Turbo boost vs Supercharger boost
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Joined: Apr 2004
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From: Miami, FL
Originally posted by Silver-Bolt
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Tim are you saying that the size/output volume has nothing to do with the end result?
Interesting that two different sized turbos run at exactly the boost produce very different number on the dyno.
Clarify please.
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Tim are you saying that the size/output volume has nothing to do with the end result?
Interesting that two different sized turbos run at exactly the boost produce very different number on the dyno.
Clarify please.
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Joined: Dec 2000
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From: Portland, Oregon. USA
Originally posted by Tim Skelton
Yes. I am saying that size/output volume has no direct effect on power. But other factors -- like where that blower is on its efficiency curve -- certainly can.
Eaton at 20 PISG. KB at 20 PSIG. No contest. The Eaton is being run well outside of its efficient range and is just making heat.
Now do the same comparison with a (theoretical) larger Eaton, being spun half as fast. The contest is now much closer because it is now operating closer to its efficient range.
The only way that the two turbos at the same boost make different power is if one is being run closer to its efficient range that the other -- or is simply more efficient to begin with.
Your rhetorical question was "What's better, 50ci volume at 20psi or 100ci volume at 10psi?" The answer is: no way to tell. But a tiny turbo or centrifugal can whip the pants off of a much larger roots blower. They move small amounts of air per rev, but crank out a lot of revs. At 15,000 PRMs, an Eaton is being overspun. But a turbo is barely moving.
Yes. I am saying that size/output volume has no direct effect on power. But other factors -- like where that blower is on its efficiency curve -- certainly can.
Eaton at 20 PISG. KB at 20 PSIG. No contest. The Eaton is being run well outside of its efficient range and is just making heat.
Now do the same comparison with a (theoretical) larger Eaton, being spun half as fast. The contest is now much closer because it is now operating closer to its efficient range.
The only way that the two turbos at the same boost make different power is if one is being run closer to its efficient range that the other -- or is simply more efficient to begin with.
Your rhetorical question was "What's better, 50ci volume at 20psi or 100ci volume at 10psi?" The answer is: no way to tell. But a tiny turbo or centrifugal can whip the pants off of a much larger roots blower. They move small amounts of air per rev, but crank out a lot of revs. At 15,000 PRMs, an Eaton is being overspun. But a turbo is barely moving.
I am not buying your theory. If you have two turbos side buy side. One big one small. If one revolution of the big turbo moves 10 cfm and the small turbo one revelution moves 5 cfm how is volume not a factor? Granted the small turbo will spool faster but both turbos operating within their effiency window at the same boost pressure to big turbo is moving more air.
With turbos I can change the size of either the hot side or cold side on the same turbo and affect the horsepower.
Go too big and lag becomes a factor go to small and the turbo can't keep up with the demand. Volume.
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From: The People's Republic of Los Angeles
Originally posted by Silver-Bolt
Tim,
I am not buying your theory. If you have two turbos side buy side. One big one small. If one revolution of the big turbo moves 10 cfm and the small turbo one revelution moves 5 cfm how is volume not a factor? Granted the small turbo will spool faster but both turbos operating within their effiency window at the same boost pressure to big turbo is moving more air. . . .
Tim,
I am not buying your theory. If you have two turbos side buy side. One big one small. If one revolution of the big turbo moves 10 cfm and the small turbo one revelution moves 5 cfm how is volume not a factor? Granted the small turbo will spool faster but both turbos operating within their effiency window at the same boost pressure to big turbo is moving more air. . . .
In your new example, is the pressure ratio (i.e., boost) constant for both?
Are both operating at the same efficiency (i.e., heating the air and taking the same drive HP)?
If the answer to those two questions is yes, then the engine does not know the difference between them.
In this example, the bigger turbo can't be moving more air. The two are moving exactly the same amount of air. If the bigger one were moving more air, it's boost would be higher (i.e., more air would be backing up in the intake manifold), as the engine's airflow is a constant. But the boost can't be higher, because we have assumed equal boost.
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I don't chime in often, but I thought I'd take a stab at explaining boost, hp, turbos v superchargers. Diesels vs Spark Ignition engines is a totally different subject.
First thing you have to understand is that boost is bad and HP is good. Boost is the byproduct of forcing more air into an engine then it wants to injest naturally. The pressure ratio, is infact the volume of air being pushed into the engine vs the volume of air the engine would take in if it was naturally asperated. Boost is also the byproduct of a less dense (hot) flow of air. Think of a hot air ballon, there are no more air molecules in the balloon when its up in the air, then when its on the ground, but the balloon has more pressure when its flying. So you can run into situations where the same engine could be running more boost, but produce less power, because the charge is less dense.
Second thing is that energy is not created or destroyed. Compressing air requires energy. Heating air requires energy. You don't mind spending some hp to compress air, but you want to keep the hp you spend heating it to a minimum. So an ineffiecent compressor bites you twice. Once creating the overly heated boost and another time because the charge is less dense.
Third, some forced induction devices create boost, others dont. Roots SCs simply move air, it is compressed by the pressure ratio in the manifold. While this works, its very inefficent (hotter charge, steals more power to create the boost). Turbos, centrifugal SCs, twin screw SCs create a boost internally, before releasing that air into the manifold. However the manifold pressure is usually different then this internal pressure, this causes the new air entering the system to change its pressure. When the new air entering the system is at a lower pressure then the manifold, the air is further compressed just like a roots sc (but to a lesser degree, because it already has a headstart) in the manifold. When the new air is at a higher pressure, it releases its pressure almost perfectly (ie the temp goes down like a perfect gas). Unfortunetly that air wasn't compressed perfectly effiecently, so it carries extra heat with it. This is why when you look at a turbo effiency map there is a sweet spot. To the left of the sweet spot, the charge was overboosted. To the right, the charge is being further compressed like a roots blower (ineffiecently). Turbo flow map for a T3-40:
Now we can finally talk about turbo boost v sc boost. It all depends on a) when you want to pay the bill and b) how well the compressor is matched to the situation.
Air doesn't compress itself, so it costs you power to compress the air. A turbo in theory uses wasted energy to compress the air, so it should be free. However, a turbo is going to cause exhaust back pressure, sometimes approaching the pressure of the boost its creating. Remember how we talked about the pressure ratio of the manifold being the air pushed in vs the air the engine wants to breath? How well would a NA engine breath with 15psi of exhaust backpressure? Probably not too well, but in the turbo case you're not stealing created hp to compress air. Instead you're creating less hp because your engine doesn't flow air as well. In the SC case, your engine should flow air like it would NA, but HP is being taken from the crank to compress the incomming air.
All of this is for not if you don't match the compressor to the system. If you pick a turbo or a SC that isn't compressing air internally to closely match the manifold pressure of the engine, the whole deal is less effienent (and that bites you 2x).
Ahhh... that all said, in real world the turbo usually wins because you can more closely match the compressor to the system and because of the wastegate, you can stay in the sweet spot of the compressor's effeicency for longer then you can with a SC.
I hope this rambling helped some. This was the crash course version, people have written 100s of pages in books to fully explain whats going on.
First thing you have to understand is that boost is bad and HP is good. Boost is the byproduct of forcing more air into an engine then it wants to injest naturally. The pressure ratio, is infact the volume of air being pushed into the engine vs the volume of air the engine would take in if it was naturally asperated. Boost is also the byproduct of a less dense (hot) flow of air. Think of a hot air ballon, there are no more air molecules in the balloon when its up in the air, then when its on the ground, but the balloon has more pressure when its flying. So you can run into situations where the same engine could be running more boost, but produce less power, because the charge is less dense.
Second thing is that energy is not created or destroyed. Compressing air requires energy. Heating air requires energy. You don't mind spending some hp to compress air, but you want to keep the hp you spend heating it to a minimum. So an ineffiecent compressor bites you twice. Once creating the overly heated boost and another time because the charge is less dense.
Third, some forced induction devices create boost, others dont. Roots SCs simply move air, it is compressed by the pressure ratio in the manifold. While this works, its very inefficent (hotter charge, steals more power to create the boost). Turbos, centrifugal SCs, twin screw SCs create a boost internally, before releasing that air into the manifold. However the manifold pressure is usually different then this internal pressure, this causes the new air entering the system to change its pressure. When the new air entering the system is at a lower pressure then the manifold, the air is further compressed just like a roots sc (but to a lesser degree, because it already has a headstart) in the manifold. When the new air is at a higher pressure, it releases its pressure almost perfectly (ie the temp goes down like a perfect gas). Unfortunetly that air wasn't compressed perfectly effiecently, so it carries extra heat with it. This is why when you look at a turbo effiency map there is a sweet spot. To the left of the sweet spot, the charge was overboosted. To the right, the charge is being further compressed like a roots blower (ineffiecently). Turbo flow map for a T3-40:
Now we can finally talk about turbo boost v sc boost. It all depends on a) when you want to pay the bill and b) how well the compressor is matched to the situation.
Air doesn't compress itself, so it costs you power to compress the air. A turbo in theory uses wasted energy to compress the air, so it should be free. However, a turbo is going to cause exhaust back pressure, sometimes approaching the pressure of the boost its creating. Remember how we talked about the pressure ratio of the manifold being the air pushed in vs the air the engine wants to breath? How well would a NA engine breath with 15psi of exhaust backpressure? Probably not too well, but in the turbo case you're not stealing created hp to compress air. Instead you're creating less hp because your engine doesn't flow air as well. In the SC case, your engine should flow air like it would NA, but HP is being taken from the crank to compress the incomming air.
All of this is for not if you don't match the compressor to the system. If you pick a turbo or a SC that isn't compressing air internally to closely match the manifold pressure of the engine, the whole deal is less effienent (and that bites you 2x).
Ahhh... that all said, in real world the turbo usually wins because you can more closely match the compressor to the system and because of the wastegate, you can stay in the sweet spot of the compressor's effeicency for longer then you can with a SC.
I hope this rambling helped some. This was the crash course version, people have written 100s of pages in books to fully explain whats going on.
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Joined: Apr 2005
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From: Jonesboro AR
Boost
Boost as related t what you see on your gauge is from the air that isn't going anywhere. If your heads suck and your exhaust is too small then boost will build quicker and hp will be less if engine size is equal. If you run the supercharge at the same speed but add ported heads and better exhaust or cam etc. the boost will be lower but you will make more horsepower. If you want to get the same boost now you have to spin faster because the air is being passed through the motor more efficeintly and and back pressure is less. Which is better? The turbo cars always seem to be quicker and faster?