atmospheric), and starting temperature will be higher (depending on inter-cooler and turbo efficiency). And here where the PV=nRT comes in effect as the combustion chamber can be considered closed system. Disregarding detonation, more 'n' = more power.

Here is some mental gymnastic:
initial state:
PV=nRT
V is constant (piston BDC+combustion chamber)
R is constant
n1, P1, T1 are initial states of N/A engine
n1= P1V/RT1
n2= P2V/RT2

so n2/n1 = (P2V/RT2)/ (P1V/RT1) = (P2V/RT2) * (RT1/P1V) = P2T1 / P1T2

so lets assume the following numbers:

P1 = 0.9bar (no boost)
T1 = 40'C
P2 = 1.9bar (~0.9bar boost)
T2 = 70'C (average intercooler)

so n2/n1 = 1.9 * 40 / 0.9 * 70 = 1.2 mass increase

If the post intercooler temperature is dropped down to 50'C, then there is ~1.7 increase of mass, which would translate to little bit under of 1.7 increase of power.

[Grrrrrrr!]

In case of internal combustion, the boost is the difference between flow rates of engine and compressor. When you have boost pressure the compressor supplies more air than engine can pump through.

Incorrect, mass flow thru the turbo and mass flow thru the engine are equal at constant boost, otherwise pressure would just keep increasing and you'd blow something apart, or if you had a car made of unobtanium you'd end up with a super dense mass called a black hole in your intake manifold.


As fivebob said, the manifold pressure is resistance to flow. If the turbo forces more flow in, the pressure goes up, but this forces more air thru the restriction (engine) until they reach a new equilibrium at a higher flow rate (and higher pressure).

Apart from that, what you said is correct as far as I can see.

[sergei]

In case of internal combustion, the boost is the difference between flow rates of engine and compressor. When you have boost pressure the compressor supplies more air than engine can pump through.

Incorrect, mass flow thru the turbo and mass flow thru the engine are equal at constant boost, otherwise pressure would just keep increasing and you'd blow something apart, or if you had a car made of unobtanium you'd end up with a super dense mass called a black hole in your intake manifold.


As fivebob said, the manifold pressure is resistance to flow. If the turbo forces more flow in, the pressure goes up, but this forces more air thru the restriction (engine) until they reach a new equilibrium at a higher flow rate (and higher pressure).

Apart from that, what you said is correct as far as I can see.

it reaches equilibrium due to following factors:
due to compressor flow increase is in non linear relationship to pressure.
wastegate.
in case of supercharges are rpm limited.

if there would not be a wastegate the turbocharger would reach much higher boost, but it will never rise indefinitely due to thermodynamcs.

the boost is there because compressor outflows the engine at atmospheric pressure hence the pressure in the system rises.

[Grrrrrrr!]

it reaches equilibrium due to [bunch of things that result in flow in = flow out]

[fivebob]

Ok, now put theory into practice and show how much you really know ?

Using this as a guide;



What are the respective characteristics of the following compressors?

[fielderz]

You've just disproved your own point with those compressor charts, notice all of that positive compressor boost pressure/mass flow rate efficiency gradient... hence the positive relationship of boost to MASS flow rate.

Allbeit at gradually dimishing returns as the boost pressure increases, the relationship still remains positve however.

So well done showing how much YOU know... Not much.

[Grrrrrrr!]

The t04e is mor suited for a smaller displacement engine running higher boost pressures, with good efficiency up at the higher pressure ratios. Wont flow quite as much (outright) as the t04b , but would make positive pressure a bit earlier (assuming same hotside) , and is more efficient pretty much everywhere.

the T04B would be better matched to a larger displacement motor flowing more at lower pressures. Expect surge problems on a smaller displacement engine without aggressive cams.

[fivebob]

You've just disproved your own point with those compressor charts, notice all of that positive compressor boost pressure/mass flow rate efficiency gradient... hence the positive relationship of boost to MASS flow rate.

Allbeit at gradually dimishing returns as the boost pressure increases, the relationship still remains positve however.

So well done showing how much YOU know... Not much.

Unfortunately you've totally missed the entire point of the thread.

It's not about turning up the boost on a particular vehicle, it's about comparing the returns from different set ups.

No body would be so silly as to say that you won't always get more from winding the boost, limits of the motor not withstanding.

So why don't you answer the question posed and prove how smart you are?

[Dell'Orto]

Stay on topic please, this is a pretty good thread so far

[fielderz]

You've just disproved your own point with those compressor charts, notice all of that positive compressor boost pressure/mass flow rate efficiency gradient... hence the positive relationship of boost to MASS flow rate.

Allbeit at gradually dimishing returns as the boost pressure increases, the relationship still remains positve however.

So well done showing how much YOU know... Not much.

Unfortunately you've totally missed the entire point of the thread.

It's not about turning up the boost on a particular vehicle, it's about comparing the returns from different set ups.

No body would be so silly as to say that you won't always get more from winding the boost, limits of the motor not withstanding.

So why don't you answer the question posed and prove how smart you are?

Really? Well thats exactly what you said here:

Boost is a measure of restriction at a given flow rate. That is all, it is not a measure of potential HP. Having more boost does not always mean you'll make more HP.

While this is true, if the only change is increased boost, then the result is more power.

No, if the only change is increased boost then there is a power decrease.

pV=nRT. (V)olume is fixed, R is a constant, so an increase (p)ressure without an increase in (n)umber of molecules results in an increase in (T)emperature only.

If however flow is increased along with boost then there may be a power increase, providing that the increase in temperature is not too great.

It's all about flow, boost is just easier to observe

Your whole thread has been about "turning up the boost" with all other things held constant, where you claim this would result in a net decrease in power, as you stated above. Yes mass flow rate is what you need to get power, but you imply that increasing the boost will not increase flow, which is clearly not true as I have explained in previous posts.

I'm not some retard with an opinion, I'm in the third year of a fluid/thermal dynamics degree, and before this I was/am a refrigeration engineer so I know whats going on with fluids/gasses.

Stay on topic please, this is a pretty good thread so far

If this was directed at me, my previous post was 100% on topic, I suggest you read the thread again if you believe otherwise.

[iOnic]

It was directed at me trolling as you were

[Lith]

It's not about turning up the boost on a particular vehicle, it's about comparing the returns from different set ups.

Ahh cool, glad you mentioned that - I've been enjoying the discussion and seeing where different people are at and how they approach this, never know if or what one may learn from someone with different experiences and knowledge (some may understand more about some parts, but not others - and we don't always appreciate what we don't know until faced with it). I was't entirely sure where it was heading, or meant to be heading.

Tuning (as really this ultimately is all about - at least from my standpoint, even if the focus here is on forced induction) is a really really fascinating topic if you are geek-inclined, SOOOO much can affect what happens and there is so much to take into consideration. A lot of what is mentioned here is beyond a lot of the conversation you often hear or see on this topic, it's refreshing as to see

There are some really interesting dynamics which go on, and I've definitely learnt a massive amount more (or maybe put more understanding to what I theoretically knew) after doing a bit of tuning and trying to find more HP with both NA and turbocharged cars and it has definitely made me realise that I still knew SFA and figured I should really get a better understanding of how it all works... its been a few years now and I am just as bewildered by how much there is I need to learn and understand, despite the fact I feel like I've learnt rather a lot compared to when I started.

The focus in this thread seems to be about the ability to build pressure (or perhaps shove lbs of air would be more accurate) through the valves, and things seem reasonably consistent with my understanding on things. What I am interested to see is if the topic is going to drift into discussing what happens on the exhaust manifold side of things, and arguably most importantly - how the gases that are flowing through behave when things get REAL interesting in the cylinders themselves when someone decides to set fire to shit.

Rule of thumb is often considered that you may make 10hp per lb/min air moved on a turbocharged car - however moving 48lb/min in a car tuned without being octane limited does not necessarily guarantee 480hp. Do it shit, you could make WAY less than that despite having the air and fuel mass required to do so. Do it AWESOME and you could make much much more. LOTS of variables, so much fun to play with them

[fivebob]

....

I apologise for not making myself clearer earlier on in the thread. so I will make it clear now
This thread is not about turning up the boost on a particular vehicle, it's about comparing the returns from different set ups.

Please try to use some common sense, I can't be arsed with a flame war right now.

[touge_ae101]

ok I shall enter the firey bounds of this thread to add some perspective. I know zilch about turbochargers and boosted engines essentially but I work with pumps and like designing shit in general and most things in cars car be broken down to simple mechanical actions and reactions. (not wiring/electricity though - that shit is just magic! )

To explain what fivebob mentioned about 'just turning the boost up' not necessarily meaning more power you need to think about what a turbo is doing.. essentially it is a pump which by spinning the impeller (by using exhaust gases) it is increasing the pressure into the engine over and above atmospheric..
for a generic standard *turbo A* increasing the boost pressure the wastegeate witholds before venting can run the turbo outside of its efficiency. you just end up heating the air too much as it passes through the turbo as there is slip created in the fluid moving through the impeller (sorry to use pump speak but it is easy to understand) slip means more work put into the air moving through which means disproportionately more heat for an increase in shaft RPM and small increase in boost.
(if any) or cfm whichever way you want to look at it

this is because the angle (I believe turbo speak is trim) of the blades on the impeller are tuned to produce pressure at a certain rpm given the pressure drops before and after the turbo in a standard scenario. this is of course driven by cfm flowed through the exhaust housing..

the angle of the blades (and probably a whole lot of other factors but lets keep it simple) will determine at what RPM that turbo creates the most efficient boost pressure by accelerating the air being fed into it, a shallower angle will create boost at a slower IMPELLER RPM whereas a steeper angle will need more impeller RPM's to work efficiently but has the potential to flow more CFM for the same impeller RPM once in its efficient range. of course there will be a cusp on this relationship between angle and RPM but as a simple way of thinking about standard vs well engineered turbo for increased power by making more boost..

also you have to remember there is no non-return valve in a turbo, once there is positive pressure behind the impeller it needs to be maintained. once the turbo starts slowing then the blowoff valve will release pressure to stop the air from trying to go backwards through the impeller (less pressure outside --> mass of air wants to get outside and equalize)


of course this isn't taking into account the exhaust wheel etc as it would have the same effect of load vs rpm but in the opposite. the balance between impeller and exhaust wheel would be (I can imagine) a very interesting but quite indepth science which is why every man and his dog can't just slap generic chinese turbo onto car and make bih hp as well engineered turbo ratios suited to an engine (more specifically a certain CFM) will always be more efficient (less heat for same pressure) at making power

hopefully if you've read and understood my post then you should understand what Fivebob is saying and you need to remember that pressure is being created by a mechanical action - ideal gas laws and density etc are the result of a mechanical action (in the case of a turbo anyway). the key to making power from a boosted vehicle is to increase the CFM of air into the engine whilst minimizing the effect of the ideal gas law and extra heat going into the air moving through. more air + more fuel at given air-fuel ratio = bigger bang if done right.

flame away

[gt4dude]

In fact having higher boost at the same flow usually means you'll make less HP than an engine that achieves the same flow at a lower pressure.

Not sure if this refers to car fitted with aftermarket cams or turbo
^-- this is a no brainer. ofcourse it will make more power with less boost, for cams allow more air into the cyls, and bigger turbos push larger quantities of air. the idea of making more power with less boost usually comes at the cost of low end torque and spoolup


or standard car with the boost turned up.
^-- in this case, usually due to a factory conservativeness, theres a little power to unlock by winding up the boost. in some cars more so than others.

so maybe not a T28 nissan turbo (2560) but the ct20b on a celica gt4 can kick out flow up to 1.3bar (past the stock injectors) above the stock 0.9bar with its 62mm compressor wheel, an evo with a 68mm compressor wheel can kick out flow up to 1.7bar. so to some people who say more boost is more power, they are correct. most of the time the factory injectors are the limiting factor before the turbo, this is all they're going to know in their closed little world.


this stuff is fairly straight forward, obviously certain turbos can only put out so much puff and when choosing an aftermarket turbo, its good to do your research and figure out what you want and what suits the goal.


also, alot of this hot air stuff is simply that, hot air. when hot air becomes the limiting factor (that people commonly say when they experience the symptom of turn the boost up, no extra power) then the real limiting factor is intercooling and fuel octane/knock well before the point that no more oxygen can physically be extracted from the inlet pipe

[sergei]

ct20b on a celica gt4 can kick out flow up to 1.3bar (past the stock injectors) above the stock 0.9bar with its 62mm compressor wheel, an evo with a 68mm compressor wheel can kick out flow up to 1.7bar.

The 0.9bar is not stock boost for GT4. It is defined as much wider range.
If you look at repair manual the stock boost will be defined as "82 - 115kPa", or roughly 0.82 to 1.15 bar, anything above > 1.15 bar stock ECU will induce a "boost cut". There is simply not much difference between 1.3bar or 1.15bar, to make a dramatic effect on the flow of the turbo charger in question. ct20b when left to its devices will overboost well above 2.0bars.
BTW stock engine will not outflow stock injectors with stock boost (even on higher end, eg 1.1bar) and minor mods (intake+exhaust). You will need cams for that.

[gt4dude]

ct20b on a celica gt4 can kick out flow up to 1.3bar (past the stock injectors) above the stock 0.9bar with its 62mm compressor wheel, an evo with a 68mm compressor wheel can kick out flow up to 1.7bar.

The 0.9bar is not stock boost for GT4. It is defined as much wider range.
If you look at repair manual the stock boost will be defined as "82 - 115kPa", or roughly 0.82 to 1.15 bar, anything above > 1.15 bar stock ECU will induce a "boost cut". There is simply not much difference between 1.3bar or 1.15bar, to make a dramatic effect on the flow of the turbo charger in question. ct20b when left to its devices will overboost well above 2.0bars.
BTW stock engine will not outflow stock injectors with stock boost (even on higher end, eg 1.1bar) and minor mods (intake+exhaust). You will need cams for that.

Sergei??????? What? You missed the entire point. If you're on a dyno tuning, sorry, If I'M on a dyno tuning, since i still own a tuned Celica GT4, you would be continuing to make power way beyond what you're saying or what the manual says IF the injectors were bigger than 540cc. Fact.

And probably not 2bar, it will hit some peak and fall very flat at 6000 rpms...

[sergei]

ct20b on a celica gt4 can kick out flow up to 1.3bar (past the stock injectors) above the stock 0.9bar with its 62mm compressor wheel, an evo with a 68mm compressor wheel can kick out flow up to 1.7bar.

The 0.9bar is not stock boost for GT4. It is defined as much wider range.
If you look at repair manual the stock boost will be defined as "82 - 115kPa", or roughly 0.82 to 1.15 bar, anything above > 1.15 bar stock ECU will induce a "boost cut". There is simply not much difference between 1.3bar or 1.15bar, to make a dramatic effect on the flow of the turbo charger in question. ct20b when left to its devices will overboost well above 2.0bars.
BTW stock engine will not outflow stock injectors with stock boost (even on higher end, eg 1.1bar) and minor mods (intake+exhaust). You will need cams for that.

Sergei??????? What? You missed the entire point. If you're on a dyno tuning, sorry, If I'M on a dyno tuning, since i still own a tuned Celica GT4, you would be continuing to make power way beyond what you're saying or what the manual says IF the injectors were bigger than 540cc. Fact.

And probably not 2bar, it will hit some peak and fall very flat at 6000 rpms...

I personally seen CT20b boosting over 2bars (due to free boost problem) on a stock gen1 3SGTE, and that was around 5000rpm, I was not brave enough to let it continue to see what sort boost it get at 6000rpm . Additionally one of the first setups I had made 178kW at the wheels at 16psi, with 440cc injectors with 10:1 fuel ratio on the dyno. So for 178kW at four wheels, 440cc injectors were not the limit. My latest setup made 195kW with 540cc injectors (still gen1 block) at 16psi, with plenty of fuel (safe tune, which would be on richer side).
What sort of power are you talking about? If you are talking about 250kw+ at the wheels, then you should not be using CT20b to start with (or any CT incarnation, be that stock or modified).

Yes you can push CT20b to silly pressures, but it would not be very efficient there. You will make a little bit more power than at 1.2bars (maximum which I would push a steel CT20b), but your efficiency will drop right down. Perhaps you are maxing out 540cc injectors because you are running too hot inlet temperatures and need extra fuel to combat detonation?

You would be making more power if you would replace CT20b with a more suitable turbo charger for power level you are after.

power gain = P2T1 / P1T2 where P2 boost+1bar after increase, P1 boost+1bar before increase, T1 is inlet temp before increase, and T2 is inlet temperature after increase. Here is an example:

P2 = 2.3bar
P1 = 2.0bar
T1 = 70'C
T2 = 100'C


The power gain is = (2.3 * 70) / (2.0 * 100) = 0.8 (or 80% of what you are started with), even though you are increasing boost, due to extra inlet temperature you are in fact losing power.

And yes, it is possible to go from 70'C to 100'C inlet when the turbo charges falls off efficiency range. The over spun turbos can push out temps as high as 160'C

[fivebob]

power gain = P2T1 / P1T2 where P2 boost+1bar after increase, P1 boost+1bar before increase, T1 is inlet temp before increase, and T2 is inlet temperature after increase. Here is an example:

P2 = 2.3bar
P1 = 2.0bar
T1 = 70'C
T2 = 100'C


The power gain is = (2.3 * 70) / (2.0 * 100) = 0.8 (or 80% of what you are started with), even though you are increasing boost, due to extra inlet temperature you are in fact losing power.

And yes, it is possible to go from 70'C to 100'C inlet when the turbo charges falls off efficiency range. The over spun turbos can push out temps as high as 160'C

I think you've got that wrong. Temps should be in °K not °C so the result is;
(2.3*343)/(2.0*373) = 1.06, i.e. a 6% increase.

[gt4dude]

My latest setup made 195kW with 540cc injectors (still gen1 block) at 16psi, with plenty of fuel (safe tune, which would be on richer side).

what dyno? i made 200kw on 21psi and maxed my 540cc injectors with gt2860rs on soichis dyno. Originally 220kw but dialed back since it knocked when heatsoak set in