Why larger turbos make more power
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Staging Lane
Joined: Sep 2004
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Why larger turbos make more power Everyone here knows that 15psi is 15psi, so the question is-- why does a bigger turbo pressurizing the intake manifold to 15psi make more power than a smaller one? If the volume is the same, and the pressure is the same, then theoretically, the engine should make the same amount of power.
I understand that larger turbos don't heat up the air as much, but is 76% efficiency on a larger turbo more efficient than 76% on a smaller one? I wouldn't think so, but I could be wrong.
Also, I know that there is less of a restriction in the exhaust with a larger turbocharger, which I'm guessing is where most of the advantages of a larger turbo come from. Even so, why would two turbos with indentical turbine housings and turbine wheels make different power -- or would they make different power if the compressor wheels were the same trim, and the housings were different sizes?
The only thing that I could think of is that the larger turbo replaces the air taken in by each cylinder faster. I imagine that as soon as the intake valve opens, the runner doesn't have a full 15psi anymore. Maybe with a larger turbo, it is producing more air, so the runners can keep a higher pressure, even when the engine is consuming the charge air.
Anyone with any in depth knowledge of turbochargers please chime in! I really want to know. Thanks!
--Bryson
I understand that larger turbos don't heat up the air as much, but is 76% efficiency on a larger turbo more efficient than 76% on a smaller one? I wouldn't think so, but I could be wrong.
Also, I know that there is less of a restriction in the exhaust with a larger turbocharger, which I'm guessing is where most of the advantages of a larger turbo come from. Even so, why would two turbos with indentical turbine housings and turbine wheels make different power -- or would they make different power if the compressor wheels were the same trim, and the housings were different sizes?
The only thing that I could think of is that the larger turbo replaces the air taken in by each cylinder faster. I imagine that as soon as the intake valve opens, the runner doesn't have a full 15psi anymore. Maybe with a larger turbo, it is producing more air, so the runners can keep a higher pressure, even when the engine is consuming the charge air.
Anyone with any in depth knowledge of turbochargers please chime in! I really want to know. Thanks!
--Bryson
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Staging Lane
Joined: Sep 2004
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Well yes, but flowing more theoretically shouldn't make a difference if the intake is pressurized to the same amount. The only thing I can think of, like I said before, is that a larger turbo can "refill" the air consumed by the engine quicker.
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From: Wichita Falls, TX
ok 15psi is 15psi but at what rpm. The larger the turbo will create the 15psi at a high rpm level. More air is required at a higher rpm to create the same pressure. Also psi is just a number that is it. The larger turbo can grab more air and push more air...
just my thoughts
Ricky
just my thoughts
Ricky
FormerVendor
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From: WWW.SPEEDINC.COM
Look at a compressor map. Peak compressor efficiency is similar for most turbos, but at different airflow levels.
A GT61 compressor wheel is at 76% efficiency at 45lbs/min (approx 450hp), at a 2.2 p/r. Maxed out it's a bit over 60lbs/min, but at 60% efficiency.
A GT76 compressor wheel is at 78% efficiency at 60lbs/min at a 2.2 p/r. Maxed out it's about 90lbs/min at 60% efficiency.
And yes, a larger turbo would also usually have larger turbine wheels and exhaust housings which would reduce backpressure.
A GT61 compressor wheel is at 76% efficiency at 45lbs/min (approx 450hp), at a 2.2 p/r. Maxed out it's a bit over 60lbs/min, but at 60% efficiency.
A GT76 compressor wheel is at 78% efficiency at 60lbs/min at a 2.2 p/r. Maxed out it's about 90lbs/min at 60% efficiency.
And yes, a larger turbo would also usually have larger turbine wheels and exhaust housings which would reduce backpressure.
Teching In
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From: Spartan Country!
Originally Posted by bryson
Well yes, but flowing more theoretically shouldn't make a difference if the intake is pressurized to the same amount.
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its all about flow. PSI is resistance to flow, so a smaller, less efficent turbo will flow less air mass at the same psi as a larger efficent turbo. psi is not a measurement of air mass, its a measurment of restriction to flow.
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Staging Lane
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First, let's take the turbine side out of the picture, because that seems to be the most obvious. Also, I understand about the turbos themselves flowing more air and stuff, but think about just the intake manifold. What is different about an intake manifold pressurized to 15psi from a large turbo and 15 psi from a smaller one? The point about the efficiency at different airflow rates is a good one. Does a smaller turbo just not have the capability to flow enough air to keep the intake manifold pressurized when a larger engine is consuming a lot of air? This is a good discussion -- thanks guys!
--Bryson
--Bryson
On The Tree
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From: Tempe, AZ
Efficientcy does come mostly from the compressor wheel itself but some of the new GT port shrouded compressor housings will make some free power.
Another thing to keep in the back of your mind is that turbos have some rotational and air friction losses involved that actually add up to a lot. A single 120lb/min turbo will make more power and spool up the same if not quicker than a pair of 60lb/min turbos.
Back in the day just for fun some friends built a tri turbo supra, the turbos add up to about 100lb/min and the car has a lot of lag, but it's never made over 700whp. Where pair of turbos on a Supra that add up to 100lb/min will make 850+whp. And a single 100lb/min turbo on a Supra will make 1000+whp. All three will lag about the same but the big turbo gets it done.
Another thing to keep in the back of your mind is that turbos have some rotational and air friction losses involved that actually add up to a lot. A single 120lb/min turbo will make more power and spool up the same if not quicker than a pair of 60lb/min turbos.
Back in the day just for fun some friends built a tri turbo supra, the turbos add up to about 100lb/min and the car has a lot of lag, but it's never made over 700whp. Where pair of turbos on a Supra that add up to 100lb/min will make 850+whp. And a single 100lb/min turbo on a Supra will make 1000+whp. All three will lag about the same but the big turbo gets it done.
Teching In
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From: Phoenix
in a closed system 15psi is 15psi....correct
but..
an engine is not a closed system....it is free flowing with air going in and out
the faster you can get the air in, and the faster you can get it out...will make more power. exactly the same way we put K&N filters, intake manifolds, headers, high flow cats, cat back exhausts....to get more air in..and allow it to get out faster to make more power.
A larger turbo will have a bigger compressor wheel, but also a larger turbine wheel on the exhaust side...to help get rid of the exhaust pressure "faster".
And on the question about efficiency...well you almost answered your own question. Efficiency is given as a percentage %. that is exactly what it is...an arbitrary number that means nothing without some other number to go with it. 76% of abiatic efficiency means at said boost level, said airflow will be heated by this percent of a set constant.
I don't want to get technical, it just means that in laymens terms, 76% of 500 is less than 76% of 1000, we all know this, and unless we are all engineers, all we really need to do is know in a basic function, the more efficient the better
but..
an engine is not a closed system....it is free flowing with air going in and out
the faster you can get the air in, and the faster you can get it out...will make more power. exactly the same way we put K&N filters, intake manifolds, headers, high flow cats, cat back exhausts....to get more air in..and allow it to get out faster to make more power.
A larger turbo will have a bigger compressor wheel, but also a larger turbine wheel on the exhaust side...to help get rid of the exhaust pressure "faster".
And on the question about efficiency...well you almost answered your own question. Efficiency is given as a percentage %. that is exactly what it is...an arbitrary number that means nothing without some other number to go with it. 76% of abiatic efficiency means at said boost level, said airflow will be heated by this percent of a set constant.
I don't want to get technical, it just means that in laymens terms, 76% of 500 is less than 76% of 1000, we all know this, and unless we are all engineers, all we really need to do is know in a basic function, the more efficient the better
Originally Posted by Ken in AZ
I don't want to get technical, it just means that in laymens terms, 76% of 500 is less than 76% of 1000, we all know this, and unless we are all engineers, all we really need to do is know in a basic function, the more efficient the better