lets look at it this way. Fact is Fact. A bigger compressor wheel flows more volume at any given pressure measurement than a small one.
JESUS CHRIST!!!!!!!!!!!!

 

maybe if they think voltage and current. You can flow 12v (pressure) through a 22ga. (little turbo) wire and 12v through a 2ga. (big turbo) wire. but you can't flow 150amps (cfm) 22ga. wire that you could through the 2ga wire. Or the same reason you can get 12volts through any of your fuses in a fuse box, but some will flow 30amps and some won't without popping. they all can supply 12 volts (pressure) but not all can support the same current (cfm). the current (cfm) flowing through these can be different at the same voltage (psi). Jesus. end of story I hope.

 

This is exactly right. The nozzle being the intake valve which is the restriction into the cylinder. How can a bigger turbo flow more air past the same valve (nozzle) restriction and still be at 8 PSI back pressure behind the valve? You can't. Once you hit a restriction in air flow (intake valve) pressure will build up behind your restriction. The only way to increase air flow beyond that restriction is to increase pressure or cool the charge so the air is denser and a larger volume is surpassing that restriction at 8PSI.

 

Wow. Just.... Wow... I think I'll just say goodnight gracie

 

Because pressure is only one value of the system, and volume flow rate (ft^3/m) is also not very accurate. Larger turbochargers have a much larger mass flow rate (kg/s, lb/m), which means they are moving a much larger mass of air through the system at the same pressure.

Engines do not burn pressure or volume, they burn oxygen mass. The end.

 

Turbos work on MASS FLOW, not VOLUMETRIC FLOW.

 

The topic is PSI vs. CFM. CFM is a volumetric flow capacity by rate (time). FYI
You can't compare a force to a volumetric unit of measure. There is no tie.

Now, a turbos main purpose is to maximizes the engine's VE. VOLUMETRIC EFFICIENCY! Increased airflow and compression = greater MASS entering the engine.

Ever wonder why they call the cold side of a turbo the COMPRESSOR? It compresses the air, which causes heat (friction) but yields more mass per volume. More mass = more power potential.

You guys need to get PSI out of your heads. PSI is a restriction and should only be used when measuring the delta across the engine to determine efficiency. A 4G63, LSX and a B18 have very different VE. So, when buying a turbo you need to know your setup and what will work best.

 

Why are y'all still trying to explain something they obviously will never understand.

 

Thanks for explaining it guys.
I'm not trying to be awkward, I'm genuinely trying to understand. I appreciate the people that took the time to post and explain.

I get it now.

I was thinking that the psi figure meant the pressure in the cylinder.
The psi figure is the restriction in the system, created by the inlet valve.
The psi will remain the same with a large or small turbo, but the physical mass of the air entering the cylinder will be greater, so the higher the volumetric efficiency.

FYI the garden hose vs the fire hose analogy was great.

 

I'm glad you understand now.

 

Assuming that is correct please explain this for me. How does one turbo get more air past the same restriction (inlet valve) than another turbo?

The arguments I'm hearing in this thread is because a larger turbo moves a larger volume or mass of air. That's fine but once you've hit the limits of how much air can pass said restriction how does an air source of larger capacity get more air past the exact same restriction without any additional pressure or force (PSI) pushing the air past that restriction?

In other words a certain amount of air volume or mass can pass a restriction point. Once you supply more air than what will pass that point it starts to build up pressure behind it right? We measure that in PSI. The PSI or excess pressure is because the turbo is supplying more air volume than what can pass that restrictive point. So now we are already supplying such an excess of air that we are seeing 8 PSI of pressure behind the restriction. Swapping to a larger air source but keeping it at 8 PSI will still flow the same air past that restriction. It can't magically just get more air past it. Now a larger air source and generate less heat and supply a cooler denser charge at 8 PSI.

The hose example proves this point exactly too.
If you have a garden hose with a fixed nozzle (intake valve) and have 8 PSI of pressure you will get X volume of air or water past that nozzle. Now change out the garden hose for a fire hose or heck put a 3' diameter city water supply pipe on there. Still at 8 PSI the EXACT same volume of air or water will pass that nozzle (intake valve). I don't see how you guys are arguing that more water will flow out the nozzle still at 8 PSI just because the hose is larger in diameter.

 

Air is a very compressable media compared to water, oil, etc. Because the lager turbo compresses the air and makes it have more mass per a given volume you will pass more through the valve area. Stop getting hung up on your thought process.

I'm done trying to break it down for someone who is stuck on their own fucked up logic. Good luck to those that care enough to help these people out....

 

I high lighted in bold above what the crux of your explanation is. That is the first time I've grasped that concept. If it was in the thread before I missed it.
No need to get snippy, I'm not trying to argue with you. I'm really trying to understand.

And actually our explanations for more air are basically identical. You are saying the larger turbo will have more air mass because it compresses the air more. I was saying the larger turbo would have more air mass because of a cooler charge. I may have been slightly off but not to far. In fact a denser charge would have to be cooler right? Air can only be a certain density and a certain temperature otherwise the pressure would change.

So essentially it's a denser air mass (which has to be cooler right?) is the reason a larger turbo will produce more power at the same given PSI. I don't think anyone ever disagreed on that.

I think the confusion was coming in from people saying it's flowing more volume or pushing more air past the restriction point. Also towards the beginning of the thread someone points out that a larger turbo produces a cooler charge (denser air) which is why you'd get more power at the same PSI and then others go on to argue that's incorrect which lead to more confusion.

So in summary a larger turbo compresses air more efficiently generating less heat which results in a cooler denser air mass which will produce more power at the same given PSI than a smaller turbo would.

 

You are incorrectly assuming that the intake valve is solely responsible for the restriction and hense pressure. The entire system is an air pump, you need to move as much air out as you are moving in. A larger turbo will have a larger turbine capable of flowing more air out, decreasing backpressure and intake pressure. Stop concentrating on just the compressor and look at the whole system. With less restriction the compressor can move more cfm at lower pressure.

 

Everything but the heat part, correct. Its amount of heat generated is a little less than you think. The more you compress, the higher the increase in actual heat generated..

That's it.. not that deep. Tepid1 isn't snippy, he's just elaborating a point.

 

Yes you are correct in that the whole system is an air pump and there are many restrictions. I've seen this first hand on my Mustang. It has a supercharger moving a fixed volume of air based on engine RPM and when I changed the stock exhaust manifolds for long tubes I removed a restriction in the system and my PSI dropped because the engine was now able to consume more air. HP went up though because the car was now flowing more air. I totally get that.

However for the sake of this thread it's stated that simply using a larger turbo produces more power than a smaller turbo at the same given PSI. We are not talking about removing any restrictions in the motor or changing it's abilities to flow more air. All else been equal just using a larger turbo.
That said the larger turbo makes more power because it compresses the air more efficiently and produces a cooler denser air charge which would get more air mass into the cylinder. Is that not correct?

 

Turbos definitely generate a lot less heat. My experience over the years has all been superchargers. I've been through so many of them, roots blowers, twin screws, ported, pullied etc. I can tell you that to small of a supercharger been spun really hard on my cobra producing 17 PSI was also generating such high IAT2s that HP was suffering. The intake air charge was just simply so hot and inefficient. When I'd switch to a more efficient larger blower at only 14 PSI I'd see the same HP and IAT2s where way down. Larger blower was more efficient and produced a much cooler denser air charge.

Going from a smaller turbo to a larger turbo should also generate cooler charge. I get what you're saying though just not as much of a difference as I'm used to seeing on superchargers.

 

Thank you for bringing up the supercharger example. I think its an easy analogy to understand and the same concept applies to this thread.

No its moving MORE air mass now that you've decreased restriction with the better header.
Pumps flow more volume with less restriction - fuel pumps, superchargers, turbos.


Yes we are removing restriction and changing the engine's ability to flow more air with a larger turbo. A turbine wheel is a huge exhaust restriction. Where is intake air going to go if it cant move out the exhaust? Its going to back up, increasing PSI.

 

compared to Superchargers, of course they generate less overall. But even going to a larger turbocharger with 'less boost" still doesn't account for lack of response , usability and need than a smaller one. If the turbo is tuned, it kinda doesn't matter unless you're going for higher pressures over 25+ psi of boost pressure.

 

Good grief I totally missed that. I'm so used to superchargers which have no bearing on exhaust flow when swapping them out that I was only thinking of the compressor side of the turbo.
I take it the wastegate does not provide enough air flow around a smaller turbo's turbine to keep up with the exhaust air flow capabilities of a larger turbo?