Warrtalon

iTrader: (6)

Glad you're convinced of something that's not true. I can't explain volumetric efficiency to you, but I find it amusing that you didn't actually know why people buy bigger turbos then suddenly because of wikipedia you thought you knew everything.

So you still think larger turbos don't flow more air volume, but rather make the air more dense? Okie dokie.

urslo4n8

iTrader: (5)

Think of it like this bro.....say you have a water pipe of 4" diameter with water rushing through it at 22psi. Now lets say you have a 6" pipe with the same water rushing through it at again; 22psi. You have the same psi with much more volume. Now replace the water with air.....more air+more fuel = more power@ same psi and yes theres way more to it than that but in a nutshell more volume of air from a larger turbo can mean more power

W_evo8

iTrader: (22)

bernoulli would slap you if he were alive.

spdracerut

iTrader: (2)

Joehunk, you are completely correct in that regardless of turbo size, they are all basically pumping the same CFM into the motor as measured at the intake manifold.

CFM is CUBIC Feet per Minute. That is a unit of VOLUME. Volume is dictated strictly by displacement. If you took the turbo off our motor, it would still pump the same CFM as measured at the intake manifold at 7k rpms as a GT40 turbo pumping 50psi (assuming the same volumetric efficiency of course). Now, the mass will be completely different which is where all the confusion is.

On compressor maps, the CFM given is all at STP (standard temp and pressure for all of those that remember 10th grade chemistry). So yes, at the compressor inlet of the turbo, the bigger turbo does flow more CFM.

If you guys don't understand any of this, go grab that chemistry book from high school.

sharkm87

iTrader: (29)

i see where his mind set is.......if in the IM, there is constant pressure of 21psi, whether its from a turbo, whether you have a bike pump......but if the pressure in the IM is always 21, than why is it necessary to switch the source of that pressure to make the same 21psi.......now i could understand if you were to change the size of your UICP and LICP to flow the more valume, but if that remained a constant, as in the examples, I dont see a difference either at peak boost...unless the argument is spool up time

KevinD

iTrader: (56)

wow. just wow!!!!!


warrtalon, usually i enjoy reading your posts but your are way off the mark on this one.


joehunk is looking for an engineering answer because hes, well, an engineer.

so hopefully this will help you some charles:

you are correct. a huge turbo is going to flow the same CFM into the motor as a tiny turbo, or even the same sized engine without a turbo, assuming the same volumetric efficiency. reason being, the CFM (or cubic feet per minute, i.e. a VOLUME rate) is entirely dictated by the bore and the stroke of the motor (and Volumetric efficiency). a 2L motor is going to suck in about 2L of air every two revolutions. adding a bigger turbo will not suddenly make it 3L, it will still be 2L of air coming in.

heres were having the different sized turbos makes a difference.

DENSITY. a more efficient compression cycle will mean the air coming out of the turbo will be colder. the colder the air, the more dense you can make it. the higher the density, the higher the MASS flow rate of the turbo will be. this is why you will see some turbo compressor maps labeled in lb/min or other mass flow rate units. yes, a bigger turbo does flow more air, but the CFM is relative to the engine, not the turbo. the Lb/min is greater with a bigger turbo, and is dictated by the efficiency of the turbo as well as the size of the motor.


the fan example a few people gave is completely wrong. if you have a 20in box fan, and a 30in box fan, and point them at a 3inch hole in a wall, your only going to get what that 3in hole can flow. making it a 100in box fan still wont make a difference. now lets say you have the 20in box fan somehow compress the air to 20 psi and push it through the 3 in hole. a ton more air is going to pass through correct? correct. if the 30 in box fan was completely the same in efficiency at compressing the air, your still only going to get the same 20psi through the 3 inch hole. now lets add a 10in box fan that is far more efficient at compressing the air, its outlet temps are colder, the charge is way more dense, but even at 20psi you have many more air molecules per square inch then the 20, and the 30inch box fans. now when aimed at the 3inch hole, more air is going to pass through even at the same 20psi, because it is more dense. thats how a bigger turbo makes more power then a smaller one.

one other addition to this is the turbine backpressure. it was already mentioned that less backpressure has less parasitic loss on the motor. that is correct as well. its like the difference between a test pipe and the stock cat. less back pressure helps the motor put more power to the ground, rather then pushing it through the exhaust.

SWOLN

iTrader: (6)

Best thread I've seen in a while.

Now if we can just make a 2 or 3 stage compressor to compact all those little air molecules even more, and alter the laws of physics to so that compression does not make heat, we would be faster and richer.

Maxdelta

Dude Great break down thank you

Mmelmann

iTrader: (1)

Let's simplify things... break it down barney style.

My understanding is through using a snorkel analogy. If you had to breathe through a drinking straw (small turbine diameter) you get a higher psi in the tube but it's not very efficient. Then, if you have to breath through say a divers snorkel with about a 1.5" diameter (larger turbine diameter) you will get a smaller psi in the tube but it's way more efficent at letting you breath. Would that be a correct analogy? Correct. You could even just focus on the exhaling part of the breath... i.e. the compressor side of the turbo.

Maxdelta

hmmm not an engineer but wouldnt a bigger diameter make it harder to pressurize or at least take more work to make the same PSI?

SoundBoy4

Take a closer look at the ideal gas law and the relationship between pressure, volume, and temperature. pV=nRT where p is pressure, v-volume, n-mass(molar), R is the gas constant and T is temperature.

The simplest thing to take out of this is the pressure/volume inverse relationship and the direct relationship between pV and Temperature.

Now imagine two compressors - Compressor A and B. The only difference is that B has a better compression ratio and flows more air. Now if you were to keep the same mass as you understand it (the 'n' value in the ideal gas law) then the pressure would have to decrease. But compressor A and B are both run at the SAME PSI, the temperature of compressor B may be slightly smaller, The gas constant value remains well...constant, and it means you HAVE to have a greater concentration of molecules in compressor B.

Boy, i am a horrible teacher. All i can say is research Boyle's law, Charle's law, and Lussac's law for simple equations that relate it all.

Cliff note: (pressure) x (volume) = (constant number) x (molecules) x (temperature), A larger turbo at the same psi as a smaller turbo flows a greater volume of air which has to result in more air molecules for combustion.

gixxerman11

iTrader: (10)

ill give my .02 look its like a bicycle tire the are at 80psi bt a car tire is at 35psi its the area of the psi not how big it is

PessimiStick

iTrader: (2)

Greater mass, same volume.

douggsx

iTrader: (2)

Great question asked! The OP made me take a step back for a second and rethink why a larger turbo made more power for the same amount of pressure.

Its funny how some people jumped to the conclusion of bigger turbo = more air. Yes a bigger turbo will move more air, but you can't move more air into the same volume without increasing the pressure! Come on guys.......

Thanks KevinD for the detailed explanation!

Joehunk

Kevin, thanks for being the first person willing to come in here and talk to me on the level I was asking for. Thanks also to the other posters who did the same.

Also SoundBoy thanks for busting out the equations. I have a degree in ChemE so I was obviously tempted to do the same, but someone once told me you lose 10 audience members per equation I think it was Warrtalon who suggested that charge air temp could not possibly be enough to account for the increase in power from larger turbos, but temp has the exact same weight in the equation as pressure and volume as everyone can now see.

Anyway this is all largely academic for me since a larger turbo is likely not in the cards for me any time soon. But it was a gap in my knowledge of tuning turbocharged cars that I wanted to fill. Despite the noise on this thread, I am glad to have it filled, and also impressed that Wikipedia had it basically right!

EDIT: Also, one other factor no one has mentioned yet (even my by-now-over-touted Wikipedia lol) is that cooler charge air also means you can heat the air more before you detonate. So more air mass, more burning fuel, more cranking it up before you detonate = more expansion = more powah! (and that last part I even figured out all on my own )