Ive read a few articles and lots of posts that say that two different turbos at 10psi does not always equal the same airflow. This doesnt make any sense to me, given the engine airflow is fixed (no porting differences). yet there are dyno's, showing 13Bs making 300whp at 10psi, which is pretty much impossible on teh stock turbo

How can different compressor wheels cause different airflow at the same PSI, given they are both run on the same engine? I would think that airflow, keeping boost pressure fixed, is dependent on how much air your engine can flow, not on how much air your turbo can push. Boost pressure is just a measure of how resistant your engine is to airflow... I can see how 10psi is a lot more airflow on a bridgeported engine than on stock ports, because the BP engine can move a lot more air.

the only difference between the airflow of 2 different turbos at the same boost pressure on the same engine is temperature. a turbo run out of its efficiency range will heat teh air, lowering the airflow per psi. This is the only difference i can understand. am i wrong??

 

Keep reading...

The restriction is not the engine.
The restriction is the turbo.


-Ted

 

And yes, efficiency plays a huge roll too. The air temps are significantly different from tiny turbo's out of the efficiency range straining and heating the crap out of the air verses a larger frame turbo pumping out reasonably cool/dence air while in the 70~78% efficiency range.

~Mike............

 

Its all about volume of 10psi, a bigger turbo is gonna put out more 10psi versus a small turbo.

 

For a specific compressor pressure ratio, which is the ratio of pressures between the compressor outlet vs the inlet ((10psi+14.7psi)/14.7psi) at 10psi boost, each turbo has a different mass flow rate of air.

Take a look at two different compressor maps and it will become obvious. Imagine a ridiculously large turbo flowing 10psi of boost vs a really tiny turbo flowing 10psi of boost and it is more intuitive.

That means that just because you say a turbo is operating at 10psi, it doesn't tell you anything about how much air the turbo is providing for the engine to burn. It gives you a rough idea, but it is different for different sized turbos.

In addition, the turbos are designed to operate in a specific pressure ratio and flow range for optimal efficiency. Smaller turbos are more efficient at smaller flow rates, and larger turbos are more efficient at higher flow rates.

What efficiency in a turbo does for you is that it allows you to pressurize the ambient air with minimal energy used to heat the ambient air, and allows you to utilize the exhaust energy coming in, leading to more power and cooler intake temps.

Kevin

 

I can see how a bigger turbo can flow more air at a set psi than a smaller. But picture this example:

blow into a straw until you have 10psi pressure built up in your mouth. now, what happens when you blow harder?? does the airflow increase? yes, but can you increase the airflow without increasing pressure in your mouth? I would think not. The only way to get more airflow through the straw at a given psi, is to increase the straw size, not blowing harder. meaning, seems to me airflow is directly related to engine flow (straw size) at a fixed boost pressure, not turbo flow (blowing power)

 

Another factor (and perhaps the most important) when comparing different turbos on the same engine is the exhaust backpressure created by the different turbos.

This will make a huge difference on flow of the engine as a whole as we have a generous amount of overlap even in stock form. The more overlap through porting the more difference backpressure will make.

If you have 10psi boost and 20psi of exhaust backpressure on a stock set of twins and 10psi boost and 5psi backpressure on a large single turbo on the exact same engine the later will flow much more and make much more power.

The above example is extreme, but illustrates the point.

BTW, when you do get boost pressure higher than exhaust backpressure you have complete scavenging and the power will really go up.

 

Your missing the point about density from compression.

-S-

 

Density from compression is taken into consideration when considering mass flow rates.

Kevin

 

I was referring to the straw example, you can't use it to explain what you just stated. The thing that was touched on is the simple fact that regardless of what the engine can move, it has to go through the turbo first. So the straw shouldn't be thought of as the engine.

-S-

 

ah that makes sense, i didnt even think about that

well in my analogy, a smaller turbo would be the having weak lungs and not being able to move as much air. not sure what youre trying to say, the airflow isnt restricted by the turbo unless youre in vacuum. if a turbo is restrictive internally, it will just have to spin faster to create the same boost pressure, not changing the amount of airflow

what im getting at, is that the only difference between turbos running the same boost pressure is intake temps and exhaust backpressure

 

It is not a matter of weak lungs, carbonR1 put it about as good as it will get. Also the turbo is a restriction. Think one with a 50mm inducer is not more restrictive than one with 80mm? It can only pull in so much air. Same goes on the backside. You answered your own question almost. Having to spin the compressor faster to cram more air in to get the same boost pressure will generate heat. More heat equals less density which is exactly what carbonR1 hinted at with effiency. Running a larger turbo will require having to compress the air less to get the same amount of air moving through.

-S-

 

No, you're still assuming the engine is the restriction - that's the wrong assumption.
The turbo is the straw.


-Ted

 

Your lungs would just be a blower and not a compressor. A turbo physically compresses the air as it passes through it. It isn't just merely moving it from one place to another like your lungs would. A roots style supercharger has no internal compression much like your lungs. Any compression that takes place is within the manifold or engine itself. If we take 2 different sized roots superchargers and installed them on an engine, if we had 10 psi with both, we'd also have the same flow with both. We'd just have different efficiency levels and the larger blower would be turning slower to achieve the same flow. There is no internal compression.

In a turbo, twin screw supercharger, or a centrifugal supercharger, there is internal compression. It will compress the air even if there is no engine behind it. Of course the pressure will equalize again with the outside world right after if there is nothing there such as an engine but it still compresses it. If you free spin a roots blower with no engine around, there was no pressure change anywhere internally or otherwise.

Imagine having a closest full of paper. It's full to the top. Your goal is to fill it up. The closet is the engine and the paper is the air. You are the forced induction device. the paper doesn't all want to fit and is holding the door open. You are trying your hardest to close the door. You are pushing it and are forcing it to compress on itself until you get the door closed all the way. That takes alot of effort and isn't a very inefficient way of doing things. This is how a roots blower or even your lungs would do it. You didn't compress the paper first. You are beating it into submission through brut force. Now imagine taking these pieces of paper and wadding them up before you throw them into the closet. When it comes time to close the door, you can easily do it. It was more efficient. What if 2 different people do the same thing with 2 different closets? They have the same amount of paper and the same sized closets to work with. One of them wads the paper up coarsely and throws it in the closet. His neighbor wads the paper up tightly which means that there is still the same amount of paper but now in a smaller area. When it comes time to close the door, the more finely wadded up paper takes up less room. Even though one was compressed more, they still both ended up moving the same amount. This crude example is much like how turbo sizing works and how you can have different pressures and the same flow rates. There are always other little things in there as well but for the sake of discussion we'll just leave it at that.

Now that you can see that there can be a pressure change with no engine being present, you can also see how boost is not necessarily related to total flow and can happen at different amounts. Efficiency, exhaust backpressure, etc all play their roles in power as well but from a compressor only standpoint you can have 1 total flow rate but 2 different pressures through 2 different turbos.

 

I spent 14yrs working with paper and you just confused me




This sounds profound

-S-

 

PV = nRT

P (pressure) = 10 psi = constant
V (volume) = manifold = constant

n = amount of air (dependent on temp)
R = constant
T = temperature

Only thing that changes is temperature which causes the change in the amount of air. More temp, less air, that's the only way it could stay equal to PV which is a constant. So the more efficient the more air it gets in.

However, the exhaust said allows the engine to breath easier which changes what 10 psi is. 10 psi is a difference from turbo to engine. An engine taking in 15psi + 10psi = 25psi, which is more than a more restrictive engine that only takes in 13psi + 10psi = 23psi. Same as if you had a higher compression ratio, the 10psi you put into the engine will make more power because the engine itself becomes more efficient.

 

Please, stop using the Ideal Gas Law in these types of cases, cause it's only for static environments.


-Ted

 

I'd say the ideal gas law does a good job of explaining basic relationships. He wasn't looking to estimate the exact power difference so why throw in way more variables than you need? He just wanted to know what caused the bulk of the difference. Plus if all that changes is the turbo it's pretty close to a static envirnment. He was looking at peak horsepower so it's pretty much at a set point. You could take it down to figuring out what the difference in the amount of surface friction if you wanted to, but it's not going to make a big enough difference to matter and be worth mentioning unless you're trying to generate some real numbers. Everybody learns different and the law is just one more way to represent info without getting over complicated.

 

The ideal gas law is the foundation of speed-density efi systems used in millions of vehicles, so it couldn't be too far off!

As for the original question, consider an internal combustion engine as an air pump. To get it to consume more air, we pressurize the intake. 10 psi is 10 psi, no matter what device is being used to create that level of boost. If all other operating parameters were the same, power output would also be the same regardless of the badge on the turbo. However, there are obvious differences in output at the same boost pressure with different turbos, so how can these be accounted for? Assuming basic engine configuration is unchanged, there are two primary reasons which were mentioned or alluded to in earlier posts:

1.) Higher manifold air temperatures with stock turbo reduces air density, thereby reducing the mass flow of air. This is the ideal gas law in effect here.

2.) Higher exhaust restriction with stock turbo effectively cancels some of that boost pressure. A more efficient hot side will reduce that restriction, providing a greater pressure difference across the engine (intake pressure minus exhaust pressure) which will increase engine output. It's actually that pressure difference that is of greatest interest, but is not often measured at the auto enthusiast level. In turbocharged aircraft applications it's possible to see power output actually increase with altitude so long as a constant manifold pressure can be maintained and the intercooler(s) can keep the air temps in check. This power increase is due to the reduced exhaust back-pressure effectively pulling more air through the engine. This potential power increase is not often realized though, as lack of intercooler capacity often results in rising manifold air temps with altitude.

 

Regardless of the badging on the turbo? Do you mean size wise or same efficiency? I'm confused! How can the power output be the same at 10psi, if one turbo is more efficient then the other one? Sure you have your 10psi reading on both but if one is pressuring up a cooler charge at 10psi, that's the one that will make more power.

 

A lower efficiency compressor will result in higher air temps, which is item #1 in my post.

 

Read the rest of his post. Seems like you read the first paragraph and turned your brain off from then onwards. Turbo effiency plays major roll which, if you been reading I and many stated earlier means cooler air charge which equals greater charge density. More over as stated several times now... turbine effiency. A turbo in its effiency range requires LESS energey to turn over/spin at that flow/pressure verses one that is out of effiency. There by less back pressure. Larger frame turbos by nature have lower back pressure from the get go also most aftermarket turbos (not all) use newer more effiencient wheels to go along with better designed manifolds (not always the case) and who the hell upgrades a turbo without an upgraded exhaust which plays HUGE roll in spool and peak power. Its several factors that make a tiny out of range turbo thrashing and heating air at 15 psi verses a large huffer at 78% effiecency with a large effiencient turbine and littler restricted exhaust make gobs more power then the stocker. If you notice and look at a lot of dyno results, the middle frame turbos make the same power as the larger ones at moderate boost levels (1 bar or 14.7 psi). Example would be 60-1,62-1, t61, gt35R, GT40r, T78, T66, T04R all make roughly 400 RWHP at 1 bar of boost. They all are roughly at 74~78% effieciency range (on a mild ported 13b). Its boost above 1 bar were the 60-1, 62-1 fall flat and the other larger turbo's begin to wake up and make some power. The smaller turbo's spool faster. This is why I don't understand those throwing BIG *** turbos on there rides when they only run pump gas and limit boost to 15 psi or so. There making boost later then smaller turbos and making same peak power. In otherswards, they'd loose the race (less power under the curve). I guess a lot of guys use big as turbos to make up for other short commings?? Maybe they like the fact that if they throw some race gas in the tank and turn the wick up they know they'll have the huffer to back it (though this cenario rarely happens, and when it does its for a dyno plot to brag over)??

Sorry for the rant, its late and I have had more then 1 cocktail

~Mike...........

~Mike...........

 

renns answered the question:

i am a physics major and based off of thermo and mechanical theory i've learned the guys (i wont say who) saying air velocity at the intake port of the motor is higher at a certain psi than a small turbo making the same psi is not true.
the air velocity at the intake port depends on the engine. assuming intake temp and pressure are the same (and something else not the motor is driving the turbo) it doesnt matter what compressor is being used.
as long as the psi and temperature at the intake port are the same (and the compressor exducer is not right next to the port) the air velocity at the intake port depends on the motor and port size.

but renns answered my question hands down and i suspect this is the real reason for hp differences (assuming the intake plenum psi and temperature is the same on larger and smaller turbo environments):

in a rotary engine (and piston engine) not all exhaust gases leave the chamber between the exhaust port and intake port (or between exhaust valve lift and intake valve lift in piston)

so, if there is higher exhaust back pressure then there will remain more exhaust gases in this transition point and therefore more exhaust gas remains in the chamber when comes time to intake air from the intake port.

sooo, with higher exhaust back pressure the motor cannot intake as much air and the port air velocity is lower compared to a motor with lower exhaust back pressure.

sooooo, IF a larger compressor is more efficient (less energy needed) at pumping air and a larger turbine is more efficient at converting exhaust gases to rotating energy at high flow (high rpm) AND the motor has less exhaust back pressure at high rpm because of a more efficient turbine THEN air velocity at the intake port is higher at a certain psi and rpm (more powerful) than the same motor on a small turbo at the same psi and engine rpm!

a motor with a big turbo makes more power at 7k rpm and 10psi than a motor with a small turbo at 7k n 10psi!
BUT a motor with a small turbo will make positive psi with less air flow and therefore will have a more usable powerband at lower engine rpm!

all things considered, a small turbo motor will be more gas efficient at real life everyday driving but a large turbo motor will be more efficient at higher power and engine rpms.

 

First off, great thread, very informative.

Simply Stated - More in More out = more flow.

So, Reducing exhaust backpressure will help the turbo spin more freely, and less exhaust back pressure in/at the turbo = higher efficiency range?

On a stock turbo, I've read you can clip the exhaust turbine wheel to reduce backpressure and increase the efficiency. Would that be correct?

Blue TII quote: "BTW, when you do get boost pressure higher than exhaust backpressure you have complete scavenging and the power will really go up."

Please, do tell more...

Renns and BEX your posts are easy to comprehend. Thanks

 

Clipping older style turbines can help, clipping modern turbines such as GT wheels is pointless they come "clipped" essentially in their profile to start.

~S~