What difference does turbo size make?
 

Okay guys, humor the SC guy for a minute...

I read lots about 7 psi from turbo x is way different the 7 psi from turbo y. What specifically makes them different? Is it charge temp/density? My limited understanding is that with the heads/valves being the same, the effective opening is still the same therefore 7 psi of manifold pressure is going to move the same amount of air through the opening regardless of how its produced.

I think the same stuff is valid for SC cars as I see some of the same variations blower to blower, it's just that we have fewer options to compare.

Is any of the difference a "tuning" thing? By that I mean do the guys with the bigger turbos, also have better engine management and can therefore extract more power from the 7 psi than someone else?

And maybe 7 psi isn't the best example, does it have to be 15 psi and then compressore efficiency comes into play? But that puts me back to my first question, why does it matter how the pressure was generated?

1. If all things remain equal 7 psi from a big turbo is the same 7 psi from a small turbo. The difference is temperature of the air, and efficiency at which the turbo compresses it. The other difference is optimal operating range of the turbo. Small turbos work great off the line, large turbos work well toward redline.

2. All things being equal if I run a megasquirt on a t25 and some other guy runs a Hydra with a T25 we should be making the same power if both cars are properly tuned. Tuning plays a role in how much power you make, but it can either be tuned or not.

3. It could be 7 psi and it could be 15, it depends on the turbo itself and its compressor map. It matters how the pressure was generated because you dont want to spin a turbo outside its efficiency range, you get a turbo suited best for your application.

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Guys correct me if im wrong somewhere.

Think of it as CFM instead of pressure. 7psi through a 2 inch pipe is not the same as 7psi through a 3 inch charge pipe. What you really want to compare is CFM and Charge Temp

Yes. Its all about the charge temp. The colder air is, the more dense it is.

this is a good comparison:

http://www.msprotege.com/forum/showp...51&postcount=8

1 is way wrong. the difference is flow.

small turbo is like blowing through a straw.

big turbo is like blowing through a big pipe.

you can do both at the same pressure, but one will flow a lot more than the other. that's why the bigger turbo works better at higher rpm where the engine requires more mass flow rate of air.

but you also get some of that higher flow benefit at lower rpm if you can get the turbo to spool.

That's how I make >250 rwhp at 9 psi and someone with a 2554 would have to be probably over 15 psi to get the same power.

ditto. if #1 were true, we could run K03's with a huge FMIC and make 300whp at 5psi.

Thats why I love this forum. Thanks

it might also help to compare the turbo/SC to an injector. a 230cc and a 650cc both operate at the same pressure, but one sprays a lot more fuel.

Not disputing you, but here is where I get lost. If you blow throw the straw or through the pipe and pressurize the intake manifold to 7 or 15 or whatever PSI, you still have the same pressure and the same size opening for that pressure to go out of when the valves open. So what am I missing?

They're saying the difference in turbos is not the pressure, it's the volume of air that it can push. It's easiest to just look at the figures. Y8s turbo at 9 psi put him at like 259 HP. My small T25 at 9 psi puts me at more like 190 HP. It's because of the larger turbo pushing more CFM that he gets more power and the temps are lower.

if you blowing through that straw at 7psi, and blow through the pipe at 7 psi, more air is moving through the pipe than the straw. But it takes longer to reach that goal whatever psi.

Chris, here's another way to say the same thing.

Take your average 1.6litre Miata. Naturally aspirated, the engine pumps 1.6litres of air every other revolution. At 1000rpm, that makes 500x1.6=800litres of air.

But say you're boosted (1bar=14.7psi to make it easy). So, if you're running 1000rpm at 14.7psi, that's 500x3.2litres=1600litres of air per minute. At 7200rpm, that's 3600x3.2=11,520litres of air.

11520litres=407cfm.

If your turbo can push out at 407cfm, then you can make 14.7psi on a 1.6litre motor at 7200rpm.

As others have said, it's not about how much PSI a turbo will make, but how much AIRFLOW it will make. Engine size and RPM are just as critical in this equationas PSI of boost. The biggest problem when making a small turbo overwork itself (such as making a small T25 push 15+psi) is that the compressed air gets inefficiently hot (hot air=thin air=low oxygen).

Here's a great read:
http://forums.nasioc.com/forums/arch.../t-359824.html

it's more of a dynamic pressure. and there's 8 valves to choose from. so even if some are closed, another pair is open.

and there's different amounts of compression going on in the motor. the psi on your gauge is only at one point--an average in the plenum. as the air enters the ports, the pressure isn't constant.

so the local pressure at the port goes up, the mass flow rate at the port goes up, the cylinder fills with more air than a smaller turbo that can't provide the increased flow through the ports.

I'm sure some form of bernoulli's equation covers exactly how to figure it, but i'm not gonna.

Okay some of it is seeping in. Thanks for the patience.

Let me see if I can restate what I think I learned.

The larger turbos have more CFM capacity, and a greater efficiency so they can
a) produce the same pressure but add less heat to the charge
and
b) maintain that pressure more uniformly as the valves open and cause a pressure differential within the intake tract
and
c) that the difference in power output for a given indicated psi is the sum effect of a and b above.

Is that mostly right?

That sounds right to me.

So, lets ad another piece of the puzzle... spool. Since the turbo is powered by exhaust gas, and it takes more exhaust to spin the turbine of a larger turbo vice smaller, you're going to spool later on a larger turbo. Assuming WOT, your average T25 will start to spool around 1800rpm and be fully spooled by 3500. Your average T3 will start to spool in the low 3's and be spooled mid-4's. A T4 will start to spool mid-4's and maybe be fully spooled by redline.

So when talking about the POWER a turbo makes, also think about the powerband it produces. I like my T25 because of the great mid-range torque. Somebody who drives on the track a lot might be more interested in a smaller T3 which spools well but has a lot more top-end.

im going to repost this for emphasis:

http://www.atpturbo.com/root/release...2871r_dyno.gif

Same psi, timing, air fuel ratio?

Chris, you're absolutely correct. If you have 7 psi in the intake manifold and the air temperature there is the same - you will have the same air flow through the engine regardless of what size turbo, or a supercharger, or even a person blowing through a straw is doing it. Same psi and temperature - you'll get the same flow (CFM).

But, this is if the exhausts were the same - and with the different turbos they are not. Different size turbos, put different restriction in the exhaust and that restriction in the exhaust changes the the air flow ... the same 7psi in the intake manifold will be different air flow depending on the exhaust restriction (the turbo), the least restrictive (highest flow) being with a supercharger (because it is creating zero restriction in the exhaust compared to a turbo). Alas, the supercharger gets the highest flow, but uses some of the produced by the engine power to compress the air (via the belt), so it adds one more variable to the system ... ;)

From what I understand, manifold pressure is actually a measurement of the DIFFERENCE in pressure between the intake manifold and the exhaust manifold. When you see a 12psi "difference", or what most people refer to as 12psi of boost, on a small turbo (technically a small turbine/turbine housing), you don't need to flow as much air to maintain that 2psi of difference. With a big turbo that allows pressure to escape from the exhaust manifold faster, you need to flow more air to maintain the same 12psi of difference (boost) in the intake manifold.

Same difference, but more air with the larger turbo, and more power. This is why people like Y8S can make 257whp on 9psi, but it takes me 15psi to get to 219whp with my smaller turbo (GT2876R vs GT2554R).

Can't be the same, but it's probably similar. Each turbo will run less timing at their respective torque peaks, and the air-fuel will be tuned for the flow of each turbo.

Having said that, Jeremy at FM has said that he is able to run more timing the more efficient the turbo is, which makes sense.

from his thread: Audi A4 Quattro 1.8T at 22 psi. The only things changed were the turbo and tuning (same boost level, etc.).

Since the OP's question has been answered, I have a question that could be related to this topic.

How important is compressor housing A/R?

doesn't make a lick of difference.

Uhm, no. In relative terms (like on a boost gauge) it's the difference between the pressure in the intake manifold and the ambient pressure outside. In absolute terms (MAP sensor) it's the difference between intake manifold pressure and a complete vacuum. In no case does the pressure inside the exhaust manifold play any part whatsoever.

That's how it's measured, but what causes the restriction? If you think about it, the coffee straw vs. garden hose analogy isn't very good; the size of the turbo doesn't change the intake volume of the IC/pipes/intake manifold. Speaking directly out of my ass, doesn't it make sense that the backpressure in the manifold/turbo system itself is the real cause as to why bigger turbos flow more air at equal pressure?

Its just a number for comparison purposes. Its a ratio between the housing and the inducer, or something like that.

Heres a question since the ops questions has been answered.

Why cant cars make boost in neutral? I have several thoughts, but rather have someone explain once and for all.

Actually, Saml01 was right on the money when he said "If all things remain equal 7 psi from a big turbo is the same 7 psi from a small turbo. " The key point being "if all things remain equal". Like temperature.

When we talk about "more air" it's important to bear in mind that this refers to the mass of air, not the volume. For a given engine, all turbos are acting against the same restriction, namely the engine itself. They're trying to blow air into a closed space. So all else being equal, the more pressure they can build within the intake system, the more individual air molecules will find their way into the cylinder during the intake cycle.

Problem is that the density of air decreases with temperature. If you heat the air up, then for a given volume at a given pressure you have a smaller mass of air.

This is why "bigger turbos flow more air." As a broad generalization, turbochargers on the larger end of the scale are more efficient at moving large volumes of air than are smaller turbos. Meaning that they can move large volumes of air without heating the air as much as a smaller turbo would when doing the same work.

Imagine that we have an imaginary intercooler which is 100% efficient, meaning that the air coming out the coldside of the intercooler is equal to the ambient temperature in Carlsbad, CA under all conditions. We'll call it the i-cool.

With the i-cool installed, it doesn't matter what turbo we are using or what boost pressure we are running, the air going past the throttle plate will always be exactly 74°F. Under this condition, 10PSI out of a GT2554 at 5,000 RPM will truly be equal to 10PSI out of a GT2871 at the same engine speed, assuming that the pressure measurement is made at the intake manifold (the usual location.)

Now, if you were to measure the pressure between the turbocharger and the inlet of the i-cool under those conditions, you'd find that the pressure coming out of the GT2554 would be higher than that coming out of the GT2871. Because of course the air would be much hotter at that point, and the smaller turbo would actually be moving a greater volume of air than the larger turbo. The fact that they're both achieving the same pressure at the same temperature after the magic i-cool means that they're both flowing the same mass of air, but the smaller turbo is having to work harder to do it, heating the air more in the process, and thus ironically ends up moving a greater volume of air than the larger turbo.

Unfortunately, the i-cool is fiction. And thus, in the real world large turbos do typically wind up making greater power for equivalent boost, and 10 PSI winds up not being equal to 10 PSI, at least insofar as the mass of air that both figures represent.

but joe, bigger turbos are bigger. bigger exducer, bigger compressor outlet. bigger. more mass flow for a given pressure.

the size of the intake pipes is all subject to bernoulli as well. the pressure changes every time the flow area changes.

Yup, I think so too. If the two turbos achieve the exact same psi and temperature in the intake manifold, the difference in the airflow depends entirely on the backpressure difference of the two. Whichever of the two has less backpressure, that one moves more air mass if the intake pressure and temperature are te same.

That's definitely a big piece of the puzzle-- bigger turbos tend to not just have a bigger compressor, but also a bigger turbine. Larger turbine a/r = slower spool, but also less exhaust restriction and more ultimate power potential at the top end. On the flip side, smaller turbine a/r will spool faster but create a bigger exhaust restriction and hold you back on power at the top end...

Mine will vent the BOV in neutral. Audible BOV noise = boost.

The turbo has inertia and spins even at idle. It spins faster when you rev the engine quickly and with inertia it takes time to slow down. Thus, positive pressure, albeit a small amount, when you slam the throttle plate shut.

Thats just vacuum opening your BOV and whatever the turbo is pushing is vented out. May not even be 1psi.

I think it doesnt make boost because your engine equalizes much faster then when its under load. No load on the engine means whatever as fast as air gets in, it just as fast gets out. But with load on the engine and the hot exhaust gas spinning it even faster it has more ability to squeeze in air during the intake cycle.

I can make a good 2-4psi reving up on my T3.

if you hold the motor at 7 grand and snap the throttle plate closed, you'll get boost in the intake for a split second.

Don't you mean that you will get positive pressure pre TB? Post TB will just continue to consume what is left in the manifold(instantaneously) before relying mostly on the IAC and returning to an idle state.

You CAN get positive pressure while free rev'ing on a small enough setup. This is not to mention that if I really want positive pressure, then I would just adjust the anti lag to give me as much as I want. For instance, under no load, using my Boost Logic tubular header with my 2876, I can build 25+ psi on anti lag sitting in neutral.... all by 4k. This is with the AEM ecu. Theoretically, there is no load on the engine itself, but by creating enough exhaust energy(key to turbocharging) by timing the combustion event to occur extremely late in the process, you can build positive pressure.

My bone stock 07 Mini Cooper S can also build positive pressure while free revving.

Justin

If it wasn't POURING rain right now I'd go rev the snot out of my car, piss off the neighbors, and post the datalog. :)

before we modified my gt2560r it'd make 5 lbs of boost revving in neutral.

yeah sorry I didn't specify what part of the intake. i meant the part between the turbo and TB.

what scheme does the AEM use for antilag?

It can actually do it a number of ways... Rally style(for killing turbos very quickly), and drag race style... Each way uses a different algorithm for either retarding timing and injecting extra fuel based on TPS, deceleration delta, rpm, etc. The AEM is pretty robust when it comes to the versatility. It can also use an additional injector in the exhaust if you so choose, however, if only using antilag on occasion, there is no need. As I mentioned, I can build as much boost as I would ever need sans an exhaust injector. Exhaust valves can take a beating after a while though... I recommend no longer than 3 seconds of antilag at a time. Since the Miata is my road race car, I only use antilag when I am extremely bored and feel like hearing some banging and popping.

Justin

The main reason is that turbo work off load. When the car is at idle there is little to no load so the turbo doesn't strong volume of exhuast. I think the most boost I built at idle was 4# with my 20g