I am trying to get a little better understanding of turbos. I understand the concept of how they operate and all but I was curious about this.

Alot of people go with the saying bigger is better, but is that always true. For this discussion I will just use generic terms to avoid the differences in manufacturers.

We have turbo A (small turbo) and turbo B (big turbo). For the sake of keeping all things equal lets say that both turbos are efficient to at least 15psi, and in this scenerio we are only going to be running at 15.

Not here is where I am trying to fully understand. Is 15 psi the same 15psi for both of them in which case the smaller turbo would be better because it would spool faster therefore reaping is benefits sooner, or since generally speaking a larger turbo is capable of running higher boost pressures more efficiently the air charge would be cooler than the smaller turbo (since it would be nearing it's peak efficiency and therefore be heating the air more) and therefore more dense in effect pushing more air at that same level of 15psi.

I hope you know what I am attempting to say.

I never claimed to be a physics genius or anything but just close to genius on an IQ level

Depends on what you want to do with your car....

The bigger turbo is going to generally spool later, but will provide more air at the same boost pressure. Great for top end power. The small turbo will spool quicker, but will not produce the same HP numbers as the bigger one becasue it's not pushing as much air.

Generally the smaller turbos are better for autocrossing, road racing, and (normal) street use. the bigger turbos are great for producing lots of power and lend themselves to drag racing.

Personally, I'd probably never put anything bigger than say a 20g on something driven daily. I'm sure some will disagree... a lot of it's personal opinions. Really comes down to what you want to do with the car.

So then if you take a car and put turbo A on it at 15 psi and dyno it you will have less horsepower than turbo B at 15 psi on the same car. Is that because of the air temp or something else.

BTW I am not talking about my specific car just turbo fundamentals in general.

As Rob said and I will reiterate, turbo B at 15psi will push a larger volume of air than turbo A will at the same 15psi.

I heard that (so to speak ) but explain why

you guys should leave psi out of the discussion and just talk about air flow rates like cfm.

the larger turbos have bigger compressor wheels to push more air volume, the lag is the extra force required to spin something physically larger. But once it's spinning it can move a lot more air.

Maybe a good analogy would be thinking of big house fan versus one of those pocket deals that runs off of a double a battery. Using a big turbo at a low manifold pressure is like running a house fan on slow. Conversely, if you hotwire the pocket fan, there's only so much those wimpy blades can do.

Good analogy Kris.

The reason I included psi was because at a given pressure you should not have any more or less air unless the air density is different......right????

Thats kind of like saying if I use a bigger shovel I can get more than five pounds of sh!t in this five pound box.

Do you understand what I am trying to say?

Not exactly the right way to think about it. Back to the house fan analogy. Lets say both fans have a high and a low setting. The low setting is the same voltage for both fans as well as the high setting being the same for both fans. Low RPM and High RPM. The pocket fan takes little effort to go from Low to High and on High, it pushes 100cfm. The house fan takes about twice as long to go from Low to High speed but on high it pushes 600cfm.

Anybody that has lived through the summer without A/C can tell you, the bigger the fan the better. A small fan at 110volts vs. a big fan at 110volts, which one would you choose? The bigger fan takes longer to get up to speed but when it's there you feel like you're driving an open cockpit jetplane. The small fan gets up to speed faster but you only feel like you're driving your brother's big-wheel down the sidewalk.

Small fan=small turbo, big fan=big turbo, 110volts=energy reqired to spin the turbine blades (or exhaust gases coming from the combustion chamber)

Think of it this way, a fire hose at 15psi will have more water coming out of it than a garden hose at 15 psi.

You can move more water volume using the bigger hose(larger turbo) than a small hose (small turbo) at the same psi

Just try it Scorp. It's astonishing feeling the power at lower psi levels on a large turbo versus the power level I'm used to at higher psi on the small turbos even when the physics are understood. It is crazy sweet!

OK understood. I was looking at it from a closed pressure system as opposed to a system that blleds off pressure to allow air to keep moving.

Now a little more specific.......... with the same car set up to handle either and set at the same psi...........if I made a run in the 1/4 with say a big 16 then took it off and replaced it with say a turbo that flowed more but took 2000-3000 rpm longer to hit the same psi, which would net a better time, would the extra airflow have enough time to make up for the extra lag.

I wasn't able to type up a really good response as I was in a hurry for work...

As I can see it, here's the part where it gets tricky. Turbo efficiency.

If a big turbo pushes more air at slower wheel speeds then it would seem to make sense to just set it at a lower pressure. So if a t25 flows 20 lb/min of air at 18psi and a 50 trim flows 20 lb/min at 7 psi, then it would seem to be a lot better to set a 50 trim at 7 and not overly stress a small turbo and risk getting compressor surge or other such nastiness. To go back to my fan analogy, it would be better to put the big fan on the low setting than crank up the juice on the small fan and risk frying the circuit. But this is not the case at all.

You've got to consider the mass of the wheel and the energy it takes. If you look at a turbo map, you'll see what look like islands on a topological map that represent turbo efficiency. Of course more efficiency is better, so you want a turbo with a broad map that relates to the air flow you'd like to see and the operating range you'll be within. So to relate this to the previous analogy, lets say that you are given only a certain amount of juice to feed the fan (which would be the case with a small displacement, specific output engine). Now a large fan isn't going to get the most out of that energy, but a small fan could excel in this situation due to it's low inertia (less mass for fan blades, or compressor wheel).

The thing I still don't quite understand to this day is how a rotary engine gets by running what I would consider a ridiculously low amount of boost and still having a very solid power output and similar dyno charts to a conventional piston engine running more boost. I just read an article about a rotary with a GT42 pushing 30psi, and on evom, there's a huge thread about AMS's 800+ hp customer car with a GT42 at 45psi. So what gives? I doubt the FD in the first article was putting down that kind of power, but why bother with a large turbo if you're not going into it's efficiency range? Do rotaries distort turbo maps or do they just use an entirely different portion?

Here's some turbo efficiency maps to look at.
http://not2fast.wryday.com/turbo/maps/

Here's a much better, technical explanation worth reading. It covers very fundamental topics like volumetric efficiency and gas laws that are crucial to properly understanding these concepts.
http://www.gnttype.org/techarea/turbo/turboflow.html

Easy, a 13B rotary is the equivalent of a 2.6 liter piston engine, plus it probably rev's higher than AMS's EVO. And, the 'peak' effeciency island of a 'little' 94mm compressor wheel GT42 is from 12.5psi to just over 30psi, while the peak effeciency island of a 'big' 104 mm compressor wheel GT42 is from 7psi to 23.5psi. Just because a turbo is bigger doesn't mean it is more efficient at higher boost levels, but it can flow more air that a larger/higher revving/more efficient engine might need. Interestingly, the efficiency island on the larger GT42 stretches much farther and allows the big bad 104mm to top 50psi and still be moderately effecient, while the little guy is stuck around 45psi.

Boost numbers mean NOTHING between different setups on the same engine, let alone between different engine layouts/sizes. Just because a turbo CAN flow X air @ X psi doesn't mean it will on your motor. Also, the compressor efficiency maps have nothing to do with how hard the wheel has to be spun to compress the air, they only have to do with how much heat is placed into the air by the act of compressing it.

If you had a hypothetically perfect intercooler(such that the charge air temp was cooled to ambient after being compressed), and a T-25 and a 50-trim with the same exact exhaust housing and wheel, you'd see almost exactly the same horsepower PSI for PSI until the T-25 chokes out on the compressor side, somewhere around 14psi in the upper RPM ranges.

Transitioning into real life, some of the gains you see at the same boost level when upgrading turbos come from gaining effeciency in the compressor side, while most of the gains typically come a more efficient exhaust housing that sacrifices spool for power. That's why the small 16G and big 16G make very similar power at the same boost levels on nearly stock cars, as they have the exact same turbine wheel and housing.

As an example, in the Buick world there is a range of 3 turbos that all use the same compressor housing and wheel, the TE44, PT51, and PT53. The TE44 using a T350 70-trim exhaust wheel is rated at around 580hp at the crank, while the TE53 uses a slightly larger T04B 69-trim exhaust wheel and is rated at over 610hp at the crank.

There's also a TE60 turbo, which shares the same exhaust wheel as the TE44, but has a larger compressor wheel. Most vendors rate them at exactly the same power level.

Great info guys........thanks