Forgive me if this has been answered, but some cursory searching turned up nothing. I have a basic question: how does a larger turbo give you more power, if you aren't running any more boost? I run 23-ish psi on the stock turbo and make about 305 whp. I hear about people running 20-22 psi on after-market turbos and making a lot more. Now I realize it's not really valid to compare one dyno to another, but I'll assume that these people spent a couple grand on a new turbo kit for SOME reason. From what I have seen, the stock turbo is good up to above 25 psi or more if you can feed it enough gas (or methanol).

Quite simply, why do you want a larger turbo, if not to run more boost?

 

Because a larger turbo pushes more air at a given psi. PSI is not a measurement of power.

 

I may need a more detailed explanation. Is there a good place on the web I can read about this? To my mind, a piston is going to create the same amount of volume on the intake stroke no matter what. That volume is the same amount of air at a particular psi, no matter what. It's the engine that's moving air ultimately, not the turbo, right? The turbo just pressurizes the manifold so the same volume of air is more actual air molecules.

So from my understanding of things, psi really *is* power, given a constant bore/stroke, intake valve cycle, etc. And if as you say that's not really the case, then something is flawed about my understanding and I would like to know what.

 

The larger turbo pushes more volume at the same psi. There is an awsome thread on here somwhere about this.

 

Allow me to answer my own question. I knew I should have checked Wikipedia for posting here. Anyone else that wants a detailed answer, just search for "turbocharger" on Wikipedia. A larger turbo makes more power at a given psi for 2 reasons:
1. The turbine is larger and therefore less restrictive. Less exhaust back pressure = less parasitic power loss due to back pressure. This portion has nothing to do with moving more air.

2. A larger compressor means cooler charge air at a given psi. In other words, when I said above "That volume is the same amount of air at a particular psi," I should have added "and temperature." Cooler air means more air at the same psi.

So a larger turbo does not flow more air volume, but it does flow more air molecules at a constant volume and pressure, because the charge air is cooler.

 

Yes, a larger turbo DOES flow more air. Not sure why you're arguing that. Go read about CFM now if you'd like. Less backpressure and cooler air charge are lesser reasons for greater power than the fact that a larger turbo flows more air at the same psi.

If you don't think a GT42R flows more air at 25psi than a 16g, then I don't know what to say. We're talking double the horsepower, and you don't get that simply from a cooler intake charge and less backpressure. It flows a TON more air at 25psi than a 16g.

 

I'm not arguing it, I am detailing the issue as I understand it. If I am wrong, please explain why, and please don't just say "that's how it is" (which is effectively what you've said so far). I am not interested in how it is; I am interested in why it is.

I understand that more CFM of atmospheric-pressure, ambient-temperature air is going *into* a larger turbo's compressor. But the same CFM *must* be coming out of the compressor (and again if I am wrong, please explain why), because the engine will flow the same CFM of air at a given RPM and manifold pressure. However the *temperature* of the air coming out of the compressor is cooler, hence same CFM == more air. Since the temp. and pressure of the atmosphere is constant (more or less), you get more CFM coming in for the same CFM coming out. Again, as I understand it. And that means as I understand it, charge air temp and lower back pressure are still the only 2 factors at work in a larger turbo making more power (and this is not my inference; it's what I got from Wikipedia).

Once again, I am not arguing. These are the facts as I understand them. You have yet to even attempt to convince me otherwise. It may be that we are both saying the same thing...you are saying more CFM in, I am saying same CFM out -- to-MAY-to to-MAH-to -- and we are both right. If that's the case, I think we're done. If not, please set me straight, and as any good algebra teacher says, please show your work

 

sorry posted twice pc is screwin up

 

psi is psi but the colder the air the more it will compress which is why ppl spray their intercoolers with nitrous systems rather than injecting it into the motor one of these set ups can add up to 50 hp almost instantly

 

Example: lets say 16g is 2" diameter suction/discharge, & the gt42 is 3" suction/discharge. Both at 20psi, which has more flow?

 

Here is a simple example : Just keeping it simple.
One 20" fan
One 30" fan
Both blowing at the same speed. Which one will cool a room faster and efficiently ?

Of course. Aftermarket turbos come with diff. turbine/compressor wheels, housings/hotsides a/r ect...But if you want a simple explanation..there you go.

 

I'd say the gt42 (EDIT: and the 30" fan for the other example), as long as there's no engine on the other end

Seriously though, whether there's a 2" pipe, a 3" pipe, or an interdimensional rift on the other side of the turbo, I still don't understand how any turbo is gonna flow any more air (I'm talking volume *out* of the compressor) as long as that air is going nowhere except through an engine that moves a constant volume of air at given RPM and manifold pressure. Which unless someone tells me different, is how I understand engines work (got that off Wikipedia too!).

Anyway I am pretty sure I have my answer. I was never trying to suggest that big turbo at X psi doesn't make more power than small turbo at X psi. I have seen data that says bigger turbos make more power at the same psi, and I believe it. And that can only happen if more air (as in air molecules, not volume of air) is making it into the engine. I know a thing or two about thermodynamics and I'm pretty confident my understanding of what's going on is correct.

 

here read all of this dude.
http://www.enginelogics.com/cmaps.html

 

i forgot this one to it will help ya.
http://www.stealth316.com/2-3s-compflowmaps.htm

 

the engine ISNT flowing the ame amount of air with a larger turbo.

the bigger turbo forces more air into the motor which in turn makes the engine flow more.

the best explination i have seen in here is the fan one. it is pretty basic but gets the point across.

to make more power you need more fuel. to add more fuel you need more air. to get more air you use a larger turbo.