Yeah, that's the point that we're getting at... something like a GT42 at 16 psi produces more power than a stock turbo or 20g does at 24 psi. That's what was confusing to the OP at first till he figured it out. If we were to push 24 psi out of the GT42 and 24 psi out of the stock turbo, we get way more power out of the GT42... all because of the additional cfm's it's producing at the same psi.

I may have said that wrong or confused myself because the inlet manifold is still the same size.

 

Yeah I apologize. It was bad style on my part to come in here and ask the question then answer it myself. I should have done the research first, then come in here and said "This is what I found; am I right?" Doing it the wrong way seems to have attracted some non-constructive posts to the thread that it would have been cleaner and more to the point without. Anyway thanks again, hopefully despite the noise this will serve as an answer to future people with the same query.

 

oops!

 

This has got to be the funniest thread ever

KevinD definitely saved this thread, while some just tried to sound smart with these fan examples. Joehunk, thumbs up for a great question, it seems that many people learned new things from this

 

.....

 

Very well said! But CFM is cubic feet/min which is volumetric flow rate that tells you how much air is passing through one point. It is dependent on the turbo itself than the engine size. A huge turbo will have more surface area on the compressor wheel than the small turbo therefore more CFM. The CFM can be converted into lbs/min by multiplying it by air density.

Engine being a 2.0L gives us only fixed amount of volume that can be exerted into the engine itself. Like you said, a 2.0L engine will not receive 3.0L or 4.0L of air.

The density here is the key. Since the huge turbo has the ability to move air much more(faster) than the small turbo, at given amount of time when the intake valves are open and volume is fixed inside the cylinders, more air or oxygen molecules rather will be packed into the cylinder due to the velocity of the air being faster. More dense air = more oxygen molecules which means more power.

So the bottom line is, psi is a measure of pressure that is being exerted onto the intake manifold and it does not tell us how much oxygen molecule is present, much less the velocity of the air inside the manifold.
CFM which is dictated by the turbo will give us a smile by telling us a huge turbo will have more CFM which means higher velocity/density of the air therefore in that short interval when the valves are open, more oxygen molecule will be combusted inside the chamber.

I don't know if this is clear

 

So... Are we saying here that a bigger turbo only makes more power because it heats up the intake air less and restricts the exhaust less than a smaller turbo?

 

I like your use of words. "Hey man, it's hot... could you adjust the thermostat to the heat the air less mode 'cause I'm hot man."

 

that's only part of the equation.

Since we're debating why bigger turbo makes more power than smaller turbo at given pressure, with everything being equal between those two, bigger air will move air more rapidly through the intake manifold and into the engine with fixed volume than the small turbo.

Then end result is that once the valves close, the air inside the combustion chamber from the big turbo will have much more oxygen packed inside than the air from the small turbo.

The big turbo simply moves more air than the smaller one at same psi.

 

it has more dense air

 

i thought they both move the same amount of air, just that the larger turbo, with the larger turbine will move larger volumes of air and since the turbine can spin slower it doesn't heat the air as much as a smaller turbo would...causing air to be more dense

 

Where Is The Icon For Beating A Dead Horse?

 

lol just read Kevin's answer. It's what I was after. The more basic answer of "big turbo moves more air than small turbo" is also true, but is incomplete and a LOT less detail than I was looking for.

EDIT: And here you go:

 

yup you've basically repeated what i've said

however, i will agree with you that CFM is a volumetric flow rate (which is repeating what i said), however, the CFM going into the motor (not right after the compressor), is entirely dictated by the motor, not the turbo. again, the volume is fixed. its the volume of the motor. the density changes... and the turbo and intercooler are what effect that, thus thats why i pointed out the mass flow rate, which indicates that a bigger turbo does have a higher mass flow rate. the mass flow rate going into the motor is different from one turbo to the next, but the volumetric flow rate is not different.


the velocity of the air is dictated by the CFM, which is dictated by the engine displacement. so the speed of the air going into the motor is dependant on how fast the motor is going.

at higher RPMs when the stock turbo "tapers" boost, its because the mass flow rate of the turbo is insufficient to maintain that pressure at that rpm. the CFM of the motor is still increasing as the rpms increase, but the mass flow rate relative to RPM drops off.

volume rate increases (i.e. rpms go up) -> mass flow rate stays the say (small turbo limited) -> efficiency drops off (temperatures increase) -> same mass, more volume higher temperatures = less pressure, lower density

bigger turbo:
volume rate increases (i.e. rpms go up) -> mass flow rate also increases (big turbo not limited) -> efficiency is also better (temperatures are lower) -> volume rate increases, mass rate increases, temperatures remain cooler = pressure remains high, density also remains high = much more air.



one thing to consider in all this is where the big turbos make the power. everyone knows a 35r on the 2L with 93 oct and 23psi doesn't make power until deep in the RPM range, say peak power at 7-8k rpm. this is where the stock turbo runs out of steam a 35R with 23psi at 7k rpm is far more psi then the stock turbo is going to give (what, the stock turbo is lucky to give 21psi pushing as hard as it can at 7000rpm).

if you analyze the dyno graphs you will see that a stock turbo will actually makeas much or more torque then a 35R on the same motor with same gas/boost level. thats because the stock turbo makes the power earlier, and this is where horse power has a smaller multiplication from the torque because of the lower RPM. at higher rpm, if the stock turbo could maintain the 300+ftlb of torque, it would have awsome power numbers just like the 35r. 300 ftlb of torque at 7000rpm is 400 whp. 300ftlb at 3500rpm is only 200hp. you can see why a 35r has higher HP numbers all things being equal. the torque numbers generally are in the same ball park, thats because the VOLUMETRIC flow rate is the relatively the same for both motors. but the higher efficiency of the 35r (i.e. lower temperatures when the air is compressed), and the ability to maintain the mass flow rate as the volumetric flow rate increases is where you see the torque numbers hold strong at higher RPM, and thus much better HP numbers.

 

I think this might help a bit. Here are the compressor maps of a gt25 series turbo (fairly close to our 16g i think) and at gt42 series turbo from garrett's website. The glaring difference here, and the source of the power as I can see, is the difference in lb/min at a given efficiency range, not cfm.

correct me if there is anything wrong with what i said.