Bigger turbo will almost always give more power at the same boost. Bigger turbos flow more air, more efficiently.
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
explanation. please help!!!
Official Miami Dolphins Thread: Come share in the misery!
| 2012 Nissan Leaf | 1995 Jeep Wrangler |
| 2007 Toyota Prius | 2003 Pontiac Montana | 1991 Acura Legend
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?
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.
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.
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.
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?
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
Check out my Garage build thread - 08.5 GTI 2.0T TSI, APR stage2, APR intercooler, 42DD 3" Catless Downpipe, Autotech 3" Exhaust, Full Carbonio intake, AWE DV relocation, BSH Motormounts, Coolingmist CMGS stage2 Meth injection, Dieselgeek Shortshifter, Southbend Stage2 Endurance clutch, Forge Shift knob, Euro LED tails, Koni Coilovers, Miro STP3 19x8.5 rims, General Exclaim 225/35/19 Tires