equal boost but more power?

[ACCSlvCivic]

Ok,
Ive been reading a whole lot about turbos of late and i noticed something quite interesting. a car with a T66, a T78, and a T88 all at the same 15psi of intercooled boost dyno as much as 40whp diff (or so the adds and stats claim). I don't understand how a T88 @ 15psi vs. a T66 @ the same 15psi could possibly be any more powerful. Is it the housing? how cool the air is as it gets pushed through? i'm not sure, but i am certianly comfused.

Also why is it that a TT setup where the first turbo is 8psi and the second at 15psi isn't as fast as a single turbo at 15psi (i've seen single turbos own the TTs, but i can only stare confused)..

maybe it's just bad driving and bad numbers?
thanks

[drift]

it's more about total turbo flow as measured by PPM or CFM. any half assed T3 can push 20psi... but a T6 series will push 20psi with much more volume of air.

[fritz_269]

If all those engines had the exact same displacement, only major difference in those turbos outputs would be heat. There is a notion of efficiency of a turbo - a 100% efficient turbo would only add as much heat to the air as the absolute minimum that thermodynamics requires. A 50% efficient turbo would add twice that heat to the air.

Different sized and shaped turbos are most efficient at different psi and flow numbers. A small turbo may be able to push a large psi and cfm, but it will be less efficient there, and put a lot more heat into the air than a big turbo at that exact same psi and cfm. This is why it's important to size a turbo appropriately from the compressor map. The idea is to try to stay near peak efficiency as much as possible.

The cooler air has a greater density, allowing you to add more fuel and get more power. The cooler air also keeps the engine away from dangerous detonation, allowing one to advance ignition timing and maybe the IVC point, also resulting in more peak power.

[flylwsi]

okay...
the reason a big turbo owns a tt is b/c it is bigger, flows more air, even at the same boost level, b/c it is more efficient, yet has more lag. that is the downfall there.

the same psi from different sized turbos is easy...

basically, you are pushing say, 15psi through the smallest of your three. there isnt a whole lotta flow there, considering the size. if you push the same psi through a bigger turbo, it would make sense that it is moving more air, making more power, spinning faster. remember that the size of the turbo is on both sides, and if you step up to a bigger turbo that spins freer, and causes less restriction, you will see big power gains at the same 15psi.

think of it this way...

if you have three containers, each a little larger than the one before it, look at the amount of air, or whatever, that is in it. it is all at the same pressure. wouldnt you want the big one?

if one 8 oz cup holds 8 oz at room pressure, but a 12 oz cup holds 12 oz at room pressure, what would you take?

example again...
if you blow through a straw, and you move say, 15psi of air, then step up to a bigger straw, think of how much harder you would blow to move 15psi, meaning that you are moving more air at the same psi...

i hope that kinda makes sense, i know what i am thinking, but it isnt coming out...