GT500FTW

Hey guys.
I'm having difficulty understanding why power at 8psi on a small turbocharger is not the same as power at 8psi on a larger turbocharger.

The way I see it, if im at full throttle, and if I have a wastegate that opens at 8psi and a small turbo is producing enough boost to open the wastegate, then the pressure between the compressor wheel and the inlet valve would be 8psi

If I'm at full throttle with the same 8psi wastegate, but this time with a large turbo, the pressure between the compressor wheel and the inlet valve is 8psi again.

Now, giving that the cam lobe only opens the inlet cam for a brief moment, the volume of air that could enter the cylinder, at 8psi(small turbo) and 8psi(large turbo) would be the same. I.e., when the piston is at the bottom of its induction stroke, in both scenarios, the max psi in the cylinder at this time could be 8psi, so the number of oxygen atoms in the cylinder would be identical in both scenarios.

I know I've possibly made a noob error somewhere in my logic, but if someone could point out my mistake in laymans terms, I would appreciate it.

GT500FTW

So a larger turbo increases exhaust velocity, which aides scavenging? I don't quite get what you mean by just "back pressure"

Could it also be air density? In my example I was presuming no air density loss using a smaller turbo - obviously I understand that a smaller turbo will increase air temp and reduce density, but surely this wouldn't cause the huge power differences?

wantboost

iTrader: (2)

No no no

A large turbo moves more air at a given psi than a small one..

A gt28 at 8psi and a gt42 at 8psi move entirely different amounts of air which means 2 different power levels. A bigger turbo will force more air into the motor at a given pressure than a small turbo will

It doesn't matter how long the valve is open... turbos force air into the motor, hence the positive pressure in the intake manifold.

gabocastano

The psi is the restriction in the system and the smaller turbine will get the same restriction (8 psi in your case) with less flow.

SPOOLINmatt

think about it this way, 6 psi on a big *** dragster slick makes the tire look fully inflated and full, but 6 psi in our small tires makes the **** start looking flabby.

turbohatch96y7

iTrader: (15)

picture a fire hose with a tiny nozzle at the end... Before the nozzle you have pressue.. but only a certain amount off water can escape so fast from the hose..

Now picture the same hose, but with a slightly larger nozzle at the end.. same pressure in the hose but more water is escaping..

drummerdude

But if the bigger turbo supposedly forces MORE air in the cylinder, the psi would increase. The way its been explained to me is that the bigger turbo produces lower IATs, which is what gives more power at the same psi. If the density and temps are the same 8 psi should make the same power regardless of turbo size.

drummerdude

http://www.physicsforums.com/archive.../t-193403.html
If both turbos are creating 19 psi in the manifold, they must be producing the same cfm at that given pressure, as cfm would directly be related to psi... an increase in cfm should have a corresponding increase in psi. Bigger turbo, higher possible cfm, higher possible psi. I may be way off base here, but from a physics standpoint it has to be, and only be, the larger turbos efficiency that gives more power at the same psi.

spectacle

Whoever explained it to you, explained it to you wrong.

drummerdude

The bigger turbo is more efficient. Assuming, for my example, that all IATs are the same at the compressor outlet, if the bigger turbo created more cfm, it would increase the pressure... if you can't understand that, then I can't help you. Think about it, if the volume of air in the intake manifold (where we measure the psi) is increased via higher cfm, there is now more air in the same space, thus higher pressure. Turbo efficiency leading to lower IATs is the reason a bigger turbo at the same psi makes more power.

SPOOLINmatt

"How exactly does the posi trac rear end on a Plymouth work? It just does"
-Joe Dirts dad

Best way I can say it. There's all sorts of equations and physical data to read to figure it out. PSI is generally irrelevant and kinda the by product of the amount of cfm the blade is flowing at a given comp wheel rpm. The tire pressure or water hose examples explain it the best in everyday type examples.

turbo82

That is wrong. Psi is psi no matter the cfm. More power from a bigger turbo is cause it moves more air at the same psi.

drummerdude

The bigger turbo would be more akin to a bigger hose not a bigger nozzle, so both hoses at the same water pressure put out the same amount of water. Just my .02 cents

drummerdude

So by your theory, the air in the cylinder can only go up to the pressure available to it (8 psi for example). So with a small turbo 8 psi is available to the cylinder, with a bigger turbo again at 8 psi the cylinder still only has access to 8 psi. Explain to me how that equals more power. My example is explainable... is yours? More efficient larger turbo leads to lower IATs which allows more fuel to be introduced without detonation, thus increasing the power.

turbo82

Ok to make it easier using a hose as a reference. Compare a garden hose at 8 psi vs a fire hose at 8 psi. Both see the same psi but the fire hose puts out more water cause its bigger. Now apply that example to a turbo. If you can't understand that, non of us will be able to help you.

drummerdude

If the nozzles are the same size in both hoses, and both were at 8 psi they would output the same amount of water. Either way, truth is truth, and you are free to believe whatever you'd like. Email Garrett and ask them, actually I think I'll do that right now... assuming you won't use some analogy to prove them wrong.

tepid1

You're wrong drummerdude. All you're doing now is making a mess of this thread....

LocoJoe

turbo82

I never said anything about a nozzle. And you clearly have a lot of learning to do. Once you get that email back, it will say the same thing we have been saying.

Ash J. Williams

stay in school kids.

OneBadTurboCRV

I was going to type out some stuff but...

Very Basic:

http://www.modified.com/editors/0804...w/viewall.html

Confused Yet?

http://www.theturboforums.com/thread...essure-vs-flow

This thread is truly pathetic...

wilham320

iTrader: (1)

Just for compressor side of things....At the same psi and same rpm a small turbo and a large turbo will move the same volume of air. The real issue is the density or mass of air being moved.

A larger turbo will move a larger mass of air than the small turbo at the same psi and rpm (yes and therefore still the same volume). This is because the larger turbo will be moving cooler air. Cooler air is denser than hotter air (check a chemistry book – cooler air means more air in the same space provided), therefore there is more oxygen contained in a cooler volume of air then a hotter volume of air of the same volume

The other half of having a bigger turbo is a larger exhaust wheel to allow less exhaust backpressure at the same manifold pressure.

tepid1

D=m/V

You people are killing me!

GT500FTW

If I have 2 identical canisters, and fill them with oxygen.
1 is at atmospheric pressure and 1 is at 100 psi.

The number of oxygen atoms in the 100psi bottle would be vastly higher than the number of oxygen atoms in the atmospheric bottle.

If I then set them on fire, the bigger explosion would come from the 100psi canister.

Exchange canister for cylinder.
If I have 2 cylinders. One is at atmospheric pressure at BDC and one is at 100psi at BDC, the number of oxygen atoms in the second cylinder is vastly higher, so the more oxygen there is to burn.

GT500FTW

Lets look at it another way.
If I have a massive turbo on my car, with an insanely flow rate, running at 10psi.
If I change the intercooler for a larger one....an intercooler the size of a house (for argument reasons, lets assume it has no better cooling properties than the smaller intercooler)...the flow rate would remain the same, but the pressure would drop DRAMATICALLY!
Say if the manifold pressure is only 1psi now, the number of oxygen in the cylinder would decrease, and you would have less oxygen to burn.