# Thread: Is 20PSI on K04 equal to 20PSI on GT28? i think no my friend thinks yes

1. ok so here is what happened. a few days a ago i saw another B6 A4 guy drive by me so i followed

him and he pulled over and we talked about our cars for a little. he said that he had a K04 setup

running 20PSI of boost. so blah blah blah later that day im talking to my friend about his car and i

tell him yea he is running a K04 at 20PSI. so he says "so its basically as fast as your car". i drive a

B6 A4 with a GT28 running 20PSI boost. so i say no my car would be faster because it is bigger

running the same PSI. so he says 20PSI is 20PSI. we argued for a good 30 minutes and couldnt

come to a conclusion. i figured since its 20PSI in a bigger turbo it would be moving more air. Can

someone tell me if im an idiot or right or what????? ive searched google and couldnt find a clear

2. Bigger turbo will almost always give more power at the same boost. Bigger turbos flow more air, more efficiently.

3. You should have just whipped your cacks out and the question would have been answered

4. More air would flow at the same pressure through a bigger outlet... Why is this so difficult?

You should have just whipped your cacks out and the question would have been answered
This.

5. Originally Posted by BlackA4stage3
Can someone tell me if im an idiot or right or what????? :
Possibly all of the above.

Larger turbos have more capacity, it's pretty simple. If you own a turbo vehicle you should atleast understand the basics of it's operation.

6. Think of power as a function of air volume rather than boost pressure required to supply that volume.

20 PSI pumped through a straw will not flow the same volume of air as 20 PSI pumped through a garden hose... unless you've got a REALLY big straw.

7. This is going to be one of those TCL threads where it's best to look the other way and pretend you were never here.

8. Originally Posted by Shomegrown
This is going to be one of those TCL threads where it's best to look the other way and pretend you were never here.
Yeah, I agree.

9. Originally Posted by nmap
Think of power as a function of air volume rather than boost pressure required to supply that volume.

20 PSI pumped through a straw will not flow the same volume of air as 20 PSI pumped through a garden hose... unless you've got a REALLY big straw.
This. There are also other factors to consider, like the straw size, not only the ins and outs of the turbo, but the size of the intercooler piping, intercooler etc. The larger that piping is (assuming the bends are equal) the more air can get through at any given PSI. So, yeah, the post I quoted....

Chris

11. With his logic, what would be the point of upgrading a turbo then?

12. You need to get smarter friends.

13. Pressure and volume are 2 different things...

14. Originally Posted by unintended acceleration
You need to get smarter friends.
Lol

15. OK, the answers here are mostly right, but for the mostly wrong reasons!

To understand, you need to look at the basics - what is boost?

Positive boost pressure occurs because there is more air being forced through the engine than the engine can ingest using its own volumetric efficiency.

Changing to a larger turbo does not have a significant impact on VE or the volume of your intake system, nor is it the most restrictive part of the intake (that's the throttle) so the "blowing through a straw" analogy is way off base. It doesn't apply at all.

The reason larger turbos usually make more power for the same relative boost level is twofold:

1.) Compressor wheel efficiency leading to lower IAT
2.) Lower TIP (turbine inlet pressure or less exhaust restriction)

What this means is if the IAT is the same and the exhaust backpressure is the same, then 20 psi on turbo X will absolutely equal 20 psi on turbo Y even if the turbos are sized differently.

Now, both factors 1 and 2 can be controlled outside of turbo size. Let's pretend...

Car # 1 has the smaller K04 but open dump pipes and a huge FMIC with meth injection

Car # 2 has the larger GT28 but smaller intercooler and a quiet exhaust with cats

Under those conditions, it would be possible that the Car #1 performs better at 20 psi than Car #2.

Long story short, boost pressure is restriction in the engine, not the turbo. So 20 psi = 20 psi unless other properties are changed (VE, IAT, TIP, etc).

So who's not going to read the thread and be the next to post the straw/garden hose analogy?

16. The bigger turbo is capable of pumping more air at higher pressure than a smaller turbo. If the engine management is actuating the waste gate of both a bigger turbo and smaller turbo to keep the pressure in the intake manifold the same for the same throttle position and engine speed, then the power output of the engine will be the same despite the turbo size. The bigger turbo simply has more potential that is being wasted. In fact, the bigger turbo may be worse due to slightly higher turbo lag.

If you take the smaller turbo and push it near the edge of it's design limits, the charge temperature will probably be higher, reducing engine performance. This situation is probably not terribly likely with a stock setup. In a car that's running higher boost pressure than designed, it can happen.

It's likely a smaller turbo will hit the limit of it's air flow capability if it's too small, causing a drop in intake manifold pressure at higher engine speeds. So, in this case the bigger turbo is better. It's also not running at the same "PSI." The air flow is not high enough to maintain the pressure.

If one of the setups is faster, it's probably because the slower car had a turbo that couldn't maintain intake manifold pressure when the engine gets closer to red line.

If the engine management is actuating the waste gate of both a bigger turbo and smaller turbo to keep the pressure in the intake manifold the same for the same throttle position and engine speed, then the power output of the engine will be the same despite the turbo size.
Correct, but only if you assume intake air temps and engine volumetric efficiency is the same between both setups.

18. Originally Posted by Shomegrown
So who's not going to read the thread and be the next to post the straw/garden hose analogy?
I'll just sit back and wait until someone tries to "correct" you

19. no

20. thanks for the comments guys. i always thought i was right but he made me second guess myself causing this thread lol. i will be sending him a link to this thread so say hi to him. his name is rusty and he drives a g35 coupe fully bolted

21. Is the reason that IAT is lower with a larger turbo at the same PSI due to less air turbulence with a bigger inlet?

22. Basically to expand upon what Shomegrown said, two differently sized turbos (lets assume similar compressor and turbine designs just different sizes) *can* make the same power at the same boost, it is IN SPITE of the size difference. The bigger compressor will generate less heat for a given pressure, and arguably even more important is the fact that the bigger turbine side will have a lower Exhaust or Turbine pressure ratio (lower ratio=better Volumetric Efficiency).

On otherwise identical physical setups, even if you work to get IAT's on the smaller turbo down the bigger turbo will more power because of the hot-side still allowing greater VE.

23. Originally Posted by BlackA4stage3
his name is rusty and he drives a g35 coupe fully bolted
There's the problem. G35 with bolt ons, he should know the VQ doesnt do anything with simple mods. I'm totally assuming and kind of being an ass, but he probably got 10whp for \$2000.

24. Originally Posted by konigwheels
There's the problem. G35 with bolt ons, he should know the VQ doesnt do anything with simple mods. I'm totally assuming and kind of being an ass, but he probably got 10whp for \$2000.
he beats my avant with a gt28. he has put a lot of money in it but it looks great and sounds great and handles very well, so id say money well spent. plus NA means no turbo heart ache like us audi guys have sometime lol

25. Yes, more capacity, but the same amount of air flowing through two different size tubes (turbos) will produce different pressures. Less flow through a smaller opening will produce more flow through a larger opening.

The equation escapes me at the moment, I have not had physics for over 20 years. But this is straightforward physics. Your statement is suspect.

Originally Posted by rsj0714
Possibly all of the above.

Larger turbos have more capacity, it's pretty simple. If you own a turbo vehicle you should atleast understand the basics of it's operation.

26. Said simply:

A bigger turbo will have cooler, denser air (read:more oxygen) at a given PSI than a smaller turbo, all things being equal.

More oxygen means more bang bang.

More bang bang means more zoom zoom.

27. Originally Posted by BlackA4stage3
ok so here is what happened. a few days a ago i saw another B6 A4 guy drive by me so i followed
Did you cup his balls?

28. Originally Posted by silverA4quattro
Did you cup his balls?
yes. i was on my way to class and saw the only other modded audi in statesboro GA so i had to creep. ended up skipping class. thats Audi dedication

29. You will just build your 20psi faster and reach peak hp quicker thats all 20psi is the same whether it be a small or large turbo. your just pushing more air to build 20 psi before the smaller turbo, its a pretty simple answer...

30. I thought a turbo forced an exhausted mixture which has very little O2? Most would have been exhausted in the initial cylinder detonation. I wonder if the difference in exhausted gases would even be a factor? I thought the pressurized exhaust from the turbo provides the energy, not the O2?

Originally Posted by DIAF
Said simply:

A bigger turbo will have cooler, denser air (read:more oxygen) at a given PSI than a smaller turbo, all things being equal.

More oxygen means more bang bang.

More bang bang means more zoom zoom.

31. Originally Posted by LhW
I thought a turbo forced an exhausted mixture which has very little O2? Most would have been exhausted in the initial cylinder detonation. I wonder if the difference in exhausted gases would even be a factor? I thought the pressurized exhaust from the turbo provides the energy, not the O2?
No, the exhaust spins the hot side and then continues out the tailpipe.

On the other side of a nice bearing is the cool side, which pulls air in through the intake.

Intake air is not exhaust air. You are thinking of DEI.

:lol:

32. Ahhh - I got it. The illustration is classic!

So, 02 does play a factor. News to me. Thanks for the info.

Originally Posted by DIAF
No, the exhaust spins the hot side and then continues out the tailpipe.

On the other side of a nice bearing is the cool side, which pulls air in through the intake.

Intake air is not exhaust air. You are thinking of DEI.

:lol:

33. PSI = a measure of resistance, so "boost" does not equal horsepower. What you want is CFM. That's a measurement of flow, and all a motor is, is an air pump. The more you can push through it, the more HP you can make.

A small turbo can reach 20psi, yes, and a large turbo can also reach 20psi. Difference is that with a larger housing, you can flow more cubic feet of air per minute than a smaller housing. Think of it this way: 20Psi of air through a 3ft diameter pipe flows more air than 20Psi of air through a 1/4" pipe

Anyone that tells you 20 psi = 20 psi has no clue what they are talking about

34. Originally Posted by LhW
Ahhh - I got it. The illustration is classic!

So, 02 does play a factor. News to me. Thanks for the info.
There are three things that matter in motor - fuel, air, and spark (hidden fourth would be timing).

If you add more of the three proportionally, you get more power.

35. Originally Posted by Shomegrown
So who's not going to read the thread and be the next to post the straw/garden hose analogy?
wait for it...

Originally Posted by Slayer
Think of it this way: 20Psi of air through a 3ft diameter pipe flows more air than 20Psi of air through a 1/4" pipe
...and there it is.

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