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?