Here's a hypothetical:

Identical KA24DE engines, identical setups (downpipes, injectors, fuel, ECU, intercooler/piping), but different turbos. One has a T3 super 60 boosting 10 psi, the other has a t3/t04e 50 trim boosting 10 psi, at the exact same RPM, same gear, measured from the same place (intake manifold). Can anyone give me a concrete reason why 10 psi on a bigger, more efficient turbo makes more power than 10 psi on a smaller turbo? I hear that "10 psi is 10 psi is 10 psi," but it clearly isn't. Otherwise, Orion wouldn't be making 240whp @8 psi on a T3/T04E, almost maxing out his fuel injectors and MAFS, whereas I'm not maxing out anything at 10 psi from a T3 super 60.

I thought the bigger turbo would make more power for two reasons:

a) efficiency-a larger turbo than the one I have would be more efficient at a certain PSI for the KA24DE than another smaller turbo.

b) air flow-a larger turbo simply flows more air at each respective RPM than a smaller turbo.

I could be wrong on both. But even if a larger turbo does flow more, if it's at the same psi, HOW does it make significantly more power than the smaller one? The engine's head can only ingest and expel so much air....is it because the larger turbo that flows more air pushes in air faster at the same RPM, allowing for greater energy once the combustion process ocurrs?

This has bothered me for a while, and I can't stand how much more power people make with identical setups to mine except with larger turbo.

I was just reading the AEM EMS KA-T that made almost 380whp @ only 14 psi....with conservative timing and a/f ratios. I'd be lucky if mine were making 240 whp @ 10 psi. So that KA-T is making 140whp more with only 4 more psi....there has to be something to account for that, and I think a big thing is the turbo (equally big is tuning, of course--but again, his a/f's and timing were conservative).

Anyway, all you turbo experts can now disabuse my ignorance!

Quote »b) air flow-a larger turbo simply flows more air at each respective RPM than a smaller turbo.[/quote] Bingo.

you answered your own question as best as i could tell. better efficiency=less wasted energy making the 10psi of boost also larger turbo will make 10psi easier because its designed to flow more air

10psi is 10psi @ the manifold, so it doesn't matter what compressor you use. the difference maker is backpressure.

take a t3 turbine with a t3 compressor and spin it to 15psi. your going to choke the engine at higher rpms because the turbine is spinning too fast to allow the exhaust gases to escape.

take the same t3 turbine and switch to a t4 compressor and spin it to 15psi. your turbine doesn't have to spin as fast to create the same 15psi so you have less restriction.

less backpressure = more power.

-demetrius

The picture is actually much bigger than you think. And bigger turbos don't necessarily move more air at a given pressure. It depends on the efficiency at the boost and flow rate. But for the most part, a larger turbo tends to run more efficiently than a smaller one. Backpressure is a big aspect of helping make additional power. Not only to larger turbines allow more flow through the turbine, a more efficient compressor requires less shaft torque to compress the air, so less pressure is needed in the exhaust manifold. Larger compressor wheels generally do not need to spin as fast either so again, less pressure in the manifold is required. On the compressor side, a more efficient turbo (when operating in the higher part of it's efficiency) will heat the air less for the same given boost. This will provide denser air which does several things. When it goes through the intercooler, it ends with a lower temp at the manifold so you actually get more air in the combustion chamber, or you can run an intercooler that gives up some efficiency for flow resulting in a lower pressure drop. Having denser cooler air helps to be less prone to detonation so you can possibly run more timing advance. It also heats the intercooler less so it gives more headroom before heat soak can set in. This is typically only important when boosting frequently. Usually something along the lines of running on a road course.

10psi is not 10psi...

You need to take into accout how hot or cold the air could be.

A good intercooler design could change allot... There is more factors we need to account for.

Same with my Dyno pulls... Am I running rich? or was a lack of power due to the fact that the intercooler had no air running across the fins to draw the heat away... Thus killing my power.

Anyways.

But yeah less backpressure should help in the quest for power.

EDIT: Damn you... I took to long writing mine... talkin to my bro..

I like C-kwiks explanation the best so far, but there are quite a few variables, such as air temp, humidity, IC efficiency, hell everything down to spark plug gapping would have to be exactly the same.

But a bigger turbo will almost always make more peak power then a smaller one. However, the useable powerband is the issue that most people forget. I can use a T88 on my car and have boost from 6700RPM to 7400RPM and make a ton of peak torque and power. But its completely useless.

Brian

ok, not to be a thread jacker, but i have a question thatis kinda along the lines of this thread.

at equal psi levels a larger turbo produces morepower than a smaller one(understood that)now, lets just say for example sake.if the stock internals can only take about 10 psiwould it still be the same psi for both sized turbos?or would the motor not be able to handle the 10 psi fromthe larger turbo?

There are a lot of factors that go into that. Primarily, the factors that cause detonation. A less efficient compressor will have a hotter charge and will be more likely to detonate. A less efficient and /or smaller turbine will also have more backpressure which will cause higher exhaust valve temperatures and can leave a residual amount of hot exhaust gas still in the combustion chamber which can also promote detonation. While a more efficient turbo will have a cooler charge at the same psi, it willalso be denser. Combined with more fuel, it would have more overall combustion energy. Perhaps a higher peak pressure as well. But most of the additional energy will be spread ouot throughout the combustion stroke and so it probably won't contribute to detonation as much as the factors that affect a smaller turbo. Looking at it this way is a bit more complicated.

But if you were to compare the same motor with two differnt motors that have the same peak power, then the motor with the more efficient turbo would have less stress overall.

Red-KAT wrote:10psi is not 10psi...

You need to take into accout how hot or cold the air could be.

no you don't.

10psi is just 10psi regardless of the temperature. you may have a denser charge from the larger compressor, but your engine doesn't care. it's most basic goal is to move air. that's all. how well it is able to do so is the basis for making power, and that's why larger compressors will always produce more power. as C-Kwik already stated, larger compressors require less work from the turbine...which in turn becomes less of restriction to your exhaust flow which in turn allows more air to enter your engine.

-demetrius

Hm, well, gas laws are pretty simple. Its all doubling and halving. So, assuming volume is constant (since IC piping, IC, intake mani, etc all don't expand), if pressure in 2 setups is 10 PSI, the one that is hotter will be less dense. If its twice as hot, itll be half as dense. Denser air lets you cram more oxygen per liter into the engine, so i owuld think that this would be very important.

"Efficiency refers to the temperature of the air that the turbocharger compresses to create boost pressure. A natural aspect of compressing air is heat: as the turbo compressor pressurizes the air, heat is created from squeezing, or compressing, the air molecules into a smaller volume. Friction between the molecules is then released as heat."

http://www.hahnracecraft.com/h...y.htm

so basicly a bigger turbo spins slower to move the same amount of air. because it spins slower it doesnt generate as much heat & friction and it doesnt heat the air as much. There are other factors involved in this too.

in a closed container such as the intake manifold we can use the ideal gas law:[pressure] * [volume of container] = [some constant] * [temperature]

so when u increase the temperature of a gas, the pressure increases proportionally. A turbo system operates on pressure, so when the pressure is increased it will try to decrease it using the only method that it can: decreasing the amount of air moved in. SO if u increase the temperature of the intake charge then you will decrease the volume of air flowing in proportionally to the rise in temperature!

standard pressure is 273 K or 0 Ca change from 80 C to 70 C in intake temp is the same as a 343K to 353K which is a 3% change in volume. At 200 hp thats 6 hp for ya!

if one turbo is 10% more efficient than another im guessing that u can safely conclude it will make 10% more hp in the same scenario, am i right?

The difference in temperature is only part of the benefits of operating a compressor at a higher efficiency. One of the reasons the efficiency of the compressor is higher is because at that speed, boost and airflow, the compressor is more aerodynamic than it is when it operates at a lower efficiency point on the map. So not only is it not heating up as much air, it is easier for the turbine to drive the compressor. So there is a possibility that more than a 10% increase can be made with a 10% change in efficiency. The opposite is true as well. It depends on a lot of factors.

And jumping from 70% efficiency to 80% efficiency at the same flow, and boost will actually result in a 12.5% decrease in temperature. 12.5% of 70 is 10.

u misunderstand what i said, when i said 10% more efficient i mean 1.10 times as efficient, bad wording on my part. I think we all get the point anyways