So my cousin and I were having a discussion over a couple of emails and now I'm confused.

Contrary to how I thought turbos work, my cousin says otherwise.

I thought that a smaller turbo and larger turbo at the same boost pressure would result in the larger turbo creating more power in the engine.

My cousin says no. Both turbos at say 15psi would provide the same amount of charged air in the intake.

I thought it was just like fans. Larger fans have a higher flow rate than smaller fans at the same RPM.

Would someone care to explain?

I remember reading a few posts on this forum about how a 16G or T28 at say 15psi would provide you more power than a T25 or 14B at the same boost level.

I also recall reading something similar to this all over the place (tuners, other DSM sites).

Cheers

Here's his email...

Begin quote//

The problem with guys using a 16g and running fast times is that (and I know I’ve been out of the scene for a while) they are not doing it with pump gas. If I run race fuel and turn my bleeder up to 23+ psi I’m sure with some practice I can run close to low 13s (possibly high 12s). Slapping on a 16g will not make you any faster if you run it at the same boost you have now (in theory). If your turbo can make and sustain the boost (say 15psi) to redline then a bigger turbo will not get you anything (discounting the effect of a more efficient exhaust side on a bigger turbo which actually does have a big effect). The only time you will get more power is if you turn up the boost or make the intake air colder and that’s where the big turbo comes in. It can spin slower (less heat) and provide the boost to whatever pressure and up to redline all on pump gas.

If I had one of those FP35 series turbos and assuming the lag wasn’t too bad I would run it at 25psi on the street and if I were racing I would turn it up to 30-35psi and hope my engine could handle it.

//end quote

For a given pressure, bigger turbo = less spinning = less heat = colder air = more air = more power.

Which he seems to understand, so why does he contradict himself?
Slapping on a 16g will not make you any faster if you run it at the same boost you have now (in theory).
Wrong! It'll be faster.

Your theory is right Michael, your cousin has definately been out of the scene

A bigger turbo at 15psi will push more air than a smaller turbo at 15psi, and it will do it more efficiently. A larger turbo flows more air at the same boost level, will require more fuel and WILL make more power.

Back in the days, and people still today, slap on a bigger turbo and don't go faster because they have not done any supporting mods or tuning.

With that said I ran mid to high 11's all day on a 16g on pump gas and meth injection and 30 psi in a full weight car.
Also ran two 10.9s @ 132ish on my FP3065 last year on pump gas and meth injection.



Most people do this and HOPE that it can handle it, rather than trying to tune it for high boost and actually make the engine handle it.

That's exactly what I thought.

Jeez, I thought I'd read everything wrong for a second.

This is going to be long, but I thought I'd include the whole conversation...his part.

Begin quote//

This was one of the first things I had to come to grips with when I got into the scene. If you have 15psi worth of air in your intake, as far as your engine is concerned it does not matter what kind of turbo is supplying it. The amount of air is the same so the power will be the same. In reality you will get a bit more power with a bigger turbo at the same pressure but not what you think. The extra power will come from a cooler charge air and easier exhaust release (less backpressure).

If you think about it, if your little turbo is supplying 15psi what is the difference when a big turbo is supplying 15psi? Think of the intake tract as a fixed volume and you pressurize it to 15psi, the number of air molecules is the same, the only variable is the temperature which will increase the number of air molecules if it is lower. The benefit of the bigger turbo is the ability to have a cooler charge of air compared to a smaller turbo at the same pressure and volume enough to supply the pressure at higher boost levels. That’s it (other than exhaust efficiencies).

It’s one thing to hear about guys going fast with a particular setup but it’s quite another to actually try and do it. These days the formula is more precise then when I was into it but the fundamentals don’t change. Back in the day someone with a 14B in Ohio ran high 12’s and tried to mark it off as relatively simple to do, he listed his mods which were the same as what I had but with a fuel pressure regulator. I thought when I go to the track I should get 13’s easily cause a lot of other people with a lot less were deep into the 13s, I wasn’t even close. Getting low 14s was tough to do with my setup. No one talks about how much weight they removed or what kind of gas they used and how much boost they actually used or how much they had to abuse their car to get those times.

I can tell you from my experience that with a 14B and exhaust and intake mods at 15psi, the best I could do was low 14s. I measured power with a GTECH PRO and datalogger from dsm.org to be around 260-275hp. No matter what I did I could not get a better time or make more power until I touched the boost. The only time I was able to run 13s is when I bumped the boost to 18psi and got my power up closer to 300hp. That’s when I started looking into what actually makes power and discovered how much misunderstanding there was with bigger turbos. Everyone thinks that all they have to do is slap on a bigger turbo and do some supporting mods and they will be fast but I think they’d find out how wrong that thinking is.

A 16G will not flow more air than a T25 at 15psi (assuming everything else is the same, i.e. temperature and backpressure and assuming that the T25 can maintain 15psi to redline). In a fixed volume 15psi is 15psi, there is no difference. Google the universal gas law (PV=nRT), it helped me.

<break>

Yes a bigger turbo will flow more air, all that means is it will pressurize your intake faster. Once you hit your set pressure you have no more air molecules than if it were pressurized with a smaller turbo thus the same power. As mentioned the benefit of the bigger turbo is to support air volume at higher boost pressures, smaller turbos cannot keep up the volume and boost pressure drops.

//end quote

So my question is, what is the boost level a measure of?

I think it's the charged air pressure level entering the engine, but not the amount of air, oui?

which is what you want on any car. The colder you can get that intake air, the more dense the air is. Which results in more fuel needed to burn it, which makes your power.

The smaller turbo will flow less volume at the set psi. Think of you blowing 5psi of water out of your straw, and blowing 5psi out of a garden hose, same pressure, more volume.

....I think I got that right

Your cousin has a wrong basic misconception of turbos and boost. At the same psi he is stuck on volume as being the same. Of course it is if you press the pause button. Mass of the air is not the same.

A bigger turbo flows more air at the same boost level. E.g. A FP3065 will flow maybe 50lbs/min at 30 psi , a 16g will flow 40lbs/min at 30 psi, and a 70mm triple ball bearing turbo would probably flow 75 lbs/min at 30 psi.

He is stuck on 15psi being 15psi regardless of turbo. He doesn't know that each turbo flows a different amount of air.

Its not the high boost level that makes a car go faster, its the volume of air flowed in a given amount of time that does.

If his analogy was correct, go tune a 14b car at 19psi on 550cc injectors. Without changing anything else, slap on a GT40r and roll around at 19psi and let me know what happens :

Yeah, ask him what he knows about CFM

The pause button and garden hose anologies are good

I think it's the whole concept of BOOST and what's pressurized that's being missed here.

My cousin thinks the intake has a fixed volume, which makes sense, but he thinks that the air that is charged in the intake is 15psi, in terms of volume...so in otherwords there is 15psi 'worth of air' in the intake and you can't fit any more.

15psi of boost is the charged air's pressure, or the rate at which it's being forced into the intake, but not the volume, or amount of air going in.

This is a bad example but should clarify things for myself and others I guess.

If we could make a teeeeeeeny little turbo the size of a pop can or something and flow air that is charged at 15psi through it into a cylinder or something that would DEFINITELY be less air than a regular sized turbo on a car doing the same thing.

So what is '15psi'? That is the question. He's misinterpreting that I believe.

Shoot why do I always doubt myself on 'for sure' things?

We're talking about air pressure and volume of air here, it's an obvious thing no?

I think I doubted myself because he's older and supposedly wiser than I. Haha, I'm the baby of the family at 28. lol


Edit: Fans and Turbos = same concept no? In terms of amount of air passing through them, spin rate/size/turbulence etc.

Modify that a bit.

15psi is the air's pressure.
CFM (cubic feet per minute) or lbs/min as in Reza's example, is the rate at which the air is being forced into the intake.

You need to consider at least both of those to get an air mass.
Bigger turbo, more CFM at the same boost = more air, and your motor is happy to keep eating those 15psi over and over again. It's not just "fill the intake up once and we stop", it's perpetual.

I am no expert, but I am not sure I agree with this....
If nothing else changes, then think of it as trying to fill a sealed container. If you have a certain amount of pressure in a certain size vessel, there will be the same amount of air, whether you fill it with a garden hose or a straw. The reason you have "boost" or "pressure" is because you have a restriction....you are providing more air than the engine can use. Unless you change the amount of air the engine uses, then it is going to require a certain amount of air to maintain 15psi in the intake piping regardless of what turbo put the air there, no? If you want to put in MORE air, that is going to by its very nature mean more pressure...

Isnt the mass of the air determined by volume times pressure? If that is the case, then the mass cant change unless either the volume or the pressure changes..

But 15psi is only what's stuck in the intake. The engine is consuming the air in a cycle and 15psi says nothing to how fast the air is moving.

Exactly, the air "stuck" in the intake. Doesn't 15psi say that is the amount of air that the engine has yet to accept? Wouldn't that mean that as long as the turbo is able to maintain this "buffer" of 15psi, who cares whether it is coming through a straw or a garden hose (ignoring the difference in temperature)? The only time it would matter, if the air was going SO FAST that the smaller turbo could not keep up, but then you would no longer have 15 psi...the pressure would drop...

Someone tell me if I'm out to lunch here, but isn't it (more air mass) the same reason we must re-tune a speed-density setup if the turbo is changed? Something to do with a change in volumetric efficiency?

Picture the bigger turbo pushing the air faster. The wastegate continues to maintain the same boost pressure, but you are filling the cylinder faster at the same engine RPM. You're not over-boosting, so the air must be going somewhere, right? It's exploding and powering the motor providing you give it the right amount of fuel.

That's my guess anyways.

This is exactly what happens with smaller turbos, the engines basically sucks the air out of them, eventually causing boost to fall off, before the turbo heats up the air as it trys to keep up.

Yes this does even happen at 15 psi on a 14b, but it is still able to keep up to the engines demands at that level. A 16g at 15psi, won't heat up as much as a 14b and flows enough to keep the engine happy at a more efficient level.

At the same time I think you have to look at the turbo and whole engine as a system. They work together. You have to look at other variables as well. If you just wanted to run only 10 psi on a stockish car, I would not install a GT35r, instead stick with a 14b or 16g. It will spool faster, making a more fun car.

There are so many variables, and if you wanted to get really technical about it a 14b should be able to push 35 psi and perform just as good as a GT35R at 35 psi. In reality it can't and never will for obvious reasons.

Correct Rob , more air mass is why you would retune a speed density setup, or any setup for the matter.

There are some mixed terms in here...

The point of a turbo is to provide more air *mass* to the engine.

Given that the volume of the combustion chamber is fixed, the way to provide more air mass is to increase the density. Density can be increased by changing either the pressure or the temperature.

So it is definitely is possible for a more efficient (larger) turbo to provide increased density at the same pressure (psi) by keeping the temperature lower.

If you look at a few compressor maps side-by-side it's pretty clear that different turbos can flow different amounts at the same PSI: http://www.ztechz.net/id7.html

A turbo requires energy to make it turn. This energy comes from the exhaust leaving the engine. It makes sense that a small turbo will spin much easier to generate a positive air pressure within the intake - quick spool, simply because of it's size. However, once the turbo begins spinning so fast that it's out of it's efficiency range, (given by the compressor MAP), it is going to be producing more heat and transferring less of that energy into spinning the turbine faster. That heat also reduces the amount of air mass in the intake and therefore contains less energy when mixed with fuel and burned. Meth is a great way to extend the compressor map by reducing that heat generated.
On the other hand, a larger turbo will take more power to get moving (lag) but once it gets going within it's most efficient range it will push more air than the smaller turbo.
To understand which turbo will push more air at a given pressure you would need to compare the compressor maps for the two turbos in question. What you will find is that if your looking for max power typically we are running in a pressure ratio where the larger turbo will make more power with the same pressures, within reason. I don't think you'll see a 42R making more power than a 14b at 10psi on a 2L engine.
As stated, there are many more variables present but compressor efficiency is a big one.

<Jeff beat me to it but said it much more efficently>

I would agree that TEMPERATURE is a big part of the overall equation....but in discussing the theory of turbo size and PRESSURE, I still don't get the difference. Of course there are all the OTHER variables....but how does the engine know you have unbolted the 14b and bolted on a GT42 so it should use more air?
What I am saying, is that everything else being equal, and nothing else being changed, other than the TEMPERATURE difference 15psi is 15psi. IF you want to add more air, you need to change either the temperature(which we were ignoring for this discussion), the volume(which I think we can agree is fixed if you don't change anything in your intake piping/engine), or the pressure...

The engines knows you have put on a new turbo, because all of a sudden there is a sh!t load of more air being forced into it at the same boost level.

Turbos are measured in CFM , not psi.

I've been using simple terms and examples so that the original poster can go back to his cousin and and explain without getting all fancy.

Here's a more simpler example:
Think of your impact gun air tool and 20 gallon compressor tank at 90 psi. Your impact gun is the engine. How well the impact operates is related to the volume and mass of air in the tank.

Now take your same impact gun and hook it up to a small 1 gallon air tank filled to 90 psi, and lets see how well it operates.

Question, they were both at 90 psi, so why doesn't it run well off the small tank, your still using the same impact gun (or engine) ?

I am fully agreed with Ziggy. This is how I understand!

Because small tank will loose its 90psi after first few seconds of operation! If the pumping motor was fast enough to keep-up with the gun you won't notice the difference.

and the same thing applies to small turbos, they don't have enough CFM.....

Now find me a 14b that will run and hold 30psi, and a pumping motor that can produce enough CFM and 90 psi with a 1 gallon tank

I think what people are ignoring is how the motor consumes the air so dynamically. 15psi is just a measurement at the intake manifold - an average at some moment of time. That air is moving and there are a whole lot of other things going on!

What I'd like to know is, why was my car faster at 20 psi on the FP3065 than the 16g at 20psi ? All else the same. Obviously it needed more fuel at the same boost level with the bigger turbo.

I'd like someone to explain that to me.

Did the 16g hold to redline?
The 16g must have spooled faster too, right?
So that means there was more time at 20psi on the 16g. By their logic, the 16g should have been faster!!!

Unless efficiency (temperature) plays a far greater roll than I'm giving it..
But that's fine too, with the argument being only that bigger turbos do make more power with all else being equal.

Yes it did hold 20psi to redline on the 16g, and 20 psi was built by 3000 rpm on the 16g.

On the FP3065, 20 psi was built by 4200 rpm, and once it spooled it felt like the 16g did at 30 psi.

Rule #1

Pressure doesn't flow!

Hum, this is changing the way I look at things...

I think I was looking at it backwards, I was thinking that at the same psi, the turbo flows more CFM.
INSTEAD I should be thinking at the same CFM the bigger turbo can run less psi.

How much the engine will flow (CFM) seems to be a static amount.
So when a bigger turbo can push the same CFM at less pressure (psi) as a smaller turbo, it is essentially being more efficient at the amount of PSI you want to run, being the bigger turbo will not create as much heat runnign the same cfm.
While the bigger turbo will sacrificing low end/rpm efficiency (More exhaust gas to get the turbo spinning)

So I guess it is all about where you are running your car at... are you doing 20psi or 30psi?

Am I out to lunch or does that make some sort of sense?

Can't it be both?

And you're right, it's all about what you're doing with the car and how the rest of the motor is set up.
It's definitely possible to have the wrong big turbo on the motor and end up less efficient / less power over all.

.

Pressure = Force (i.e. forced induction)

An engine is a pump, piston goes down and sucks air in. Pumps are rated by how much they can move (lb/min). Can't squeeze a liquid but guess what, you can squeeze a gas! And wait air is a gas. So let’s put a turbo in front of it and compress the air before it goes into the pump because if we can get more air in than we can put more gas in there and cause a really big explosion forcing that piston to go down quicker!

So your pump theatrically flows a certain amount if it was 100% efficient and could suck that amount of air in one revolution but guess what?, it’s not!! So let’s put that turbo to use and force the air in to make it more efficient! Remember pressure = force. Turn up the force, increase the efficiency of the pump. Turn it up to much and the force breaks things.

To be continued.......i'm at work.

That is how i am seeing it also..but if you increase the CFM that you are pumping into the intake "system", unless the pump (engine) "takes away" more air, you are going to have more pressure (boost - psi). The fact there is 15psi "stuck" in the intake means the engine can't accept any more air, no matter HOW big a turbo you push with...push in more CFM, and that 15 psi will climb...

Obviously, the bigger turbo is capable of more volume or more cfm, but that should only make a difference WHEN the small turbo cant keep up. If they are BOTH providing a "buffer" of 15psi of air that the engine can't accept, then the ONLY difference is back to temperature...

Obviously also the bigger turbo will be capable of flowing the larger volume of air to sustain a higher boost pressure....but if BOTH turbos are running at 15psi, I still cant see how there is ANY difference other than the temperature.

The impact gun /compressor / tank analogy is good....but the problem there is that the compressor wont maintain 90psi in the 1 gallon tank as you are using the air. After a few seconds, the pressure in the tank will drop. Similarly, too small a turbo won't MAINTAIN 15psi in the intake at high RPM....but then you have a pressure drop. IF the turbo can maintain the pressure, I don't see how it is any different than the larger turbo..(except temperature, again)

Ziggy,

I think the main difference here (besides temperature) is the velocity of the air flowing into the engine, which is proportional to the volume flow rate, which people have been referring to as "CFM".

The best analogy i can think of without getting too in depth is to compare a lake to a river.
If you're standing in a lake waist deep you don't feel too much of a force on your body. Where if you're waist deep in a fast flowing river you'll likely be pushed over. However the pressure in both situations is the same, approximately atmospheric.

The same forces can be applied to the air. So even though you have the same pressure for a large turbo and a small turbo say 15psi, the large turbo will have a much higher speed, which then can force more air into the engine.

I'm not exactly sure if that analogy makes sense. But it does make sense in terms of the flow into the engine.

Fun topic, I missed four pages already.

I've always assumed that the gains of the larger turbo at the same pressure was mainly from the increased efficiency, but also to a lesser extent from the increased flow rate.

My assumption regarding the benefit of the increased flow capacity is that since it's a dynamic system, the manifold pressure is actually oscillating very rapidly and these oscillations in pressure coincide with the intake strokes for the four cylinders. When you use a larger flowing turbo, the amplitude of the oscillations are reduced, resulting in better filling of the cylinders. This is also why doing flow related head-work yields more power; better cylinder filling.

I doubt that any boost controller gauge/display has a fast enough response or accurate display to show the pressure changes. It does sound like a fun project though. Might see about connecting an oscilliscope to my map sensor and see what really is happening.

I could be wrong though

I can sort of see that.....but still...the velocity of the air filling the "dead space" in the intake before the engine...
It seems to me, again, how does the engine know how fast that space is being filled? Or why would it care?
Even with the small turbo, there is a 15psi "backlog" of air that the engine can't handle. If the engine was CAPABLE of accepting more air.....wouldn't it "permit" some of that 15psi backlog to enter, therefore reducing the backlog from 15psi?

Remember now, we are not talking about changing anything in the engine like porting for better flow, and we are ignoring the temperature effect.

The only way this makes any sense to me, is if Tashko's theory is correct...and the intake pressure is constantly oscillating. That would mean that what we read on a boost gauge is actually just sort of a "temporary peak boost" reading, and the ACTUAL boost is constantly falling below that. Of course, then the amount of pressure drop during those oscillations would be a lot less with a larger turbo than a smaller one, and the larger turbo WOULD stuff more air into the engine. But then we really shouldn't TECHNICALLY be saying we have 15psi boost in both cases.

Basically what I said back here, I thought that we all already knew that



When you have boost leaks and all the turbo even works extra harder to maintain that same boost level, but producing even more heat.

^^^
But we're talking about a BIG turbo that can keep up, so you never see the boost drop.
The bigger turbo is capable of ramming more air into the cylinder for the very short moment that the intake valves are open in the cycle, resulting in more bang.

It's not just pressure - it's pressure and flow over time.

How about another analogy?
Put some buckets on a conveyor belt moving at a constant speed.
Above the buckets put a garden hose and one of those big pool vacuum hoses, each at the same psi.
Does the bigger hose put more water into the buckets? I think so..

I'm not sure if temperature has too much to do with it actually.
If you were running a car without an intercooler then that would make a lot of sense, but with the intercooler, the hotter air would be cooled more than cooler air. I'm not sure if anyone has ever measured the temperature of the air coming out of the intercooler, but I doubt there would be that much variation in the temperature.

The density of air only varies slightly with temperature, from 0C to 40C there's only about 13% difference in density.
http://www.engineeringtoolbox.com/air-properties-d_156.html

That's why I believe it would come down to the velocity of the air. But it's such a complex system it's hard to tell.
However when everyone is thinking about pressure they're talking about it as if you have a tank full of pressurized air. I don't think we can actually consider the intake manifold to be a tank, since it's probably about 1-2L in volume. Meanwhile at idle (750rpm) the engine is flowing 12.5L/s so the plenum would get emptied 6 times a second.

The other problem is the fact that we don't actually know what speed the compressor is operating at. Based on the compressor maps, at a certain pressure the compressor can flow a large range of volumes and unless we're measuring/controlling the speed of the compressor we can't tell which curve we're operating on.

Ok, this is starting to make some sense to me...but I DON'T buy Rob's bucket analogy. In the engine, the "resistance" is the limited amount of air the engine will accept. The buckets offer no resistance.

If you ran that garden hose and that vacuum cleaner hose, each at the same pressure, to plates that each had a quarter inch hole in them (representing the unchanging resistance of the engine) would one still get more water in the buckets than the other??

It's not just the density of the air, it's also the effect the charge temperature has on the timing you can run.