Cars define turbos, turbos don't define cars

[Zenja]

I agree with the theories talked about in this thread but many people would disagree and say that it's all just theory and good math but it doesn't hold up well in real life. This may well be true because people claim that they get quite a bit more power out of a 50trim than a 14b (for example) at the same PSI at moderate (let's say 16) psi.

I thought of a little theory which may very well be false. If you have a 1" hole in something with 10psi on it, you'll flow a certain amount of air. Now, let's say that you put a 1" open ended tube where the hole used to be, and you had various restrictions in that tube. You would need more than 10psi at the start of the tube to end up with 10psi at the end. The flow in the restricting area would create more pressure on the starting side and less would reach the exit. This is relevant because the air reaches various restrictions on the way to the combustion chamber. The more air you can pump quickly the more you might be able to neutralize these restrictions.

Another idea I have (which is probably more realistic than the last one) is that air and water do indeed act differently so when the intake valves open up, the air doesn't smoothly flow in as one big mass like water would. A pressure drop gets created and the air rushes into the combustion chamber. Between the area where the air was filled up under pressure before the intake valve opened and the combustion chamber there is now quickly rushing air which is far less than a big solid mass. The velocity is much greater so the air is spread out thinner. Imagine an air compressor blowing through a tube. It won't blow a steady massive stream of air but turbulent, thin air going through the tube. Now if you made a higher flowing compressor, like a bigger turbo, then you WOULD be able to flow a more massive stream of air. All the air would be able to reach the end of the tube much quicker.

It would be the same concept in the engine. When the valves open up, it takes time and sustained pressure for all that air to quickly rush into the combustion chamber. If you have a small turbo then the air would start rushing in but it would quickly run out and for the rest of the time that the intake valves are open there wouldn't be much air left to flow into the combustion chamber. A bigger turbo would supply this air and let it all come in during the precious time that the intake valves are open.

If the turbo is too small then not only would you not be able to flow much air into the combustion chamber efficiently, but you wouldn't even be able to flow enough to keep up to a certain pressure. In other words, before the turbo could create enough pressure in the system, the intake valves would already open and use up all the air and the turbo wouldn't be able to keep up. If you CAN supply the pressure, then you just won't be able to efficiently get as much air into the chamber. The pressure is measured at how much pressure there is in the piping waiting to get into the combustion chamber. This doesn't mean that there's necessarily the same amount flowing into the chamber.

Keep in mind I don't have much to back me up so I'm wondering what some of you with some kind of degree or education in this has to say. I wouldn't be surprised if the first paragraph could be shot down completely; I should have just took it out of the post. I think there obviously is some reason why a bigger turbo would perform better at the same psi even though they are very close in efficiency range and the temperatures would be minimally different.

 

[crankbender]

I agree with the theories talked about in this thread but many people would disagree and say that it's all just theory and good math but it doesn't hold up well in real life. This may well be true because people claim that they get quite a bit more power out of a 50trim than a 14b (for example) at the same PSI at moderate (let's say 16) psi.

There are 2 reasons that they will get more power with the larger turbo on that setup. First off it must be assumed they they have done other work to the car. With a well planned out exhaust and intake a 14b is actually getting out of its efficiency at 16psi. This means that when you strap on the 50 trim you will get cooler air (especially at higher RPMs). Cooler air is more dense and allows for more power.

secondly increasing from a 14b to a 50 trim is radically changing the exhaust HOUSING. this along with the change in wheels to one that is more fitting for this car (sorry a 14b is not good for 16psi on a well planned out car) will allow more flow.

That said often they are getting more power because they had the smaller turbo on and it didn't fit the car at the time or they were incorrectly tuned. Additionally don't trust the seat of the pants meter...you just spent money and it is going to feel faster even if it isn't because subconciously you want it to be.

I thought of a little theory which may very well be false. If you have a 1" hole in something with 10psi on it, you'll flow a certain amount of air. Now, let's say that you put a 1" open ended tube where the hole used to be, and you had various restrictions in that tube. You would need more than 10psi at the start of the tube to end up with 10psi at the end. The flow in the restricting area would create more pressure on the starting side and less would reach the exit. This is relevant because the air reaches various restrictions on the way to the combustion chamber. The more air you can pump quickly the more you might be able to neutralize these restrictions.

If you are saying "with constant flow a larger pressure drop occurs with increasing resistance" and "with a constant pressure drop flow decreases with increasing resistance" then yes you are correct.

Another idea I have (which is probably more realistic than the last one) is that air and water do indeed act differently so when the intake valves open up, the air doesn't smoothly flow in as one big mass like water would. A pressure drop gets created and the air rushes into the combustion chamber. Between the area where the air was filled up under pressure before the intake valve opened and the combustion chamber there is now quickly rushing air which is far less than a big solid mass. The velocity is much greater so the air is spread out thinner. Imagine an air compressor blowing through a tube. It won't blow a steady massive stream of air but turbulent, thin air going through the tube. Now if you made a higher flowing compressor, like a bigger turbo, then you WOULD be able to flow a more massive stream of air. All the air would be able to reach the end of the tube much quicker.

Yes air acts differently than water...but air is air no matter what turbo is pushing it. The local pressure in the intake manifold does change based on valve openings but the stagnation pressure (a measure of its energy) does not change. Do some reading on bernulli and you will understand this a bit better. where you are getting your compressed air does not matter if you don't change any of the restrictions it will flow the same through the intake. you can not compare a turbo to a piston compressor...look at it this way if you have a 50 gallon tank on that compressor and you put a pin hole in it the same ammount of air will flow out (during the first few seconds which is much longer than a valve is open) of it as will out of a 100 gallon tank.

It would be the same concept in the engine. When the valves open up, it takes time and sustained pressure for all that air to quickly rush into the combustion chamber. If you have a small turbo then the air would start rushing in but it would quickly run out and for the rest of the time that the intake valves are open there wouldn't be much air left to flow into the combustion chamber. A bigger turbo would supply this air and let it all come in during the precious time that the intake valves are open.

As stated before if the air ran out you would not be able to hold the boost level.

If the turbo is too small then not only would you not be able to flow much air into the combustion chamber efficiently, but you wouldn't even be able to flow enough to keep up to a certain pressure. In other words, before the turbo could create enough pressure in the system, the intake valves would already open and use up all the air and the turbo wouldn't be able to keep up. If you CAN supply the pressure, then you just won't be able to efficiently get as much air into the chamber. The pressure is measured at how much pressure there is in the piping waiting to get into the combustion chamber. This doesn't mean that there's necessarily the same amount flowing into the chamber

If the air is all at the same temperature and you are not changing any of the restrictions so that the combustion chamber initial conditions are the same then yes the pressure directly controls how much air gets into the combustion chamber. Therefore if you can supply the same pressure and the same temp (neglecting the exhaust side) then 2 turbos will flow exactly the same ammount of air.

Keep in mind I don't have much to back me up so I'm wondering what some of you with some kind of degree or education in this has to say. I wouldn't be surprised if the first paragraph could be shot down completely; I should have just took it out of the post. I think there obviously is some reason why a bigger turbo would perform better at the same psi even though they are very close in efficiency range and the temperatures would be minimally different.

This thread is here so people like yourself can learn. Glad to help ya on this.

#1 a bigger turbo is not going to be close to the same efficiency (or even close) to a small turbo at the same pressure (remember changes in rpm too)

#2 temperature is always a factor...5 degrees = 1% power gain (so 10 degrees on a modded car is 6-10 hp)

#3 they do perform better when you have incorrectly selected the smaller turbo...you can go too big.....


As previously stated the only effect the turbo has on flow (at constant pressure) is its changes in resistance (exhaust side) which imho is more based on the housing and the nozzle inside, and its changes in intake temps.

 

[Zenja]

I understand the difference in temperature and the actual airflow restriction in the exhaust turbine, but I still think it might be possible that what I was saying (pretty poorly) might be true.

I'll try to exaggerate to paint a picture. Let's say the intake valves are closed. The turbo is pumping air and pressurizes the whole intake system. The valves now open and that pressurized air rushes in. Now let's exaggerate and say that the turbo is so weak it can't supply much air at all. The air would rush into the combustion chamber and as it's removed from the piping the pressure drop becomes less and less. Eventually the air is barely flowing into the chamber (because the turbo can't pump much - again, an exaggeration) but the intake valves are still open for a while longer and this time is being used on nothing. As soon as the valves close again, the turbo pressurizes the system and you have your pressure that you read. In contrast, a bigger turbo would be able to pump more air and keep the air flowing in smoothly the whole time rather than running short, but not so short that you can't supply the amount of boost that you're aiming for.

I realize that the scale of each of these elements might be completely different in a real world setup but it should still affect things.

Now, it's late at night and I haven't really thought about how the valves open and in what order. If there is basically always some intake valve that is open then what I just said is irrelevant. I think that's not the case though - aren't our cylinders paired and the intake stage is only one out of the four stages? If so then there should be time that the intake is closed.

I'm taking engineering next year. I love stuff like this and it will be interesting to understand it better.

 

[crankbender]

I'll try to exaggerate to paint a picture. Let's say the intake valves are closed. The turbo is pumping air and pressurizes the whole intake system. The valves now open and that pressurized air rushes in. Now let's exaggerate and say that the turbo is so weak it can't supply much air at all. The air would rush into the combustion chamber and as it's removed from the piping the pressure drop becomes less and less. Eventually the air is barely flowing into the chamber (because the turbo can't pump much - again, an exaggeration) but the intake valves are still open for a while longer and this time is being used on nothing. As soon as the valves close again, the turbo pressurizes the system and you have your pressure that you read. In contrast, a bigger turbo would be able to pump more air and keep the air flowing in smoothly the whole time rather than running short, but not so short that you can't supply the amount of boost that you're aiming for.

#1 there is basically overlap on the intake valves.
#2 if the pressure dropped in the intake manifold you would see it in your boost gauge and you would not hold pressure.
#3 you don't sample pressure only when the valves are closed.

Turbo systems on cars do NOT work like a compressor and tank for air tools.

The systems are paired for ignition purposes but do overlap on the intake valves being open (basically there is always atleast one open usually more with 272 cams and such).

 

[kpt4321]

Originally posted by Zenja
I agree with the theories talked about in this thread but many people would disagree and say that it's all just theory and good math but it doesn't hold up well in real life.

It does hold up well in real life.

 

[Groomz]

Nice work Crankbender. Love reading the posts.

 

[painmaster]

So is sizing the turbo for an engine based off the CFM through the engine? That stealth site talking about compressor maps says you cant flow more air through your engine without uping your VE. But doesn't uping your boost pressure cause more CFM to flow through your engine, regardless of VE?

 

[DirtyBirdRacing]

Hi,

I dont mean to bring this back from the dead, but I wanted to tie up some of the previous posts "loose ends."

So is sizing the turbo for an engine based off the CFM through the engine?

Sizing a turbo is based off of the desired pressure ratio you want, the cfm necesssary for your goals and the efficency that you want to run the compressor at.

The airflow required from the turbo is:

Required CFM = pressure ratio*((displacement*rpm*0.5*volumetric efficeny)/1728)

Pressure Ratio = (Desired Boost+atm. pressure)/atm. pressure

That stealth site talking about compressor maps says you cant flow more air through your engine without uping your VE.

In a boosted engine you can increase your flow by increasing your displacement, increasing your rpm's, increasing the turbochargers boost, and by increasing its volumetric efficency. Remember an engine is just and air pump really, so anything that makes the pump pump more is going to increase the engines airflow. If displacement, rpm, and turbo boost are held constant then yes the only other thing that can be changed to increase engine air flow is increasing your volumetric efficency.

But doesn't uping your boost pressure cause more CFM to flow through your engine, regardless of VE?

Yes, based off of the previous equations.

Also,
CFM stands for cubic feet per minute. Its a unit for measuring volumetric flowrate. The turbo is moving that amount of volume in one minute.

PSI is a unit of pressure, it stands for pounds per sqaure inch. As can be seen pressure = force/area.

When the air is compressed by the turbocharger its pressure increases but so does its temperature, counter acting each other in way (why we use intercoolers to cool the intake charge) and to take full effect of the pressure increase, since this is what is increasing our cfm as well as our charge density. The higher our charge density the more fuel we can burn, thus the more power we can produce.

If it is still confusing you can think of it like this. The volumetric flowrate (cfm) discribes how much stuff is moving during a period of time. The pressure (psi) describes how much the air in this case is "pushing back" on its container(the intake manifold perhaps).

Bill